The Chinese nationwide air quality monitoring network provides surface O3 and NO2 data. Since 2015, more than 1400 sites have covered more than 330 cities across the country. The nationwide hourly O3 and NO2 concentration data in Chinese cities in 2014–2021 were obtained from the National Urban Air Quality Real-Time Publishing Platform (https://air.cnemc.cn:18007/, last access: 1 September 2022).

O3 and NO2 measurements were reported in micrograms per cubic meter and the mass concentrations for all years were unified to the reference state (298.15 K, 1013.25 hPa). Extensive data quality controls were applied to eliminate unreliable observed outliers following the method of previous studies (Lu et al., 2018). The average of daily NO2 and the daily maximum 8 h average (MDA8) O3 concentration for all nationally controlled sites in a city was regarded as the city's daily NO2 and O3 levels.

Given the short lifetime of NOx, the high NO2/NOx ratio in the boundary layer and the fact that HCHO is an intermediate in the oxidation reactions of various VOCs and is approximately proportional to the total rate of VOC reactions with OH radicals (Duncan et al., 2010), current satellite-based spectrometers have provided continuous global observations for two species indicative of O3 precursors, namely, NO2 for NOX (Lamsal et al., 2014; Martin, 2003) and HCHO for VOCs (Shen et al., 2019; Zhu et al., 2014).

Satellite products of tropospheric NO2 and HCHO vertical columns are retrieved from two satellite instruments: the Ozone Monitoring Instrument (OMI) the and Tropospheric Monitoring Instrument (TROPOMI). The high spatial resolution (24×13 km2 for OMI and 5×3.5 km2 for TROPOMI) allows for the observation of fine details of atmospheric parameters. They provide daily global observations, and the overpass time (13:40–13:50 and 13:30 local time) is well suited to detect the O3 formation sensitivity when O3 photochemical production peaks. The boundary layer is high, and the solar zenith angle is small, thereby maximizing instrument sensitivity to HCHO and NO2 in the lower troposphere (Jin et al., 2017).

The TROPOMI data from the Earth Engine Data Catalog are based on the algorithm developments for the QA4ECV (Quality Assurance for Essential Climate Variables project) reprocessed dataset for OMI and have been further optimized. The TROPOMI data, available for 2019–2021, are processed with a spatial resolution of 1113.2 m (about 0.009∘ within China). The same chemistry transport model for HCHO and NO2 is better suited for analyzing their ratio than products developed with different prior profiles.

The OMI data with longer time horizons (2013–2021) are used to study the long-term changes in NO2 and HCHO and track changes in emissions of NOx and VOCs. Tropospheric NO2 vertical columns are obtained from the Aura OMI Level-3 NO2 products (OMNO2d 003) with a grid resolution of 0.25∘×0.25∘, while HCHO total columns were obtained from OMI/Aura Level-3 HCHO products (OMHCHOd 003) with a grid resolution of 0.1∘×0.1∘. Since HCHO mainly presides in the troposphere, its total column can be regarded as the tropospheric column (Duncan et al., 2010).

The high spatial-resolution TROPOMI data are used to derive the HCHO/NO2 threshold values marking transitions in O3 formation regimes and to diagnose the current ozone–NOx–VOC sensitivity in China. TROPOMI data are aggregated by sampling gridded NO2 and HCHO columns consistent with the ground-based observations of O3. O3 concentrations are averaged at all available sites every half-month for the warm season (April to September) from 2019 to 2021. O3 measurements at 13:00 to 14:00 local time are selected to match the TROPOMI overpass time. Considering the data volume and the need to reduce retrieval noise, TROPOMI-retrieved NO2 and HCHO columns are sampled every half-month over O3 sites for the same period, yielding nearly 100 000 paired observations. In addition, considering the number of available data and topographic conditions, China is divided into nine regions to evaluate the satellite-based HCHO/NO2 and to study the O3 production regimes, as shown in Fig. S2.

The ozone concentrations averaged from 367 cities with monitoring sites in China from 2015 to 2021 are shown in Fig. 1a. The three metrics include the maximum 6-month MDA8 ozone running average (MMA6-MDA8) and the 90th and 99th percentiles of the annual distribution of MDA8-O3 (MDA8-90th, MDA8-99th), following the definition in the World Health Organization global air quality guidelines (World Health Organization, 2021) and Chinese ambient air quality standards (GB 3095-2012). The three metrics increased by 2.3 (2 %), 2.5 (2 %) and 2.3 (1 %) µg m-3 yr-1 in 2015–2021. Moreover, they significantly increased by 4.6 (5 %), 5.6 (4 %) and 5.4 (3 %) µg m-3 yr-1 (all with p<0.05) in 2015–2019, based on the linear trend estimate.

The time series of the monthly mean MDA8 ozone in 2015–2021 is shown in Fig. 1b. Based on the monthly anomalies (the difference between the observed monthly mean and the 7-year monthly mean for the same month), MDA8 ozone levels increased by 2.3 µg m-3 yr-1 (3.0 % yr-1, p<0.01) averaged for all cities. Rapid increases are also exhibited before 2019, with 3.5 µg m-3 yr-1 (4.5 % yr-1, p<0.01). The daytime and nighttime mean ozone levels show similar trends. In 2015–2019, the maximum and minimum hourly average concentration increased from 97.1 to 117.9 µg m-3 and from 37.3 to 44.3 µg m-3, respectively, based on the mean April to September ozone diurnal variation, as shown in Fig. 1c. Overall, the average ozone concentration shows an increasing trend from 2015 to 2021, although it has decreased in the last 2 years compared to the peak in 2019.

The spatial patterns of MDA8-90th and their trends for approximately 367 cities in 2014–2021 are shown in Fig. 2. The cities with high ozone levels are widely distributed in eastern and central China, where intensive anthropogenic emissions are located. Distinct exacerbation of ozone pollution can be observed from 2014 to 2019. The locations and definitions of the key regions are shown in Fig. S1 in the Supplement. In 2014–2015, sites with MDA8-90th greater than 160 µg m-3 were concentrated near the Beijing–Tianjin–Hebei and the Yangtze River Delta (YRD) megacity clusters. By 2019, ozone hot spots extended westward to the Fenwei Plain and southward to the central Yangtze River Plain. During the 6 years, the ozone concentration has increased almost everywhere in China, particularly in the North China Plain (NCP) and the central Yangtze River Plain. In 2020 and 2021, ozone pollution was alleviated, the level of which is equivalent to the level of ozone pollution in 2017–2018. O3 concentration decreases in major key regions, including the North China Plain, Pearl River Delta (PRD) and Sichuan Basin (SCB).

The percentage of sites with MDA8-90th ozone higher than 160 µg m-3, the Chinese grade II national air quality standard, increased from 15.0 % in 2015 to 39.3 % in 2019 and then decreased to 23.0 % in 2021. The percentage of sites with increasing ozone trends was 83.1 % in 2015–2019 and 71.8 % in 2015–2021. From a national perspective, widespread surface ozone increases occurred in Chinese cities during 2015–2019. Moreover, decreases have been observed in the last 2 years.

Based on the non-linear O3–NOx–VOC relationship, changes in ozone precursors can directly affect ozone levels. Ground-based measurements of NO2 and satellite-based NO2 columns are used to indicate the changes in NOx emissions. As shown in Figs. 5a and S6, NO2 is concentrated over urban areas and industrial cities, particularly in megacity clusters, such as the NCP, YRD and PRD and large cities, such as Wuhan (in Hubei) and Xi'an (in Shaanxi). Satellite observations show large NO2 decreases from 2013. This finding is consistent with previous studies (Wang et al., 2019; Lin et al., 2019). Moreover, the decrease is more than 2.8 % yr-1 in half of the grids. Figure 5b shows that the average annual reduction in satellite NO2 in eastern China is 2.96 % (0.11×1015 molec. cm-2, p<0.01). Figure S8 shows similar trends in surface NO2 concentrations and satellite-based NO2 columns averaged in April–September. More than 75 % of the cities show negative trends in surface NO2 levels (and 22 % with p<0.05), and nearly 64 % of the grids show decreases in satellite NO2 columns. Strong and consistent decreases in high-value areas, particularly the NCP, as well as YRD, PRD and SCB, have been observed in the last 9 years. The mean NO2 in NCP has decreased by 0.37×1015 molec. cm-2 yr-1 (4.59 %, p<0.01) from 2013 to 2021.

The variations in VOCs primarily drive the temporal patterns of satellite HCHO columns over China (Shen et al., 2019). The changes in satellite HCHO can roughly indicate changes in VOC emissions, which has been applied in several previous studies (K. Li et al., 2020; Shen et al., 2019; Zhu et al., 2014). Figures 6a and S7 compare the HCHO columns from April to September 2013–2021. As expected, a clear gap exists between the HCHO in southeast and northwest China. The mean HCHO is high over megacity clusters in the southeastern regions, such as the North China Plain, Sichuan Basin, Yangtze River Delta, Pearl River Delta and the Central Yangtze River plain. More than 45 % of the grids show positive trends in satellite HCHO columns in 2013–2021. HCHO columns increase insignificantly by 0.90 % yr-1, averaged for all grids in 2013–2019 in eastern China. This finding is consistent with the increases in anthropogenic VOC emissions (Zheng et al., 2018; Zhang et al., 2019). In 2020–2021, satellite-based HCHO decreases in most of eastern China compared to 2019.

In addition to anthropogenic and biogenic emissions, the long-term changes in HCHO are driven by several other factors (Zhu et al., 2017): NOx reductions can lower the HCHO yield from isoprene oxidation (Souri et al., 2020; Wolfe et al., 2016), and, in addition, HCHO columns may show interannual variability driven by interannual variability of meteorology, particularly temperature as shown in Fig. S9 (Duncan et al., 2009; Abbot et al., 2003). Nevertheless, HCHO has not declined as dramatically as NO2, particularly until 2019. In the NCP, where VOC-limited chemistry exists most widely, the gap between HCHO and NO2 changes is even larger.

The changes in VOC and NOx emissions presumably have the same disparity as the changes in satellite HCHO and NO2. The anthropogenic NOx emission has been decreased considerably since 2013. However, VOC emissions have not been effectively reduced or even slightly increased until 2019. The significant NOx reduction alone without effective VOC control has not worked or has even aggravated O3 pollution in recent years because of the VOC-limited (or NOx-saturated) O3 production regimes in major city clusters. Considering that biological VOC also increased (L. Li et al., 2020), it is clear that the reduction in anthropogenic VOCs is not sufficient to bring down the total VOC emissions. The slight decrease in HCHO and the corresponding slight decrease in O3 concentrations in 2020–2021 are more indicative of the effect of the lack of VOC emission reductions on the increase in O3 over the last 9 years.

The synergistic control of NOx and VOC, with VOC control as the focus, can help reduce ozone pollution in major cities. The reduction ratio of VOC/NOx can be indicated by the slope of the ridge line of the classic ozone isopleths (Ou et al., 2016; Wang et al., 2019). The relationship between O3 concentrations and the satellite HCHO and NO2 columns reflected by the approach in Fig. 3a is similar to the O3–VOC–NOx isopleths. Thus the peak of the fitted curve of O3 and HCHO/NO2, corresponding to the ridge line of O3 isopleths, is the ratio of HCHO to NO2 reduction for the fastest decrease in O3, i.e., ΔHCHO/ΔNO2.

This approach is used to provide preliminary scenarios for synergistic VOC and NOx reduction in three developed cities: Beijing in the NCP, Guangzhou in the PRD and Chengdu in the SCB. All of these are dominated by the VOC-limited regime. As shown in Fig. 7, the optimal decrease ratio of HCHO to NO2 is between 2:1 and 4:1, which is 3.4–3.9 for Beijing, 2.2–2.6 for Chengdu and 2.3–2.8 for Guangzhou. The results obtained by this method roughly reflect the optimal abatement pathways for the two precursors by using the satellite HCHO and NO2 to represent VOC and NOx. The short-term future O3 control becomes much more promising in terms of enhanced VOC control in urban cities dominated by VOC-limited regimes.