key: cord-0832108-lm14dmf0
authors: Collivignarelli, Maria Cristina; De Rose, Claudio; Abbà, Alessandro; Baldi, Marco; Bertanza, Giorgio; Pedrazzani, Roberta; Sorlini, Sabrina; Carnevale Miino, Marco
title: Analysis of lockdown for CoViD-19 impact on NO(2) in London, Milan and Paris: What lesson can be learnt?
date: 2020-12-25
journal: Process Saf Environ Prot
DOI: 10.1016/j.psep.2020.12.029
sha: 6043294377a4eebe9916d19d91e4a043500b5ac2
doc_id: 832108
cord_uid: lm14dmf0

Nitrogen dioxide (NO(2)) can have harmful effects on human health and can act as a precursor for the formation of other air pollutants in urban environment such as secondary PM(2.5) and ozone. The lockdown measures for CoViD-19 allowed to simulate on a large scale the massive and prolonged reduction of road traffic (the main source for NO(2) in urban environment). This work aims to selectively assess the maximum impact that total traffic blocking measures can have on NO(2). For this reason, three megacities (London, Milan and Paris) were chosen that had similar characteristics in terms of climatic conditions, population, policies of urban traffic management and lockdown measures. 52 air quality control units have been used to compare data measured in lockdown and in the same periods of previous years, highlighting a significant decrease in NO(2) concentration due to traffic (London: 71.1 % - 80.8 %; Milan: 8.6 % - 42.4%; Paris: 65.7 % - 79.8 %). In 2020 the contribution of traffic in London, Milan and Paris was dropped to 3.3 ± 1.3 µg m(-3), 6.1 ± 0.8 µg m(-3), and 13.4 ± 1.5 µg m(-3), respectively. Despite the significant reduction in the NO(2) concentration, in UT stations average NO(2) concentrations higher than 40 µg m(-3) were registered for several days. In order to reduce the pollution, the limitation of road traffic could be not enough, but a vision also aimed at rethink the vehicles and their polluting effects should be developed.

In March 2020, the World Health Organization declared a pandemic state (WHO, 2020) and, in many countries total lockdown was imposed (Collivignarelli et al., 2020b; Lau et al., 2020) . The heavy restrictions on movements imposed to limit SARS-CoV-2 contagion have made it possible to significantly reduce the impact of CoViD-19 on national health systems by limiting the otherwise exponential number of victims (Lau et al., 2020; Remuzzi and Remuzzi, 2020; Saez et al., 2020; Sjödin et al., 2020; Tobías, 2020) . However, these measures allowed to simulate situations only hypothesized or tested on a small scale till now but never realized on a large scale, such as the massive and prolonged reduction of vehicular traffic in urban environments.

The effects of road traffic on air quality in large cities have been studied for years and the main pollutants emitted by vehicles are: particulate matter with a diameter lower than 10 μm (PM10) (Heydari et al., 2020; Ionescu et al., 2013; Pant and Harrison, 2013; Thorpe and Harrison, 2008) , black carbon (Ali et al., 2020; Invernizzi et al., 2011) , CO2 and nitrogen dioxide (NO2) (Agudelo-Castañeda et al., 2020; Degrauewe et al., 2019) . In the literature there are many works on the quantification and modelling of the dispersion of pollutants from their sources. Some of these have highlighted the spaciotemporal heterogeneity of the emissions (Zhang et al., 2019; Zhang et al., 2020) .

Several studies highlighted also that high concentrations of these pollutants for prolonged periods, particularly NO2, can have harmful effects on human health (Bahrami Asl et al., 2018; Curtis et al., 2006; Strak et al., 2017; Zhao et al., 2020) . For instance, according to Khaniabadi et al. (2017) , exposure to high levels of NO2-containing air or proximity to a busy road increases the likelihood of lung cancers among ex-smokers and non-smokers. Moreover, NO2 can act as a precursor for the formation of other air pollutants such as secondary PM2.5 (Chen et al., 2017) and, in the presence of J o u r n a l P r e -p r o o f solar irradiation, of ozone (O3) (Escudero et al., 2019) . While other traffic pollutants (e.g. PM10) may also have a different type of origin (e.g. domestic heating, industrial sector, etc.) (EEA, 2012) , the emission of NO2 in major European cities is almost exclusively attributable to road transport (Degrauewe et al., 2019) .

For these reasons, attempts have been made to limit the access of private vehicles in large urban areas in favour of public transport, through various measures such as the creation of low emission zones (LEZs) or the introduction of tolls for entering in the city centre (Gehrsitz, 2017; Jiang et al., 2017; Mussone, 2017; Trivellato et al., 2019) . However, in large cities, road traffic is still the main polluting source for NO2 (Degrauewe et al., 2019) .

Several researches relating to the impact of the lockdown in major cities are available in the literature (Collivignarelli et al., 2020a; Hashim et al., 2021; Mahato et al., 2020; Nakada and Urban, 2020; Tobías et al., 2020) . They clearly demonstrated a significant improvement in air quality in the period of greatest limitations, but they were mainly focused on the combined effect of the reduction of all NO2 sources. However, the total lockdown gives also the unique opportunity to evaluate the effect on air quality of the prolonged reduction of vehicular traffic, specifically.

This work aims to selectively assess the maximum impact that total traffic blocking measures can have on NO2. For this reason, three "megacities" (London, Milan and Paris) were chosen that had similar characteristics in terms of climatic conditions, population, policies of urban traffic management and lockdowns. 25 urban traffic (UT) air quality control units, where pollution level was influenced mainly by traffic emissions, and 27 urban background (UB) air quality control units, where pollution level was influenced by the integrated contribution of all sources, have been used to collect data measured in lockdown and in the same periods of 2019, 2018 and 2017. Since weather conditions have a great influence on air quality, the temperature, precipitation and wind speed were also studied and discussed. The results made it possible to formulate proposals and future perspective for a better management of the urban environment in terms of reducing traffic pollution.

The study involved three European "megacities": (i) London, (ii) Milan and (iii) Paris. In order to obtain reliable results, it was decided to consider the cities as a whole beyond the municipal borders and not just a limited portion of them ( Figure 1 ). "Greater London" and the entire territory of the "Metropolitan City of Milan" were considered. Finally, for Paris the "Départements" of Paris, Val-de-Marne, Seine-Saint-Denis and Hauts-de-Seine were considered. The aim was to develop the study on different cities that presented similar conditions to evaluate the repeatability of the results.

According to recent data, they had similar population presenting over 8,500,000 (London) (ONS, 2020), 3,250,000 (Milan) (CMM, 2020) and 6,500,00 (Paris) inhabitants (INSEE, 2016) . These three cities were also selected due to their similar conditions in terms of urban traffic management, with LEZs divided into different bands that impose increasingly restrictive requirements on vehicles as they approach the central area (MoM, 2020; MoP, 2020; TfL, 2020) and in terms of climate conditions with temperate/mesothermal characteristics according to Köppen climate classification (Köppen, 1936) . Moreover, these cities were subjected to similar lockdown measures.

For each of the cities, different periods were considered depending on the entry into force of the restrictive rules that imposed a total lockdown. Table 1 shows the total lockdown periods considered.

[Please, see Reports (Google LLC, 2020) . In the selected periods, data on movements associated with public transport, retail and recreation (e.g. restaurants, shopping centres, museums, and cinemas), residential and workplaces have been studied. As reported by Google (Google LLC, 2020) , the reference was the median value, for a given day of the week, for the fiveweek period from January 3 to February 6, 2020.

Moreover, information on mobility trends related to COVID-19 for London, Milan and Paris have been collected from Apple © reports (Apple, 2020 ) that assumed the 13 January 2020 as reference.

Data on temperature, rainfall, and wind speed for London, Milan and Paris have been collected by the U.S. National Oceanic and Atmospheric Administration (NOAA-NCEI, 2020). For each city, one meteorological control unit located within the boundaries of the area, have been selected (Figure 1 ). The daily averages (24h) monitored during the lockdown were compared with those of the same period of 2019, 2018, and 2016. For rainfall values, the comparison was also made on the total contribution in the periods considered.

Data collected of nitrogen dioxide (NO2) concentration were obtained from the local air quality agency (AIRPARIF, 2020; ARPA Lombardia, 2020; UK AIR, 2020). All urban traffic (UT) and urban backgrounds (UB) air quality control units located in London (10), Milan (15), and Paris (27) [Please, see Figure 1 ]

The daily averages (24 h) of the NO2 pollutant concentration for each city have been calculated and the relative percentage variation of the mean concentrations throughout the periods was examined. The European legislation requires that the NO2 annual average concentration of 40 µg m -3 is not exceeded (EP, 2008) . This value was chosen as a reference to better understand the effect of the lockdown. Days in which the average exceeded 40 µg m -3 were counted and this value was compared with what was recorded in the same period of previous years. Moreover, for each city, the daily ratio between the average daily measurement of NO2 in the UT stations and that measured in the UB stations has been calculated (Equation 1) and compared with that of the previous years (2019, 2018, and 2017).

(1) (2)

Where "i-n" represents the period considered. Finally, results were compared with NO2 measured in the same periods of previous three years.

As expected, according to data provided by Google LCC (2020) , during the total lockdown, movement to entertainment venues (e.g. cinemas, museums and restaurants), workplaces and transit through public transport hubs dropped. In London they were reduced by 80.2 ± 0.9%, 70.5 ± 2.5%, and 76.8 ± 0.8%, respectively. Similar data were recorded in J o u r n a l P r e -p r o o f 79.9 ± 1.1%, respectively) ( Figure 2 ).

This had an impact on road traffic, which in urban environments was significantly reduced. To verify the almost total absence of traffic in the three cities considered, and therefore to motivate the research conducted, the data provided by

Apple © (Apple, 2020) on the mobility trends of people in the three periods considered were also studied. According to data based on changes to requests for directions on Apple © Maps, during lockdown the reduction in driving activity was massive in all three cities: -66.1 ± 1.4% in London, -83.2 ± 1.0% in Milan and -82.5 ± 1.3% in Paris.

[Please, see Figure 2 ]

Considering that meteorological events generally have a strong influence on the concentrations of pollutants in the air (Baklanov et al., 2016; Borge et al., 2019; Zhao et al., 2019) , the meteorological data collected in cities have been studied (Table 2) .

Regarding London, an overall equivalence of the average temperature and wind speed in the periods studied was found.

Total rainfall during lockdown in 2020 ( These results are useful for explaining the trend of the average NO2 concentration in the urban environments considered over the various years.

[Please, see Table 2 [Please, see Figure 3] [Please, see Table 3 ]

The average daily NO2 concentration of 40 µg m -3 was chosen as a reference to better understand the effect of the lockdown. The days in which the average exceeded this value were counted and compared with those recorded in the same period of previous years. All cities show a marked improvement in air quality with a drastic decrease in the number of exceedances in both UT and UB stations (Table 4 ). However, cities nevertheless showed that the daily average of 40 µg m -3 was exceeded, in particular in the UT stations (London: 4/48; Milan; 9/54; Paris: 16/55).

[Please, see Table 4 ]

In the lockdown, the ratio between NO2 measured in UT and UB control units, except for London, remained

substantially unchanged compared to that measured in previous years (Figure 4 ). In fact in London, the NO2 ratio was [Please, see Figure 5 ]

Comparing the 2020 data with those recorded in previous years, a marked decrease in NO2 concentration was highlighted ( Figure 3 ). The general improvement of air quality in metropolitan areas during the lockdown has already been widely discussed in several studies. For example, Bao and Zhang (2020) In this work, the analysis focused on the comparison between urban traffic stations and urban background stations, highlighting a reduction in the concentration of NO2 in both types of stations (Table 3) . These results agree with the study of Baldasano (2020) who estimated an average reduction of 62% and 50% of NO2 in Madrid and Barcelona during the total lockdown.

While in UT stations the decrease is attributable to the strong traffic reduction, in UB control units the lower NO2 recorded in the lockdown was also caused by the lower secondary emissions (e.g. from industries). In Paris, the average J o u r n a l P r e -p r o o f temperature of 2020 was higher than in previous years (Table 2 ) and may have further reduced the use of residential heating. This could explain the higher NO2 abatement in UB control units compared to other cities.

During the lockdown, these countries were hit hard by the pandemic (Figure S1 ), and the presence of strict limitation rules has led to an almost total reduction of the movements of people (Figure 2 ). Analysing the NO2 ratio in the UT and UB stations in Milan and Paris, the percentage of NO2 due to vehicular traffic has remained substantially unchanged while only a slight decrease in London was highlighted in London (Figure 4) . These results can be attributed to the simultaneous and decisive lowering of the NO2 present in the background in the urban environment due to reduced emissions, for example in the industrial sector. The results obtained show that the lower NO2 in urban areas registered during the lockdown was largely due to the reduction of vehicular traffic, especially in London and Paris. In fact, the absolute decrease was decidedly more evident in the UT stations than in the UB stations (Table 4) data. In this case, the lower reduction could be attributed to the meteorological conditions. In fact, in 2020, the low rainfall (Table 2) ). In general, a more rational and coordinated localization of the main public and private offices and services would allow to significantly optimize urban travel allowing to reduce pollutants emission. In this perspective, also the decentralization of cultural tourism in large cities has been discussed ( Barrera-Fernandez et al., 2016; Koens et al., 2018) , identifying historical, artistic and cultural points of reference even in peripheral areas, so as to distribute between the centre and the periphery the vehicular traffic connected to tourism and therefore also the relative emissions.

However, this study also highlighted the possible limitations of traffic limitation interventions. Despite the significant reduction in the NO2 concentration, Milan and Paris still showed several days with average NO2 concentrations higher than 40 µg m -3 (Table 4 ). In order to reduce the pollution, the limitation of road traffic could be not enough, but a vision also aimed at rethink the vehicles and their polluting effects should be developed. Regarding this aspect, the potential of the sector is very high considering that at least 40% of the vehicle fleet is fuelled by diesel (65% in Paris) (Table S1) .

Only in Milan, according to the most recent data (early 2018), more than 1,000,000 vehicles that produced high NOx emissions were still in circulation (MoM, 2019).

There are several experiences of automotive industries reconverted for production following to the "green automotive"

concept (Koronis et al., 2013; Zailani et al., 2015) , according to the environmental challenge launched by the European Union with the aim of achieving the reduction of CO2 emissions within eco-sustainable limits by 2030 (EC, 2020). As reported in literature, on this point there was also the consensus of the stakeholders (FC, 2019). Due to CoViD-19, the automotive sector and the related supply chains entered a crisis, with a sharp decline in sales until the end of May 2020 (Fernandes, 2020; Gillingham et al., 2020) , so the good intentions intended to produce non-polluting vehicles arrested.

Nonetheless, it would seem to be excluded that this negative impact definitively prejudices the path of those good intentions, to which, among other things, factors of economic utility were not and are not extraneous. Therefore, a comparison between the stakeholders of the automotive industry is strongly suggested, in order to verify the consequences that the economic crisis caused by the pandemic has determined within them and can draw the consequent findings.

This work aims to selectively assess the maximum impact that total traffic blocking measures can have on NO2. 65.7 % -79.8 %). In 2020 the contribution of traffic in London, Milan and Paris was dropped to 3.3 ± 1.3 µg m -3 , 6.1 ± 0.8 µg m -3 , and 13.4 ± 1.5 µg m -3 , respectively. In the opinion of the authors think solutions that rationalize and make the use of means of transport less indispensable is necessary. In this sense, for instance, initiatives to reduce the travel time that a citizen has to make during the day, by decentralizing services are strongly suggested. Despite the significant reduction in the NO2 concentration, in UT stations average NO2 concentrations higher than 40 µg m -3 were registered for several days (London: 4/48; Milan: 9/54; Paris: 16/55). In order to reduce the pollution, the limitation of road traffic could be not enough, but a vision also aimed at rethink the vehicles and their polluting effects should be developed.

About this aspect, there are several experiences of automotive industries reconverted for production according to the "green automotive" concept, a comparison between the stakeholders of the automotive industry is necessary. For each city, the period of total lockdown, selected in the study, is highlighted in grey. Lighter staining represents residual activity while more intense staining is associated with more significant activity than baseline values. n: number of data.

J o u r n a l P r e -p r o o f Table S1 .

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Arrêté du 16 mars 2020 complétant l'arrêté du 14 mars 2020 portant diverses mesures relatives à la lutte contre la propagation du virus covid-19

Arrêté du 17 mars 2020 complétant l'arrêté du 14 mars 2020 portant diverses mesures relatives à la lutte contre la propagation du virus covid-19

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Milan (B) and Paris (C). a: only data from 2017 to 2018 were available; b: The station located in Igor Stravinsky square was moved to the Nelson Mandela gardens at the end of 2019, always remaining classified as UB. The figure has been realized with QGIS (2020) while the layer of the map is to be attributed to © OpenStreetMap contributors (OpenStreetMap, 2020)

Reduction of NO2 concentration in UT and UB air quality control units during lockdown in comparison with previous years

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