Impact of SARS-CoV-2 on Ambient Air Particulate Matter in Tehran

The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has indirectly produced both positive and negative effects on the environment, particularly in terms of air quality. Our study aimed to determine these effects in the city of Tehran by comparing the ambient PM 2.5 and PM 10 levels recorded at 22 air quality monitoring stations during the outbreak (20 February–2 April 2020) with those from the corresponding period last year (20 February–3 April 2019). Contrary to expectation, the average concentrations of both the PM 2.5 and the PM 10 were markedly higher during the former, increasing by 20.5% and 15.7%, respectively, for the first month of the outbreak (20 February–19 March 2020) and by 23.5% and 20.0% for the subsequent Nowruz New Year holidays (from late March till early April), which resulted in overall increases of 20.5% and 16.5% for the entire period. The non-integrated responses to the pandemic, including the failure to close administrative centers and, in particular, the recommendation to maintain social distancing by reducing public transportation use (prompting citizens to travel by private vehicle), have worsened the ambient air quality in Tehran, providing an exceptional opportunity to evaluate the direct/indirect influence of air quality policies and emission control measures on PM 2.5 and PM 10 . Because of the significant association between the lethality of coronavirus disease 2019 (COVID-19) and exposure to ambient air pollution, the rise in airborne PM 2.5 during this outbreak may increase the mortality rate of SARS-CoV-2.


INTRODUCTION
The first official announcement of death in Iran due to the coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was reported on 19 February 2020 Takian et al., 2020). As of 20 February, restricting social contact, reducing public transportation, closing down administrative centers, universities, schools, businesses and religion public places were recommended by the Iran Ministry of Health and Medical Education, but we observed two or three weeks' time lag to implement the aforementioned recommendations (not lockdown) in Iran, specifically in Tehran megacity. # These authors contributed equally to this work.
Compared to other megacities around the world, particularly in China, Italy, Spain, France, Germany, England and USA (Frontera et al., 2020;Tobías et al., 2020;Zambrano-Monserrate et al., 2020), lockdown measures were not adopted during the SARS-CoV-2 outbreak in Tehran. As a result, Tehran with the highest confirmed COVID-19 cases and associated deaths experienced an emergency situation during this outbreak and ranked the first place among all cities in Iran . Tehran, with the daytime population of more than 12 million people and about 9 million residents, is the capital and most populous city of Iran. The city has faced high levels of ambient air PM 10 and PM 2.5 during the past few decades due to transportation of more than 4 million vehicles in the megacity, as well as its proximity to a large number of industrial sources (Heger and Sarraf, 2018;Shamsipour et al., 2019;Yousefian et al., 2020). During the SARS-CoV-2 outbreak in Tehran in particular over the first month, the reduction of using the public transportation recommended by public health bodies at the national and subnational levels to minimize the close contact led to a remarkable increases of private transportation. The aim of this study was to compare the concentrations of ambient air PM 10 and PM 2.5 in Tehran during the SARS-CoV-2 outbreak and over the same period of last year.

METHODS
We obtained hourly air quality data of PM 10 and PM 2.5 , as the most common ambient air pollutants in Tehran, from 22 active ambient air quality monitoring stations (AQMSs; Fig. 1) belonging to the Tehran Air Quality Control Company for the period of 20 February-2 April 2020 as the SARS-CoV-2 outbreak and 20 February-3 April 2019 as the same period of last year. The air quality data processing was conducted based on our previous publications elsewhere (Faridi et al., 2018Yousefian et al., 2020). After air quality data processing, 15 and 16 eligible AQMSs were used to calculate the averages of ambient PM 2.5 and PM 10 in Tehran at the city level, respectively. The results were compared in three defined scenarios as follows: 1) the period between 20 February 2020 and 19 March 2020 (the first month of SARS-CoV-2 outbreak in Iran) in comparison to the same period of last year (20 February-20 March 2019); 2) Nowruz Iranian New Year holidays from 20 March 2020 to 2 April 2020 compared to 21 March-3 April 2019 and 3) during the whole study period, 20 February-2 April 2020, in comparison to 20 February-3 April 2019. In addition to the comparing of the results at the city level, we investigated the aforementioned scenarios at two stations with the highest (Shad abad) and lowest (Zone 2) ambient air particulate matter concentrations . To validate the results of the impact of SARS-CoV-2 on ambient air PM 2.5 and PM 10 in Tehran, the most important meteorological parameters, including wind speed, wind direction, temperature, precipitation and the number of rainy days, were investigated at three mentioned scenarios during the SARS-CoV-2 outbreak in comparison to the same period of last year. In fact, meteorological parameters were used for qualitative interpretation of the effect of SARS-CoV-2 on the ambient PM 2.5 and PM 10 level. The purpose of this comparison was to make sure the unfavorable or stagnant meteorological conditions did not indirectly increase the ambient air PM 2.5 and PM 10 concentrations during the outbreak in the city (Dantas et al., 2020;Kerimray et al., 2020).

RESULTS AND DISCUSSION
Tables 1-3 compare the ambient air PM 2.5 and PM 10 concentrations at three different scenarios. As shown in Table 1, the average concentration of PM 2.5 and PM 10 increased from 24.0 and 57.5 µg m -3 during the same period of last year to 28.9 and 66.5 µg m -3 over the first month of SARS-CoV-2 outbreak in Tehran, respectively. In fact, an increase of 5.0 µg m -3 (20.4%) and 9.0 µg m -3 (15.7%) in average concentrations of ambient PM 2.5 and PM 10 was experienced during the first month of SARS-CoV-2 outbreak in comparison to the same period of last year in Tehran. As the most polluted AQMS in Tehran according to our previous study , Shad abad ambient air monitoring station recorded the average concentrations of ambient PM 2.5 and PM 10 equal to 37.6 and 89.5 µg m -3 in the first month of SARS-CoV-2 outbreak, whereas these figures for ambient PM 2.5 and PM 10 were 28.4 and 70.1 µg m -3 in the same period of last year, respectively. Zone 2 AQMS in north of the city which normally recorded the lowest amount of pollutants ) experienced a noticeable increase of nearly 75.5% and 49.0% in the average concentrations for ambient PM 2.5 and PM 10 over the first month of SARS-CoV-2 outbreak compared to the same period of last year. SARS-CoV-2 outbreak caused a significant increase equal to 3.0 and 5.0 µg m -3 in ambient PM 2.5 and PM 10 average concentrations   (Table 2). In fact, the SARS-CoV-2 outbreak led to a rise of about 23.5% and 20.0% in the ambient air PM 2.5 and PM 10 concentrations in comparison to the last two Nowruz New Year holidays. Moreover, the SARS-CoV-2 caused a growth of 3.0 and 5.0 µg m -3 in the ambient air PM 2.5 and PM 10 concentrations at Shad abad AQMS, whereas the average concentrations of ambient PM 2.5 and PM 10 increased from 8.1 and 21.5 µg m -3 to 13.3 and 32.5 µg m -3 at Zone 2 AQMS, respectively ( Table 2). As can be seen in Table 3, the average concentrations of ambient PM 2.5 at Shad abad and Zone 2 AQMSs increased by 4.1 (20.5%), 7.1 (30.1%) and 9.5 (72.5%) µg m -3 during the SARS-CoV-2 outbreak in the third scenario (the whole study period) in comparison to the same period of last year. In general, over the SARS-CoV-2 outbreak in Tehran, the ambient PM 10 concentrations increased from 46.9, 34.2 and 57.9 µg m -3 to 54.6, 51.9 and 72.6 µg m -3 for Tehran city, Zone 2 and Shad abad AQMSs, a considerable rise equal to 16.4%, 51.7% and 25.4%, respectively. Detailed information (25 th , 50 th and 75 th percentiles) regarding the ambient PM 2.5 and PM 10 concentrations in Tehran during the SARS-CoV-2 outbreak days is presented in Tables 1-3. Detailed information on the meteorological parameters (wind speed, wind direction, temperature (minimum, average, maximum), precipitation and the number of rainy days) during the investigated scenarios is summarized in Table 4. As shown in Table 4, the average temperature during the three scenarios of SARS-CoV-2 outbreak was 12.3°C, 12.2°C and 12.2°C, while it was 9.4°C, 11.4°C and 10.1°C during the same period of last year, respectively. Additionally, the average of wind speed during the three scenarios of SARS-CoV-2 (3.0, 4.0 and 3.2 m s -1 for Scenario I, II and III, respectively) was approximately similar to those figures over the same period of last year (3.0, 3.3 and 3.4 m s -1 ). The number of rainy days during the three scenarios of SARS-CoV-2 outbreak was 8.0, 8.0 and 16.0 days, while it was 5.0, 6.0 and 11.0 days at the same period of last year. Lastly, the prevailing wind direction during the periods under investigation was from the west of Tehran city (Fig. 2). This study critically highlights the negative indirect effect of SARS-CoV-2 outbreak on the average concentration of ambient air PM 2.5 and PM 10 as the most notable air pollutants in Tehran. We investigated the changes in ambient air PM 2.5 and PM 10 concentrations due to the SARS-CoV-2 outbreak compared to the same period of last year through three different time period scenarios. The first scenario of investigation was across the first month of SARS-CoV-2 outbreak from  Table 4. Meteorological parameters during the SARS-CoV-2 and the same period of last year at three scenarios.

SARS-CoV-2 outbreak
The same period of last year (Scenario I) 20 February 2020 to 19 March 2020. Second and third scenarios were over the Nowruz New Year holidays as the longest holiday in Iran (two weeks from late March to early April each year) and the whole study period (20 February-2 April 2020), respectively. In addition to studying the average concentrations of ambient air PM 2.5 and PM 10 in Tehran at the city level, their concentrations also were compared at two AQMSs (Zone 2 and Shad abad) located at the highest and lowest socioeconomic regions of the city (Rohani-Rasaf et al., 2018). Based on our analysis, the average of ambient air PM 2.5 and PM 10 concentrations noticeably increased at the city level during the three scenarios of SARS-CoV-2 outbreak in comparison to the same periods of last year. Furthermore, we observed that the increase of ambient air PM 2.5 and PM 10 concentrations at Zone 2 AQMS was higher than those of Shad abad station. Nevertheless, Zone 2 station was ranked as the least polluted location throughout Tehran based on our previous publication . In fact, the growth of ambient air PM 2.5 and PM 10 concentrations at Shad abad AQMS (as the most polluted location in Tehran; Faridi et al., 2019) during the SARS-CoV-2 outbreak was much lower than the Zone 2 station. The average temperature during the three scenarios of SARS-CoV-2 outbreak was higher than those figures during the same period of last year (Table 4). Additionally, the average of wind speed during the three scenarios of SARS-CoV-2 was approximately similar to its figure over the same period of last year. The number of rainy days during the three scenarios of SARS-CoV-2 outbreak was higher than the same periods of last year. Lastly, the prevailing wind direction during the periods under investigation was exactly the same. These results show that the meteorological conditions did not affect the ambient PM 2.5 and PM 10 concentrations during the SARS-CoV-2 outbreak in Tehran compared to the same period of last year. In fact, the increase of ambient PM 2.5 and PM 10 concentrations during the SARS-CoV-2 outbreak can be attributed to the numerous potential socioeconomic factors and governmental recommendations. For example, some countries recommended and immediately implemented lockdown measures to reduce or prevent the SARS-CoV-2 outbreak (Dutheil et al., 2020;Frontera et al., 2020;Tobías et al., 2020;Zambrano-Monserrate et al., 2020); however, Iran faced two or three weeks' time lag to implement the restricting precautions (not lockdown measures) recommended by public health bodies at national and subnational levels. It was also endorsed by authorities/policy-makers to avoid public transportation (specifically metro lines and Bus Rapid Transit system with daily usage of between four and five million passengers as the most crowded public transportation sectors in Tehran; Heger and Sarraf, 2018) and the use of private cars might potentially lead to worse ambient air quality status during the SARS-CoV-2 outbreak in Tehran. Previously conducted studies have reported that there is a strong association between short-and long-term exposures to ambient air pollution, particularly PM 2.5 , and high lethality of SARS-CoV-2 (Conticini et al., 2020;Dutheil et al., 2020;Frontera et al., 2020;Yongjian et al., 2020). Consequently, we worry that the observed rise in the ambient air PM 2.5 and PM 10 concentrations might increase the lethality of SARS-CoV-2 outbreak in Tehran. We did not evaluate the increase in the number of private cars directly, but the potential reason for such increase in PM 2.5 and PM 10 level could be attributed to the usage of private cars. In agreement with the mentioned reason, previously conducted studies in Tehran have reported that mobile sources and associated secondary aerosols as the most notable sources of ambient PM 2.5 air pollution are responsible for approximately 75-80% of PM 2.5 emissions in Tehran (Taghvaee et al., 2018a, b (Xu et al., 2020a). In fact, the adopted and immediately implemented appropriate policies led to reduced ambient air pollutants in central China during the SARS-CoV-2 outbreak. Jain and Sharma (2020) showed that the implementation of social and travel lockdown directed to a decline in the ambient air pollutants (PM 2.5 , PM 10 , NO 2 and CO) in megacities of India. Xu et al. (2020b) showed that lockdown related to COVID-19 pandemic in Anqing, Hefei, and Suzhou (China) declined the ambient air pollutants of PM 2.5 , PM 10 , SO 2 , CO and NO 2 equal to 46.5%, 48.9%, 52.5%, 36.2%, and 52.8% compared to the same period of last three years. Similar to our study, Mohd Nadzir et al. (2020) stated the increase of ambient PM 2.5 and PM 10 concentrations during the Movement Control Orders (MCOs) as a lockdown measure in Kota Damansara, Malaysia. They reported that the ambient PM 2.5 and PM 10 concentrations increased up to 60% and 9.7%, respectively (Mohd Nadzir et al, 2020).

CONCLUSIONS
Contrary to expectation, the levels of ambient PM 2.5 and PM 10 in Tehran did not decrease during the SARS-CoV-2 outbreak. As we did not assess the change in concentration for gaseous air pollutants, further studies are necessary to determine the overall status of ambient air quality in this city. Crises such as this pandemic offer the opportunity to evaluate the direct/indirect effects of environmental policies and socioeconomic factors on emissions and air quality.