key: cord-0772753-6j9pk1ij
authors: Ulpiani, Giulia
title: On the linkage between urban heat island and urban pollution island: Three-decade literature review towards a conceptual framework
date: 2020-08-18
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.141727
sha: de8232584261a4bb70657f068fd581f3ff50eacc
doc_id: 772753
cord_uid: 6j9pk1ij

Abstract With the doubling of urban population within the next two decades and the disproportionate growth of megacities, it is critical to explore the synergism between urban heat and pollution. In this paper, a systematic review is conducted on the existing knowledge, collected since 1990, on the link between urban heat island (UHI) and urban pollution island (UPI). Results from 16 countries and 11 Köppen-Geiger climatic zones are perused and compared to delineate methodological and experimental trends, geographical dependencies and research gaps. Detailed content analysis is conducted according to five prominent topics: i) the role of UHI on temperature-dependent chemistry, ii) the daytime/nighttime variability in the UHI-UPI interaction, iii) the role of urban geomorphic types, forms and growth schemes, iv) future trends and v) primary and secondary effects of UHI mitigation on urban air quality. Different approaches and observations are eventually harmonized to outline opportunities and challenges towards the disentanglement and/or the two-way mitigation of both phenomena. This will help governments and urban planners to deliver coping strategies and precautions towards a more salutogenic urban design.

interaction, iii) the role of urban geomorphic types, forms and growth schemes, iv) future trends and v) primary and secondary effects of UHI mitigation on urban air quality. Different approaches and observations are eventually harmonized to outline opportunities and challenges towards the disentanglement and/or the two-way mitigation of both phenomena. This will help governments and urban planners to deliver coping strategies and precautions towards a more salutogenic urban design. 

The method by Pickering and Byrne [48] was used to screen the existing knowledge on the link between UHI and UPI in search of patterns, methodological trends and climate dependencies, drawing on five main databases (ScienceDirect, Scopus, Google Scholar, PubMed and Web of Science). The search was systematically conducted by looking for keywords obtained by coupling the urban-related and air quality-related taxonomy reported on the left and right side of  specifically addressing the link between UHI and UPI. Urban temperature and urban forcing were considered proxies for UHI when the heat island was not explicitly quantified. Health-focused manuscripts, only mentioning the heat island effect, were excluded;

 contemplating experimental observations in real urban scenarios. Wind tunnel or lab investigations were excluded;

 in case of multiple publications by the same authors or research groups, only one/two studies conducted from substantially different perspectives were retained;

 for multi-city studies, the largest one was used for reference whenever easily identifiable.

According to the above criteria, a total of 40 manuscripts was selected for in-depth analysis 

The considered papers were published in 29 journals. In detail, 12 increased social concern (escalation of heat waves, weather extremes and mortality rates). The reported average UHII varies between slightly negative values and 6 °C, with an average of 1-J o u r n a l P r e -p r o o f 2°C, while the peak UHII touches 12 °C in [49] fluctuating on mean values of about 3-5°C. Nine papers refer to surface urban heat island (SUHI), while all the others deal with atmospheric urban heat island (AUHI). This is highlighted in Table A1 . Most papers detailed accurately the panorama of land use and land cover (LULC) in the considered geographic settings. Some of them discretized the domain in more than 15 LULC types [11, 33, 50] . Most papers focused on summer conditions (47.5 %) and among them more than 40 % limited the analysis to clear sky conditions. Another 40 % developed year-round assessments, while few instances covered other seasons, such as winter (2), autumn (2) , spring (1) and wet/dry season (1) . The reader is referred to Table A1 in Appendix for further details, including the list of observational data reported in each of the analyzed manuscripts. [51] . Other than sensitivity analyses on the inputs, several parametric analyses are conducted, mostly on scenarios of i) emission control and urbanization [38, [52] [53] [54] , ii) heat flux change (usually obtained by replacing urban land with cropland or by integrating anthropogenic sources or by changing the soil moisture level) [15, [55] [56] [57] [58] , iii) different circulation, vertical diffusion [39, 58] or wind intensity [59, 60] and iv) future UHI/UPI exacerbation and/or mitigation [54, [61] [62] [63] . Kingdom, yet to a lesser extent (2.5/2.6 %). From a climatological perspective and considering the updated Köppen-Geiger classification by Peel et al. [65] , the interlacement between urban heat and pollution appears to find fertile ground in temperate oceanic climates (Cfb) across

Europe, most likely due to the extensive urbanization, the frequency of heat waves and the complex interaction between land and sea breeze. On the other hand, humid climates with subtropical and Mediterranean nuances (Cfa and Csa classes) stimulated almost half of the revised investigations. These climates are typically characterized by extremely hot summers, anticyclonic conditions, low inversion and low wind speeds conducive to both heat and pollutants entrapment. Another 12.5 % of the observations is related to type A climates (Aw, Af and Am) in the low-latitude belt around the Earth where high solar irradiation is recorded Fig. 3 Geographic distribution, climatic classification and population data for each analyzed city:

Details A, B, C are close-ups on United States, Europe and East Asia, respectively.

In this section, each of the 40 publications selected for detailed analysis is accurately discussed according to the dominant topic ranging from daytime/nighttime variability, urban form and growth patterns, future drivers and countervailing effect of heat mitigation techniques.

In 1990, Cardelino and Chameides posed the basis and the method for the analysis of temperature-induced direct and indirect effects over ozone pollution, focusing on: i) enhancement of ozone photochemistry owing to temperature-enhanced thermal decomposition of peroxyalkyl nitrates [68] ; ii) enhancement of BVOCs emissions from trees in their growing season (highly reactive olefinic hydrocarbons) [69] ; iii) enhancement of evaporative anthropogenic hydrocarbon emissions from motor vehicles [70] and nitrogen oxide emissions from air conditioning [32] . Empirical evidence from 9 years of observation for the city of Atlanta confirmed a positive linear association between temperature and ozone. The temperature during ozone-event days (ozone exceeding the 120 ppvb NAAQS limit) was higher than on nonevent days by 3.1 °C, on average. By numerical modelling, the authors simulated 5 scenarios of emission control and urbanization effects. The base case was a 50 % VOCs reduction. When a 20% natural emissions reduction was additionally considered (scenario 2) the ozone production was strongly mitigated and always stayed below the limit. The counteracting effect of the average summertime UHI (2 °C) was investigated by progressively adding the 3 afore-mentioned temperature-induced mechanisms (scenario 3 to 5). Scenario 5 totally compensated the benefit achieved in scenario 1. This entails that a 2°C UHI can cancel out the ozone benefit of a -50% J o u r n a l P r e -p r o o f VOC policy. The simulations were later repeated by considering a different base case (NO x reduction in place of VOC reduction). Significantly lower ozone maxima were identified, suggesting that NO x abatement could be a more effective strategy. As remote sensing started being extensively used in climatology, further evidence was collected and other players in the UHI-UPI interaction were identified (e.g. water vapor). For instance, in [33] , the authors examined the interlink between heat and pollution islands in Shanghai by combining land skin temperature, albedo AOT, water vapor, cloud fraction and land cover satellite data (MODIS Germany [11] . Notably, LST and air temperature were used as proxies for surface and atmospheric UHI (SUHI and AUHI, respectively) whereas PM10 and AOT were proxies for near-surface and atmospheric pollution (NSUPI and AUPI, respectively). The urban-rural difference was investigated in terms of incoming solar radiation and atmospheric longwave radiation. Prominent findings can be summarized as follows: i) AUHI and NSUPI were negatively correlated as higher temperatures strengthened the turbulent dispersion and enhanced the boundary layer height in agreement with [61, 77] , ii) the response of urban surface materials to the AOD-induced change in net solar radiation was strong in the nighttime and weak in the daytime due to lower energy redistribution factor and longwave component dominance in the balance (as in [67] ), iii) the UPI-induced different net solar budget reflected in up to +12 % higher SUHI, iv) SUHI was higher in the daytime and lower in the nighttime, contrary to AUHI.

The study considered only clear days and low wind speeds (<3 m/s). Hence, more stable wintry conditions with weaker turbulent mixing were not considered and advection of heat and pollution was disregarded.

The source of pollution is also a key player in such dynamics. Jonsson et al. [75] As a rule of thumb, the UHI-UPI interaction shows pronounced differences in the daytime and nighttime [79] . Nocturnal ozone can be depleted by NO x titration (NO+O 3 NO 2 +O 2 ) and dry deposition in the nocturnal boundary layer but not in the nocturnal residual layer, which J o u r n a l P r e -p r o o f emphasizes the importance of vertical mixing on surface-residual layer coupling [80, 81] .

Conversely, in the daytime, the reaction tends to be balanced by NO 2 solar photolysis [82] , except in the immediate vicinity of very large emissions of NO (e.g. power plants). Indeed, one of the pivotal factors driving photochemical reactions is high energetic shortwave radiation [83] .

Dry and wet deposition phenomena are critical too, as they represent the main sinking processes through which the atmosphere cleans itself. The former occurs at the ground surface (e.g. soil or vegetation), and the latter at the contact with water droplets (e.g. rain or fog) [84] . To elucidate these time-specific mechanisms, in [15] , Sarrat et al. coupled a mesoscale meteorological and chemical model (Meso-NHC) with the Town Energy Balance (TEB) to verify how nocturnal and diurnal UHI and other urban effects impinged on primary and secondary regional pollutants. A 2-day anticyclonic episode with abundant solar radiation and low-speed wind of about 2 m/s was considered, ideal scenario to analyze the atmospheric pollution dynamics over a large urban area.

Two heat flux scenarios were considered: URBAN (with TEB, urban surface), NO-URB (without TEB, natural surface). In the nighttime, UHI and pollution events were primarily of thermal origin, associated with a convergence area south-west of Paris. The effect of the synoptic wind was to displace the UHI-induced plume of warm air above the surrounding suburban areas.

In the daytime, the urban processes were dominated by thermal and dynamical effects. The NO x spatial distribution was sensitively impacted by the modification of the atmospheric boundary layer induced by the UHI and the roughness of the urban fabric. Much milder was the effect on ozone concentration. In the urban areas, the dominant nocturnal budget term for NO was the kinetic energy (destructive term) that prevented its near-ground accumulation, whereas for O 3 the dominant mechanisms was deposition (destructive term) followed by chemical processes on par with turbulent mechanism (destructive and constructive terms, respectively). In agreement with J o u r n a l P r e -p r o o f several studies [85] [86] [87] , the authors reported how the day-after spatial distribution of the ozone fields was strongly influenced by the nocturnal pockets contained in the residual boundary layer.

Finally, they calculated the VOC/NO x ratio concluding that, in Paris, the regime of ozone production is NO x -limited in the urban plume and in the suburban areas and becomes VOCslimited in the rural areas, thus paving the way towards targeted pollution abatement strategies.

Overall, the study highlights the positive effect of urban turbulence on pollutants dispersal under low-speed winds and clear days. [81] . This was already pinpointed in 1994 by Neu et al. who showed that half of daytime pollution events in the Swiss plateau were associated with the downward mixing of ozone from the residual layer [85] proving that vertical mixing processes could outweigh chemical production and advection. Similarly, in [60] an attempt was made to delineate the influence of UHII and mixing height on local air quality by studying the diurnal variation of secondary pollution levels in the Indian city of Chennai, affected by heavy traffic emissions, poor dispersion, and regional transport of pollutants. [38] . The study was then expanded to link air quality, UHI and respiratory hospitalizations in 29 districts [45] . GIS was used to determine the spatial links. The heat stress and worsened air quality mostly affected youngsters' health. GIS had already been used by Dousset and Gourmelon to analyze the physical processes that determined the nearsurface heat fluxes and their impact on ozone concentration in Paris by combining statistics of thermal infrared images, near-infrared and visible SPOT-HRV images and in-situ data [4] . The results show the contrast between intense nocturnal UHI and many distinct microclimates during J o u r n a l P r e -p r o o f the day associated with local surface properties. Upward LST trends were paralleled by ozone concentration variations.

The role of mixing height and turbulence induced by the urban heat island on the up/downtrends of urban pollution is extensively covered by the studies of Huszar et al. [58, 91] . In a recent comprehensive publication, the authors designed a suite of simulations all over Europe (focus on the cities of Prague and Berlin) and all over the seasons by coupling the Regional Climate Model for Southern California, by means of WRF/Chem+UCM [95] . Analogous results are also reported for the Pearl River Delta Greater Bay Area in China as a result of the mutual interconnectedness among cities [96, 97] . Notably, in [97] , the authors investigated and 

In this subparagraph the link between UHI and UPI is discussed in relationship with the local landscape (continental, coastal, valley-like, mountainous).

Several studies aimed at investigating how geolocalization impinge on UHI/UPI. In 1996, Yoshikado and Tsuchida [38] presented strong evidence of exacerbated pollution episodes when wintertime small-scale sea breeze coupled with urban heat island circulation into a persisting convergence zone over Tokyo Bay. The authors examined air temperature, wind and pollution (PMs, NO 2 , O 3 ) data from a number of urban, suburban and rural sites. Heavily urbanized areas tended to be systematically more polluted, while the temperature difference between less urbanized sites tended to level out. A positive correlation was established between vertical stability and urban pollution. Warm southwesterly winds were associated with both higher UHI and poorer air quality, while northern winds (winter monsoon) exerted a cleansing effect near the surface. On highly polluted days the UHI intensity could exceed 5 °C. Ozone increased in response to vertical air exchange and divergence between seaward breeze and westward inland wind, while very similar patterns were identified between PMs and NO 2 with maxima generally recorded in the morning and in the evening. PMs and NO 2 peaked in response to traffic density and the establishment of a stable layer in evening hours. The afternoon decrease was jeopardized on sea breeze days. Two types of severe air pollution events were discerned and associated to the J o u r n a l P r e -p r o o f presence of sea breeze: a synchronous type that occurred in the frontal area over the urban center and a delayed type that occurred on the second or a later day due to the accumulation of pollutants in the air mass floating over the bay before the onset of the sea breeze. This is in agreement with [87] . The authors concluded that -the winter sea breeze possesses a characteristic of the heat island circulation‖ thus creating a convergence zone that persists for several hours.

The role of sea breeze is also the focus of a 2011 study by Chen et al. [56] . The authors adopted 70% of the long-term and short-term summer variation in peak ozone concentrations could be described by changes in meteorological conditions. The interaction between HIC and sea breeze was identified as the major driver for daily variations in ozone distribution (see Fig. 5 ). Tokyo's HIC strongly prevented the penetration of the sea breeze inland and favored severe ozone accumulation. Furthermore, when the sea breeze was stronger, as during the morning, the highozone air masses formed over the urban area moved inland, sweeping the northwestern regions.

This study complements the wintertime results by Yoshikado and Tsuchida, previously mentioned [38] .

In [55] the authors analysed the complex UHI-UPI nexus in southern hemisphere subtropical climates (Brisbane, Australia). Local natural and man-made features promoted sea-breeze circulations, mountain valley airflows and the heat island effects, all contributing to transport, entrapment and dispersion of pollutants. A mesoscale hydrostatic model, (described in detail in [104] ) was used to track the air flows generated by differential surface heating and terrain irregularities and was modified to incorporate anthropogenic heat fluxes and heterogeneous surface characteristics. Anthropogenic heat release was dominated by motor vehicles. Three numerical experiments were conducted on a typical summer day potentially conducive to high pollution (sunny, light winds): 1) disregarding UHI and anthropogenic heat, 2) reintroducing UHI, 3) contemplating both UHI and anthropogenic heat. Only in experiment 3) good agreement was achieved with the measurements at two sites (urban/industrial and suburban). In this scenario, during hours of intense anthropogenic heat release, i) a strong convergence zone formed over the urban core, ii) the sea breeze moved inland faster towards the city and iii) winds over the city were sustained during the night. Cumulatively, the UHI increased by 5°C.

Apparently, a vicious loop was established between UHI, convergence and pollutants emission. 

Beyond the geographical context, urban design and urban growth are key players in the mutual intensification of UHIs and UPIs. Indeed, as the number and distribution of urban patches changes along with urbanization, so does the temperature distribution within the city and intercity [66] . To name one mechanism, sprawling cities characterized by high-rise, packed buildings favor stagnation and thus aggravation of both UHI and pollution [51] . This was J o u r n a l P r e -p r o o f cases could experience up to 62% more high-ozone days. As well stressed in the abstract, -this relationship was found to hold when controlling for population size, average ozone season temperatures, and regional emissions of nitrogen oxides and volatile organic compounds,

suggesting that urban spatial structure may have effects on ozone formation that are independent of its effects on precursor emissions from transportation, industry, and power generation facilities‖. This significant result may support the hypothesis that the urban spatial texture

impacts on air quality also through non-emissions-related mechanisms [110] . In the same vein, in [117] , the authors analyzed the ramifications on UHI, pollution and health of the intensive LULC change the city of Atlanta underwent between 1973 and 1998 to accommodate almost twice the original population. The 119 % increase in the periurban low-density urban use produced a distinct increase in surface temperature and UHI while reducing the Normalized Difference Vegetation Index (NDVI). These impacts were strongly correlated to VOCs and NO x emissions and, in turn, to enhanced ground-level ozone production. VOCs and NO x closely tracked the J o u r n a l P r e -p r o o f spatial patterns of surface temperature and NDVI, increasing significantly with the former and decreasing with the latter, whereas ozone accumulated when a UHI-induced convergence zone formed over the urban areas. UHI was also found to initiate and/or enhance urban precipitation (with urban pollutants acting as condensation nuclei) owing to roughness-induced surface convergence and instability caused by urban heating [118, 119] . This is in line with [120] , where the fast-paced development and growing heat island in suburban Atlanta was associated with a 40 % increase in heavy rainfall events in 50 years.

At local-scale, anthropogenic activities and their urban distribution also play a role. This is described in [121] . The authors integrated Spectral Mixture Analysis and Endmember Remote

Sensing Indices to investigate the relationships between land use/land cover, land surface temperature UHI and air pollution, using ASTER thermal remote sensing satellite images and eight ground-based PMs measurement stations. The Iranian city of Tabriz was discretized in 8

LULCs. The two most intense UHI zones were identified at the Tabriz' petrochemical industrial site and at the bazar market area in the central part of the city. The same areas, although inverted in order, were associated with the highest PM levels. Hence, a high correlation between highly air-polluted areas and UHI zones was established, denoting how high population density, fervent commercial activities and heavy traffic were major drivers of severe pollution concentration.

Moreover, in [59] , the authors presented an experimental and numerical approach to determine how urban morphology, changes in microclimate and air quality relate within a city center. wave episodes in the Baltimore metropolitan region [124, 125] . In this perspective, decentralization and sprawl may also expand the influence area of UHI-associated pollutant downwind transport.

All future climatological scenarios point towards remarkably higher temperatures, increased downward solar radiation, lower likelihood of precipitation and higher likelihood of air stagnation, which converge into stronger UHI-UPI interactions [126] [127] [128] . Besides, higher temperatures and irradiance result in increased emission of biogenic VOCs and enhanced J o u r n a l P r e -p r o o f photochemical processes that prelude to ozone formation [129] . Another well-acknowledged future trend is the increase in magnitude and frequency of heat waves. The annual number of days falling into this category is escalating [130] [131] [132] and even conservative outlooks suggest an increase of associated mortality and morbidity [133] . This tandems with the forecasted increment of pollutant emission levels, especially ozone [134, 135] . Furthermore, heat waves impact not just on higher energy consumption and consequently higher emissions of greenhouse gases, but also on forest fires, wilted crops, reduced photosynthesis flux, droughts, boundary layer anomalies and thereby, on air quality [136, 137] . Extremely hot, persisting events reinforce temperature inversions that trap pollutants near the surface, as well documented for the European summer heat waves of 2003 [138] [139] [140] [141] and 2006 [142] .

In this perspective, Wilby investigated the future urban heat island and peak ozone concentration showed the highest sensitivity, increasing by 25%-38%, followed by NO 2 (14%-29%). In contrast, ozone's increment didn't surpass 12%. The minima were recorded at the suburban stations. These findings complemented the results of a previous study in the city of Volos [147] .

Again, the authors found that the major pollutions events in 2007 summer coincided with heat wave events, when also UHI reached a high. A causal link was established between the presence of low-speed sea breeze and higher aerosol concentration (by up to 31 %) due to i) the J o u r n a l P r e -p r o o f development of an internal boundary layer in the marine air mass that hampered any efficient vertical diffusion of pollutants and ii) the increased production of secondary aerosol from gas-toparticle conversion processes [148] , strongly enhanced by sea breeze-like mesoscale circulations.

These findings are in line with those by Colbeck et al. [149] . A study by Theoharatos et al. [150] confirmed the negative effects of the synergy between extreme high temperatures and excessive pollution levels during the same heat wave also for the city of Athens. Toxicological analyses revealed that accumulation of health-threatening species (e.g. metals and polycyclic aromatic hydrocarbons) was likely associated with temperature stress.

Bushfires are among the calamitous events that are envisioned to escalate in the future due to global and local climate change. The drawbacks in terms of UHI and UPI were recently investigated by Ulpiani et al. [153] . The authors carried out an environmental and air quality monitoring campaign in the inner west of the city of Sydney, during the massive bushfires that afflicted Australia between December and January 2020. Under a combination of extreme pollution, heat waves and drought, a suite of dependencies could be established: i) near-surface particulate matter tended to accumulate within specific thermohygrometric ranges adverse to deliquescence as found in [154, 155] and under the transport of sea breeze; ii) high PM concentration were likely recorded in the nighttime/early morning, especially after daytime heat J o u r n a l P r e -p r o o f waves; iii) PM-rich atmosphere attenuated UV radiation maintaining the UV index under healththreatening levels in line with [156] [157] [158] ; iii) heavy rain splashing generated the most intense concentration of dust due to the ejection of sub-micrometre organic matters from the soil surface into the air [159, 160] ; iv) a steady and accentuated UHII emerged, in contrast with the historical trends due to the change in radiative forcing and the disappearance of any cool island events. It was thus demonstrated that during days of high pollution, rich in particulate matter, the UHI was exacerbated, providing fertile ground for increased mortality and morbidity rates. Some of the above results are displayed in Fig. 6 .

J o u r n a l P r e -p r o o f 

In light of the above considerations, the quality of life and the health conditions of urban populations, especially high-sprawl ones, would plummet in a BAU scenario [39] . Mitigating the UHI effect curtails the demand for cooling energy in commercial and residential buildings with reduction in emissions of CO 2 and smog [161] . Since ozone is formed when its precursors (NO x and VOCs) photochemically react with heat and solar radiation, actions taken to mitigate urban overheating are in principle strongly beneficial in terms of air quality too [162] . Nonetheless, well-established mitigation technologies and techniques (e.g. highly reflective surfaces, greenery, water features) usually do not limit their action to sensible cooling, but may variously alter the local thermal and chemical balance. By way of example, Table 1 demonstrates the intricate and countervailing effects of tree planting on UHI and or UPI and vice versa: changes in albedo, evapotranspiration, roughness and canopy alter air temperature, relative humidity, solar budgets, wind speed and chemistry at the local scale. The product of the interaction between vapor pressure deficit, air temperature, stomatal conductance, tree hydraulic status, soil water availability and wind speed must be carefully evaluated [163] . This applies to most mitigation strategies. 

temperature-dependent ozone-forming chemicals, iv) enhancement of wet deposition. On the other side, evapotranspiration rates increase with temperature, hence trees may exchange ~30% more water on paved urban areas than in vegetated areas [168] . However, excessive ambient overheating caused trees to close their stomata and suppress water loss even in well-watered conditions [169] . Strategic planting of deciduous trees Significant reduction of summer solar exposures of structures, parking lots and streets. This limits re-emitted long-wave radiation that triggers ozone photochemistry and reduce evaporative emissions from vehicles (VOCs and NO x ). On the other side, evergreen species are most effective at dry deposition. Growth

The benefits of urban trees increase with time (e.g. larger crowns, denser foliage, carbon storage) especially when periodically replacements with new mature trees are scheduled (storage capacity increased by up to 33%). On the other side, maintenance activities may require large inputs of energy from fossil fuels, thus affecting the overall budget. Albedo change Replacement of manmade materials (concrete, asphalt) with trees increases the reflectivity and reduces the heat absorption capacity.

The physical barrier breaks the wind force and depreciates mixing, stimulating accumulation of pollutants.

Tree emit biogenic VOCs (especially isoprene) that contribute to ozone and CO formation. The emission rate depends on the species (beefwood and eucalyptus on the front line, elm, crape myrtle, cedar and Judas trees on the bottom line). Anyway, some high-emitting species exert also the higher cooling effect [170] .

Several papers tackle the controversial question of whether urban heat island mitigation impacts positively or negatively on local pollution. EPA has teamed up with NASA and LBNL since 1998 to untangle the knot [162] . Negative impacts are intimately related to the reduced vertical mixing that comes with air cooling. As air layering is more stable, primary pollutants (SO, NO) tend to remain close to the ground sources from where they are emitted. Nonetheless, the net effects are strongly connected to the specific mitigation strategy.

Trees are generally found to deflate both primary and secondary pollutants in cities. Already in 1996, Taha The temperature drop amounted to 1.5 °C on average with peaks of 3.5 °C. A net ozone reduction was achieved with low-emitters only. Planting medium and high-emitting trees negatively impacted the air quality. At peak smog hour (3pm) a 1 % increase of low-emitting species reduced the population-weighted ozone exceedance exposure by 11 % (NAAQS threshold considered), while an equal increase of medium-emitting species upturned the benefit into a +9 % penalty. This has implications for the future selection of species used for biofuels or urban tree planting [171] . In the same vein, in [3] , the authors explored the impact of street trees on mitigating daytime temperatures and on improving the air quality in Bangalore. Second fastest growing city in India with technologically intensive industries, highest national vehicleto-person ratio and tropical climate, Bangalore is facing escalating UHI and UPI. The monitoring campaign focused on ten major roads in different parts of the city, having tree cover for about 150 m along one segment and no tree cover along the adjacent. Variation in ambient air temperature, road surface temperature, relative humidity, air pollution (PMs and SO 2 ) and traffic intensities were synchronously recorded for paired segments on summer sunny working days.

Street trees lowered i) air temperatures by up to 5.6 °C, ii) road surface temperatures by up to 27.5 °C and iii) SO 2 levels by up to 65%. Furthermore suspended PMs stayed within prescribed limits for 80 % of the vegetated roads, while surpassed twice the limit in 50 % of the nonvegetated roads. Greenery was linked to urban climate and pollution also by Makhelouf [172] .

His study was based on a four-year monitoring campaign on temperature, humidity, precipitation and pollution in green and built-up areas of the city of Paris, France. The influence of different types of gardens and extensions of parkland was highlighted. A maximum temperature difference of 3 °C was observed between green and inhabited zones, which was enough to J o u r n a l P r e -p r o o f Journal Pre-proof engender a local breeze that pushed away local pollutants (especially SO 2 ). The extent of this washing action was largely a function of the park size and was negligible in less-than-1-hectare gardens. On the above grounds, the author suggested to avoid built-up areas longitudinally to extensive green areas and to place large avenues facing the parklands at the crossing with neighboring inhabited spaces to take maximum advantage of the local pollutant-dispersing and fresh breeze. Substantiated protocols for precinct ventilation design and characterization are, indeed, increasingly needed [173] .

Another widely applied UHI countermeasure relies on the use of high-reflective materials. Cool roofs and pavements reduce temperature-dependent emissions of ozone precursors and PMs and curtail greenhouse emissions by cutting buildings' cooling needs [174, 175] . By lowering the ambient temperature, they also reduce the rate of VOC evaporation and ozone formation [176] . Cardelino and Chameides [32] ). Finally, the photochemical model CAMx was adopted to verify the effects of a 20 % stepwise emission reduction in NO x , then in reactive organic gases (ROG) and then in both. A suite of isopleth diagrams was derived, showing the response in the 1-h peak ozone. It was found that, in order to achieve a 1 ppb reduction, ROG emission had to be reduced by up to 9.3 % and 8.5 % in Central and Southern California, respectively.

In most of the studies concerning high-albedo materials, UHI and UPI it is assumed that cool surfaces would not alter the UV reflectance. Yet, UV reflectance can significantly modify the rate of ozone production. Enhanced pollution is reported, for instance, in the Uintah basin as a consequence of the snow cover [179] . A first comprehensive study is reported by Fallmann et al. Conversely, the turbulent term was always positively correlated with ozone concentration during the morning and the evening, when ozone was higher in the residual layer than at ground level (less titration, less deposition). Around noon the boundary layer was well mixed and the vertical ozone gradient was attenuated, while during the night ground-level ozone was strongly depleted by NO titration. The decrease in near surface temperature, due to mitigating strategies, resulted in an increase of primary pollutants (NO, CO). Extra vegetation led to 5 % increment of both compounds, while the increase of albedo led to +25 % NO and + 9 % CO (mean concentrations).

Ozone was decreased by 8 % and 6 %, respectively. While urban density had negligible impacts, greenery was steadily effective in curbing ozone at any time of the day due to the stronger cooling over vegetative surfaces than over impervious surfaces. When assuming an increase in the urban albedo, peak ozone concentration showed a tendency to increase (+12 %) between 2pm and 4pm. This was ascribed to accelerated photochemistry (higher photolysis rates) owing to enhanced intensity of reflected shortwave radiation from highly reflective surfaces (up to 170 % increase in reflected shortwave radiation). However, on average, the O 3 mean concentration could be reduced by 5-8 % [61] in response to a 1.1/1.7 °C UHII mitigation with greenery and increased albedo, respectively. Despite no direct correlation could be established between primary pollutants and air temperature, a linear regression could be observed when relating ozone and temperature. The authors concluded that daily average ozone concentration could be more efficiently decreased by low-emitting urban vegetation than highly reflective surfaces, as advocated also in [180] . These results contrast with the results by Taha [177] who reported a Results of this study motivate the careful consideration of future rooftop and pavement solar reflectance modification policies‖.

Overall, the impacts of high-albedo urbanscapes on global scale and climate-dependencies remain unsettled in literature [181] , especially if aging and long-term performance loss are taken into account in contrast with the time-growing benefit of greenery (Table 1) . On the other side, there is no doubt that the cost of inaction would be much higher [182] .

Already in 2004, Crutzen [10] advocated the importance of exploring the consequences of combined urban heat and pollution island effects, since, despite the strong entanglement, UHI and UPI are generally investigated separately [11] . Recently, holistic approaches are gaining ground to enable the conceptualization of double-acting countermeasures [10, 92] , as demonstrated by the European Project MEGAPOLI [183] and dedicated conferences [184] .

Assessing the interlink between UHI and UPI is no trivial matter. The setup and maintenance of a dense sensor network is indispensable to investigate near-surface parameters, but expensive.

Alternatively, unmonitored areas can be assessed by geo-statistics based on relatively few direct observations [50] or by applying complex algorithms on remotely acquired images [185] .

Multiyear assessments should also be encouraged when looking at trends and not occurrences, to make sure anomalous years are smoothed in their contribution. Nevertheless, meaningful information are collected also from shorter experiences, especially if weather extremes such as heat waves, droughts or fires are under scrutiny. Numerical modelling encounters several challenges as well due to the uncertainty on chemistry mechanisms, weather forecasts and J o u r n a l P r e -p r o o f emission inventories, especially for long-lived species [58] . The exclusion of anthropogenic heat is what depreciates the validity of most proposed models, since, reportedly, this term can increase the near-surface temperature by nearly 1-3°C [186] [187] [188] with the highest impact overnight [189, 190] . Resultantly, even high-detail architectures coupling WRF with Noah and multilayer urban representations fail at depicting the nighttime surface air temperature, with underestimation in the order of 2-4 °C due to the lack of consideration for anthropogenic sources [57] . Furthermore, despite relative humidity plays a relevant role in transforming the reactive pollutants and increasing the secondary pollutants concentrations, its inclusion and its role is sparsely covered [60] . Additionally, as expressed by Huszar et al. [58] , special care should be devoted to balance the uncertainties on the vertical eddy diffusion. This term strongly influences pollution dispersal, with extremely complex dynamics when it comes to secondary pollutants or pollutants having secondary components. In this vein, detailed large scale eddy simulations (LES) recently proved to have an edge over mesoscale models in representing urban-induced turbulence [191] .

Despite the investigatory difficulties, the most important determinants of both UHI and UPI have been progressively identified over recent years. In a nutshell, the probability of UHI and UPI formation increases with proximity to the city center and/or heat/emission sources, air stagnation, clear anticyclonic conditions, warm/hot seasons, nocturnal conditions, city size and/or population.

The impacts affect climatological aspects like sunshine, precipitation, temperature and air quality.

Under specific contingencies, the positive correlation between UHI and UPI can reverse owing to the stimulation of vertical transport, mixing and inversion breaking. Further, since the budget terms in UHI and UPI formation and magnification relate to the mechanisms of advection, chemical production/loss tendency, turbulence, dry deposition and anthropogenic emissions, the J o u r n a l P r e -p r o o f net tendency might differ between nighttime and daytime [15] . As a rule of thumb, nocturnal chemistry substantially depends on altitude and local orography. Air from different elevations interact and mix, each with its own chemical history. Nocturnal ozone maxima are reported in a variety of studies owing to the vertical mixing from higher levels (-leaky‖ residual layer, as dubbed by Hu et al. [192] ) or horizontal transportation through local and mesoscale wind systems like low-level jets [193] . Major gaps exist in the understanding of the underlying causes, as reported by Tong et al. in their review on urban ozone and its evolution upon change of boundary layer characteristics [194] . The mechanisms through which the availability of pollutants for a given day affects the next day's is also a grey area and a major uncertainty in atmospheric pollution models [15] . It is however acknowledged that while the ozone contained in the atmospheric boundary layer tends to be titrated near the surface at night, it remains available at the higher residual layer. As turbulent kinetic processes develop during the day, especially in presence of rough urban fabrics, the convective boundary layer starts eroding the residual layer and makes its ozone pockets available for mixing near the ground [195] . Further, due to thermal and chemical processes occurring in the city, the urban residual layer is higher, deeper and richer in ozone compared to the rural surroundings [15] .

The urban canopy meteorological forcing (by and large represented by higher Bowen ratio), is repeatedly found to significantly impact the spatial and vertical distribution of air pollutants. The enhanced updraft can reduce the surface concentrations of particulate matter and NO x , which, in turn, lessens titration and triggers the production of near-surface ozone, with important variations at different altitudes [53, 58] . Another key-player is the HIC, namely the mesoscale urban breeze generated by the UHI and characterized by a surface convergent flow onto the city center [196] .

This phenomenon is enhanced by urban sprawling. Urban sprawl has been vastly documented to J o u r n a l P r e -p r o o f elevate the economic and environmental cost of services and daily routines by incrementing the average travelling distance and traffic congestion from/to expanded urban fringes/city center [197, 198] with a broad spectrum of health-related implications [199] . The revised literature additionally points out a positive association between sprawl and ozone exceedances, possibly due to the hypothesis that lower density, but spatially extended patterns of urbanization generate more surface that heats up compared to compact textures. Hence, sprawling cities may experience severer air quality deterioration due to a mightier and more extended HIC. From observed results and considering the present trend towards congested city centers with less parking lots and expanded traffic lanes, the issue of establishing thresholds for minimum airflow within the pedestrian street should be sought [59] . Additionally, advecting plumes of ozone precursors could travel further downwind thus exposing the periurban areas and neighboring smaller cities to poorer air quality. As suggested by Stone, strategies like -urban growth boundaries‖ and -form-based codes‖ should replace traditional zoning ordinances to limit peripheral growth and to promote higher density.

Generally speaking, the patterns of UHI and UPI and, even more, their linkage gets more complex in case of coastal cities, because of a variegated interrelation between land and sea breezes with the heat island circulation and convergent fronts [200] . Typical characteristics of the sea breeze observed in dense coastal metropolitan areas include formation of suburban stagnant regions, frontal intensification, and delay of inland advance [201] . Sea breeze and synoptic forcing typically interfere destructively thus producing weaker winds over urban areas conducive to stagnation. The ability of sea breeze to counteract background winds is especially vivid during heat waves when the synoptic circulation is lowest [52] . As a result, ozone precursors emitted from a coastal city and advected to the sea by the land breeze transform into J o u r n a l P r e -p r o o f ozone directly over the water body. From there O 3 -rich air masses are advected back to the city by the sea breeze, where they merge and combine with the HIC creating stationary systems of stalling polluted air. In most cases, the HIC interfering effect outweighs the UHII driving effect over sea breeze propagation due to larger thermal gradients [38, 51, 56 ], yet multi-directional sea breezes (e.g. land-bay and land-gulf circulations) further complicate the interaction [56] . Equally complex is the HIC role in valleys, where the temperature inversion tends to decouple the wind fields above and below thus restricting the vertical motion, the growth of the canopy boundary layer and the subsidence from the atmosphere aloft. As a result, the ozone trapped within the shallow decoupled surface layer is depleted by titration. This persists in rural and lightly urbanized contexts. The inversion breaking induced by intense urbanization couples back the two fields thus freeing the transport of pollutants out of the valley [53] .

In the future, heat waves, droughts, bushfires and other weather extremes are expected to progressively loosen their sporadicity and expose the fragility of urban infrastructures and resources. The built form tends to amplify and distort the natural hazard by multiplying the UHIs and UPIs and by strengthening their interaction [39] . There is little controversy worldwide on the potential occurrence of more hot anticyclonic summer episodes conducive to capping temperature inversions and hence to pollutants trapping near the surface. According to Lee, a unitary increase in ambient temperature would be associated to enough catalyzing sunshine to increase the surface ozone concentration by 14% in London [182] . Higher temperatures would likely increase VOC emissions too by triggering VOC production, by increasing the volatility of gaseous compounds and by altering the local flora and fauna [35] . On the other side, adaptation strategies may come into play: for instance, some tree species drop their leaves under drought conditions, which results in less emissions, less wind breaking but also less evapotranspiration J o u r n a l P r e -p r o o f [202] . Predicting these impacts is especially difficult, as our understanding of VOC emissions in extreme environments is still very limited [35] . On top of that, catastrophic bushfires are occurring more and more frequently with intense pollution emission and substantial erosion of the green lungs worldwide [153] . Bushfires were also found to alter UHI patterns towards more intense and stable episodes [153] . In this scenario, PM levels are of special concern since the related mortality rate escalates during hot days, due to the physiological stress and evermore altered response to toxic agents [203] .

Since UHI and UPI share most of the driving forces, UHI mitigation strategies appear to be a valuable measure against urban pollution too. The effects of albedo modifications manifest themselves in multiple ways: on temperature, cloudiness, wind, moisture, mixing, all potentially related to intensity and characteristics of both UHI and UPI [54] . Competing effects on photolysis rates, titration rates due to enhanced shortwave radiation and reduced temperature have been vastly reported [61, 62] . The ozone increase due to highly reflective surfaces is generally limited to peak values [61] and is however strongly dependent on the UHI mitigation intensity.

Greenery appears to be especially promising, provided that low-BVOC-emitting species are selected [63] . Results for Sacramento in the early 90s pointed to a 35% and 40% air conditioning load reduction from feasible measures of tree planting and albedo increase, with several 0 kWh and hence zero emission days [204] . On the other side, many conurbations suffer from a substantial lack of development space or excessively steep terrains which hinder the inclusion of green features or other mitigation interventions. This is reported, for instance, for the city of Hong Kong [185] and for Singapore [205] . For these urbanscapes and for desert cities as well [206] , even small pockets of greenery within the congested inner city are of immense J o u r n a l P r e -p r o o f environmental importance, hence strategic planning in view of the natural topographic setting, climatic pattern and building layout is pivotal.

Overall, for most conventional mitigation strategies, different positive and negative effects have to be traded off against each other. Other than that, urban pollution control strategies based on emission reductions proved very and widely successful over the past decades, especially in terms of NO x and nonmethane hydrocarbons. However, the application of exhaust control regulations in the industrial sector and of diesel emissions in the transport sector proved much less effective against photochemical oxidants [51] . The reason is that reaction-enhancing phenomena, whose magnitude was mild in the past are exacerbating, UHI at the forefront. Furthermore, transboundary air pollution from large city clusters (e.g. from East Asia) introduce complex long-range patterns which can seriously deteriorate the air quality over hundreds of kilometers.

Overall, the current paradigms for pollutants formation, accumulation and transport need to be expanded to consider i) the vertical structure of the mixing layer and its dependence on atmospheric stability, ii) the preservation of the decoupling from the residual layer overnight, iii) the relative collocation of emission sources and the role of urban sprawl in different topographic contexts [81, 207] . The conceptual framework in Fig.7 attempts to recap most of the above considerations to guide researchers and practitioners in interpreting the UHI and UPI linkage, the arena of potential key factors and thereby the room for counteraction.

J o u r n a l P r e -p r o o f heat islands and convergent air circulations characterized by reduced wind speeds but enhanced vertical eddy mixing, which, in turn affect air chemistry, transport and inversion layer coupling/breaking mechanisms. As such, constructive and destructive interferences may establish depending on local topography, geography, climate and local-scale specificities.

According to the revised literature, synoptic conditions characterized by high temperature, low relative humidity, low wind speed and cloudlessness are conducive to UHI development and severe pollution episodes at the same time [87] . Urban breeze with low-level inversion and lowlevel convergence worsens both local overheating and air quality, especially in the nighttime.

The effect of enhanced turbulence in the urban boundary layer is of paramount importance in determining the mutual relationship between UHI and UPI. Generally, both synergistic and antagonistic interactions are established depending on the considered class of pollutants [58] .

When designing mitigation scenarios, high site-specificity comes into play. If it is true that higher temperatures boost atmospheric chemistry cycles, it is likewise true that higher convective turbulence and vertical mixing is vital for pollutants dispersal. If it is true that higher albedo increases photochemical ozone production, it is likewise true that the temperature drop likely overcompensates. If it is true that greenery cleans and cools the air, it is likewise true that BVOC-emitting species might upend the balance.

Overall, a one-fits-all recipe is hard to ascertain [10] ,  awareness: a strong disabler to UHI and UPI prevention is the level of consideration citizen and urban planners tend to devote to climate-responsive design over aesthetics, economics and beautification of the city. To stimulate a participatory and fast-learning approach, citizen science projects are aiming at mobilizing and sensitizing citizens on the challenges and opportunities of living the built realm [210] ;

 abatement strategies: from a UHI perspective, NO x abatement strategies appear to be more promising than VOC abatement strategies given the minor sensitivity to temperature increases [15, 32] , yet the choice is strictly site-specific. On a general note, knowing the ozone chemical regime (ozone sensitivity to NOx and VOCs unit change) is crucial to plan effective abatement strategies [52] not just in large conurbations but also in downwind smaller cities [57] . The regime could be estimated with the Sillman method which establishes at 0.28 the threshold over which the ratio of HCHO/NO y (sum of the total nitrogen species) produces NO x -limited regime [211] .

The concomitant action on all of the above points is suggested for future counteraction and disentanglement of heat and pollution islands. Citizen-oriented technologies and information platforms should become a priority in shaping the cities of tomorrow [212] . Against this backdrop, scientific breakthroughs in the field of materials sciences are expected to pave the way to new mitigation technologies. Among them, passive daytime radiative cooling [213] and elastocaloric cooling [214] hold great promise.

All things considered, the various impacts of climate change, urbanization and changes in lifestyles (e.g. changes in occupancy and consumption patterns, emission control strategies, J o u r n a l P r e -p r o o f LULC conversions) should be harmoniously combined and holistically investigated to give plausible and consistent narratives of the intricate nexus between UHI and UPI [215] . The topic is envisaged to receive much more attention in the future, as perceptions of unhealthy and stressful urban environments have been recently emphasized by the COVID-19 pandemic [216] [217] [218] [219] and bushfire season [153] , thus enforcing the paramount need for improved urban environmental quality, improved coping strategies and improved monitoring towards a more salutogenic approach to urban life [185] . This review attempts to be a first step in this direction.

J o u r n a l P r e -p r o o f Appendix   Table A1 .

List of referenced works in chronological order of publication and corresponding city/cities of investigation. Observation data and UHII, territorial and temporal frames. Bold and italic are used for mean and peak UHI to highlight when the analysis refers to SUHI and AUHI+SUHI, respectively. The abbreviation n.s. stands for non specified. J o u r n a l P r e -p r o o f Vegetation biomass distribution data from Horie et al. [222] 2 scenarios of increased urban vegetation: from 16 to 17% and from 16 to 18%. 3 levels of BVOC-emitting species: lowemitters (<2μg/gh of isoprene and <1μg/gh of monoterpenes), medium-emitters (4μg/gh of isoprene and 2μg/gh of monoterpenes on average) and highemitters (20μg/gh of isoprene and 5μg/gh of monoterpenes on average). [ 

Remote sensing of the urban heat island effect across biomes in the continental USA

Potential Impacts of Climate Change

Effect of street trees on microclimate and air pollution in a tropical city

Satellite multi-sensor data analysis of urban surface temperatures and landcover

Analyzing the heat island magnitude and characteristics in one hundred Asian and Australian cities and regions

Urban Climate Mitigation Techniques

On the impact of urban heat island and global warming on the power demand and electricity consumption of buildings-A review

Cooling the buildings -past, present and future

The American landscape in the 21st century

New Directions: The growing urban heat and pollution ‗-island‖' effect-impact on chemistry and climate

Interaction between urban heat island and urban pollution island during summer in Berlin

Mitigation of the urban heat island of the city of Kuala Lumpur, Malaysia

Practical issues for using solar-reflective materials to mitigate urban heat islands

Cool communities: strategies for heat island mitigation and smog reduction

Impact of urban heat island on regional atmospheric pollution

Improving the microclimate in a dense urban area using experimental and theoretical techniques -The case of Marousi, Athens

Spatial distribution of aerosol pollution based on MODIS data over Beijing

The energetic basis of the urban heat island (Symons Memorial Lecture

Biosphere/atmosphere transfer scheme (BATS) for the NCAR community climate model

Potentials of meteorological characteristics and synoptic conditions to mitigate urban heat island effects

Studying the effects of aerosols on vertical photolysis rate coefficient and temperature profiles over an urban airshed

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

Aerosol relationships to warm season clouds and rainfall at monthly scales over east China: Urban land versus ocean

Observations and Modeling of Downward Radiative Fluxes (Solar and Infrared) in Urban/Rural Areas

Global radiation attenuation by air pollution and its effects on the thermal climate in Mexico City

Urban-rural solar radiation loss in the atmosphere of Greater Cairo region

Contrasting characteristics of the surface energy balance between the urban and rural areas of Beijing

Warming trends in Asia amplified by brown cloud solar absorption

Photochemistry of Air Pollution. IX

Natural hydrocarbons, urbanization, and urban ozone

Satellite-observed urbanization characters in Shanghai, China: Aerosols, urban heat Island effect, and land-atmosphere interactions

Isoprene emission from plants

Plant volatiles in extreme terrestrial and marine environments

Vascular plant controls on methane emissions from northern peatforming wetlands

The Toronto heat island and pollution distribution

High levels of winter air pollution under the influence of the urban heat island along the shore of Tokyo Bay

Constructing climate change scenarios of urban heat island intensity and air quality

Global radiative forcing and megacities

Quantifying precipitation suppression due to air pollution

Thermal comfort implications of urbanization in a warm-humid city: the Colombo Metropolitan Region (CMR)

Urban forests and pollution mitigation: Analyzing ecosystem services and disservices

Urban heat island and air pollution-an emerging role for hospital respiratory admissions in an urban area

Temperature modifies the acute effect of particulate air pollution on mortality in eight Chinese cities

Interactive short-term effects of equivalent temperature and air pollution on human mortality in Berlin and Lisbon

The benefits of publishing systematic quantitative literature reviews for PhD candidates and other early-career researchers

On the impact of temperature on tropospheric ozone concentration levels in urban environments

GIS-based identification of spatial variables enhancing heat and poor air quality in urban areas

Analysis of the Relationship between Changes in Meteorological Conditions and the Variation in Summer Ozone Levels over the Central Kanto Area

On the impact of J o u r n a l P r e -p r o o f urban surface exchange parameterisations on air quality simulations: The Athens case

Effects of urbanization on the temperature inversion breakup in a mountain valley with implications for air quality

Meteorological, air-quality, and emission-equivalence impacts of urban heat island control in California

Effect of heat island on the meteorology of a complex urban airshed

A numerical study of interactions between surface forcing and sea breeze circulations and their effects on stagnation in the greater Houston area

Impact of Shanghai urban land surface forcing on downstream city ozone chemistry

Urban canopy meteorological forcing and its impact on ozone and PM2.5: Role of vertical turbulent transport

Impact of urban geometry on outdoor thermal comfort and air quality from field measurements in Curitiba, Brazil

Urban Heat Island (UHI) influence on secondary pollutant formation in a tropical humid environment

Secondary effects of urban heat island mitigation measures on air quality

Air-quality implications of widespread adoption of cool roofs on ozone and particulate matter in southern California

Modeling impacts of increased urban vegetation on ozone air quality in the South Coast Air Basin

Urban sprawl and air quality in large US cities

Updated world map of the Köppen-Geiger climate classification

Temporal and spatial variation relationship and influence factors on surface urban heat island and ozone pollution in the Yangtze River Delta

Urban heat islands in China J o u r n a l P r e -p r o o f enhanced by haze pollution

Smog chamber studies of temperature effects in photochemical smog

The effect of environmental factors on the emission of biogenic hydrocarbons from live oak and slash pine

Effective control of highway vehicle hydrocarbon emissions

A study of urban heat island and its association with particulate matter during winter months over Delhi

Aerosols over Delhi during pre-monsoon months: Characteristics and effects on surface radiation forcing

Urban Surface Temperature Reduction via the Urban Aerosol Direct Effect: A Remote Sensing and WRF Model Sensitivity Study

Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle

Suspended particulate matter and its relations to the urban climate in Dar es Salaam

Surface urban heat island in China's 32 major cities: Spatial patterns and drivers

Influence of mixing layer height on air pollutant concentrations in an urban street canyon, Urban Clim

Evidence of the effect of an urban heat island on air quality near a landfill

Urban climates

The Role of Vertical Mixing in the Temporal Evolution of Ground-Level Ozone Concentrations

Impacts of Mixing Processes in Nocturnal Atmospheric Boundary Layer on Urban Ozone Concentrations

Surface ozone measurements at urban coastal site Chennai, in India

Atmospheric chemistry and physics: from air pollution to climate change

Air pollution modeling. Theories, Computational Methods and Available Software

On the relation between ozone storage in the residual layer and daily variation in near-surface ozone concentration-a case study

GATOR-GCMM: 2. A study of daytime and nighttime ozone layers aloft, ozone in national parks, and weather during the SARMAP field campaign

Air quality influenced by urban heat island coupled with synoptic weather patterns

Impact of low-level jets on the nocturnal urban heat island intensity in Oklahoma city

Nocturnal secondary ozone concentration maxima analysed by sodar observations and surface measurements

A meteorological analysis and forecast of air pollution episodes in Taiwan

The impact of urban canopy meteorological forcing on summer photochemistry

Megacities, air quality and climate

Impacts of different urban canopy schemes in WRF/Chem on regional climate and air quality in Yangtze River Delta, China

Influence of an urban canopy model and PBL schemes on vertical mixing for air quality modeling over Greater Paris

Ban-Weiss, Effects of urbanization on regional meteorology and air quality in Southern California

Impacts of thermal circulations induced by urbanization on ozone formation in the Pearl River Delta region, China

A modeling study on the effect of urban land surface forcing to regional meteorology and air quality over South China

Changes in regional meteorology induced by anthropogenic heat and their impacts on air quality in South China

The impacts of urbanization on air quality over the Pearl

River Delta in winter: Roles of urban land use and emission distribution

Influence of local production and vertical transport on the organic aerosol budget over Paris

Urbanization Effect on Winter Haze in the Yangtze River Delta Region of China

Urbanization-induced urban heat island and aerosol effects on climate extremes in the Yangtze River Delta region of China

Climatology of discomfort index and air quality index in a large urban mediterranean agglomeration

Verification analysis of the University of Virginia three-dimensional mesoscale model prediction over south Florida for

Numerical study of urban impact on boundary layer structure: Sensitivity to wind speed, urban morphology, and rural soil moisture

An urban surface exchange parameterisation for mesoscale models

Urban climate and air quality in Trier Germany

Advection caused by the urban heat island circulation as a regulating factor on the nocturnal urban heat island

Local winds in a valley city

Rapid urban growth

Meteorological and environmental aspects of one of the worst national air pollution episodes

Effects of shear, stability and valley characteristics on the destruction of temperature inversions

Thermal valley inversion impact on the dispersion of a passive pollutant in a complex mountainous area

A numerical study of inversion-layer breakup and the effects J o u r n a l P r e -p r o o f of topographic shading in idealized valleys

Temperature inversion breakup with impacts on air quality in urban valleys influenced by topographic shading

Measuring sprawl and its impact

Land-Use and Land-Cover Change, Urban Heat Island Phenomenon, and Health Implications: A Remote Sensing Approach

Summary of urban effects on clouds and rain

Urban heat islands and summertime convective thunderstorms in Atlanta: Three case studies

Interepochal changes in summer precipitation in the southeastern United States: evidence of possible urban effects near Atlanta, Georgia

Examining Urban heat Island relations to land use and air pollution: Multiple endmember spectral mixture analysis for thermal remote sensing

Tropospheric chemistry and transport, Science (80-. )

Impact of megacity Shanghai on the urban heatisland effects over the downstream city Kunshan

Upstream urbanization exacerbates urban heat island effects

Impact of upstream urbanization on the urban heat island effects along the Washington-Baltimore Corridor

Potential regional climate change and implications to U.S. air quality

Impact of climate change on surface ozone and deposition of sulphur and nitrogen in Europe

Time series analysis of ambient air-temperature during the period 1970-2016 over Sydney

Impact of temperature on oxidant photochemistry in urban polluted rural and remote environments

Cleland, others, Emissions pathways, climate change, and impacts on California

More intense, more frequent, and longer lasting heat waves in the 21st century

Rising heat wave trends in large US cities

Stochastic diffusion models to describe the evolution of annual heatwave statistics: A three-factor model with risk calculations

Reizer, others, Effects of climate change on ozone and particulate matter over Central and Eastern Europe

Enhanced near-surface ozone under heatwave conditions in a Mediterranean island

A review of the european summer heat wave of

Europe-wide reduction in primary productivity caused by the heat and drought in 2003

The predicted number of air pollution related deaths in the UK during the

Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: 3. Evaluation by means of case studies

Impact of the extreme meteorological conditions during the summer 2003 in Europe on particulate matter concentrations

European surface ozone in the extreme summer

Formation and transport of photooxidants over Europe during the

Assessing ozone-related health impacts under a changing climate

Past and projected trends in London's Urban heat island

Characterizing the urban heat island in current and future climates in New Jersey

Air quality and thermal comfort levels under extreme hot weather

Temperature, comfort and pollution levels during heat waves and the role of sea breeze

Influence of spectral solar irradiance on the formation of new particles in the continental boundary layer

Formation and transport of atmospheric aerosol over

Synoptic conditions, bioclimatological assessment, air quality levels and health effects

Evaluation of land-use regression models used to predict air quality concentrations in an urban area

A review of land-use regression models to assess spatial variation of outdoor air pollution

Experimental evidence of the multiple microclimatic impacts of bushfires in affected urban areas: the case of Sydney during the 2019/2020 Australian season

Particulate matter < 10 μm (PM10) and total suspended particulates (TSP) in urban, rural and alpine air in Switzerland

Effect of wind speed and relative humidity on atmospheric dust concentrations in semi-arid climates

Application of satellite and groundbased data to investigate the UV radiative effects of Australian aerosols

Smoke aerosol and its radiative effects during extreme fire event over Central Russia in summer 2010

Vertical redistribution of zooplankton in an oligotrophic lake associated with reduction in ultraviolet radiation by wildfire smoke

Airborne soil organic particles generated by precipitation

Formation of soil organic matter via biochemical and physical pathways of litter mass loss

On the energy impact of urban heat island in Sydney: Climate and energy potential of mitigation technologies

Air Pollution Prevention Through Urban Heat Island Mitigation: An Update on the Urban Heat Island Pilot Project

Responding to the Urban Heat Island : A Review of the Potential of Green Infrastructure

Urban Heat Island and Climate Change : An Assessment of Interacting and Possible Adaptations in the Camden

The effects of urban trees on air quality

Pollutant uptake by plants

Dry deposition of air pollutants on trees at regional scale: A case study in the Basque Country

Urban tree transpiration over turf and asphalt surfaces

A 3-year study of water relations of urban street trees

Efficiency of parks in mitigating urban heat island effect: An example from Addis Ababa

A highly spatially and temporally resolved inventory for biogenic isoprene and monoterpene emissions: Model description and application to Great Britain

The effect of green spaces on urban climate

Enhancing urban ventilation performance through the development of precinct ventilation zones: A case study based on the Greater Sydney

Evolution of cool-roof standards in the US

Potential benefits of cool roofs on commercial buildings: Conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants

Regional climate consequences of large-scale cool roof and photovoltaic array deployment

Meso-urban meteorological and photochemical modeling of heat island mitigation

Episodic performance and sensitivity of the urbanized MM5 (uMM5) to perturbations in surface properties in Houston Texas

High winter ozone pollution from carbonyl photolysis in an oil and gas basin

Development and Application of an Urban Tree Air Quality Score for Photochemical Pollution Episodes Using the Birmingham, United Kingdom, Area as a Case Study

Ten Hoeve, Effects of urban surfaces and white roofs on global and regional climate

Climate change and air quality in London

MEGAPOLI: concept of multi-scale modelling of megacity impact on air quality and climate

Special Issue of Journal of Urban Climate: Modelling of urban air pollution and climate interactions, Urban Clim

Assessment of urban environmental quality in a subtropical city using multispectral satellite images

Impact of anthropogenic heat on urban climate in Tokyo

Influence of airconditioning waste heat on air temperature in Tokyo during summer: numerical experiments using an urban canopy model coupled with a building energy model

Impacts of urban land-surface forcing on ozone air quality in the Seoul metropolitan area

Modeling the impacts of anthropogenic heating on the urban climate of J o u r n a l P r e -p r o o f Philadelphia: a comparison of implementations in two PBL schemes

Numerical simulation of the anthropogenic heat effect on urban boundary layer structure

Comparing turbulent mixing of atmospheric oxidants across model scales

Impact of the vertical mixing induced by low-level jets on boundary layer ozone concentration

Summer nocturnal ozone maxima in Göteborg

A review on ozone evolution and its relationship with boundary layer characteristics in urban environments

Evidence of dynamical coupling between the residual layer and the developing convective boundary layer

Towards better scientific communication in urban climate

Costs of sprawl

Changing urban form and transport CO2 emissions: An empirical analysis of Beijing

Urban sprawl and public health: Designing, planning, and building for healthy communities

Predicting the magnitude and the characteristics of the urban heat island in coastal cities in the proximity of desert landforms. The case of Sydney

Numerical study of the daytime urban effect and its interaction with the sea breeze

Amazonia and global change

Interactions between particulate air pollution and temperature in air pollution mortality time series studies

Mitigation of Urban Heat Islands: Materials, Utility Programs, Updates

Analysis of the urban thermal environment with LANDSAT data

Experimental and Theoretical J o u r n a l P r e -p r o o f analysis of the urban overheating and its mitigation potential in a hot arid city -Alice Springs

On the relation between ozone storage in the residual layer and daily variation in near-surface ozone concentration -A case study

Water mist spray for outdoor cooling: A systematic review of technologies, methods and impacts

Notin-kind cooling technologies: A quantitative comparison of refrigerants and system performance

Public engagement in urban microclimate research

The use of NOy, H2O2, and HNO3 as indicators for ozone-NO x-hydrocarbon sensitivity in urban locations

Citizen-Oriented Technologies in the Cities of Tomorrow

Recent progress in daytime radiative cooling: Is it the air conditioner of the future?

Elastocaloric cooling: roadmap towards successful implementation in the built environment

Recent progress on urban overheating and heat island research. Integrated assessment of the energy, environmental, vulnerability and health impact. Synergies with the global climate change

Summer Heat Islands, Urban Trees, and White Surfaces

Pallavicini, others, SARS-Cov-2 RNA Found on Particulate Matter of Bergamo in Northern Italy: First Preliminary Evidence

Exposure to air pollution and COVID-19 mortality in the United States

Longitudinal survey of microbiome associated with particulate matter in a megacity

Heat waves and urban heat islands in Europe: A review of relevant drivers

A European daily high-J o u r n a l P r e -p r o o f resolution observational gridded data set of sea level pressure

Inventory of leaf biomass and emission factors for vegetation in California's South Coast Air Basin

Modeling of concentration of atmospheric secondary aerosol

J o u r n a l P r e -p r o o f