Climate impacts of emission reductions in China during 2013–2017

. China has implemented a sequence of policies for clean air since year 2013 and the aerosol pollution has been 20 substantially improved, but ozone (O 3 ) related issues arose. Here, climate responses to changes in aerosols and O 3 related to the emission reductions over China during 2013–2017 are investigated using the Community Earth System Model version 2 (CESM2). The overall decreases in aerosols produced an anomalous warming of 0.09 ± 0.10 ℃ in eastern China (22°N– 40°N, 110°E–122.5°E), which is further enhanced by the increase in O 3 in the lower troposphere, resulting in an accelerated warming of 0.16 ± 0.15 ° C in eastern China. Reductions in industrial emissions contributed the most to the aerosol-induced 25 warming, while emission reductions from residential sector induced a cooling effect due to a substantial decrease in light-absorbing black carbon aerosols. It implies that switching residential sector to cleaner energy is more effective to achieve climate and health co-benefits in China.

Given that aerosol and O3 are important short-lived climate forcers, a reduction in emissions of air pollutants for clean air always comes with climate consequences. The climate effects have been demonstrated in North America and Europe during the past decades when clean air actions were taken (Leibensperger et al., 2012a(Leibensperger et al., , 2012bTurnock et al., 2015). 55 Reductions in aerosol emissions in U.S. exerted a direct radiative forcing (DRF) by 0.8 W m -2 and an indirect radiative forcing (IRF) by 1.0 W m -2 over eastern U.S., resulting in a 0.35 °C warming between 1980 and 2010 (Leibensperger et al., 2012a,b). Similarly, decreases in aerosols resulted in a DRF of 1.26 W m -2 over Europe between 1980s and 2000s, and increases in O3 exerted a radiative forcing of 0.05 W m -2 in the meanwhile (Pozzoli et al., 2011). The clean air actions in Europe have been estimated to warm the surface air by 0. 45 ± 0.11 °C between 197045 ± 0.11 °C between and 201045 ± 0.11 °C between (Turnock et al., 2015. 60 The clean air actions in China have been reported to potentially affect radiative balance and regional climate in recent studies. Dang and Liao (2019) found that the reductions in aerosols led to a regional mean DRF of 1.18 W m -2 over eastern China in 2017 relative to 2012 using the chemical transport model GEOS-Chem. Zheng et al. (2020) also reported that the decrease in aerosol emissions in China from 2006 to 2017 exerted an anomalous ERF of 0.48 ± 0.11 W m -2 and further caused a warming of 0.12 ± 0.02 °C in East Asia. Along with the decline of aerosols, O3 concentrations also changed due to 65 the clean air actions. The combined impacts of aerosol and O3 changes on regional climate over China associated with clean air actions have not been studied. In addition, for the physical basis of climate policy decision making, it is valuable to know the relative roles of the sectoral sources contributing to the aerosol-induced climate change.
In this study, we examine the climate responses to emission reductions in air pollutants over China due to clean air actions from 2013 to 2017, with the consideration of both aerosols and O3 changes, using the Community Earth System 70 Model Version 2 (CESM2) with its atmospheric component Community Atmosphere Model version 6 (CAM6). The climate impacts of aerosol emission reductions from individual sectors are also investigated through emission perturbation experiments.

Materials and methods
In this study, we perform simulations using the CAM6, the atmospheric component of CESM2 (Danabasoglu et al., 75 2020). In CAM6 major aerosol species, including sulfate (SO4 2-), black carbon (BC), primary organic matter (POM), secondary organic aerosol (SOA), mineral dust, and sea salt, are represented by a modal aerosol scheme  with four lognormal modes (i.e., Aitken, accumulation, coarse, and primary carbon modes). PM2.5 is calculated as the sum of SO4 2-, BC, POM, and SOA in this study. A comprehensive consideration of aerosol/O3-radiation and aerosol-cloud interactions are included in the model. Since the standard configuration in CAM6 does not include a gas chemistry package, 80 global three-dimensional tropospheric O3 concentrations for years 2013 and 2017 are adopted from simulations using GEOS-Chem model v12.9.3, a global model of atmospheric chemistry driven by the MERRA-2 (Modern-Era Retrospective analysis https://doi.org/10.5194/acp-2022-27 Preprint. Discussion started: 25 January 2022 c Author(s) 2022. CC BY 4.0 License.
for Research and Applications Version 2) meteorological fields with the same aerosol and precursor gas emissions as used in CAM6.
Default anthropogenic and open biomass burning emissions of aerosols, aerosol precursors and O3 precursors are 85 obtained from the CMIP6 (the Coupled Model Intercomparison Project Phase 6) (Hoesly et al., 2018;van Marle et al., 2017).
Because CMIP6 emissions did not fully consider the emission reductions of clean air actions in China    concentration decreases with maximum decreases in the range of 12-18 μg m -3 . The low biases in CAM6 are caused by 115 many factors including the lack of nitrate and ammonium representation, the absence of natural aerosols in the calculation of modeled PM2.5 concentrations, strong aerosol wet removal, and uncertainties in new particle formation, which have been reported in many previous studies (Yang et al., 2017a, b;Zeng et al., 2021;Ren et al., 2021). In contrast to the aerosol decreases, the simulated annual mean O3 concentrations increased the most between 25°N and 135 45°N averaged over 110-125.5°E from 2013 to 2017 (Fig. 3b), partly because the reductions of aerosols can lead to a slowdown of the sink of HO2 radicals in aerosol chemical processes and thus more radicals to accelerate the O3 production.
Over eastern China, O3 concentration increased from the surface to about 800 hPa. Meanwhile, O3 concentrations decreased in the mid-and upper troposphere in eastern China. The reason why O3 concentrations decreased there is that the mid-and upper troposphere are relatively cleaner than near the surface, which are the NOx-limited regime as NOx emissions decreased 140 (Dufour et al., 2018). However, in the lower troposphere over eastern China, O3 concentrations are limited by VOCs, which increased with reduced NOx emissions.

Climate impacts of emission reductions in China
As short-lived climate forcers, aerosols and O3 exert considerable impacts on climate through perturbing the radiation budget of the Earth. Along with the reductions in aerosol and precursor gas emissions due to clean air actions in China, the 145 decreases in aerosol concentrations lead to an anomalous ERF of 1.18 ± 0.94 W m -2 at TOA over eastern China in year 2017 https://doi.org/10.5194/acp-2022-27 Preprint. Discussion started: 25 January 2022 c Author(s) 2022. CC BY 4.0 License. relative to 2013 (Fig. 4a), which can potentially cause a regional warming effect. The anomalous ERF was largely induced by the aerosol-radiation interactions (ERFari, 0.79 ± 0.38 W m -2 ) and the aerosol-cloud interactions also contributed to the ERF anomaly (ERFaci, 0.44 ± 0.87 W m -2 ) (Fig. 5). Note that due to the large uncertainties involved in the aerosol-cloud interactions (IPCC, 2021), changes in ERFaci and thus total aerosol ERF are not as statistically significant as ERFari. 150 As a result of emission reductions in O3 precursors, the O3 concentrations increased in the lower troposphere and decreased in the mid-and upper troposphere, resulting in a net ERF anomaly of 0.81 ± 0.92 W m -2 at TOA over eastern China during 2013China during -2017 (Fig. 4b). The positive ERF anomaly related to the near-surface O3 increases enhanced the positive ERF produced by the aerosol decreases, leading to a total ERF anomaly of 1.99 ± 1.25 W m -2 over eastern China (Fig. 4c).
Owing to the emission reductions, surface air temperature increased in China during 2013-2017, as the consequence of 155 less solar radiation reflected to the space and more thermal radiation captured within the atmosphere. Over eastern China, surface air temperature increased by 0.09 ± 0.10 ℃ induced by anthropogenic aerosol emission reductions alone from 2013 to 2017 (Fig. 6a) and the intensified O3 pollution exacerbated the temperature increase by 0.07 ± 0.09 ℃ in the meantime (Fig. 6b). The total aerosol and O3 emission reductions from 2013 to 2017 induced a 0.16 ± 0.15 ℃ warming over eastern China, with statistically significant warming in the range of 0.3-0.5 ℃ between 30-40°N (Fig. 6c). 160 The regional surface air temperature changes over the seven sub-regions in China due to emission reductions of air pollutants are provided in Table 1. In FWP, temperature increased by 0.35 ± 0.06 ℃ between 2013 and 2017, equally attributed to the changes in aerosols and O3. Temperature in NCP and SCB increased by 0.22 ± 0.09 ℃ and 0.26 ± 0.08 ℃, largely attributed to changes in aerosols and O3, respectively. Decreases in both aerosols and tropospheric O3 above the surface caused a net surface cooling by 0.14 ± 0.12 ℃ in the Northeast Plain (NEP) in China. Note that, in this study we only 165 focus on the climate responses over central-eastern China. Although temperature also increased or decreased in western China and outside China likely related to feedbacks or natural variability, there are few observational sites of air pollutants over these regions to verify the simulated pollutant changes and therefore large uncertainties exist in the simulated climate responses over these regions.
Although the air pollutants can influence precipitation through multiple microphysical and dynamical ways, the 170 complicated aerosol-cloud interactions produced large uncertainties in the precipitation responses to the changes in air pollutants. Over eastern China, the reductions in emissions of air pollutants between 2013 and 2017 lead to the annual mean precipitation change by -0.06 ± 0.23 mm day -1 (Fig. S1). Neither the precipitation responses to changes in aerosols nor the O3 are statistically significant at 90% confidence level over eastern China. In the simulations of this study, only fast climate responses are included with fixed SST at the climatological mean. Precipitation change is also driven by land-sea 175 temperature differences over monsoon regions. Fixing SST in simulations can induce biases to the estimate of precipitation responses, which can be revisited using a fully coupled model configuration with both fast and slow climate responses included in future studies.

Climate impacts of aerosol reductions from individual sectors
To explore which emission sector contributed the most to the aerosol reduction-induced regional warming over eastern 180 China, Fig. 7 shows the changes in column burden of PM2.5, ERFari and surface air temperature averaged in eastern China due to emission reductions of anthropogenic aerosols and precursors from individual sectors, and Table S2 summarizes the values. Among all the sectors, industrial emissions contributed the most to the column burden decrease of PM2.5 in eastern China, accounting for 67% of the total burden decrease, followed by 27% due to emission reductions from the energy sector.
The ERFari changes due to aerosol emission reductions in individual sectors from 2013 to 2017 are roughly in linear 185 proportion to the burden changes but in the opposite direction. Reductions in aerosols from industrial and energy sectors exerted ERFari anomalies of 0.50 W m -2 (72% of the combined ERFari anomaly from all sectors) and 0.20 W m -2 (29%) and temperature anomalies of 0.063 and 0.025 ℃, respectively, over eastern China. Declined surface transportation emissions introduced an ERFari anomaly of 0.05 W m -2 (8%) and a temperature anomaly of 0.007 ℃, offset by the change in solvent usage. It is interesting that, different from most sectors, residential emissions reductions lead to a net cooling (-0.03 W m -2 190 and -0.004 ℃) in the context of the aerosol burden decreases over eastern China. It is because the residential heating sector releases a large amount of BC aerosol, which absorbs solar radiation and warms the atmosphere. With residential emissions reduced, decreases in BC resulted in less radiation trapped in the atmosphere and a negative ERFari anomaly, although this effect was largely offset by the decreases in other scattering aerosols. Previous studies have found that switching residential energy to cleaner energy prevented millions of premature deaths in China (Zhang et al., 2021). We suggest that the use of 195 cleaner energy in the residential sector with less BC emissions is more effective to achieve climate and health co-benefits in China in the near future.

Conclusions and Discussions
Since year 2013, China has implemented a sequence of policies for clean air, which could have led to climate impacts through interactions between the changing air pollutants and radiation and clouds. In this study, the climate responses to 2017. An additional ERF of 0.81 ± 0.92 W m -2 by the increases in O3 in the lower troposphere accelerated the climate warming by 0.07 ± 0.09 ℃, leading to an anomalous ERF of 1.99 ± 1.25 W m -2 and a total 0.16 ± 0.15 ℃ warming in eastern China due to the changes in aerosols and O3. It indicates that the recent growing O3 pollution has strengthened the climate warming caused by aerosol emission reductions. Among all emission sectors, emission reductions in the industry sector contributed the most to the aerosol reduction-induced warming (72%), followed by the energy sector (29%). It is noteworthy that, associated with the reduced residential emissions, decreases in BC resulted in less solar radiation trapped in the atmosphere and caused a cooling effect, implying that switching residential sector to cleaner energy with less BC emissions is more effective to improve air quality and mitigate climate warming.
Different models have different climate responses to emission reductions due to uncertainties associated with the different physical, chemical and dynamical parameterizations and feedbacks. Table S3 compares the results in this study 215 with those from previous studies in the literature. Dang and Liao (2019) reported that reductions in aerosols from 2012 to 2017 had led to a regional mean DRF of 1.18 W m -2 in eastern China using GEOS-Chem model, which is higher than the ERFari of 0.79 ± 0.38 W m -2 in this study. They also showed a much weaker O3 DRF of 0.08 W m -2 than the ERF of 0.81 W m -2 estimated here, which is probably because we only adopted tropospheric O3 concentrations from GEOS-Chem but they used total column O3 for the radiation calculation. With the coupled climate model CESM1, Zheng et al. (2020) found that 220 emission reductions exerted a smaller ERF anomaly of 0.48 ± 0.11 W m -2 and a stronger warming of 0.12 °C in East Asia during 2006-2017 compared to the 1.18 ± 0.94 W m -2 and 0.09 ± 0.10 ℃ in this study averaged over eastern China during 2013China during -2017 As shown in this study, aerosol emission reductions in 2017, compared to 2013, led to a regional warming in China and the increased tropospheric O3 pollution further enhanced the warming, hindering climate warming mitigation goals. The 225 connection between regional warming and emission reductions of air pollutants indicates the importance of a balance between air quality improvements and climate mitigations. Our results on sectoral contributions to climate impacts suggest that the residential sector is a good target for emission reduction to improve air quality and mitigate climate warming simultaneously yet reducing aerosol emissions in other sectors, especially the industry sector, is likely to accelerate the regional warming in China. 230 There are some limitations and uncertainties in the study. Firstly, only fast climate responses are considered in our study, while the emission reductions could also influence climate response through slow oceanic processes and air-sea interactions, which can be improved by conducting fully coupled atmosphere-ocean simulations in future studies. Secondly, we did not take into account the changes in greenhouse gas emissions during the clean air actions in the simulations, which also affected climate in China. Thirdly, nitrate and ammonium aerosols, which are not treated in current version of CESM2, 235 also changed from 2013 to 2017 Xu et al., 2019) and should have impacted on climate. Finally, only one model is used in our study, a potential model dependence of climate responses to aerosol reductions needs further investigation using multi-model ensemble simulations. Furthermore, several interesting issues can be investigated in the future. For example, our results just illustrate the impacts of China's aerosol emission changes on China's regional climate, but regional climate changes in China can respond to emission changes outside China, e.g

Competing interests 260
The authors declare that they have no conflict of interest.

Author contribution 265
YY designed the research; JG performed the model simulations and analyzed the data. All authors discussed the results and wrote the paper.    and O3 are calculated as the differences in net radiative fluxes at the top of the atmosphere between Base and AClean simulations (AClean-Base), between AClean and AClean_O3 (AClean_O3-AClean), and between Base and AClean_O3 (AClean_O3-Base), respectively. Differences in areas that are statistically significant at 90 % from a two-tailed t test are stippled. Regional average and standard deviation of the change in eastern China (22°N -40°N, 110°E - Figure 6. Spatial distributions of differences in surface air temperature (℃) due to the changes in (a) aerosols, (b) O3, and (c) both aerosols and O3 between 2013 and 2017, calculated as the differences between Base and AClean simulations (AClean-Base), between AClean and AClean_O3 (AClean_O3-AClean), and between Base and AClean_O3 (AClean_O3-Base), respectively. Differences in areas that are statistically significant at 90 % from a two-tailed t test are stippled. Regional