Worsening urban ozone pollution in China from 2013 to 2017 – Part 2: The effects of emission changes and implications for multi-pollutant control

. The Chinese government launched the Air Pollution Prevention and Control Action Plan in 2013, and various stringent measures have since been implemented, which have resulted in significant decreases in emissions and ambient concentrations of primary pollutants such as SO 2 , NO x , and particulate matter (PM). However, surface ozone (O 3 ) concentrations have still been increasing in urban areas across the country. In a previous analysis, we examined in detail the 10 roles of meteorological variation during 2013–2017 in the summertime surface O 3 trend in various regions of China. In this study, we evaluated the effect of changes in multi-pollutant emissions from anthropogenic activities on O 3 concentrations during the same period, by using an up-to-date regional chemical transport model (WRF-CMAQ) driven by an interannual anthropogenic emission inventory. The CMAQ model was improved with regard to heterogeneous reactions of reactive gases on aerosol surfaces, which led to better model performance in reproducing the ambient concentrations of those gases. The 15 model simulations showed that the maximum daily 8-hour average (MDA8) O 3 concentration in urban areas increased by 0.46 ppbv per year (ppbv a -1 ) (p = 0.001) from 2013 to 2017. In contrast, a slight decrease in MDA8 O 3 concentrations by 0.17 ppbv a -1 (p = 0.005) in rural areas was predicted, mainly attributable to the NO x emission reduction. The effects of changes in individual pollutant emissions on O 3 were also simulated. The reduction of NO x emission increased the O 3 concentration in urban areas due to the non-linear NO x -volatile organic compound (VOC) chemistry and decreasing aerosol effects; the slight 20 increase in VOCs emissions enhanced the O 3 concentrations; the reduction of PM emissions increased the O 3 concentrations by enhancing the photolysis rates and reducing the loss of 2 in photolysis rates. The adverse effect of the reductions of NO x , SO 2 , and PM emissions on O 3 abatement in Beijing, Shanghai, Guangzhou, and Chengdu would have been avoided if the anthropogenic VOCs emission had been reduced by 24%, 23%, 20%, and 16%, respectively, from 2013 to 2017. Our analysis revealed that the NO x reduction in recent years has helped to contain the total O 3 production in China. However, to reduce O 3 concentrations in major urban and industrial areas, VOCs emissions control should be added to the current NO x -SO 2 -PM policy. 35 to conduct simulations for the summer months (June, July, and August) from 2013 to 2017 with anthropogenic emissions. The shipping emissions were kept unchanged in the 5-year simulation, due to a lack of data for recent years. In Part 1 of our work (Liu and Wang, 2020), we showed the effect of changes in total anthropogenic 150 emission on O 3 changes by comparing the O 3 concentrations in 2013 simulated using anthropogenic emissions from different years. In this study, three additional sets of modeling experiments were established. The first was designed to quantify the responses of O 3 concentrations to changes in individual pollutant emissions from 2013 to 2017, with the simulation in 2013 being regarded as the baseline experiment. The anthropogenic emissions of NO x , VOCs, SO 2 , CO, NH 3 , PM (comprising PM 10 , PM 2.5 , and its components), black carbon (BC), organic carbon (OC), and combined NO x /VOCs in 2013 were changed 155 individually to those for 2017 in each sensitivity experiment (total number of experiments = 10), and the results were compared with those in the baseline experiment (Table S2). The second set of experiments was designed to investigate the effect of changes in aerosols on O 3 concentrations via altering the photolysis rates and heterogeneous reactions (Table S3). The individual effects of aerosol were deleted in each sensitivity experiment, and the results were compared with those in the also investigated the effects of changes in photolysis rates and heterogeneous reactions on O 3 concentrations, using the GEOS-Chem model incorporating heterogeneous reactions of nitrogen oxides and HO 2 . They quantified the effect 335 of changes in photolysis rates by scaling the aerosol-extinction rate using the satellite-based aerosol optical depth changes, and the effect of changes in heterogeneous reactions by scaling the aerosol surface area using the measurement-based PM 2.5 changes from 2013 to 2017. They concluded that the increase in O 3 concentrations due to changes in PM concentrations could be largely ascribed to the decrease in the effect of HO 2 heterogeneous reaction. Using a regional model and adopting different experimental settings, our work uncovered a similar and substantial effect of HO 2 uptake on increases in O 3 concentrations 340 due to changes in PM concentrations. In addition, with more heterogeneous reactions implemented in the CMAQ model, we found that the uptake of O 3 on aerosol surfaces was also important, following HO 2 . , Toward a general parameterization of N2O5 reactivity on aqueous particles: the competing

. By comparing the response of the 2013 O3 concentration to various VOCs emissions reductions, the 170 required reduction of VOCs emissions was quantified.

Comparison of the simulated and observed reactive gases
The simulated concentrations of reactive gases that are subject to significant heterogeneous reactions were compared with the observed values for the gases O3, NO2, NO3, N2O5, HONO, ClNO2, HO2, OH, and H2O2 (Table S1). Except for O3 and NO2, 175 which are measured by the regular national air monitoring network, the other gases were measured only in research-focused field campaigns. We compiled literature-reported summer concentrations of these gases for various years and compared these with the model-simulated values for 2013.
The uptake of NO2 on wet aerosol surfaces can produce HONO in the atmosphere, which is an important source of OH radicals via photolysis. After the update of the CMAQ model, the predicted average NO2 concentration in China decreased from 19.2 180 ppbv to 16.6 ppbv, which came close to the observed value (15.1 ppbv). As a product of NO2 uptake, the HONO concentrations increased significantly and approached the observed values in Beijing (Wang et al., 2017a) and Guangzhou (Qin et al., 2009;Li et al., 2012b). The decrease in NO2 concentrations and increase in HONO concentrations were attributed to the increase in heterogeneous reaction rates of NO2 due to the effects of relative humidity and sunlight intensity in the updated CMAQ model (Fu et al., 2019). Table S1 also presents the observed HONO concentrations at two coastal sites in Hong Kong (Li et al., 2018b;185 Xu et al., 2015), but their concentrations were substantially underpredicted because capturing such coastal characteristics is challenging for the model, due to its low horizontal resolution (36 km).
The simulated NO3 concentration decreased slightly (~1 pptv) due to the decrease in NO2 concentrations and the increase in concentration moved closer to the observed value in Shanghai  after the heterogeneous reactions in the model were updated.
The parameterization of 2 5 remains unchanged in the revised model. However, the decrease in NO2 concentrations described above resulted in a decrease in N2O5 concentrations, and thus a decrease in ClNO2 concentrations. The simulated maximum N2O5 concentration at the Wangdu site decreased by ~50% and thus agreed much better with the observed value 195 (Tham et al., 2016). The simulated maximum ClNO2 concentration decreased slightly, by a margin much smaller than the biases between the simulation and observation. Table S1 presents the observed N2O5 and ClNO2 concentrations at a highaltitude site on Mount Tai (Wang et al., 2017c) and a coastal site in Hong Kong Tham et al., 2014). Large differences between simulations and observations were found due to the complex terrains at these two sites, which are difficult for our model to simulate.

200
The CMAQ model predicted slightly lower the concentrations of HO2 and OH radicals after the incorporation of their heterogeneous uptakes. The changes were small, probably due to the scavenging effects of aerosols being counteracted by the increase in radical sources generated by HONO photolysis. The measured value for the concentration of HO2 contains an In the original CMAQ model, the MDA8 O3 concentration was overestimated by 11.4 ppbv. The bias was reduced to 6.8 ppbv with the updated heterogeneous reactions. In addition to the greater uptake of O3 on aerosol surfaces, the updated model also 210 includes other heterogeneous gas-aerosol reactions, weakening the atmospheric oxidation capacity and thus inhibiting O3 formation.
The H2O2 concentration decreased substantially from ~0.8 ppbv to ~0.2 ppbv, and the simulated value agreed well with the values recorded in Wangdu   In summary, after updating the heterogeneous reactions in the CMAQ model, the simulations agreed better with the observations, especially for concentrations of NO2, HONO, O3, and H2O2.

Variations in the urban and rural O3 concentrations
As most of the 493 air-quality monitoring sites established in 2013 are located in urban areas (refer to Fig. S1 in Part 1 (Liu (e.g., Xing et al., 2011). The increase in O3 concentrations in urban areas due to NOx reductions can be explained by two factors. First, most urban areas are in the VOCs-limited regime, where the reduction of NOx emissions reduces the NO titration effect on O3, resulting in increased O3 concentrations. Second, the decrease in NOx emissions can reduce the NO3concentration and increase the O3 concentration via weakening the aerosol effects.

255
In the simulation of VOCs emission changes, the spatial distribution of the O3 concentration closely tracked the changes in VOCs emissions (Fig. 3b). Specifically, the increase in VOCs emission caused an increase in the MDA8 O3 concentrations NOx emissions that primarily contributed to the changes in O3 concentrations (Fig. 3c). In the simulation of changing CO emissions, the reduction of CO emissions reduced the O3 concentrations across China (Fig. 3d). A particularly large decrease in O3 concentrations was found in the NCP region, where both the CO emissions and their corresponding reduction were large.
The CO emission reductions led to a decrease of 0.41 ppbv in the MDA8 O3 concentrations in urban areas (Fig. 4a). CO is an 265 important O3 precursor and plays a similar role to VOCs in O3 formation, but the changes in its emission have rarely been discussed in previous studies of the causes of variations in O3 concentrations. In fact, our results indicated that the reduction of CO emissions was the only government-implemented measure that reduced O3 concentrations in recent years.
In addition to the effects of O3 precursors, the emissions of other pollutants can also affect O3 concentrations by altering photolysis rates and the loss of reactive gases from heterogeneous reactions. The reduction of SO2 emissions increased the O3 270 concentrations across China, particularly in northern China and the Sichuan Basin (SCB) (Fig. 3e). Quantitatively, SO2 emissions reductions led to an increase of 0.75 ppbv in the MDA8 O3 concentrations in urban areas (Fig. 4a), which was the largest cause of O3 concentration increases among all the pollutant emissions changes considered in this work. The SO2 emission was reduced by more than 60% from 2013 to 2017, which resulted in a significant decrease in ambient SO4 2concentrations, and increased O3 concentrations by increasing the photolysis rates and retarding the loss of reactive gases from 275 heterogeneous reactions. The reduction of NH3 emissions, an important precursor of ammonium, increased the O3 concentration across China in a similar way to the reduction in SO2 emissions (Fig. 3f), but to a small extent, as the NH3 emission was only reduced by 4%. Specifically, the increase in the MDA8 O3 concentrations in urban areas due to the reduction of NH3 emissions was only 0.06 ppbv (Fig. 4a), which was an insignificant fraction of the total increases in O3 concentrations.
The reduction of primary PM emissions also enhanced O3 formation across China, especially in the NCP and SCB regions 280 (Fig. 3g). The MDA8 O3 concentrations increased by 0.72 ppbv due to the PM emission reduction in urban areas (Fig. 4a).
The effect of the changes in PM emissions on O3 concentrations was comparable with that of the changes in SO2 emissions, which indicated the significant O3-promoting role played by reductions in both primary and secondary aerosols. BC and OC https://doi.org/10.5194/acp-2020-53 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License.
are among the components of direct aerosol emissions, and reductions in both were found to increase the O3 concentrations ( Fig. 3h and i). Although the reduction of BC emissions was smaller than the reduction in OC emissions, the increase in MDA8 285 O3 concentrations due to the former was more significant. BC is an especially strong absorber of visible solar radiation in the atmosphere (Ramanathan and Carmichael, 2008), and therefore greatly retards photolysis rates by reducing the solar radiation reaching the earth's surface.
The dominant cause of O3 concentration increases due to emission changes varied among regions. Fig. 4 shows the average changes in O3 concentrations due to changes in individual pollutant emissions in four megacities, Beijing, Shanghai, 290 Guangzhou, and Chengdu (refer to Fig. S1 in part 1 for their locations), which are the representative cities in the Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD), and SCB regions, respectively. In Beijing, NOx and PM emission reductions were the two largest causes of rising O3 concentrations, followed by SO2 emission reductions. Air quality in the BTH region is a major concern and strict emission-control measures have been implemented since 2013. As a result, the emissions of NOx, PM2.5, and SO2 in BTH were reduced by 25%, 44%, and 65% from 2013 to 2017 (Fig. S4), 295 respectively, which were generally larger reductions than occurred in other regions (Fig. 2). In Shanghai, the increase in the

The effects of aerosol on the variations in O3 concentrations
Aerosols in the atmosphere derived from direct emission and secondary formation can reduce photolysis rates and scavenge reactive gases from heterogeneous reactions, thereby inhibiting O3 formation. Fig. 5 shows the spatial distribution of changes in the MDA8 O3 concentrations due to the changes in the radiative and heterogeneous chemical effects of aerosols from 2013 https://doi.org/10.5194/acp-2020-53 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License.