key: cord-0851702-9f2kv1so
authors: Tobías, Aurelio; Carnerero, Cristina; Reche, Cristina; Massagué, Jordi; Via, Marta; Minguillón, María Cruz; Alastuey, Andrés; Querol, Xavier
title: Changes in air quality during the lockdown in Barcelona (Spain) one month into the SARS-CoV-2 epidemic
date: 2020-04-11
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.138540
sha: 5540074d087a30edd6325caefea46451614d7aae
doc_id: 851702
cord_uid: 9f2kv1so

Abstract Lockdown measures came into force in Spain from March 14th, two weeks after the start of the SARS-CoV-2 epidemic, to reduce the epidemic curve. Our study aims to describe changes in air pollution levels during the lockdown measures in the city of Barcelona (NE Spain), by studying the time evolution of atmospheric pollutants recorded at the urban background and traffic air quality monitoring stations. After two weeks of lockdown, urban air pollution markedly decreased but with substantial differences among pollutants. The most significant reduction was estimated for BC and NO2 (−45 to −51%), pollutants mainly related to traffic emissions. A lower reduction was observed for PM10 (−28 to −31.0%). By contrast, O3 levels increased (+33 to +57% of the 8 h daily maxima), probably due to lower titration of O3 by NO and the decrease of NOx in a VOC-limited environment. Relevant differences in the meteorology of these two periods were also evidenced. The low reduction for PM10 is probably related to a significant regional contribution and the prevailing secondary origin of fine aerosols, but an in-depth evaluation has to be carried out to interpret this lower decrease. There is no defined trend for the low SO2 levels, probably due to the preferential reduction in emissions from the least polluting ships. A reduction of most pollutants to minimal concentrations are expected for the forthcoming weeks because of the more restrictive actions implemented for a total lockdown, which entered into force on March 30th. There are still open questions on why PM10 levels were much less reduced than BC and NO2 and on what is the proportion of the abatement of pollution directly related to the lockdown, without meteorological interferences.

The first confirmed cases of SARS-CoV-2 in Spain were identified in late February 2020 (Saglietto et al., 2020) . Since then, Spain became, by the end of March, the third most affected country worldwide after the United States and Italy, and recorded the second-highest number of deaths due to the SARS-CoV-2 pandemic after Italy (One World Data, 2020). Since March 14 th , lockdown measures were in place in Spain, restricting social contact, reducing public transport, and closing businesses (MPRCMD, 2020a) . The restriction measures have been mainly oriented on flattening the epidemic curve, but at the same time confinement of the population, reduction of public transport, and most of the economic activity let to a considerable decrease in road traffic, and consequently, in levels of urban air pollution. We aimed to assess the changes in air quality during the implementation of the lockdown measures in the city of Barcelona (Spain) one month into the SARS-CoV-2 epidemic.

Data have been collected from February 16 th to March 30 th , 2020 on the particulate matter with a diameter of less than 10 (PM10), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ) and ozone (O 3 ) for the city of Barcelona, provided by the Xarxa de Vigilància i Previsió de Atmospheric Pollution (XVPCA), of the Generalitat de Catalunya (XVPCA, 2020) . In this study, we select one traffic station (TR, located within the urban center and directly affected by traffic emissions) and one urban background station (UB, far from emission sources and representative of the levels of contamination of the urban background). Additionally, a second urban background station, the research supersite co-located with the XVPCA Palau Reial, was used to collect data on black carbon (BC). Daily averages (24 h) have been calculated for the periods before (February 16 th to March 13 th ) and during the lockdown (March 14 th to March 30 th ), assessing the variation in the mean concentration (g/m 3 ) between both periods, and their relative change (%). Days with African dust outbreaks over the study area were identified from the Spanish Ministry for the Ecological Transition (MITECO, 2020). The Meteorological Service of Catalonia (Meteocat) supplied meteorological data recorded at Fabra Observatory.

Remote sensing NO 2 data, measured by the Copernicus Sentinel-5 Precursor Tropospheric

Monitoring Instrument (S5p/TROPOMI) developed by the European Space Agency (ESA), has been used to assess tropospheric NO 2 background levels in a high resolution (3.5×7 km) continuous area (Veefkind et al., 2012) . To this end, a script has been written to retrieve, calculate mean levels, and plot over a map the NO 2 data using Google Earth Engine (Gorelick et al., 2017) .

March 14 th to 30 th . Thus with the data of the Fabra Observatory, it is evident that the first period had less marked fluctuations in temperature, wind speed, relative humidity and insolation, and without rainy days (0.2 mm). In contrast, a high variability occurred in the lockdown period. With low, but higher rainfall (3 mm), and colder, more humid and less sunny days (-1.5 o C, +6.6% RH, and -655 W/m 2 ) ( Figure S1 and Table S1 ). Furthermore, during the lockdown, maximal daily insolation values were recorded during the lockdown, and windy days reached averages of 8m/s (max of 5.5 m/s in the prior period). African dust outbreaks of moderate intensity affected Barcelona before (February 28 th to March 1 st ) and during the lockdown (March 18 th to 24 th ).

After two weeks of lockdown, urban air pollution decreased with substantial differences among pollutants (Figure 1 ). PM10 averaged concentrations decreased by -28% and -31% in the traffic and urban background stations, respectively (Table 1 ). In the lockdown period, the World Health Organization Air Quality Guideline (WHOAQG) daily reference value of 20 µg/m 3 was not exceeded at the UB site and slightly exceeded at the TR. For BC, the reduction was larger, -45% in the urban background, and similar to the one of NO 2 (-47 and -51% for UB and TR sites, respectively). The low SO 2 concentrations recorded (around 1.0 to 2.6 µg/m 3 as averages of the different sites) in the study period and the slight changes (-0.2 and +0.1 µg/m 3 , for UB and TR sites, close to the detection limit) does not allow evidencing a definite trend.

Concentrations of O 3 markedly increased (+29 and +58%, and +33 and +57% of the daily averages and 8h average daily maxima concentrations for the UB and TR sites, respectively). In this case, and considering that we did not reach the usual high O 3 period (May-August), relatively high 8 hours average daily maxima were recorded during the lockdown period (56-73 µg/m 3 ), although lower than the WHOAQG (100 µg/m 3 ), compared with the pre-lockdown (27-57 µg/m 3 ).

The major changes described above for NO 2 , are clearly shown by satellite measurements of background tropospheric NO 2 concentrations supplied by TROPOMI-ESA (in this case not only in and around cities but also in the major highways) when comparing the before and during the lockdown and the later with the same period of 2019 ( Figure 2 ). Averaged TROPOMI NO 2 loads over the Barcelona Metropolitan Area (2000 km 2 with Barcelona city in the center, Figure   2 ) decreased during the lockdown by -57% compared with the reference period. The same comparison has been made for the same periods for 2019, and -22% was reduced in the second period, in this case, by meteorological effects.

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

As it could be expected, the lockdown, including restricted social contact, closing of restaurants, shops, and a large number of companies and administrative centers, temporarily reduced levels of specific air pollutants, mostly the primary dominated ones. The role of the meteorology is also evident and not quantified in this study. The most significant variation was observed for NO 2 . Urban NO 2 is emitted from combustion processes, mostly road traffic in urban areas, especially diesel and, to a lesser extent, gasoline, vehicles, industry, power generation, and shipping. Although the main source of PM10 in the urban background of Barcelona is road traffic (around 30 % of the annual mean) (Amato et al., 2016) , other important sources are industrial sources, harbor emissions, construction works, dust resuspension, and Saharan dust episodes. The latter occurred before and during the lockdown period, from February 28 th to March 1 st and March 18 th to 24 th , respectively, and this has influenced the variation of PM10. This might have reduced the decrease of PM10 when compared with that of NO 2 , but in addition, other causes could also contributed. Thus, a relevant proportion of PM10 has a regional background-origin, mostly of secondary PM, and regional air mass transport might have influenced PM10 to the point of reducing the effects of local emission abatement.

Barcelona is a low SO 2 city, with most of this pollutant arising from shipping emissions. These are large cargo ships, ferries, and cruises. The low reduction observed might be due to the detection limit of the instruments, but also to the low SO 2 emissions from cruises, the type of ships most reduced in the lockdown period.

Finally, levels of O 3 markedly increased into the city as a consequence of three possible combined causes. Firstly, the decrease of NOx in a VOCs-limited environment (as most urban areas of Europe are) might cause urban O 3 to increase, as opposed to the behavior at the ruralregional background, which is mainly NOx-limited (Monks et al., 2015) ; secondly, the decrease of nitrogen oxide (NO) reduces the O 3 consumption (titration, NO+O 3 =NO 2 +O 2 ), and causes an increase of O 3 concentrations; and thirdly, the usual increase of insolation and temperatures from February to April leads to an increase in O 3 , especially during Saharan dust episodes (from March 18 th to 24 th ), when the maximum O 3 was recorded.

In any case, by staying at home, personal NO 2 exposure is expected to be reduced by 40% in comparison to outdoor exposure, as a study carried out for 39 schools in Barcelona evidenced J o u r n a l P r e -p r o o f (Rivas et al., 2014) . Thus, in addition to the abatement of outdoor NO 2 levels, this exposure reduction should be taken into account.

Unfortunately, the current lockdown was not able to stop the rising of the SARS-CoV-2 epidemic. For this reason, more restrictive actions were implemented for a total lockdown, which entered into force on March 30 th (MPRCMD, 2020b).

Finally, we would like to express that here we characterized the changes produced on air quality during the lockdown. We do not pretend to attribute specifically, neither quantifying the effects of the lockdown since other factors might have influenced the changes, such as meteorology and regional and long transport of pollutant. An in-deep analysis is required to obtain this information accurately.

In conclusion, we expect that air quality will keep improving for PM10, PM2.5, BC, and NO 2 down to minimal levels during forthcoming weeks because of the more restrictive actions to reduce the population's mobility and shut down of a large number of industries. The air pollution effects of the lockdown will be a unique opportunity to evaluate the effects of the reduction of different emission sources and to assess further air quality policies. However, we should clearly state that air quality policy is a long-distance race and that the effects of past emergencies (e.g., the 2008 financial crisis) for further air quality and climate policies have always been adverse.

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

AIRUSE-LIFE+: a harmonized PM speciation and source apportionment in five southern European cities

Xarxa de Vigilància i Previsió de la Contaminación Atmosférica (XVPCA) . [Accessed

Google Earth Engine: Planetary-scale geospatial analysis for everyone

Real Decreto 463/2020, de 14 de marzo, por el que se declara el estado de alarma para la gestión de la situación de crisis sanitaria ocasionada por el COVID-19

Real Decretoley 10/2020, de 29 de marzo, por el que se regula un permiso retribuido recuperable para las personas trabajadoras por cuenta ajena que no presten servicios esenciales, con el fin de reducir la movilidad de la población en el contexto de la lucha contra el COVID-19

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COVID-19 in Europe: the Italian lesson. The Lancet

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The present work was supported by the Spanish Ministry of Agriculture, Fishing, Food and Environment, Madrid City Council and Madrid Regional Government, by the Ministry of Economy, Industry and Competitiveness and FEDER funds under the project HOUSE (CGL2016-78594-R), and by the Generalitat de Catalunya (AGAUR 2015 SGR33), and by the Spanish