Beginning in January 2020, a novel coronavirus (COVID-19) began rapidly spreading throughout China, first in Wuhan, and then in other major cities 1, 2 . In response, to prevent the spread of COVID-19, the first lockdown was implemented on January 23rd, suspending nonessential traffic in Wuhan, the epicenter of the outbreak. Over the following days, major cities throughout China issued similar travel restrictions and stringent lockdown measures, affecting over half a billion residents. These restrictive measures resulted in substantial reductions in human activities, which consequently induced unprecedented decreases in anthropogenic emissions of air pollutants, especially from the industry and transportation sectors 3, 4 . In particular, during the lockdown, NO2 levels in Eastern China were estimated to have decreased by ~65% compared to the same period in 2019, mainly due to the reduction of vehicle emissions 5, 6 . Similarly, other pollutants such as SO2 and CO also decreased. In contrast, during the same period, ozone (O3) increased significantly with a nationwide increase of 47.3% 5, 7, 8 .
Besides the lockdown period, China experienced persistent O3 pollutions in recent five years, especially in urban areas [9] [10] [11] . O3 is formed through non-linear photochemical reactions of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs) 12 . The O3 sensitivity regime, determined by the relative abundance of VOCs and NOx, plays a significant role in O3 formation 13, 14 . Previous studies have reported that the elevated O3 during lockdowns was mainly attributed to the enhanced atmospheric oxidation capacity (AOC) 15, 16 , reflected by the levels of major oxidants such as hydroxyl radical (OH) and nitrate radical (NO3) 17 . Specifically, NO2 is defined as the main sink of OH radical through the reaction to form nitric acid (HNO3) 18, 19 . During the lockdown, the drastic decreases in NO2 levels increased OH concentration, which then reacted with VOCs, facilitating the formation of secondary pollutants 20 . Unfortunately, the current understanding of AOC and secondary pollution is still limited [21] [22] [23] . The COVID-19 lockdown provides an important opportunity to study the interaction between emissions, AOC, and meteorological conditions.
In this study, we used observational and satellite data to identify changes in O3 levels and its associated precursors (NO2 and HCHO) during the COVID-19 lockdowns in China. The Community Multiscale Air Quality (CMAQ) model was also applied to analyze the characteristics of air quality in the same period. The roles of the O3 sensitivity regime and AOC were also discussed to provide an in-depth explanation for the increase in O3. We found that O3 elevations during the COVID-19 lockdown period in the NCP and YRD were mainly controlled by enhanced AOC, even suppressing the impacts of the O3 formation regime due to the reductions of anthropogenic emissions. In contrast, O3 decreased slightly in the PRD. The results aim to formulate more effective emission control policies, particularly focused on reducing AOC to battle the persistent O3 pollution in China. . 1a) , consistent with previous studies 5, 15, 24 . The most prominent O3 increase occurred in the NCP (Fig. 1b) , with a mean MDA8 O3 increase of 54% (from 24 ppb to 37 ppb) (Fig.1c) . In Baoding and Shijiazhuang (major cities in the NCP), O3 increased by over 100%. Moreover, in the YRD, a noticeable MDA8 O3 increase of 44% (from 26 ppb to 38 ppb) was observed. During Post-lockdown, observed O3 concentrations continued to increase in the NCP and YRD, partially due to the rising temperature (Fig. S2 ). O3 variation is more complex in the PRD, however. In general, O3 levels decrease from Pre-lockdown to Post-lockdown. But in Guangzhou, the most populated city of PRD, an increase in O3 was observed. Considering the similar temperature levels between Pre-lockdown and Lockdown over China, these variations are more related to the sudden reductions of O3
precursors.
In addition, heightened O3 pollution was observed in the NCP and YRD during the Lockdown compared to the same period in 2019 (Fig. S3 ). Although O3 precursors decreased drastically in these regions, mean MDA8 O3 levels were 14-19% higher than in 2019. In contrast, in the PRD, the mean MDA8 O3 during the Lockdown of 2019 is close to (or even slightly higher) that in 2020, and the opposite of the trend observed in the other regions. Similar temperature conditions are also found during the Lockdown in 2019 and 2020, which fails to fully elucidate the O3 differences between these two years, demonstrating that the emission reduction of O3 precursors plays a more critical role in O3 variations. Considering the O3 results in the Lockdown periods in 2019 and 2020, it is critical to deeply understand the seesaw phenomenon between elevated O3 and its reduced precursors.
Given that the ratio of HCHO to NO2 determines the O3 formation regimes, HCHO and NO2 are considered the most important precursors of O3 25 . The satellite column data and CMAQ model have revealed significant reductions of NO2 throughout much of China, especially in NCP and YRD regions ( Fig S4a) . According to the satellite data, NO2 in the NCP, YRD, and PRD regions declined by 59.61%, 63.28%, and 44.03% during the Lockdown respectively. These reductions are mainly attributed to the significant decline of NOx emissions from industry, power, and transportation sectors illustrated by the source apportionment analysis (Table. S6 ).
However, no noticeable changes were observed in the HCHO concentration during the Lockdown. The spatial distribution of HCHO, similar to that of NO2, exhibits higher levels in southeast China, whereas in western China, due to the low anthropogenic VOCs emissions, the HCHO concentration is relatively low 26, 27 . The HCHO in the atmosphere is mainly formed through direct emissions from industrial and biogenic sectors and through secondary sources such as the oxidation reaction between VOCs and OH. During the Lockdown, emissions of HCHO and other VOCs declined significantly (-37%) in China (Table.S1) and therefore might have reduced HCHO levels. However, the enhanced AOC 5 during Lockdown likely promoted the formation of HCHO from secondary sources, offsetting the impact of the decline in HCHO emissions and explaining why HCHO levels remained relatively constant. Also, the oxidation of methane, which has a long lifetime and relatively stable concentrations, plays an important role in the HCHO concentration 28, 29 , which also might help maintain constant HCHO levels as shown in our source apportionment analysis (Table. S6) during the Lockdown period.
In general, the O3 sensitivity regimes in China shifted from VOC-limited to NOx-limited during the Lockdown, as indicated by both satellite data and model simulations (Fig. 2b) . During the Pre-Lockdown period in 2020, the VOC-limited regime dominates in the NCP, YRD, and PRD regions due to the relative abundant NOx emissions from industry and transportation sectors, consistent with the previous studies 30 . However, during the Lockdown period, VOC-limited regimes transitioned to NOx-limited regimes in these regions. The percentage of NOx-limited regimes in the NCP, YRD, and PRD regions during the Lockdown period increased from 11%, 37%, and 31% to 56%, 65%, and 69%, respectively. These changes in the O3 formation regime and NO2 and HCHO concentrations were not observed in 2019 (Fig. S5a ); NO2 and HCHO concentrations during the periods in 2019 that correspond to the Pre-lockdown and Lockdown periods in 2020 remained relatively constant, explaining the lack of variation in the O3 formation regimes (Fig. S5b) .
Surprisingly, O3 levels increased in the NOx-limited regimes in the NCP and YRD regions during the Lockdown period in 2020, even though pronounced reductions of NOx levels were observed in these regions. This unusual phenomenon is contrary to previous studies 28 , indicating that the enhanced AOC might have played a more significant role in increasing O3 in these regions during the Lockdown period in 2020. The OH radicals oxidize VOCs to produce peroxy radicals such as HO2, which covert NO to NO2 without consuming O3. Then the NO2 produces O3 through photolysis reactions, leading the O3 accumulation 34, 35 .
At the same time, the NO3 radical, the primary nighttime oxidant, saw significantly increased levels in the NCP and YRD regions during the Lockdown, with respective growth rates of 0.49 and 0.29 ppt due to the relatively low levels of VOC and NO2, both of which serve as important sinks for the NO3 radical 36 (Fig. S6a ). In contrast, in the PRD region, levels of the NO3 radical declined (up to -0.21 ppt) ( Fig. 3b-d) . Given the enhanced AOC, a significant increase in O3 was observed in the NCP and YRD regions. In the PRD, however, the constant AOC was responsible for a slight decrease in O3. Importantly, in the NCP and YRD regions, the increase in O3 enhanced the AOC due to local photochemistry 37, 38 , creating a vicious cycle of heightening O3 levels.
O3 control policy implications. Based on this analysis, we have devised a conceptual scheme to demonstrate the roles of the O3 formation regime shift and enhanced AOC on O3 level during the COVID-19 Lockdown period in China (Fig. 4) . Satellite data and model simulations have revealed a pronounced shift in the O3 formation regime during the 2020 lockdown period from VOC-limited to NOx-limited, due to the significant decline in NO2. Simultaneously, throughout much of China, the AOC was enhanced, facilitating the secondary formation of O3 and HCHO in the atmosphere.
Given the synergistic effects of enhanced AOC and the O3 formation regime shift, a substantial increase in O3 has occurred during the 2020 Lockdown period in much of China, particularly in the NCP and YRD regions. In contrast, O3 levels during this period remained relatively stable in the 
in October 2017 to monitor the spatiotemporal variation of air pollutants, greenhouse gases, UV radiation, and clouds, worldwide. The onboard sensor, TROPOMI (TROPOspheric Monitoring Instrument), has a sun-synchronous orbit with a local overpass time of approximately 13:30 and near-daily global coverage [1] [2] [3] . TROPOMI data products are significantly improved compared to the still operating OMI sensor, launched in 2004; specifically, TROPOMI data products feature significantly increased spatial resolution and 1~5 times higher signal-to-noise ratios, compared to OMI products 4 . On August 6, 2019, TROPOMI began outputting data at a higher resolution than before (each near-nadir pixel size is now roughly 5.5 × 3.5 km 2 ) 5 .
TROPOMI products are available for free through the Copernicus Open Access Hub (https://scihub.copernicus.eu, last access: 30th July, 2020). Google Earth Engine (https://developers.google.com/earth-engine/datasets/catalog, last access: 30th July, 2020) has converted Level 2 TROPOMI data, provided by the Copernicus Open Access Hub, into the level 3 data used in this study through the bin_spatial operation for the harpconvert tool. In this study, we utilized two TROPOMI datasets from Google Earth Engine, tropospheric NO2 column number density, and tropospheric HCHO column number density, and re-projected them into the same domain as model simulations by using a Lambert projection As suggested by Copernicus, Google Earth Engine filters the source data to remove pixels with QA values less than 75% for tropospheric NO2 column number density datasets and 50% for tropospheric HCHO column number density. (Table   S1) 10 The simulation of RH was comparable with previous studies 12, 14, 15 . Overall, the WRF model performed well, providing the CMAQ model with reliable meteorological inputs.
The CMAQ model simulations were validated using the observations (~1500 sites) from the national air quality monitoring network (http://www.cnemc.cn/) (Table S3- The NO2 and HCHO column concentrations generated by the model were then compared with the satellite data to determine the shift in O3 formation regimes. In our study, the O3 formation regimes were categorized into VOC-limited, NOx-limited and transition regimes based on the formaldehyde nitrogen ratio (FNR) 18, 19 . Here we set FNR < 1.0 as a VOC-limited regime, FNR > 2.0 as a NOxlimited regime and FNR between 1.0 and 2.0 as a transitional regime 20, 21 .
The mean bias (MB), gross error (GE), and root mean square error (RMSE) were used for the evaluation of WRF model performance, while the mean normalized bias (MNB), mean normalized error (MNE), mean fractional bias (MFB) and mean fractional error (MFE) were used for the validation of CMAQ model performance.
where Cm,i and Co,i refer to the i th predicted and observed value, respectively. N is the number of prediction-observation pairs drawn from all measurement sites. 