Secondary PM decreases signiﬁcantly less than NO 2 emission reductions during COVID lockdown in Germany

. This study estimates the inﬂuence of anthropogenic emission reductions on the concentration of particulate matter with a diameter smaller than 2.5 µ m (PM 2 . 5 ) during the 2020 lockdown period in German metropolitan areas. After accounting for meteorological effects, PM 2 . 5 concentrations during the spring 2020 lockdown period were 5 % lower compared to the same time period in 2019. However, during the 2020 pre-lockdown period (winter), meteorology accounted for PM 2 . 5 concentrations were 19 % lower than in 2019. Meanwhile, meteorology accounted for NO 2 concentrations dropped by 23 % 5 during the 2020 lockdown period compared to an only 9 % drop for the 2020 pre-lockdown period, both compared to 2019. Meteorology accounted for SO 2 and CO concentrations show no signiﬁcant changes during the 2020 lockdown period compared to 2019. GEOS-Chem (GC) simulation with a COVID-19 emission reduction scenario based on the observations (23 % reduction in NO X emission with unchanged VOC and SO 2 ) are consistent with the small reductions of PM 2 . 5 during the lockdown and are used to identify the underlying drivers for this. Due to being in a NO X saturated ozone production regime, 10 GC OH radical and O 3 concentrations increased (15 and 9 %, respectively) during the lockdown compared to a Business As Usual (no lockdown) scenario. The increased O 3 results in increased NO 3 radical concentrations, primarily during the night, despite the large reductions in NO 2 . Thus, the oxidative capacity of the atmosphere is increased in all three important oxidants, OH, O 3 , and NO 3 . PM nitrate formation from gas-phase nitric acid (HNO 3 ) is decreased during the lockdown as the increased OH concentration cannot compensate for the strong reductions in NO 2 resulting in decreased day-time HNO 3 formation from 15 the OH + NO 2 reaction. However, night-time formation of PM nitrate from N 2 O 5 hydrolysis is relatively unchanged. This results from the fact that increased night-time O 3 results in signiﬁcantly increased NO 3 which roughly balances the effect of the strong NO 2 reductions on N 2 O 5 formation. Ultimately, the only small observed decrease in lockdown PM 2 . 5 concentrations can be explained by the large contribution of night-time PM nitrate formation, generally enhanced sulfate formation and slightly decreased ammonium. This study also suggests that high PM 2 . 5 episodes in early spring are linked to high at- 20 mospheric ammonia concentrations combined with favorable meteorological conditions of low temperature and low boundary layer height. North-West Germany is a hot-spot of NH 3 emissions, primarily emitted from livestock farming and intensive agricultural activities (fertilizer application), with high NH 3 concentrations in the early spring and summer months. Based on our ﬁndings, we suggest that appropriate NO X and VOC emission controls are required to limit ozone, and that should also help reduce PM 2 . 5 . Regulation of NH 3 emissions, primarily from agricultural sectors, could result in signiﬁcant reductions in 25 PM 2 . 5 pollution. metropolitan areas. We use f NO 2( obs,emi ) and f CO ( obs,emi ) to capture fractional changes in anthropogenic NO X and VOC emission ( f NO X ( emission ) ) and f VOC ( emission ) ) ) due to lock down restrictions, respectively. Because of the scarcity of VOC measurements, CO data was used as a proxy for anthropogenic VOC (Fujita et al., 2003; Jiménez et al., 2005; Stephens et al., 2008; Yarwood et al., 2003) and NO 2 was used as proxy for NO X . This assumption is supported by studies such as Baker et al. (2008); Von Schneidemesser et al. (2010), which show anthropogenic VOC is well correlated with CO, and Blanchard and 145 Tanenbaum (2003), which shows comparable changes in VOC and CO between weekday and weekend. Changes in biogenic VOCs are not directly affected by lockdown measures.

tion of secondary PM 2.5 sources, as well as the processes important in secondary PM 2.5 formation. Despite of significant reductions in some anthropogenic activities, natural and agricultural air pollutant sources were not affected by the COVID-19 lockdown measures. Ammonia (NH 3 ) emissions (agricultural sources) are a significant source of PM 2.5 in Germany in the 60 spring (Fortems-Cheiney et al., 2016), when lockdown restrictions are implemented. Secondary inorganic aerosols such as ammonium sulfate and ammonium nitrate are the largest contributors to PM 2.5 in Europe (Pay et al., 2012;Petetin et al., 2016).
In comparison to sulfate formation, nitrate formation is more dependent on NH 3 concentration (Erisman and Schaap, 2004;Sharma et al., 2007;Wu et al., 2008). In the winter and spring (low temperature and high relative humidity), the role of NH 3 in PM 2.5 formation is greater than in the summer (high temperature and low relative humidity) (Schiferl et al., 2016;Squizzato et al., 2013;Viatte et al., 2020). Primary components of PM 2.5 are directly proportional to primary emission but secondary components of PM 2.5 are not directly proportional to secondary precursor emissions or concentrations as they are produced by non-linear complex atmospheric chemical reactions (Shah et al., 2018). Observational and modeling evidence is required to estimate the influence of change in precursor emissions on PM 2.5 concentrations. To this end, we used ground and space-based measurements of PM 2.5 , NO 2 , O 3 , SO 2 , CO and NH 3 in conjunction with GEOS-Chem simulations to investigate the influence 70 of lockdown restrictions on PM 2.5 concentrations.
Modelling studies such as Gaubert et al. (2021); Hammer et al. (2021); Matthias et al. (2021); Menut et al. (2020) have already reported the PM 2.5 changes across Europe including Germany, during the COVID-19 lockdown period. The activity data (e.g., transportation, industrial activities and energy production) were used in the above mentioned studies to create a COVID-19 emission reduction scenario (Doumbia et al., 2021;Guevara et al., 2021). However, there are large discrepancies between 75 various activity data sets (Gensheimer et al., 2021), necessitating different approaches to estimating the actual emission reduction caused by the COVID-19 lockdown restrictions. In this study, GEOS-Chem simulations (using identical anthropogenic emission for 2020 and 2019) were used to estimate the meteorology accounted for observed pollutant concentrations changes between 2020 and 2019, which were then used as a proxy for emissions reductions caused by COVID-19 lockdown measures to create a COVID-19 emission scenario in GEOS-Chem model for simulating the lockdown pollutant concentrations (Fig. 1). 80 In addition to looking at the impact of lockdown restrictions on air pollutant concentrations (Sect. 4.1), we focus on process level analysis of the impact of changes in precursor emissions (NO X ) on PM 2.5 formation (Sect. 4.2), as well as the role of ammonia (NH 3 ) emissions in PM 2.5 formation (Sect. 4.3). //s5phub.copernicus.eu). The TROPOMI SO 2 product provides the total SO 2 column between the surface and the top of troposphere. The TROPOMI overpass occurs around 13.30 local time. At the start of the mission, the TROPOMI product provided data at a resolution of 7*3.5 km, while after August 6, 2019 the resolution improved to 5.5*3.5 km. Stricter quality filtering criteria (quality assurance value (qa) >= 0.5) was applied to the dataset. A daily mean of SO 2 is calculated by averaging these 95 values within 0.5-degree radius of the urban center.
The daily atmospheric NH 3 variability in Germany was studied using the "near-real time daily IASI/Metop-B ammonia (NH3) total column (ANNI-NH3-v3)" dataset (products-obtained from https://iasi.aeris-data.fr/catalog/). The data used are from the IASI instrument aboard the Metop-B satellite, which has a local solar overpass time of 9:30 a.m and 9:30 p.m (Clerbaux et al., 2009). We only used day-time (9.30 am) measurements in this study. Night-time measurements (9.30 pm) 100 were excluded due to their large relative errors. A daily mean is calculated by averaging the values within 0.5-degree radius of the urban center. The monthly atmospheric NH 3 variability in Germany was studied using the "standard monthly IASI/Metop-B ULB-LATMOS ammonia (NH3) L3 product (total column)" dataset. This product contains a monthly averaged NH 3 total column with a spatial resolution of 1*1 degree (products-obtained from https://iasi.aeris-data.fr/catalog/).
Temperature, relative humidity, boundary layer height and wind information are obtained from the ERA 5 product (Hersbach are used in the GC simulations for both 2019 and 2020, but with the corresponding meteorology from MERRA-2 global reanalysis product for 2019 and 2020. Natural emissions from soil and lightning are calculated for the corresponding year using mechanisms described in Hudman et al. (2012) and Murray (2016). The corresponding year's open fire emissions from 115 GFED4 (Werf et al., 2017) are used for 2019 and 2020. In the second case, the anthropogenic emission inventory were scaled down by the estimated emissions reduction caused by the lockdown restrictions for the 2020 lockdown period. The remaining (natural and fire) emissions are calculated in the same way as in the first case.

Method
The following is our methodology for estimating meteorology accounted for observed pollutant concentration changes between To disentangle the meteorology contribution from the observed pollutant concentration changes, we subtract the GC pollutant where, "obs", "GC" and "obs,emi" refer to ground-truth measurements, GEOS-Chem simulations and meteorology accounted for ground-truth measurements, respectively.
We estimate the meteorology accounted for fractional change in other pollutant concentrations analogously. Our previous study (Balamurugan et al., 2021), using the same methodology, reported the meteorology accounted for NO 2 and O 3 concentration changes for eight German metropolitan areas. Here, we reproduce the results for NO 2 and O 3 concentrations, but for ten 140 metropolitan areas. We use f NO 2(obs,emi) and f CO (obs,emi) to capture fractional changes in anthropogenic NO X and VOC emission (f NO X(emission) ) and f VOC (emission) )) due to lock down restrictions, respectively. Because of the scarcity of VOC measurements, CO data was used as a proxy for anthropogenic VOC (Fujita et al., 2003;Jiménez et al., 2005;Stephens et al., 2008;Yarwood et al., 2003) and NO 2 was used as proxy for NO X . This assumption is supported by studies such as Baker et al.  The base anthropogenic emission inventory were then scaled down by f NO X(emission) and f VOC (emission) for NO X and VOC emission, respectively, in the GC model for the 2020 lockdown period (second case), which simulates all pollutants concentrations for the lockdown emission scenario. The fractional change in emission accounted for, i.e. using scaled emission inventories, GC pollutants level during the 2020 lockdown period compared to 2020 Business As Usual (BAU), i.e., no lockdown, level is calculated as, where, "GC,emi" refers to GC simulations accounting for scaled emission and PM 2.5(GC,2020,lock) are the PM 2.5 concentrations during the lockdown period determined via the 2020 GC simulations with down-scaled emissions. We estimate the emission accounted for concentration changes of other pollutants in the same way.  (late spring) -Loose lockdown measures. Germany experienced high wind conditions due to storms in February 2020 (Matthias et al., 2021), which was used to determine the extent of meteorology's role in pollutant concentration changes. Meteorology unaccounted for mean NO 2 and PM 2.5 concentrations for February 1 to March 20, 2020 period (before the implementation of 185 lockdown) are lower than the corresponding ones in 2019 by 30 % and 42 % (f NO 2(obs) and f PM 2.5(obs) ), respectively, due to the dilution/dispersion from the high wind conditions. However, after accounting for meteorology, the difference in mean NO 2 and PM 2.5 concentrations between 2020 and 2019 for the period February 1 to March 20 (f NO 2(obs,emi) and f PM 2.5(obs,emi) ) are 8 % and 18 %, respectively. This finding is consistent with meteorology accounted for mean NO 2 and PM 2.5 changes between 2020 and 2019 for the period January 1 to January 31 ( Fig. 2 (a,b)). This highlights the importance of accounting for 190 meteorological impacts.
In the 2020 pre-lockdown period (January 1 to March 20), both meteorology accounted for mean NO 2 and PM 2.5 levels are lower by 9 % and 19 %, respectively, compared to the same period in 2019. During the 2020 lockdown period (March 21 to May 31), mean meteorology accounted for NO 2 concentrations dropped significantly (23 %) compared to the same period in 2019, which is greater than the drop in the 2020 pre-lockdown period compared to 2019 (9 %). Comparatively, mean meteorology accounted for 2020 lockdown PM 2.5 concentrations show a smaller reduction (5 %) compared to the same period in 2019, while an important precursor, NO 2 , decreased by 23 % during the same period. Furthermore, the meteorology accounted for PM 2.5 reduction during the 2020 lockdown period (5 %) is less than the meteorology accounted for PM 2.5 reduction observed during the 2020 pre-lockdown period (19 %) compared to the corresponding 2019 periods (Fig. 2). Especially in Munich and Stuttgart, meteorology accounted for PM 2.5 concentrations during the 2020 lockdown period are higher than in 2019. The 200 meteorology accounted for mean O 3 concentrations in the 2020 lockdown period are increased by 6 % compared to the same period in 2019. The increase in O 3 concentration during the 2020 lockdown period is mainly due to being in a NO X saturated regime (Gaubert et al., 2021), in which reducing NO X emission results in an increase in O 3 concentrations (Sillman, 1999;Sillman et al., 1990).

Model evidence of changes in air pollutants concentration resulting from lockdown restrictions
As mentioned in Sect. 3, we use the meteorology accounted for NO 2 and CO changes to adjust the anthropogenic NO X and 220 VOC emissions in inventories due to lockdown restriction impacts. GC model simulations are then obtained with this scaled anthropogenic emission scenario (23 % reduction in NO X emission and unchanged VOC emissions) for the 2020 lockdown period. The NO X emission reduction is within the range of estimated NO X emission reductions using activity data for Europe

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(2021) estimated 8 % reduction in VOC emissions. However, the real-time measurements at a United Kingdom station show no significant changes in many VOC concentrations during the lockdown period (Grange et al., 2020). For the NO X saturated ozone production regime regime, VOC emission reductions can decrease ozone levels, while NO X emission reductions increase them. Gaubert et al. (2021) conducted a sensitivity study of modelling work on ozone levels in response to the NO X or VOC  2021), results in slight increase in lockdown ozone levels (< 2.5 %) over only north-western Germany and slight decrease in lockdown ozone levels over other regions of Germany, compared to BAU levels. But, only reduction in NO X emission results in increased lockdown ozone levels (0-10 %) over all of Germany compared to BAU levels, which is also consistent with our results of increase in meteorology accounted for ozone levels over different metropolitan areas across Germany during 2020 lockdown period compared to 2019 levels. This implies that VOC emissions were either not reduced at 235 all or by a much smaller percentage than NO X emissions.
The emission accounted for GC lockdown NO 2 concentrations decreased by 21 % (f NO 2(GC,emi) ) while emission accounted for GC lockdown O 3 concentrations increased by 9 % compared to 2020 BAU (Fig. 3) . This is consistent with previous studies (such as Balamurugan et al. (2021); Gaubert et al. (2021)) which show that German metropolitan areas are in a NO X saturated ozone production regime in spring. The emission accounted for GC lockdown PM concentrations show small 240 decreases compared to 2020 BAU (Fig. 3). These results are consistent with previous studies (Gaubert et al., 2021;Hammer et al., 2021;Matthias et al., 2021;Menut et al., 2020), which used activity data to develop an emission reduction scenario and estimated small to no reduction in PM 2.5 , a significant drop in NO 2 and marginal increase in O 3 levels during 2020 lockdown period, compared to BAU levels, over Northern-Europe including Germany.
total PM 2.5 . Major secondary PM 2.5 components are nitrate, sulfate, ammonium and organic aerosol, which, on average, correspond to 24 %, 23 %, 15 % and 30 % of total PM 2.5 , respectively, during March 21 to May 31, 2019 (Fig. C1). Mean relative contribution of PM 2.5 species for 2020 (BAU) and 2020 (lockdown) are shown in Fig. D1 and E1, respectively. The emission accounted for GC PM nitrate levels during the 2020 initial lockdown period (March 21 to April 30) are 9.5 % lower than the 2020 BAU levels (f NIT (GC,emi) ) ( Fig. 3 (a)), however, we see NO 2 decreased by 21 % during the same period.

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The decrease in emission accounted for GC PM nitrate is also less than the decrease in NO 2 during the second half of the  Wang et al., 2013). The emission accounted for concentrations of OH and NO 3 , which drive day and night-time formation of PM nitrate, increased substantially (15 % and 12 %, respectively) during the lockdown period compared to BAU (Fig. 3). The increase in OH radicals results from German metropolitan areas being in a NO X saturated regime (Shah et al., 2020). The increase in GC lockdown NO 3 levels is predominantly at night due to a significant increase in night-time O 3 (Fig. 4 (b,e)); the reaction of NO 2 with O 3 is the most important source of NO 3 radical (Eq. R2) (Geyer et al., 2001).  Liu et al. (2020) have demonstrated that analyzing the diurnal cycle of total inorganic nitrate helps to identify the dominant pathway for the particulate nitrate production. The emission accounted for GC lockdown PM nitrate levels decreased significantly during the day, while night-time lockdown PM nitrate levels decreased slightly compared to BAU levels ( Fig. 4 (h)).
Even though GC lockdown OH levels increased, HNO 3 production from the OH+NO 2 reaction during the lockdown period is reduced due to significantly lower day-time NO 2 levels compared to BAU (Fig. 4 (d)); as a result, GC day-time lockdown PM 280 nitrate levels are significantly lower compared to BAU levels. However, higher night-time NO 3 levels result in higher nighttime HNO 3 production from N 2 O 5 hydrolysis, resulting in slightly lower night-time lockdown PM nitrate compared to BAU ( Fig. 4 (b,e,f,g)). This implies that the increase in NO 3 radical due to increased ozone partially offset the effect of reduced NO X on nitrate formation. Previous studies have also shown that N 2 O 5 hydrolysis plays important role in nitrate formation than the gas-phase day-time pathway (NO 2 + OH) (Allen et al., 2015;Chan et al., 2021;Kim et al., 2014;Liu et al., 2020;Yan et al., 285 2019). Figure 5 illustrates the conceptual model of generalized day and night-time lockdown NO X chemistry compared to BAU scenario. The oxidation of SO 2 is a major source of sulfate, and the reaction with the OH radical dominates the gas-phase oxidation of SO 2 (Zhang et al., 2015). Therefore, the enhanced sulfate formation during the 2020 lockdown period could be due to the increased oxidizing capacity of atmosphere (OH) since we observe no significant change in emission accounted for GC SO 2 concentration, compared to BAU concentration (Fig. 3). Organic aerosol (OA) formation could be affected by the It is worth noting that a significant fraction of PM 2.5 is PM nitrate. Ammonia (NH 3 ) is an important precursor for particulate nitrate formation (Ansari and Pandis, 1998;Banzhaf et al., 2013;Behera and Sharma, 2010;Wu et al., 2016). This explains the importance of monitoring and potentially regulating ammonia emissions. Therefore, the inter-and intra-annual changes in ammonia (NH 3 ) concentrations over Germany, as well as their relationship to PM 2.5 variability, are reviewed and analyzed further below. In Germany, atmospheric NH 3 levels follow a monthly pattern, with NH 3 levels peaking in April ( Fig. 6 (b) 300 and 7). NH 3 levels are also elevated during summer months. In Europe, major agricultural practices (fertilizer and manure applications) take place in the early spring (Petetin et al., 2016;Ramanantenasoa et al., 2018;Viatte et al., 2020). The higher atmospheric ammonia levels in April are attributable to agricultural practices such as fertilizer application. The high NH 3 values in summer are most likely due to warm climates (Kuttippurath et al., 2020 North-West Germany is a hotspot of ammonia emissions compared to the rest of the country. North-West Germany is known for its high livestock density (livestock farming (EUR, 2013;Scarlat et al., 2018)) and it is dominated by crop and grass land (ESA, 2017). Livestock farming and fertilizer application account for 75 % of NH 3 emissions in Europe (Webb et al., 2005).  High PM pollution episodes are likely to occur frequently during the winter due to high residential heating demand and favorable meteorological conditions (e.g., low temperature and inversion condition). However, high concentrations of PM 2.5 are apparent in German metropolitan areas in the early spring (from the second half of March to the end of April, e.g., Fig. 6 (a) for Cologne metropolitan area). On March 21, 2020, the German government imposed COVID-19 lockdown restrictions.

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However, in-situ PM 2.5 concentrations during the initial lockdown period are higher than during the pre-lockdown period in 2020. High PM 2.5 levels from the second half of March to the end of April are also consistent with previous years without lockdown restrictions. It is notable that this high spring PM 2.5 episodes are associated with high NH 3 concentrations ( Fig. 6 (b)). The high PM 2.5 events that occur in the spring have also been observed in other European cities, and they typically contain ammonium nitrate and ammonium sulfate (Fortems-Cheiney et al., 2016;Renner and Wolke, 2010;Schaap et al., 2004;Viatte 325 et al., 2020Viatte 325 et al., , 2021. Above, we show the high NH 3 levels in early spring (April) and summer months. High PM 2.5 concentrations are evident in spring, however, we did not observe high PM 2.5 episodes in summer ( Fig. 6 (a)). It is also worth noting that even   in the spring and winter PM 2.5 is not consistently high on days with high NH 3 . This reflects the complexity of the process of gas to particle conversion. Despite high NH 3 concentrations, ammonia(NH 3 )-to-ammonium(NH 4 ) conversion is mainly driven by various meteorological factors such as temperature (and relative humidity). Studies (Viatte et al., 2020;Wang et al., 2015;330 Watson et al., 1994) have shown that conditions such as temperature of less than 10 • C and a high relative humidity of more than 70 % are optimal for atmospheric gas-phase NH 3 to transform into ammonium salts, mainly due to reversible ammonium nitrate formation, which depends on temperature and relative humidity; warm and dry conditions partition ammonia back to the gas phase (Mozurkewich, 1993). In comparison to summer, the impact of NH 3 on PM 2.5 formation is considerable for winter and spring over Europe (Viatte et al., 2020(Viatte et al., , 2021 and the US (Schiferl et al., 2016). Summer weather is typically warmer 335 (and has lower relative humidity) than winter and spring, which could explain why high NH 3 concentrations are not associated with high PM 2.5 in summer or late spring. To further demonstrate this for German metropolitan areas, we consider two cases ("Simultaneous" and "Independent") for 2018 and 2019 ( Fig. 6 (d)). "Simultaneous" -Simultaneous increase in NH 3 (IASI) and PM 2.5 (in-situ) concentrations on same day. "Independent" -Increase in NH 3 (IASI) concentration not corresponding to an increase in PM 2.5 (in-situ) concentration on same day. As an example, for the Cologne metropolitan area, the temperature 340 and boundary layer height for the "Simultaneous" case (11.7±6.8 • C and 500.4±166.5 m, respectively) is lower than for the "Independent" case (13.4±6 • C and 628.9±274.3 m, respectively). In addition to low temperature, low boundary layer height results in higher pollutant concentrations and can thus result in more intense atmospheric chemical reactions. We found similar results for other metropolitan areas, but with different absolute values (Fig. 6 (d)). The regional differences are unsurprising, because other factors also influence the formation of PM 2.5 from NH 3 (e.g., other precursor concentrations such as NO X and 345 SO X ). However, these findings support previous studies and imply that low temperature and low boundary layer height are most favorable for the formation of PM 2.5 during the periods of high NH 3 . GC also simulates the high spring PM 2.5 concentrations that have been observed, with high ammonium (NH 4 ) concentrations ( Fig. 6 (c)).

Conclusions
Our study estimates the influence of anthropogenic emission reductions on PM 2.5 concentration changes during the 2020 lock-  Based on our findings, we suggest that additional emission control measures aimed at reducing ozone pollution be implemented which should also help reduce PM. A concurrent reduction of NO X and VOCs emissions should occur. Otherwise, ozone levels will rise as NO X emissions drop, increasing oxidizing capacity, until a NO X limited ozone production regime 370 is reached. We also addressed the annual spring PM 2.5 pollution episodes in German metropolitan areas, which are associ-ated with high NH 3 concentrations. North-West Germany is a hot-spot of NH 3 emissions, primarily emitted from livestock farming and intensive agricultural activities (fertilizer application), with high NH 3 concentrations in the early spring and summer months. Winter and spring meteorological conditions are more favorable for PM 2.5 formation from NH 3 than summer.
Unsurprisingly, low temperature (and low boundary layer height) is shown to be a favorable meteorological condition for the 375 formation of PM 2.5 from NH 3 . Regulation of NH 3 emissions, primarily from agriculture, has the potential to reduce PM 2.5 pollution significantly in German metropolitan areas.
In this study, a COVID-19 emission reduction scenario was created using meteorology accounted for proxy pollutant concentration changes, assuming that observed proxy pollutant concentration changes are due to the combined direct effects of emission and meteorology changes. Our GC modeling study work reflects the assumed direct relationship between changes in 380 meteorology accounted for NO 2 concentration and changes in NO X emission. This work also shows a direct relationship between changes in meteorology accounted for SO 2 (and CO) concentration and changes in SO X (and CO) emission. However, due to the non-linear feedback system in atmospheric chemistry, this assumption should be investigated further. Because of their similar sources, we use CO concentration as a proxy for anthropogenic VOC concentration. However, this is debatable because VOC is more reactive than CO. We call for further advancements in estimating the emission changes during the lock-385 down period, which would allow us to estimate the precise sensitivity of PM 2.5 to changes in emissions from various sources and comparison of VOC emission inventories with observations. This will help in the implementation of appropriate air quality regulation strategies in the future.