key: cord-0815944-zbm1kf8z authors: Zhang, Qianqian; Pan, Yuepeng; He, Yuexin; Walters, Wendell W.; Ni, Qianyin; Liu, Xuyan; Xu, Guangyi; Shao, Jiali; Jiang, Chunlai title: Substantial nitrogen oxides emission reduction from China due to COVID-19 and its impact on surface ozone and particle pollution date: 2020-09-06 journal: Sci Total Environ DOI: 10.1016/j.scitotenv.2020.142238 sha: 4210c4822bce1559ee7029acff97d599b777a663 doc_id: 815944 cord_uid: zbm1kf8z A top-down approach was employed to estimate the influence of lockdown measures implemented during the COVID-19 pandemic on NO x emissions and subsequent influence on surface PM2.5 and ozone air quality in China. The nation-wide NO x emission reduction of 53.4% due to the lockdown in 2020 quarter one in China may represent the current upper limit of China's NO x emission control. During the Chinese New Year Holiday (P2), NO x emission intensity in China declined by 44.7% compared to the preceding 3 weeks (P1). NO x emission intensity increased by 20.3% during the 4 weeks after P2 (P3), despite the unchanged NO2 column. It recovered to 2019 level at the end of March (P4). The East China (EC, 22°N - 42°N, 102°E - 122°E) received greater influence from COVID-19. Overall NO x emission from EC for 2020 first quarter is 40.5% lower than 2019, and in P4 it is still 22.9% below the same period in 2019. The 40.5% decrease of NO x emission in 2020 first quarter in East China lead to 36.5% increase of surface O3 and 12.5% decrease of surface PM2.5. The elevated O3 promotes the secondary aerosol formation through heterogeneous pathways. We recommend that the complicated interaction between PM2.5 and O3 should be considered in the emission control strategy making process in the future. NO x result in remarkable decrease in surface observed PM 2.5 concentration (Shi and Brasseur, 2020) . On the other hand, increase in O 3 and radicals due to NO x emission reduction leads to secondary aerosol formation and haze events during the lockdown period in 2020 (Huang et al., 2020; Le et al., 2020) . However, quantification of NO x emission reduction in 2020 quarte one impact on surface aerosol and ozone is still unclear. In this study, we aim to evaluate the influence of NO x emission changes for 2020 quarter one on both PM 2.5 and O 3 pollution over China. First we use a top-down method to assess NO x emissions from China for the periods before, during, and after the Chinese New Year Holiday, and we also provide a historical context by a comparison with NO x emission in 2019. Then we estimate PM 2.5 and O 3 changes due to NO x emission change through a model study. The TROPOMI onboard the S-5P mission provides tropospheric NO 2 with a spatial resolution of up to 3.55.5 km 2 . Tropospheric NO 2 vertical column density (VCD) was retrieved through a three-step procedure using the DOMINO approach (Boersma et al., 2011; van Geffen et al., 2020) . First, the slant column density (SCD) is obtained by a differential optical absorption spectroscopy (DOAS) technique. Second, the stratospheric component of SCD is separated via data assimilation using a chemical transport model (CTM/DA). Finally, tropospheric VCD is acquired by dividing SCD J o u r n a l P r e -p r o o f Journal Pre-proof by the tropospheric air mass factor (AMF) computed by the TM5 CTM. Tropospheric NO 2 VCD is sensitive to the NO 2 vertical profile shape used in calculating AMF (Lamsal et al., 2010) . The tropospheric AMF was calculated based on GEOS-Chem simulation and used in replacement of the a priori TM5 NO 2 profiles to diminish inconsistencies between model and satellite NO 2 columns (Boersma et al., 2016; Lamsal et al., 2010; Visser et al., 2019) . Tropospheric NO 2 VCDs obtained using AMF calculated from TM5, and GEOS-Chem NO 2 profiles are referred to as TROPOMI TM5 and TROPOMI GC , respectively. The difference between the two sets of tropospheric NO 2 VCD is shown in Fig. S1 . Overall, the two sets of data correlate well with each other, with the correlation coefficient r  0.90. The discrepancy between TROPOMI GC and TROPOMI TM5 is small over China (bias within  5%). TROPOMI GC is  20% higher than TROPOMI TM5 over South Korea and India. The disagreement between the two datasets can be attributed to the employment of different emission inventories within these models (Vinken et al., 2014) . For quality assurance, we use the TROPOMI data with a qa_value above 0.5, and only pixels with valid data for more than 3 days in each week were selected. In this study, we employ version v11-1 of the model to conduct a series of simulations to probe changes in NO x emissions. The nested grid version of the GEOS-Chem model has a horizontal resolution of 0.5°(latitude) × 0.625°(longitude) over East Asia (60°E  150°E, 10°S  55°N), and is driven by the MERRA-2 reanalysis meteorological field. We used the global simulation with 2°(latitude) × 2.5°(longitude) J o u r n a l P r e -p r o o f Journal Pre-proof grids to provide the boundary condition We use the mass balance procedure developed by Martin (2003) and Lamsal et al. (2011) and improved by Vinken et al. (2014) and Visser et al. (2019) to estimate the top-down NO x emissions using TROPOMI/NO 2 column and GEOS-Chem model. This method takes into account the fact that NO 2 VCD responds non-linearly to surface NO x emission change. We perform a sensitivity study by perturbing surface NO x emission by 15% to get the scaling factor β, which denotes relative change of simulated NO 2 VCD due to a 1% change in NO x emission. The influence of NO x emission on the tropospheric AMF calculation (the dimensionless factor γ) is also We conduct simulations for 2019 and 2020 first quarter with the top-down NO x emission inventories for the two years, respectively. Simulated surface NO 2 concentration is compared with observations ( Fig. S3) . The model and observation correlates well with each other with an correlation coefficient of 0.64 for the two years, but the model tends to underestimate NO 2 concentration by more than 50%. Surface NO 2 measurements are monitored by the chemilumine scenceanalyzer which may lead to overestimate of NO 2 by about 50% due to interferences from other nitrogen species (Kharol et al., 2015; Zhang et al., 2016) . If we simply cut the the observed NO 2 concentration by 50%, model underestimation will decline to 4.5% for 2019 and 13.5% for 2020. We divide the first quarter of 2019 and 2020 into four periods. The first period (P1) is 3-weeks before Chinese New Year holiday, the second period (P2) By contrast, NO x emission from EC in P3 only incrased by 9.5% than P2, and was 42.3% lower than P1. The mean NO x emission intensity from China for P2 and P3 (33.3 Gg/d) was 36.7% lower than P1. This difference was smaller than observed for NO 2 columns during the same periods of 49.8% (Fig. 3) . The discrepancy between NO x emission and NO 2 column differences can be explained by the shorter lifetime of NO 2 in February than in January. Before the Chinese New Year Holiday, NO x emissions from China was about 20% lower in 2020 than in 2019 (Fig. 2) . This change in NO x emissions reflects the long-term trend of decreasing NO x emissions in China (Bauwens et al., 2020; Field et al., 2020) , which has been an important and effective control strategy to combat air pollution. Following the holiday, differences in NO x emission from China between 2020 and 2019 were greater than 40% and as high as 54.3% during P2 and P3 periods. This large discrepancy indicates a substantial reduction in NO x emissions due to measures to combat COVID-19. to some extent, indicates that NO x emission in EC will see a further increase as the recovery of gasoline sales in April and May. In summary, the large decrase in NO x emission during the lockdown period may be the upper limit of current NO x emission control in China, and the COVID-19 has greater impact on NO x emission from the EC region than the national mean. Rebound of NO x emission from EC during P4 is weaker compared to the national mean. radical and OH have also increased (Fig. S4 ). Increase of oxidants in the atmosphere will promote formation of secondary aerosols (Huang et al., 2020; Sun et al., 2020) . Secondary aerosols form through homogeneous and heterogeneous pathways, and we find that the elevated oxidants may be more favorable for the heterogeneous pathway. We performed a sensitivity study in which the heteorogeneous formation of nitrate is isolated. The result shows that, in the STD simulation, 23.9% of nitrate is formed through uptake of N 2 O 5 on aerosols, and in the OPT simulation, the fraction increases to 28.8%. The large magnitude of decrease in NO x emission during the lockdown period helps to abate the winter time haze pollution over Central North China. However, the elevated O 3 and relevant atmospheric oxidants' concentration favors the formation of secondary aerosol, especially through the heterogeneous pathway. The outbreak of COVID-19 lead to substantial decrease of NO x emission and air Overall, NO x emission from China in 2020 first quarter is 28.1% lower than that in  NO x emission in the lockdown period is 53.4% lower than the same period in 2019.  East China received greater influence from COVID-19 than national mean level.  NO x decline leads to 36.5% O 3 increase and 12.5% PM 2.5 decrease over East China.  Elevated O 3 enhances secondary aerosols formation through heterogeneous pathway. 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Walters: Writing -Review & Editing Qianyin Ni: Resources Xuyan Liu: Investigation Guangyi Xu: Investigation Jiali Shao: Investigation Chunlai Jiang: Investigation