Spatial and temporal changes of the ozone sensitivity in China based on satellite and ground-based observations

Ground-level ozone (O3) pollution has been steadily getting worse in most part of eastern China during the past five years. The non-linearity of O3 formation with its precursors like nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs) are complicating effective O3 abatement plans. The diagnosis from space-based observations, the ratio of 10 formaldehyde (HCHO) columns to tropospheric NO2 columns (HCHO/NO2), has previously been proved to be highly consistent with our current understanding of surface O3 chemistry. HCHO/NO2 ratio thresholds distinguishing O3 formation sensitivity depend on regions and O3 chemistry interactions with aerosol. To shed more light on current the O3 formation sensitivity over China, we have derived HCHO/NO2 ratio thresholds by directly connecting satellite-based HCHO/NO2 observations and ground-based O3 measurements over the major Chinese cities in this study. We find that a VOC-limited 15 regime occurs for HCHO/NO2 < 2.3 and NOx-limited regime occurs for HCHO/NO2 > 4.2. The HCHO/NO2 between 2.3 and 4.2 reflects the transition between the two regimes. Our method shows that the O3 formation sensitivity tends to be VOClimited over urban areas and NOx-limited over rural and remote areas in China. We find that there is a shift in some cities from the VOC-limited to the transitional regime that is associated with a rapid drop of anthropogenic NOx emissions owing to the widely-applied rigorous emission control strategies between 2016 and 2019. This detected spatial expansion of the transitional 20 regime is supported by rising surface O3 concentrations. The enhanced O3 concentrations in urban areas during the COVID19 lockdown in China indicate that a protocol with simultaneous anthropogenic NOx emissions and VOC emissions controls is essential for O3 abatement plans.

(HCHO) for VOC (Palmer et al., 2003;Fu et al., 2007). NOx can be approximated from satellite observation of NO2 column 65 because of the short lifetime of NOx and high ratio of NO2/NOx in the boundary layer (Duncan et al., 2010;Jin and Holloway, 2015). HCHO is an intermediate of the oxidation reaction of various VOCs in the atmosphere. The production of HCHO is approximately proportional to the summed rate of reactions of VOC with peroxy radicals (Sillman, 1995). Therefore, HCHO can be used as a tracer for VOCs in the absence of other VOC observations (Martin et al., 2004;Duncan et al., 2010). The O3 formation sensitivity is defined by the ratio of HCHO to NO2 (referred to as FNR) (Martin et al., 2004). Duncan et al. (2010) 70 combined models and OMI HCHO and NO2 data to show certain ranges of FNR that can be useful for classifying a region into VOC-limited or NOx-limited regime. A FNR smaller than 1 indicates the VOC-limited conditions, and a FNR higher than 2 to indicate the NOx-limited conditions. A FNR in the range of 1 -2 should generally be considered as indicative of the transitional regime. These FNR thresholds defined by Duncan et al. (2010) have been widely used for various regions (Choi and Souri, 2015;Jin and Holloway, 2015;Souri et al., 2017;Jeon et al., 2018) and with different satellite instruments (Choi et al., 2012). 75 However, these prior studies linked FNR with surface O3 sensitivity in models (Martin et al., 2004;Duncan et al., 2010).
Modeled and observed HCHO columns, NO2 columns and surface O3 often disagree. Jin et al. (2017) found that the spatial and temporal correlations between the modeled and satellite-derived FNR vary over the used satellite instruments. Brown-Steiner et al. (2015) found persistent O3 biases under all configurations of the Global Climate-Chemistry Model (GCCM) with detailed tropospheric chemistry. Although FNR thresholds defined by Duncan et al. (2010) have been used previously to 80 investigate O3-NOx-VOC sensitivity in China (Witte et al., 2011;Tang et al., 2012;Jin and Holloway, 2015), their conclusions were based on the atmospheric situations in the United States and may not be suitable for the more complicated air pollution in China, concerning the different emission factors, sources, pollution levels and climatology. For example, compared with United States, most cities in China have higher aerosol levels (van Donkelaar et al., 2010;Li et al., 2019c). Secondary aerosol production may become a large sink of radicals, which could shift O3 production toward a VOC-limited regime under these 85 FNR thresholds suited to United States (Liu et al., 2012;Li et al., 2019a). It is therefore useful to describe surface O3 sensitivity using FNR thresholds derived entirely from satellite observed FNR and ground-based measurements of O3. In addition, Schroeder et al. (2017) using airborne measurements suggested that the range and span of FNR marking the transitional regime varies regionally.
In this study, we assess if space-based HCHO/NO2 ratio captures the non-linearity of O3 chemistry by matching satellite 90 observations with ground-based O3 measurements over the major Chinese cities. Thresholds suited for China between spacebased HCHO/NO2 and the ground-based O3 response patterns are derived from observations instead of model results. We focus on the spatial and temporal variability of O3 formation sensitivity using our FNR thresholds on a nationwide scale and in typical cities from 2016 to 2019.
More recently a new unique situation has occurred with the outbreak of the COVID-19 pandemic, which provided a 95 unique opportunity to demonstrate our predicted effects on O3 pollution in China. Efforts to halt the spread of COVID-19 have drastically reduced human activities worldwide (Siciliano et al., 2020;Tian et al., 2020a). As a result of these restrictions, a significant reduction in primary air pollutant emissions, especially in the concentration of NO2, has been noticed in China and https://doi.org/10.5194/acp-2020-1097 Preprint. Discussion started: 14 December 2020 c Author(s) 2020. CC BY 4.0 License.
By contrast, increasing O3 concentrations during the same period were observed in densely metropolitan areas throughout the 100 world (Siciliano et al., 2020;Zoran et al., 2020;Huang et al., 2020).
Section 2 describes the data and methods used in this study. Section 3 presents our derived FNR thresholds method and variations of O3 formation sensitivity in China. In addition, impacts of the COVID-19 outbreak on O3 levels are discussed.
Finally, section 4 gives a brief summary.

Satellite data
We use the NO2 and HCHO observations from the Ozone Monitoring Instrument (OMI) aboard the National Aeronautics and Space Administration (NASA) satellite AURA, which was launched in July 2004 (Levelt et al., 2006). In an ascending sun-synchronous polar orbit, OMI passes the equator at about 13:40 local time (LT), providing global measurements of aerosol parameters, cloud, and various trace gases (NO2 and HCHO among them) (Levelt et al., 2006). The high spatial resolution (13 110 km × 24 km) allows for observing fine details of atmospheric parameters (Jin and Holloway, 2015). OMI data are considered to be reliable and of good quality for the full mission thus far . In addition, the OMI overpass time is well suited to detect the O3 formation sensitivity during the afternoon, when O3 photochemical production peaks and when the boundary layer is high and the solar zenith angle is small, maximizing instrument sensitivity to HCHO and NO2 in the lower troposphere (Jin et al., 2017). 115 We use the OMI tropospheric NO2 and HCHO data products from the European Quality Assurance for Essential Climate Variables project (QA4ECV, http://www.qa4ecv.eu/). NO2 data are compiled by the Royal Netherlands Meteorological Institute (KNMI). The tropospheric NO2 column density, is defined as the vertically integrated number of NO2 molecules between the Earth's surface and the tropopause per unit area. We select QA4ECV NO2 daily observations with: (1) no processing error; (2) less than 10% snow or ice coverage; (3) solar zenith angle less than 80˚; (4) cloud radiance fraction less 120 than 50%. The QA4ECV NO2 monthly datasets are processed with a spatial resolution of 0.125° × 0.125°. Boersma et al. (2018) reported the single-pixel uncertainties for the QA4ECV NO2 columns are 35% -45% in the polluted regions, the monthly mean NO2 columns are estimated to have an uncertainty of ±10%.

NOx emission
Emission inventories of air pollutants are important sources of information for policy makers and form essential input for air quality models. Bottom-up inventories are usually compiled from statistics on emitting activities and their typical emission factors, but are sporadically updated (Li et al., 2017). Satellite-derived emission inventories have important advantages over bottom-up emission inventories: they are spatially consistent, have high temporal resolution, and provide up-to-date emission 135 information (Mijling and van der A, 2012). In this study, we use monthly mean NOx surface emission estimates derived from  (Ding et al., 2018). These datasets are 145 available from http://www.globemission.eu/region_asia/datapage.php.

Ground-based observations
Since 2012 the Chinese government at various levels began to establish a national air quality monitoring network, which released real-time ground-level O3 monitoring data to the public. By 2016, the establishment of more than 1,000 sites have been completed, covering more than 300 cities across the country. 150 We use hourly O3 and NO2 concentrations (in standard condition, 273 K, 101.325 kPa) from the network of ~1000 sites operated by the China Ministry of Ecology and Environment (CMEE) since 2016. CMEE revised the monitoring of pollutants to a new reference condition (298K, 101.325 kPa) since 1 September 2018 (CMEE, 2018). Daily ground-based O3 and NO2 observations are calculated from hourly observations at OMI overpass time (average of 13:00 LT and 14:00 LT). In this study, we convert the gas concentrations before 1 September 2018 from the standard condition to the reference condition. The 155 temperature dependence is according to Charles's law (1)

=
(1) where Vstd is the volume of a gas under standard condition, Vref is the volume of a gas under reference condition, Tstd (unit: K) is the thermodynamic temperature of standard condition, Tref (unit: K) is the thermodynamic temperature of reference condition.
where Cstd is the gas concentration under standard condition, Cref is the gas concentration under reference condition.
Because the Chinese national air quality monitoring network stations are mostly located in the center of cities or densely populated areas, usually the most polluted regions, we select the NaHa station located on the small island Okinawa in Japan, as a location with a clean atmosphere. The hourly O3 and NO2 observations of NaHa station are provided by the Japanese 165 Atmospheric Environmental Regional Observation System (AEROS, http://soramame.taiki.go.jp/Index.php).

CLASS model
We simulate the nonlinear relationship among O3, NO2 and HCHO using the Chemistry Land-surface Atmosphere Soil Slab model (CLASS). We performed a series of numerical experiments with the same dynamic and chemistry conditions listed in Table 1, but modified only the concentrations of NO2 and HCHO. 170 The CLASS model solves the diurnal evolution of dynamical variables (temperature, specific humidity and wind) and chemical species over time in a well-mixed, convective Atmospheric Boundary Layer (ABL) in which entrainment and boundary layer growth are considered (Vilà -Guerau de Arellano et al., 2015;van Heerwaarden et al., 2010). All these variables are assumed to be constant with height due to intense turbulent mixing driven by convection (van Heerwaarden et al., 2010).
The surface is assumed to be homogeneous in this box model. Chemistry is represented by a chemical scheme based on 27 175 reactions that control O3 formation described by van Stratum et al. (2012), with O3, NO and NO2 as most important species. This simplified chemical scheme is able to represent the evolution of chemical species in semirural areas (Janssen et al., 2012;van Stratum et al., 2012). The model has been validated under various dynamical conditions (Barbaro et al., 2014;Janssen et al., 2012;van Heerwaarden et al., 2010

O3 formation sensitivity regime classification
In Figure 1a, the CLASS model is applied to generate O3 isopleths, which illustrate O3 as function of NO2 and HCHO values. The isopleths show that O3 formation is a highly nonlinear process in relation to NO2 and HCHO. When NO2 is low, 185 the O3 increases with increasing NO2. As NO2 increases, the O3 eventually reaches a local maximum. At higher NO2 concentrations, the O3 would decrease with increasing NO2.
We first evaluate if satellite-based HCHO and NO2 columns can capture the nonlinear O3-NO2-HCHO chemistry shown by the CLASS model. In order to obtain a representative observation sample, we create monthly mean ground-based O3 and NO2 observations of 360 cities across China from the Chinese national air quality monitoring network from 2016 to 2019, and 190 the background station observations from NaHa, Japan for comparison. Temperature is also a major factor in O3 chemistry. O3 pollution is rare when the ambient temperature is below 20°C (Sillman, 2003). The seasonality of ground-level O3 concentrations also exhibited monthly variability peaking in summer and reaching the lowest levels in winter over China (Wang et al., 2017b). In addition, long NOx lifetime and low concentrations of OH and RO2 radicals would lead most regions of China to a VOC-limited regime in winter (Shah et al., 2020). Therefore, we focus in this study on May -October as the 195 summer period when meteorology is favorable for O3 formation (Jin et al., 2017). previous studies (Martin et al., 2004;Duncan et al., 2010;Jin et al., 2017). The overall O3-NO2-HCHO chemistry is also captured by satellite-based HCHO and NO2 columns in Figure 1c, which reflects the reliability of NO2 satellite retrievals.
Having established this relationship between satellite-based HCHO/NO2 columns and surface O3 concentrations, we subsequently derive the FNR thresholds marking the O3 transitional regime. The local O3 maximum can be thought of as a dividing line separating two different photochemical regimes (Sillman, 1999). According to the National Ambient Air Quality 205 Standards released in 2012, 1-hour average O3 concentration should below 160 μg/m 3 in rural regions and below 200 μg/m 3 in urban regions . We assume that the monthly O3 (daily O3 data is averaged at 13:00 LT and 14:00LT) exceeding 160 μg/m 3 has a large component that is due to local photochemical production, not meteorology or regional transport. We investigate the maximum, top 5%, top 10% and top 15% of the monthly O3 with corresponding FNRs for each city during May -October from 2016 to 2019 in Figure S1 in the supplement. The FNR calculation for each city is restricted to pixels where 210 monthly HCHO columns are higher than 2 × 10 15 molecule/cm 2 (detection limitation) and NO2 columns are more than 1.5 × 10 15 molecule/cm 2 (which are defined as polluted regions). We find that the top 10% dataset contains more than half of the total monthly high O3 concentrations (> 160 μg/m 3 ) data and more than 80% of the data in the top 15%. Therefore, we will define the transitional regime based on the monthly O3 exceeding 160 μg/m 3 in the top 10% dataset in Figure 1d. It should be noted that the actual split between NOx-limited and VOC-limited regime includes a broad transitional region 215 rather than a sharp dividing line (Sillman, 1999). Although we reduce the noise by gridding, there is a blurry transition between NOx-limited and VOC-limited regimes. The lack of sharp and clear transitions between two O3 sensitivity regimes is likely influenced by factors such as meteorology, chemical and depositional loss of O3 and noisy satellite data. Taking into account the large range of transitional regime and universality of derived FNR thresholds in China, the FNR thresholds [2.3, 4.2] marking the transitional regime, are defined as the ± 30% range from the median (3.28) covering more than half of transitional 220 regime FNRs. Three regimes can be roughly identified from the FNR thresholds we adopted: a VOC-limited regime should occur when the FNR < 2.3 and a NOx-limited regime should occur when the FNR > 4.2. The FNR between 2.3 and 4.2 reflects the transitions between the two regimes.  Figure S2 in the supplement, we see the VOClimited regimes mainly appear in the North China Plain (NCP), the Yangtze River Delta (YRD) and the Pearl River Delta (PRD), and the NOx-limited regimes dominate the remaining areas, which are consistent with results from Wang et al. (2019a) and Jin and Holloway (2015). In the NCP, the VOC-limited regimes are found in Beijing and some big cities in Hebei province, central regions in Shandong province and Henan province. Transitional regimes control the remaining regions of Shandong 230 province and Henan province and most regions of Hefei province. In the YRD, the VOC-limited regimes are found in Shanghai and southern Jiangsu province. In the PRD, the VOC-limited regimes are found in Guangzhou. Outside the NCP, YRD and PRD, the VOC-limited regimes concentrate in city centers of Shenyang, Chengdu, Chongqing, Xi'an and Wuhan, that are surrounded by transitional regimes in the suburban areas. It has been acknowledged that the urban O3 formations are generally VOC-limited due to the large amount of NOx emissions from diverse sectors, like transportation, industry, residential sector 235 and power plants (Shao et al., 2009;Wang et al., 2009;Sun et al., 2011). The NOx-limited or transitional regimes dominated O3 formation in the suburban and rural areas of eastern China (Xing et al., 2011;Jin et al., 2017).  As we know, O3 increases with increasing NOx in the NOx-limited regime and decreases with increasing NOx in the VOClimited regime. The contrast between NOx-limited and VOC-limited regimes illustrates the difficulties involved in developing 260 policies to reduce O3 in NOx polluted regions. Reductions in VOCs will only be effective in reducing O3 if VOC-limited chemistry predominates. Reductions in NOx will be effective only if NOx-limited chemistry predominates and may actually increase O3 in VOC-sensitive regions. If cities belonging to the VOC-limited regime like Beijing only focus on the reduction of NOx while ignore the control of VOC emissions, they will experience a process of rising O3 concentrations, the more NOx decrease, the greater the increase of O3 will be. 265

Observed response of ground-level O3 to chemical formation sensitivity
To validate the regimes derived from satellite observations, we also analyze the surface NO2 observations from groundbased measurements. Figure 4a   reported the increasing O3 trends in summer in megacity clusters of eastern China and the highest O3 concentrations are in the NCP, which are consistent with our results.
A complex coupling of primary emissions, chemical transformation, and dynamic transport at different scales determine the O3 pollution (Jacob, 1999). NOx and VOCs play important roles in O3 formation. Emissions of NOx and VOCs to the environment are the starting point of O3 pollution problems. During the past decade in China, ambitious steps have been taken 285 to control NOx emissions. In 2013, the Chinese State Council issued the APPAP. Stringent control measures were carried out since then, including phasing out high-emitting industries, closing outdated factories, tightening industrial emission standard, improving fuel quality (Wang et al., 2019a). However, to the other important O3 precursors, VOCs, less attention has been given in emission control strategy. Li et al. (2019b) concluded that anthropogenic NMVOC emissions in China during 1990-2017 have been increasing continuously due to the dramatic growth in activity rates and absence of effective control measures. 290 Following China's past control strategy on VOCs, we can regard VOC emissions as rising or in steady state.
The reduction of the NOx emissions for cities in the VOC-limited regime is one of the main reason for the increasing of O3. Figure

Enhanced O3 levels during COVID-19 lockdown in China
The measures in response to the outbreak of the COVID-19 lead to sudden changes of NOx emissions and anthropogenic HCHO emissions in China in the beginning of 2020 Hui et al., 2020). We analyze the change of O3 concentrations during the lockdown period to validate our method. To look into COVID-19 lockdown impacts on short-term 305 O3 level, we choose two time periods covering 357 cities across China: period I (3 -23 January, 2020) and period II (9 -29 February, 2020), to avoid the coincidence of Chinese New Year holidays (24 January to 8 February, 2020). Using our observation-based FNR thresholds method, we see that most regions of eastern China belong to the VOClimited regime during period I and II in Figure 6c and 6d. Previous studies also reported that the O3 chemistry in the urban areas in China in wintertime is in a VOC-limited regime due to the relative lack of HOx radicals (Seinfeld and Pandis, 2016). 315 During winter (VOC-limited conditions), when the concentration of NOx is high, and the level of UV radiation is low, the O3 production varies inversely with the NOx concentration (Sillman et al., 1990). The NOx reduction during the lockdown is higher than the VOC reduction (Sicard et al., 2020). Thus, a reduction in NOx leads to an increase of the O3 concentrations in most regions of eastern China during period II. Besides, reduction of freshly emitted NO in particular from road traffic alleviates O3 titration locally (Seinfeld and Pandis, 2016;Levy et al., 2014). The O3 titration occurs particularly in winter (less 320 photolysis reactions of NO2) under high NOx levels (Sillman, 1999). However, the lockdown measures result primarily in a lower titration of O3 by NO due to the reduction in local NOx emissions by road transport, which also enhances O3 levels in urban areas. On the other hand, some cities, mainly located in southeastern China, showed decreasing O3 levels. Zhao et al. (2020) concluded that the cause of O3 decline in these cities is the emission changes of NOx and VOC. In Figure 6c we see that some cities in Fujian and Guangdong provinces belong to the transitional regime. Theoretically, the transitional regime 325 should correspond to the conditions at which O3 formation is most efficient, indicating that reductions or increases in NOx and VOCs will reduce the O3 concentration.

Conclusion
Satellite-based HCHO/NO2 ratios and ground-based O3 measurements were directly connected to capture the nonlinearity of surface O3 chemistry over major Chinese cities in this study. Evaluating the FNR thresholds marking the O3 330 transitional regime in which O3 formation is less sensitive to the precursors, we found a broad transitional region, which reflects differences of factors among 360 cities, such as emissions, meteorology, and regional transport. The national FNR thresholds are defined as follows: a VOC-limited regime should occur for FNR < 2.3 and a NOx-limited regime should occur for FNR > 4.2. The FNR between 2.3 and 4.2 reflects the transition between the two regimes. Our FNR thresholds derived from satellite and ground-based observations are higher than previous reported model-based values. The nonlinear chemistry of O3 depends 335 on its precursors NO2 and VOCs with contributions from both local and regional sources (Xue et al., 2014). Modeling studies are good at simulating the response of surface O3 to an overall reduction in NOx or VOC emissions. The FNR thresholds derived with in situ O3 observations will be more indicative of the local O3 chemistry than the model, including the effect of NOx titration over urban areas (Jin et al., 2020).
We analyzed the spatial and temporal variability of O3 formation sensitivity using our FNR thresholds over China from 340 2016 to 2019. Our results showed that O3 formation sensitivity tends to be VOC-limited over urban areas and NOx-limited over rural and remote areas in China. In 2016, the VOC-limited regimes mainly appear in the NCP, the YRD and the PRD. In 2019, there was a shift in most NCP regions from the VOC-limited to the transitional regime. The area with a VOC-limited regime in the YRD and PRD also shrank. We found that O3 formation sensitivity changes in these regions were associated Emission sources of HCHO, as a tracer of VOCs, can be anthropogenic and biogenic. Shen et al. (2019) found that the OMI HCHO distribution follows their anthropogenic inventory in megacity clusters over China, while it does not follow the biogenic emissions inventory. Despite the fact that local sources of anthropogenic VOCs are difficult to identify, our FNR 355 thresholds derived from satellite-based information have the potential to provide important information to air quality planners.

Data availability
Satellite data used in this research can be obtained from public sources. The OMI tropospheric NO2 product from the QA4ECV project can be obtained from http://www.qa4ecv.eu/ecv/no2-pre/data and the HCHO product from http://www.qa4ecv.eu/ecv/hcho-p/data. 365 The monthly mean NOx emission products derived from OMI observations by DECSO v5.1qa can be obtained from http://www.globemission.eu/region_asia/datapage.php.
The hourly O3 and NO2 observations of Naha station are provided by the Japanese Atmospheric Environmental Regional Observation System (AEROS, http://soramame.taiki.go.jp/Index.php).

Author contributions 370
WW and RA provided satellite data, tools, and analysis. RA, JD, MW and TC undertook the conceptualization and investigation. WW prepared original draft. RA and JD carried out review and editing. All authors discussed the results and commented on the paper. https://doi.org/10.5194/acp-2020-1097 Preprint. Discussion started: 14 December 2020 c Author(s) 2020. CC BY 4.0 License.