Climatic Effects of Air Pollutants over China:A Review

LIAO Hong,CHANG Wenyuan,and YANG Yang

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry(LAPC), Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

Climatic Effects of Air Pollutants over China:A Review

LIAO Hong?,CHANG Wenyuan,and YANG Yang

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry(LAPC), Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

Tropospheric ozone(O3)and aerosols are major air pollutants in the atmosphere.They have also made signifcant contributions to radiative forcing of climate since preindustrial times.With its rapid economic development,concentrations of air pollutants are relatively high in China;hence,quantifying the role of air pollutants in China in regional climate change is especially important.This review summarizes existing knowledge with regard to impacts of air pollutants on climate change in China and defnes critical gaps needed to reduce the associated uncertainties.Measured monthly,seasonal,and annual mean surface-layer concentrations of O3and aerosols over China are compiled in this work,with the aim to show the magnitude of concentrations of O3and aerosols over China and to provide datasets for evaluation of model results in future studies.Ground-based and satellite measurements of O3column burden and aerosol optical properties,as well as model

tropospheric ozone,aerosols,radiative forcing,climate change

1. Introduction

Tropospheric O3and aerosols are major air pollutants in the atmosphere that have adverse effects on human health, crops,plants,and atmospheric visibility.They have also made signifcant contributions to radiative forcing of climate since preindustrial times.Ozone is a greenhouse gas, with a global mean anthropogenic radiative forcing of 0.40 (0.20 to 0.60)W m-2(IPCC,2013).Aerosols infuence climate through aerosol-radiation interactions(directeffect)and aerosol-cloud interactions(indirect effect).Anthropogenic changes in aerosols are estimated to have a global mean total direct radiative forcing of-0.45(-0.95 to+0.05)W m-2and an indirect radiative forcing of-0.45(-1.2 to 0.0) W m-2,which are comparable in magnitude to the global mean radiative forcing of 2.83(2.26 to 3.40)W m-2by the increases in CO2,CH4,N2O and halocarbons from preindustrial times(IPCC,2013).With its rapid economic development,concentrations of air pollutants are relatively high in China,suggesting that the climatic effects of air pollutants in China could be much more signifcant than those represented by the global mean radiative forcing values in IPCC(2013).

Climatic effects of air pollutants in China have unique features,because China is located within a large monsoon domain.In May to September every year,Asian summer monsoon prevails;strong cross-equatorial winds fow from the Southern Hemisphere(SH)to the Northern Hemisphere (NH),bringing warm and moist air from the oceans to eastern China.Rain belts and the associated deep convection that stretch for thousands of kilometers move from southeastern to northern China during this period.In the months from November to March,Asian winter monsoon prevails; strong northerlies in high latitudes bring cold and dry air to eastern China.While the seasonal and interannual variations in meteorological felds associated with Asian monsoon can infuence air pollutants over eastern China by infuencing their transport,chemical reactions,and deposition(Zhang et al.,2010;Yan et al.,2011;Zhu et al.,2012),aerosols have been shown to have large impacts on Asian summer monsoon(Menon et al.,2002;Ramanathan et al.,2005;Lau and Kim,2006;Bollasina et al.,2011).These complex feedbacks among chemistry and regional climate have to be considered in studies that examine the climatic effects of air pollutants.

Estimates of the climatic effects of air pollutants have to rely on climate models.The commonly used approach to quantify the climatic effects of air pollutants is shown in Fig. 1.Concentrations of air pollutants are simulated in coupled chemistry–aerosol–climate models using emissions invento-ries,and then optical properties of tropospheric O3,aerosols, and clouds are calculated depending on the chemical and physical properties of air pollutants.After the evaluation of simulated concentrations and optical properties by comparisons with measurements,climatic effects of air pollutants are obtained by examining the differences in simulated climate with or without the optical properties of pollutants in the radiative transfer scheme of climate models.During the past decade,important advances have been made in understanding air pollutants and their roles in climate change in China.Emissions inventories of NOx,CO,speciated volatile organic compounds(VOCs),SO2,NH3,black carbon(BC), and organic carbon(OC)have become available for the Chinese domain(Streets et al.,2003),allowing concentrations of O3and different aerosol species to be simulated and their climatic effects estimated by using numerical models.Increasing ground-based and aircraft measurements of concentrations of O3and speciated aerosols in China,together with satellite measurements of the physical and optical properties of aerosol particles,have provided datasets to evaluate model predictions and to improve simulations for the quantitative assessment of climatic effects of O3and aerosols(Li et al., 2007c;Li et al.,2011b).

This review summarizes recent advances in our understanding of the climatic effects of air pollutants in China.All the key datasets or parameters needed for quantifying the effects of air pollutants on climate are reviewed.The available emissions inventories for the simulation of O3and aerosols are introduced in section 2.The measured surface-layer concentrations of O3and aerosols and column burdens of O3in China are summarized in section 3.The observed optical properties of aerosols,including aerosol optical depth (AOD)and single scattering albedo(SSA),are presented in section 4.Section 5 describes the correlations between air pollutants and observed changes in meteorological parameters in China.Estimates of radiative forcing by O3and different aerosol species are presented in section 6.Modeling studies on the impacts of air pollutants on climate change in China are reviewed in section 7.Finally,the key priorities for future research on the climatic effects of air pollutants in China are highlighted in section 8.

2. Emissions inventories for the simulation of tropospheric O3and aerosols

Emissions inventories allow us to quantify the anthropogenic contribution to climate change,understand the role of each chemical species in the climate system,and eventually take measures to mitigate climate change.Emissions of CH4,NOx,CO,and non-methane volatile organic compounds(NMVOCs)are needed for the simulation of tropospheric O3,and those of SO2,NH3,BC,and OC are essential for the simulation of aerosols.Emissions of chemical species come from both anthropogenic and natural sources.Natural emissions include lightning NOx,NOxfrom soil,biogenic hydrocarbons,BCand OCfrom wild fres,dimethylsulphide, sea spray,and mineral dust.

Figure 2 shows the changes in emissions of NOx,SO2, NH3,BC and OC during the years 1850–2010 from human activities(power,industry,transportation,residential and agricultural activities)and biomass burning,which are taken from the IPCC Fifth Assessment Report(AR5)emissions inventories.The global total emission of SO2peaked in 1980 when emissions in Europe and North America were the highest,and the global total emissions of NOxand NH3peaked in 1990.Emissions of all species in eastern China have been increasing largely since 1950.In 2000,the emissions of NOx,SO2,NH3,BC and OC in eastern China accounted for,respectively,11.2%,18.8%,11.6%,17.3%,and 8.7%of global total emissions.

The NMVOCs are important precursors of tropospheric O3and secondary organic aerosol,although they are not shown in Fig.2.Bo et al.(2008)reported that anthropogenic NMVOCs emissions in China increased from 3.91 Tg in 1980 to 16.49 Tg in 2005 because of the rapid growth in the number of automobiles and solvent and paint use in China.Emissions from natural vegetation are also large sources of VOCs. The estimated annual emission of biogenic VOCs(including isoprene,monoterpenes,and other reactive VOCs)in China averaged over the period 2001–06 was 18.85 TgC yr-1(Fu and Liao,2012),which was comparable in magnitude to anthropogenic emissions.

It should be noted that emissions inventories contain largeuncertainties.For example,Streets and Aunan(2005)found BC emissions in China peaked in 1994–95 and showed reductions in 1996–2000 as a result of the reduced usage of biofuels and coal in residential and industrial sectors.However,Lei et al.(2011)reported that there were no signifcant trends in emissions of BC and OC in China during 1990–2000;emissions of BC and OC increased after 2000,with peak values of 1.51 Tg and 3.19 Tg,respectively,in 2005. Few previous studies have quantifed the impacts of uncertainties in emissions on simulated climate.

3. Observed levels of ozone and aerosols over China

Measurements of O3and speciated aerosols are important for understanding the uncertainties in emissions inventories and simulated concentrations,which can also help to improve the representation of chemical processes in models(Fig.1). To examine the climatic effects of air pollutants,long-term measurements of concentrations are needed;monthly,seasonal,orannualmean concentrations are usually used in evaluations of model simulations.Such measurements at ground stations are very limited in China,since most previous feld campaigns were designed for air quality studies,focusing on the high concentrations of air pollutants in urban and suburban regions for certain short periods of time.For aerosols, the long-term measurements that are available,managed by the Ministry of Environmental Protection of the People’s Republic of China(http://datacenter.mep.gov.cn/TestRunQian/ airdairy.jsp),are PM10mass concentrations,which can hardly be used for studies of the climatic effect of aerosols because the optical properties of aerosols are mainly determined by PM2.5instead of PM10particles.Based on the available literature,we summarize in this section the measured monthly to annual mean levels of tropospheric O3and aerosols.

3.1.Tropospheric ozone

3.1.1.Ground-based measurements

Table 1 summarizes ground-based measurements of O3in China.Observed seasonal or annual mean O3concentrations in China were mostly in the range of 20–60 ppbv. The background O3concentrations are relatively higher in western China as a result of the transport of O3from the stratosphere(Wang et al.,2006a).For example,concentrations at Waliguan(36.3°N,100.9°E)were in the range of 40–60 ppbv,which were higher than the measured values at other background stations such as Fenghuanshan(40.5°N, 124°E),Lin’an(31.3°N,120.4°E),and Long Feng Mountain (44.7°N,127.6°E).Concentrations of O3in eastern China are infuenced largely by anthropogenic emissions(Wang et al.,2011).Based on 810 vertical profles of O3measured by aircraft in different seasons during 1995–2005,Ding et al.(2008)showed that the average mixing ratio of O3in Beijing increased from about 40 ppbv at the ground to about 50 ppbv at2 km altitude.Ding etal.(2008)also compared theobserved O3concentrations in Beijing with measurements in Tokyo,Paris,and New York,and reported that O3concentrations in the lower troposphere in Beijing were generally 5–10 ppbv higher than those in Tokyo and New York.In July to September of 2001–06,the average measured O3concentrations at six urban sites in Beijing was 26.6±2.8 ppbv(Tang et al.,2009).With respect to the long-term trend of O3,surface concentrations measured in Linan,a background station in eastern China,showed that the monthly highest 5%ozone concentrations increased over the period 1991–2006(Xu et al.,2008).

3.1.2.Column burdens

Estimates of the climatic effect of O3rely upon not only surface concentrations but also vertical distributions and col-

umn burdens.Ozonesonde datasets are available only at a number of sites in China(Chan et al.,2003;Ding et al., 2008).Satellite measurements[such as those from Global Ozone Monitoring Experiment(GOME),GOME-2,SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY(SCIAMACHY),Ozone monitoring instrument(OMI),Tropospheric Emission Spectrometer(TES), and Infrared Atmospheric Sounding Interferometer(IASI)] have excellent spatial and temporal coverage,which are useful for analyses of the distributions and seasonal variations of tropospheric column O3concentration over China.Liu et al. (2006)presented the frstdirectly retrieved globaldistribution of tropospheric column ozone from GOME ultraviolet measurements during December 1996 to November 1997.The retrieved column burdens clearly showed signals due to convection,biomass burning,stratospheric infuence,pollution, and transport.Recently,using measurements from the IASI instrument aboard the European Metop-A satellite(launched in October 2006),Dufour et al.(2010)showed that maximum lower tropospheric O3(0–6 km partial column)occurs in late spring and early summer(May to June)in Beijing,and in early autumn in Shanghai and Hong Kong.Although biogenic emissions,temperature,and radiation are the highest in southeastern and southwestern China in July,O3concentrations in those regions are generally low in summer because of the summer monsoon circulation that brings clean air from the oceans(Dufour et al.,2010).

Table 1.Ground-based measurements of ozone in China.

3.2.Ground-based measurements of aerosols

Major aerosol species over China include sulfate(SO24-), nitrate(NO-3),ammonium,elemental carbon(black carbon), primary organic carbon(POA),secondary organic carbon (SOA),and mineral dust aerosols.Measured PM2.5concentrations in China are compiled and listed in Table 2.In Beijing,ambient PM2.5concentrations ranged between 59.2 μg m-3(Wu and Wang,2007)and 257.6μg m-3(Dan et al.,2004).Simultaneous measurements of PM2.5concentrations in winter and summer of 2003 in 14 cities in China indicated that the average PM2.5concentration in these 14 cities was 163.9μg m-3in winter and 71.2μg m-3in summer (Cao et al.,2007).Over eastern China,aerosol concentrations are generally high in winter and low in summer,mainly caused by the large wetdeposition ofaerosols associated with the Asian summer monsoon(Zhang et al.,2010;Gao et al., 2011).Averaged over the 14 cities,carbonaceous aerosols (the sum of OC and BC)contributed to the PM2.5mass by 44.2%in winter and 38.8%in summer(Cao et al.,2007). Sulfate(Table 3)and nitrate(Table 4)aerosols also contributelargely to the PM2.5mass in China.Nitrate aerosol concentrations have been observed to exceed 10μg m-3in many places in winter.

Table 2.Observed PM2.5concentrations in China.

Table 3.Observed concentrations of sulfate aerosol in China.

Table 4.Observed concentrations of nitrate aerosol in China.

In addition to anthropogenic aerosols,arid and semiarid regions in northern and northwestern China are large sources of mineral dust aerosol.Dust storms occur frequently in China in spring of every year(Zhang et al.,2003),which have large impacts on PM2.5concentrations over the source regions and along the transport pathway of the dust storms. Forexample,aerosol samples of PM2.5collected over38 consecutive days in March and April of 2001 in Beijing showed that the average PM2.5mineral dust concentration was 14.6 μg m-3during the observation period,with high PM2.5mineral dust concentrations of 62.4μg m-3and 54.1μg m-3on the sand-dust event days of 21 March and 10 April,respectively(Zhang et al.,2003).During a dust storm event from 21 to 26 March 2001,the total soil dust concentration(fne mode plus coarse mode)was,on average,251.8μg m-3,which was much higher than the average value of 52.1μg m-3measured on non-dust-storm days(Zhang et al.,2008).Dust concentrations depend on wind velocity,soil moisture,and precipitation in dust sources regions as well as the deposition during transport.

4. Measured optical properties of aerosols

4.1.Aerosol optical depth

The spatial and temporal distributions of aerosol optical properties are the key parameters that determine the climatic effect of aerosols.The Moderate Resolution Imaging Spectrometer(MODIS)products of AODare available since 2001. Figure 3 shows the MODIS seasonal mean distributions of aerosol optical depth over China that are averaged over years 2001–10.The highest AOD values of 0.7–1.0 were found over a large fraction of eastern China in spring,because dust storms occur in spring of every year.Throughout the year, AODs exceeding 0.5 were located over the four heavily polluted areas(northern China,the Sichuan Basin,the Yangtze River Delta,and the Pearl River Delta).Although measured aerosol concentrations are generally higher in winter than in summer(Table 2),AODs were found to be higher in summer than in winter because of the high relative humidity and hence the large water uptake by aerosols in summer.

Ground-based sunphotometric measurements are also available for understanding the columnar optical properties over China.The Chinese Sun Hazemeter Network(CSHN), launched in July 2004 by the Institute of Atmospheric Physics,Chinese Academy of Sciences,and the University of Maryland,has an extensive coverage in China(Xin et al., 2006).Based on CSHN measurements,annual mean and seasonal variations in aerosol optical properties at both urban and remote sites have been examined(Xin et al.,2007).Annual mean AOD ranges from 0.14 at clean sites to 0.7–0.9 in southeastern China(Xin et al.,2007).Du et al.(2008)compared aerosol optical properties in March to May of 2005–07 in dust and non-dust weather conditions.They found that the AOD averaged over 15 sites(including dust-source,nearsource,and distant sites)increased by 33%in the presence of dust as compared to nondust conditions.

With the establishment of Aerosol Robotic Network (AERONET)sites in China in the past decade,daily and seasonal variations in AOD have been observed at about 25 stations in China.The AOD values range from about 4 in heavily polluted episodes to about 0.2 in relatively clean conditions in northern China(Xia et al.,2006;Li et al.,2007a). Among the AERONET sites in China,only 7 sites have measurements of 5 years or longer.Since climate is measure of the average pattern of meteorological parameters in a given region,nationwide long term measurements are essential for understanding climatic effect of aerosols in China.

4.2.Single scattering albedo

The SSA,the ratio of aerosol scattering coeffcient to extinction coeffcient,is an important optical parameter that determines whether aerosols have a cooling or a warming effect.Ramanathan et al.(2001)reported that an SSA exceeding 0.95 leads to a negative aerosol forcing at the top of the atmosphere(TOA)and an SSA less than 0.85 leads to a positive forcing.Bergin et al.(2001)reported that the values of SSA were about 0.81 in northern China.Lee et al.(2007)presented a horizontal distribution of SSA values in China considering both ground-based spectral transmittance and spaceborne TOA refectance.They found that the average of SSA values over China was 0.89±0.04 at 500 nm in year 2005. More ground-based SSA measurements in China are listed in Table 5.The large differences in SSA result from the differences in chemical composition,size,shape,and hydroscopic growth of aerosol particles.

Observations show that SSA values are smaller in winter(Qiu and Yang,2008).Increases in BC emissions might have led to the decreases in SSAin Beijing during 1993–2001 (Qiu et al.,2004).Recently,Lyapustin et al.(2011)analyzed AODs in Beijing from both AERONET measurements and MODIS retrievals,and reported an increasing trend in SSA in Beijing during 2007–10 relative to the previous fve years. In addition,mineral dust aerosol can infuence SSA in China. As dust particles are mixed with anthropogenic aerosols during transport,SSA values generally show increases(Xia et al.,2005).

5. Correlations between air pollutants and observed changes in meteorological parameters in China

5.1.Reductions in atmospheric visibility

The absorption and scattering of aerosols have large impacts on atmospheric visibility.Che etal.(2007)analyzed the observed visibility at 615 synoptic weather stations in China during 1957–2005 and found that the horizontal visibility averaged over all the stations had a large decreasing trend of-2.1 km(10 yr)-1after 1990.Che et al.(2007)also showed thatmaximum reductions in visibility correlated wellwith the locations of the highestconcentrations ofairpollutants.Deng et al.(2008)found that low visibility days in Guangzhou occurred rarely(less than a few days per year)during 1954–72, but were about 150 days per year during 1980–2006.Wang et al.(2009)estimated the changes in AOD and atmospheric visibility by using the National Climatic Data Center Global Summary of Day datasets collected from about 3250 meteorological stations from 1973 to 2007.They found that AOD values over land have been increasing on a global mean basis, with large increases in AOD and hence signifcant reductions in visibility in eastern China.

5.2.Reductions in solar radiation at the surface

The impacts of absorption and scattering by aerosols can also be seen in changes of solar radiation at the surface(Li et al.,2011a;Xu et al.,2011).Analysis of data from about 200 meteorological observing stations showed signifcant decreases in sunshine duration over a large fraction of China[approximately 1%(10 yr)-1averaged over the whole China]for the period 1954–98(Kaiser and Qian, 2002).The sunshine duration was measured by a sunshine recorder,expressed to the nearest 0.1 h,in which the direct solar radiation was of suffcient intensity to activate the recorder(a rough approximation of the amount of energy in the direct beam necessary to activate the recorder is 0.12 cal cm-2min-1).Zheng et al.(2008)obtained similar results by analyzing observations at 184 weather stations across the Yunnan-Guizhou Plateau(YGP)in southwestern China for the years 1961–2005.It was found that over this 45-year period annual sunshine duration decreased largely in the northern YGP(north of 24°N),with negative trends reaching-11.8%(10 yr)-1.Increases of aerosols in the atmosphere were found to be responsible for the changes in clear-sky global and direct radiation(Liang and Xia,2005;Qian et al., 2007).Over the second half of the 20th century,while there were signifcant decreases in global radiation[-4.5 W m-2(10 yr)-1],direct radiation[-6.6 W m-2(10 yr)-1],and sunshine duration[-1.28%(10 yr)-1],the diffuse fraction exhibited increases[1.73%(10 yr)-1]as a result of aerosol scattering(Che et al.,2005).

5.3.Changes in surface air temperature

Changes in solar radiation at the surface are expected to lead to changes in surface temperature.Along with global warming,the annual mean surface air temperature averaged over China has increased over recent decades(Wang et al., 2004a;Cao et al.,2013).However,the changes in summer temperature over the period 1960–2008 showed that a negative summer temperature trend of up to-0.2°C(10 yr)-1existed in the Yangtze–Huaihe basins and a warming occurred in northern China and the western provinces(Fig.4). Several studies attempted to explain the decreases in summer temperature in the Yangtze–Huaihe basins by the increases in aerosols in these regions.Kaiser and Qian(2002)found a signifcant decrease in summertime maximum temperature over the period 1954–98 near the Sichuan Basin,where the largest AOD and decreases in sunshine duration occurred.Shi et al.(2008),by examining the distributions and variations of surface air temperature and Total Ozone Mapping Spectrometer(TOMS)AOD in March to August of 1979–2000 overeastern China,found that AOD and surface air temperature were negatively correlated.A similar negative correlation between the increases in AOD and reductions in temperature in eastern China in the past 50 years has also been reported in Chen et al.(2009).

5.4.Changes in precipitation and clouds

The radiative effect of aerosols can infuence precipitation in a number of ways.The direct effect of aerosols reduces surface temperature,evaporation,and atmospheric stability,which can lead to reductions in precipitation(Zhao et al.,2006;Duan and Mao,2008).Aerosol-induced increases in cloud condensation nuclei(CCN)lead to increased cloud droplet concentrations and smaller droplets,which can have an effect of reducing precipitation(Twomey,1959,1977;Albrecht,1989).Hansen et al.(1997)and Ackerman et al. (2000)identifed a so-called“semi-direct”effect,in which changes in the thermal structure of the atmosphere resulting from aerosol solar absorption(principally by BC)suppress cloud formation/precipitation.Furthermore,aerosol-induced changes in temperature gradient can infuence atmospheric circulation and hence atmospheric water vapor supply.

Precipitation in eastern China is associated with the Asian summer monsoon.Every year,the rain belts of summer monsoon appear frstly over southern China from April to mid-June,intensify and shift to the middle and lower reaches of the Yangtze River around mid-June,and then move to northern China in mid-July and last until mid-August.Zhao et al.(2006)examined observed precipitation,MODIS AOD, and meteorological sounding datasets over northern China, and found that precipitation in this region was signifcantly reduced during 1961–2000(the decreasing trend at some stations was larger than 0.4%yr-1).This reduction in precipitation was found to correlate with high concentrations of aerosols and was explained by a positive feedback mechanism.The aerosol-induced enhancement in atmospheric stability depresses upward motion and precipitation,further increasing aerosol concentrations.They found that the percentage of unstable days in northern China was approximately 34%during the late 1980s,and then about 17%in the 2000s.Qian et al.(2009)reported that both the frequency and amount of observed light rain(events or days with precipitation less than either 2 mm or 5 mm)decreased in the whole of eastern China over 1956–2005 as a result of the aerosol indirect effect.This trend for light rain is different from that of total rainfall observed in eastern China,which shows decreases in northern China and increases in southeastern China.Choi et al.(2008)also showed that,over recent decades,increases in aerosol concentrations correlated negatively with the trend in light rain(＜5 mm d-1)and positively with changes in moderate rainfall(10–20 mm d-1)in China. A study by Rosenfeld et al.(2008)proposed that interactions of aerosols with deep convective clouds could increase precipitation,which might be responsible for the observed increases in precipitation in southern China where aerosol concentrations were high and water vapor supply was adequate.

With respect to changes in clouds,Shi et al.(2008) showed a positive correlation between increases in AOD and those in low clouds in southern China in the past two decades. Bennartz et al.(2011)reported that cloud droplet number concentrations,derived from the Patmos-x satellite observations over the East China Sea downwind of emission sources, increased from less than 200 cm-3in the 1980s to more than 300 cm-3in 2005.Zhang et al.(2011)studied the effect of aerosols on clouds in Beijing based onin situaircraft measurements during the period of July to September in 2008. Higher aerosol concentrations were observed to lead to a larger number of cloud droplets with smaller size.The activation of aerosol particles to become cloud droplets was significantly enhanced with liquid water content(LWC)exceeding 0.5 g m-3.However,Tang et al.(2014)found positive correlations between water cloud effective radius(CER)and AOD over eastern China by using MODIS retrievals for 2003–13. Their analysis for North China Plain showed that variations in wind speed and relative humidity might have caused the positive correlations.Southerly winds carry high levels of aerosols and abundant water vapor,resulting in coincident increases in both AOD and CER in North China Plain,while the northerly winds transport dry and clean air from high latitudes,leading to decreases in AOD and CER(Tang et al., 2014).The uncertainties with correlations between AOD and CER indicate the uniqueness of aerosol-cloud interactions in China and also call for further observational and modeling studies.

6. Radiative forcing

6.1.Radiative forcing by tropospheric ozone

Few studies have examined the radiative forcing by tropospheric O3over China.Wang et al.(2005a)estimated the forcing over the Chinese domain using a coupled regional chemistry–climate model(RegCM2).They found that,at the tropopause,tropospheric O3has a shortwave forcing of 0.19 W m-2and a longwave forcing of 0.46 W m-2.The normalized net radiative forcing over China was found to be0.02 W m-2DU-1,which was lower than the global mean values of 0.03–0.05 W m-2DU-1reported in the literature (Wang et al.,2005a).Chang and Liao(2009)estimated the anthropogenic radiative forcing by tropospheric O3based on the IPCC AR5 emissions inventories;the tropopause forcing(shortwave plus longwave)averaged over eastern China (95°–125°E,18°–45°N)was found to be 0.53 W m-2.

6.2.Direct radiative forcing by aerosols

For the calculation of aerosol direct radiative forcing,the optical parameters used in the radiative transfer module of climate models are AOD,SSA,and asymmetry factor.These optical properties are dependent of the concentrations,size, and hygroscopic growth of aerosols.Earlier studies quantifed the radiative forcing by a single aerosol species.Giorgi et al.(2002)performed a coupled climate–chemistry/aerosol simulation for China for the period 1993–97.Over eastern China,anthropogenic sulfate was found to induce a negative TOA radiative forcing that varied spatially from-1 to-8 W m-2in winter and from-1 to-15 W m-2in summer. Fossil fuel BC was estimated to exert a positive TOA radiative forcing of 0.5–2 W m-2(Giorgi et al.,2002).Recently, direct radiative forcing by BC in China was estimated by Wu et al.(2008)and Zhuang et al.(2011;2013),and that by nitrate was calculated by Wang et al.(2010),Zhang et al. (2012a),and Li et al.(2014).

With more advanced models that can simulate multiple aerosol species,recent studies have estimated aerosol forcing by mixed aerosol species.Qian et al.(2003)used the offine concentrations of sulfate,OC,BC,mineral dust,and sea salt from a global transport model in the Penn State/NCAR Mesoscale Model MM5 to examine the aerosol direct effect. The direct radiative forcing of aerosols in eastern China(east of 100°E)was found to be in the range of-1 to-14 W m-2in autumn and summer,and-1 to-9 W m-2in spring and winter.The annual radiative forcing in eastern China(110°–120°E,21°–37°N)was estimated to be-5.11 W m-2(Qian et al.,2003).Han(2010),by using the Regional Integrated Environmental Model System(RIEMS),simulated the direct radiative effect of externally mixed sulfate,BC,OC,soil dust, and sea salt aerosols over East Asia(75°–145°E,15°–55°N). They showed that the direct radiative effect of aerosols had apparent seasonal variations(Fig.5).Averaged over East Asia,the annual mean direct radiative effect under all-sky condition was calculated to be-6.2 W m-2at the surface and-0.2 W m-2at the TOA(Han,2010).

Chang and Liao(2009)estimated the anthropogenic radiative forcing of sulfate,nitrate,BC,OC,and the internal mixture of all these species by using a general circulation model with online simulations of gas-phase chemistry and aerosols.The radiative forcing values averaged over eastern China(95°–125°E,18°–45°N)are shown in Table 6.Based on the IPCC AR5 emissions inventories,the simulated annual mean anthropogenic radiative forcings by sulfate,nitrate,BC, OC,and the internal mixture of these aerosols were,respectively,-2.50,-0.75,0.58,-0.13,and-2.40 W m-2at the TOA,and-2.55,-0.77,-1.50,-0.25,and-5.40 W m-2at the surface.The annual mean maximum cooling over eastern China was predicted to exceed-6 W m-2at the TOA and be up to-13 W m-2at the surface.

Some studies have also estimated forcing based on ground or satellite observations of AOD and solar irradiance (Li et al.,2007c;Xia et al.,2007a,b).Aerosol-induced reduction in surface shortwave irradiance was estimated to be about 30 W m-2at Liaozhong in northeastern China during April to June of 2005(Xia et al.,2007a).At Xianghe near Beijing,the annual and daily(24 h)mean aerosol radiative effect at the surface(24 W m-2)was only moderately lower than the cloud radiative effect(41 W m-2)(Li et al.,2007c). It should be noted that studies based on measurements usually obtained clear-sky radiative forcing by both natural and anthropogenic aerosols(i.e.,they could notobtain cloudy-sky forcing and could not separate the effect of a specifc aerosol species).

The high direct radiative forcing values by aerosols indicate their important roles in climate change.The estimated forcing values over China differ largely,because of the differences in emissions inventories,chemistry scheme,model year or month,and the different aerosol species considered.

6.3.Indirect radiative forcing by aerosols

Estimated indirect forcing values are usually dependent on the treatment of aerosol effects on cloud microphysics in climate models.Previous studies usually used one of the following two approaches in the calculation of cloud droplet number concentrations:(1)the empirical correlations between aerosol mass and cloud number concentrations obtained based on measurements(e.g.,Boucher and Lohmann,1995);and(2)frst-principle approaches(e.g., Ghan et al.,1997),which simulate cloud droplet number balance in each GCM grid cell,including processes such as the activation of aerosol into cloud droplets,evaporation and collision/coalescence affecting droplet number concentrations. Because of the lack of measurements in China,empirical correlations between aerosol mass and cloud number concentrations used in the frst approach are generally parameterized based on measurements in Europe or the U.S.,which may not be representative of aerosol–cloud interactions in China.

Giorgi et al.(2003)reported the regionally averaged TOA indirect radiative forcing due to sulfate and BC aerosols. While the direct radiative forcing showed a weak seasonal cycle(seasonal mean TOA direct forcing values ranged from-2.29 W m-2to-2.74 W m-2over eastern China),the total of direct and indirect forcing showed a very strong seasonal variation[the total negative TOA radiative forcing varied from-3.93 W m-2in December–January–February(DJF) to-11.77 W m-2in June–July–August(JJA)over eastern China].The indirect forcing was found to dominate in the warm and wet seasons of March–April–May(MAM)and JJA,when the cloud amount is the largest in a year.Huang et al.(2006)considered sulfate,BC,and OC aerosols in their simulations.Averaged over eastern China,the direct and frst indirect effects were predicted to produce a negative net radiative forcing of-4.6 W m-2at the TOA,and the sec-ond indirect effect was predicted to generate a negative net forcing of approximately-3.2 W m-2at the TOA.Huang et al.(2007)reported the surface-layer seasonal mean radiative forcing by sulfate and OC aerosols in eastern China;the forcing was found to be-1.3 to-2.3 W m-2as a result of direct scattering,and the value was predicted to be-4.2 to-9.6 W m-2with the combined direct,semidirect,and frst indirect effects.The maximum surface forcing was found to reach-30 W m-2in the areas with high aerosol concentrations.

Because of the high nitrate aerosol concentrations in China(Table 4),it is of interest to estimate radiative forcing by nitrate aerosol.Wang et al.(2010)used the RegCM3 model coupled with chemistry and aerosol schemes to calculate direct and indirect effects of nitrate aerosol in China. They reported that nitrate indirect radiative forcing is higherthan its direct radiative forcing;the annual mean direct radiative and frst indirect radiative forcings at the TOA were estimated to be-0.88 W m-2and-2.47 W m-2,respectively, as forcings were averaged over the whole of China.

Table 6.Predicted annual mean global burdens(Tg yr-1)of aerosols and O3 in 1850 and 2000,as well as the estimated radiative forcings (RF)(W m-2)at the top of the atmosphere(TOA)and surface(SRF).Ozone forcing is also given at the tropopause*.Forcing values are averaged over the globe or over eastern China(18°–45°N,95°–125°E)(Chang et al.,2009).

The indirect effects of aerosols on solar radiation found in these studies were qualitatively similar to other reports in the literature;increases in cloudiness and small cloud droplets enhance the scattering of solar radiation.The impacts of such changes in cloud properties on solar radiation are still uncertain;more physical-based methods to determine cloud droplet concentrations from aerosol concentrations and other relevant parameters should be the subject of future studies on aerosol indirect effects in China.

6.4.Comparisons with IPCC global mean forcing values

The tropopause forcing by tropospheric O3in eastern China was estimated by Wang et al.(2005a)and Chang and Liao(2009)to be 0.53–0.65 W m-2,which are close to the high end of the global mean values of 0.40(0.20 to 0.60)W m-2estimated in IPCC AR5.Radiative forcings of aerosols in China are much higherthan the globalmean direct radiative forcing of-0.45(-0.95 to 0.05)W m-2and indirect radiative forcing of-0.45(-1.2 to 0.0)W m-2reported in IPCC AR5.Radiative forcing by aerosols in China is also much stronger than the global mean radiative forcing of 2.83 (2.26 to 3.40)W m-2by the increases in CO2,CH4,N2O, and halocarbons from preindustrial times.These high radiative forcing values indicate that air pollutants in China can have important roles in regional and global climate change.

7. Simulated climatic effects of air pollutants

To understand the roles of air pollutants in climate change in China,it is important to have a clear picture of climate change in China overthe pastseveraldecades.While a warming was observed in China on an annual mean basis(Cao et al.,2013),a negative summer temperature trend existed in the Yangtze–Huaihe basins(Fig.4).The observed changes in precipitation over the past decades showed that summer precipitation decreased in northern and southeastern China and increased in the Yangtze Basin(Xu,2001).Climate change in China is associated with changes in the East Asian monsoon.According to the monsoon index defned by Li and Zeng(2002,2003,2005),the East Asian summer monsoon has been weakening since 1950.A strong summer monsoon is characterized by strong southerlies extending from southern to northern China,a defcit of rainfall in the middle and lower reaches of the Yangtze River,and high rainfall in northern China(Li and Zeng,2002;Wang et al.,2008).On the contrary,a weak summer monsoon in China features weak southerlies that cannot push the monsoon system far enough northward,resulting in high rainfall in southern China and a defcit of rainfall in northern China.These changes in the Asian summer monsoon can help to explain the observed changes in summer temperature and precipitation,but the roles of air pollutants need to be quantifed.

The climatic effects of air pollutants(either tropospheric O3or aerosols)in China have been examined by some regional or global climate models.Regional climate models have the advantage of high spatial and temporal resolutions, but they cannot account for changes in large-scale circulation if the regional climate simulations with or without air pollutants use the same boundary conditions.Although forcings by air pollutants are quite regional,they can infuence largescale circulation by altering temperature gradients;climate responses to the forcings are not local,as demonstrated by many global studies.Global simulations of climatic effects of air pollutants usually take one of two forms:(1)equilibrium climate,in which the long-term climate that would result from a fxed pollutant concentration is computed;and(2) transient climate,in which climate is simulated from a starting point,say preindustrial,with specifed annual emissions changes.Predicted changes from an equilibrium climate simulation generally exceed those from a transient climate simulation.For example,the ratio of the transient climate response(the change in surface air temperature at the time of doubled CO2)to the equilibrium response(the equilibrium change in surface air temperature from doubled CO2)lies in the range of 0.47–0.68(IPCC,1995).

7.1.Climatic effect of tropospheric ozone

A few studies have examined climate change induced by tropospheric ozone in China.Wang et al.(2004b)used thecoupled regional chemistry–climate model RegCM2(Giorgi et al.,2002)to simulate the concentrations and climatic effect of tropospheric O3in China.They found that the monthly mean column burden of O3was about 30 DU in eastern China,which led to changes in surface air temperature by-0.8 to 0.8 K.The negative changes in temperature were mainly associated with the feedback of clouds.Hansen et al.(2007)examined the climatic responses to changes in tropospheric ozone over the period 1900–2003 based on transient climate simulations with the GISS ModelE,and their predicted increases in temperature reached up to 0.5 K over eastern China.Chang etal.(2009)reported,based on a global coupled chemistry–climate simulation,that the warming due to changes in tropospheric O3over the period 1950–2000 was 0.43 K in eastern China(Table 7).These results indicate that it is important to consider the radiative effect of tropospheric O3in simulations of climate change in China.

7.2.Climatic effect of tropospheric aerosols

7.2.1.Regionalsimulationsof theclimaticeffectofaerosols

Giorgi et al.(2002)performed a number of multi-year regional simulations to assess the climatic impact of anthropogenic sulfate and BC.Using anthropogenic emissions,they found that the direct effects of anthropogenic sulfate and BC can induce a surface cooling of-0.1 to-0.7 K,highly variable at the subregional scale.Qian et al.(2003),using the offine felds of sulfate,OC,BC,mineral dust,and sea salt, reported that the negative radiative forcing of aerosols was predicted to induce a surface cooling in the range of-0.6 to-1.2 K in autumn and winter,-0.3 to-0.6 K in spring and 0.0 to-0.9 K in summer throughout eastern China.

Giorgi et al.(2003)advanced the regional study by including the frst indirect effect of anthropogenic sulfate. Averaging over eastern China(approximately east of 90°E and between 20°–40°N),the direct radiative effect of sulfate and soot was predicted to reduce surface air temperature by 0.07 K in JJA and 0.21 K in DJF.Such larger reduction in temperature in winter than in summer was also found by Chen et al.(2007).With the addition of the frst indirect effect,the impact of sulfate and soot on surface air temperature was predicted to be about-0.4 K in all seasons.Predicted changes in precipitation in DJF,MAM, JJA,and September–October–November(SON)were,respectively,-5.9%,-2.3%,-1.7%,and-4.2%,with the direct effect of sulfate and soot,and-12.6%,-7.9%,-5.3%, and-9.9%when both direct and frst indirect effects were considered.

Huang et al.(2007)used a coupled regional chemistry–aerosol–climate model to assess the direct and indirect effects ofanthropogenic sulfate and carbonaceous aerosols(OC and BC)on regional climate in East Asia.The direct and frst indirect effects were simulated to reduce solar radiation and hence decrease the surface temperature in a large fraction of eastern China,with the magnitude of the simulated changes in surface air temperature compatible with the results obtained by Giorgi et al.(2003).The second indirect effect was predicted to generate both negative solar forcing and a substantial positive longwave forcing,which led to decreases in precipitation,but because of the cancellation effect,very small changes in surface temperature(Huang et al., 2007).With the interactively calculated aerosol loading and the combined direct/semidirect/frst indirect radiative effects, the simulated precipitation was reduced by about 10%in autumn and winter and by about 5%in spring and summer.The second indirect effect was shown to have the largest impact on precipitation,which was predicted to reduce the precipitation in autumn and winter from about 3%to 20%,depending on the autoconversion scheme used(Huang et al.,2007).The semidirect effect on precipitation was found to be relatively small.

Long-term observational datasets reveal that both the frequency and amount of light rain decreased in eastern China during the period 1956–2005.Qian et al.(2009)showed by using a cloud-resolving model and satellite data that high concentrations of aerosols in eastern China can signifcantly increase cloud droplet number concentrations and reduce droplet sizes compared to pristine conditions,leading to signifcant reductions in raindrop concentrations and a delay in raindrop formation because smaller cloud droplets are less effcient in collision and coalescence processes.Results indicated that the signifcantly increased aerosol concentrations were at least partly responsible for the decreased light rain observed in China over the past 50 years.

Besides studies that have examined scattering or mixed aerosols,Wu et al.(2004,2008)examined the climatic effect of BC alone.Using the Regional Integrated Environmental Modeling System(RIEMS)and the CCM3 radiation scheme, Wu etal.(2004)performed simulations forthe months of January,April,July,and October in 2000.The radiative forcing of BC was predicted to lead to decreases in surface temperature in East Asia by 0.12–0.15 K in January,0.25 K in April,0.3–1.5 K in July,and 0.2–0.5 K in October.The maximum decreases in temperature were located over the Yangtze River drainage basin and southern China.Wu et al.(2008)further examined the direct effect of BC on the hydrological cycle in China using the RegCM3 model(Giorgi et al.,2002).Water vapor was predicted to increase by 0.6%in southern China but to decrease by up to 0.3%in northern China.The increases in water vapor content in southern China were mainly caused by the strengthening of airfow from the South China Sea.The radiative forcing of BC was found to lead to an increase in precipitation by 0.4–0.6 mm d-1in southern China.

Table 7.Summary of the average changes of selected climate variables over the period 1950–2000 in eastern China(20°–50°N,100°–130°E)(Chang et al.,2009).

7.2.2.Equilibrium climate simulations using global models

Menon et al.(2002)studied the equilibrium climatic responses to the direct and semidirect effect of aerosols over China and India using a global climate model with fxed SSTs.By considering anthropogenic aerosols,their simulation reproduced the observed increases in precipitation in southeast China and decreases in precipitation in northeast China.These changes in precipitation were attributable to the absorption(as opposed to the scattering)of radiation by the observed AOD distribution.Their model results were dependent on the assumed SSA value of 0.85,which was too low as compared to the observed SSA values in China (Table 5).

Gu et al.(2006)investigated the equilibrium climate responses to the direct radiative forcing of both natural and anthropogenic aerosols in China using the general circulation model developed at the University of California,Los Angeles(UCLA GCM).They assumed a global background AOD of 0.2,consisting of 90%maritime and 10%BC based on the observations of TOMS.Over the Chinese domain,observed changes in AODs were used to simulate the climatic effect of aerosols.Total cloud cover was predicted to decrease in northern and western China,which agreed with the observed decreases over the period 1951–94.Aerosols in China were predicted to lead to a cooling in the midlatitudes of the NH, strengthening the meridional circulation and then increasing precipitation in southern China.This predicted precipitation anomaly was similar to that presented by Menon et al.(2002). However,Gu et al.(2006)argued that the increased precipitation in southern China might be related to the increased AOD rather than the absorption of aerosol as suggested by Menon et al.(2002).

Kim et al.(2007)studied the equilibrium climatic effect of sulfate aerosol radiative forcing on spring rainfall in East Asia based on numerical simulations with the NASA fnite-volume General Circulation Model(fvGCM).The climate simulations were forced with monthly varying threedimensional aerosol distribution from the Goddard Ozone Chemistry Aerosol Radiation and Transport model(GOCART).The reanalyzed weekly SSTs from September 1986 to December 1996 were interpolated to daily values.The radiative forcing of sulfate aerosol was predicted to lead to a cooling at the land surface and a reduction in rainfall over central East Asia.The maximum reduction in precipitation was found to be located to the north of the area with the maximum aerosol loading as a result of dynamical feedback.An anomalous thermal gradient due to aerosol cooling near the land surface can reduce the baroclinicity of the atmosphere, leading to a deceleration ofthe upper-levelwesterly fow.The westerly deceleration induces,through ageostrophic wind adjustment,anomalous meridional secondary circulation at the entrance region of the East Asian jet stream,with a strong descending fow and suppressed precipitation near 30°N,but a weak ascent of air and moderately enhanced precipitation over southern China and the South China Sea.Their results suggested that aerosol radiative forcing and the associated feedback contributed to the observed trend of spring precipitation in East Asia.

A number of recent global studies have also considered the aerosol indirect effect.Takemura et al.(2005)simulated the equilibrium climate responses to aerosol direct and indirect effects with a coupled global aerosol transport–radiation–climate model.They predicted a large reduction of about 2 K in temperature and a general reduction in precipitation over East Asia.To explain the observed decreases in precipitation in southern China(114°–120°E,23°–27°N)in early summer over the past decades,Cheng et al.(2005)applied the ECHAM4 GCM with a fully coupled aerosol–cloud microphysics module to investigate the role of the aerosol second indirect effect by sulfate,BC,and OC in the drought of early summer in southern China.They separated the role of the aerosol second indirect effect from the roles of the increases in SSTs and greenhouse gases(GHGs).Model results showed that both the aerosol indirect effect and changes in SSTs/GHGs contributed to the observed increases in lowlevel cloud and decreases in precipitation over the period 1961–2000 in southern China,and the impact of the aerosol indirect effect was larger than the effect of SST/GHGs.

7.2.3.Transient climate simulations using global models

Li et al.(2007b)considered the sulfate direct radiative effect in a simulation over the period 1949–2002 using a GCM. They found that sulfate aerosol can induce a positive gradient of air temperature from the middle to the upper troposphere, which results in a northward shift of the East Asian westerly jet stream at 200 hPa and a strengthening of the East Asian summer monsoon.They simulated a surface cooling of about 1 K and increases in cloud cover and precipitation in eastern China over the period 1949–2002.

Chang et al.(2009)investigated the direct climatic effects of fve anthropogenic aerosols,including,, BC,POA,and SOA,in eastern China over the period 1951– 2000.Concentrations of,,POA,SOA,BC,and tropospheric O3for the years 1950 and 2000 were obtained a priori by the Goddard Institute for Space Studies(GISS) GCM with coupled chemistry and aerosols,and then monthly concentrations of these species were interpolated linearly between 1951 and 2000 in transient climate simulations. BC aerosol was assumed to be internally mixed with other aerosol species in all simulations.Model results indicated that simulated changes in temperature and precipitation over 1950–2000 were very sensitive to the consideration ofaerosolspecies(Figs.6 and 7).For example,the predicted sign of changes in surface air temperature by BC++POA aerosols was different from that by all aerosol species(BC++POA+SOA+).They also compared the climatic effect of aerosols with the impacts of tropospheric O3and GHGs over eastern China(Figs.6 and 7).As can be seen in Table 7,relative to the climate simulation without changes in GHGs,O3,and aerosols,anthropogenic forcings of, BC,BC+,BC++POA,BC++POA+ SOA+NO-3,O3,and GHGs over the period 1950–2000 were predicted to change surface air temperature in eastern China (100°–130°E,20°–50°N),respectively,by-0.40°C,0.62°C, 0.18°C,0.15°C,-0.78°C,0.43°C,and 0.85°C,and to change precipitation averaged over the same domain,respectively,by-0.21,0.07,-0.03,0.02,-0.24,-0.08,and 0.10 mm d-1. They concluded that aerosols,when all major aerosol species are considered,are as important as GHGs in infuencing climate change in eastern China.

7.2.4.Uncertainties with simulated climatic effect of aerosols in China

Despite the recognized importance of aerosols for the simulation of climate in China,the quantifcation and understanding of their impacts are still poorly constrained.Diffculties arise especially on a regional scale,owing to the short atmospheric lifetime of the aerosol particles combined with the limited observations available.Simulated climate responses depend on simulated aerosol concentration,size, mixing state,AOD,activation of aerosol particles to form cloud droplets,as well as the cloud schemes in climate models.Current aerosol-climate models can simulate all these key parameters and processes,butnationwide long-term measurements are needed to constrain model simulations.The model uncertainties are especially large when aerosol-cloud interactions are included in simulations.

Since China is one of the most heavily polluted regions in the world,the impacts of air pollutants on hydrological cycle are of great concern.In recent decades,there has been a tendency of“southern food and northern drought”over eastern China during the East Asian summer monsoon(Xu,2001). Many modeling studies(e.g.,Menon et al.,2002;Giorgi et al.,2002,2003;Gu et al.,2006;Huang et al.,2007;Liu et al.,2009;Zhang et al.,2012b;Jiang et al.,2013;Wu et al., 2013)have shown that the East Asian summer monsoon precipitation is affected by aerosols,but contradictory results exist.Menon et al.(2002)suggested that the radiative effect of black carbon may contribute to a“southern food and northern drought”pattern over eastern China.Liu et al.(2009) pointed outthe aerosolradiative effectofsulfate is more dominant than that of black carbon in reducing precipitation over China.Gu et al.(2006)found that such a“southern food and northern drought”pattern was due to the radiative effect of anthropogenic aerosols,which cooled the midlatitudes and led to the strengthening of the Hadley circulation.In contrast, Zhang et al.(2012b)showed that the aerosol direct radiative effect led to decreases in precipitation over southern China but slight increases in precipitation over northern China.Wu et al.(2013)found that aerosol effects on precipitation depend on the location of monsoon precipitation band and its relative location to aerosols,which may be the main factor that contributed to the discrepancy in literature regarding the aerosol impacts on the so-called“southern food and northern drought”over China.

Previous studies also reported the biases ofusing the fxed SSTs in the simulation of temperature and precipitation(Yue et al.,2011).The slow response associated with SST change caused by aerosols may play an important role in the simulated climate responses to aerosol forcing especially over ocean areas(Ganguly et al.,2012).

8. Summary and outlook

This review documents recent advances in our understanding of the climatic effects of O3and aerosols in China. All the key datasets or parameters needed for quantifying the effects of air pollutants on climate are reviewed,including emissions inventories,measurements of concentrations and column burdens,observed optical properties of aerosols,high correlations between air pollutants and the observed changes in meteorological parameters in China,estimated radiative forcing by O3and different aerosol species,as well as the simulated impacts of O3and aerosols on climate change in China.

During the past decade important advances have been made in our understanding of air pollutants and their roles in climate change in China.Emissions inventories of O3/aerosol precursors and aerosols have become available for the Chinese domain,allowing concentrations of O3and different aerosol species to be simulated and their climatic effects estimated using numerical models.Increased ground and aircraft measurements of concentrations of O3and speciated aerosols in China,together with satellite measurements of aerosol optical properties and cloud properties,have provided datasets to constrain the simulated climatic effects of O3and aerosols. However,estimates of the climatic effects of air pollutants in China are still subject to large uncertainties.Based on this review,we suggest the following key priorities for future research:

(1)Continued improvement in gas and particle emission inventories that underlie any predictions of climatological levels of pollutants.Since the emissions inventories from different research groups vary,it is necessary to quantify how the uncertainties in emissions can infuence simulated climate change,such as changes in temperature and precipitation.

(2)Nationwide long-term measurements of concentrations and optical properties of pollutants.Measurements of speciated aerosol mass concentrations,size-resolved aerosol number concentrations,and aerosol–cloud interactions are important for reducing uncertainties associated with the climatic effect of aerosols.Special attention needs to be paid to the vertical profles of concentrations and optical properties.

(3)Coupled transient chemistry–aerosol–climate simulations for interpreting climate change in China over past decades.While air pollutants can infuence climate as sum-

marized above,interannual and decadal variations in meteorological felds associated with Asian monsoon can infuence air pollutants over China by infuencing their transport, chemical reactions,and deposition(Zhu et al.,2012).Simulations also need to account for the role of SST responses to O3/aerosol-induced forcing(Yue et al.,2011).Because of the monsoon system,changes in SST have large impacts on temperature gradient,atmospheric circulation,and water vapor supply from the oceans,resulting in complex feedbacks in regional climate.It should also be noted that tropospheric O3has been found to contribute to about 0.5 K of increases in surface air temperature in eastern China in past decades (Hansen et al.,2007;Chang et al.,2009);the radiative effect of tropospheric O3should be considered in simulations of climate change in China.

(4)Improved analysis of the aerosol indirect effect in China.As in other parts of the world,aerosol–cloud interaction is recognized as perhaps the most complex and uncertain issue in addressing anthropogenic impacts on climate change. The link between aerosol concentration and cloud optical properties depends on multiple factors(i.e.,cloud droplet activation,updraft velocity etc.).The parameterizations of aerosol–cloud interactions in current climate models are generally not developed based on measurements in China,which need to be evaluated and improved to represent the chemical and meteorological conditions in this part of the world.More physical-based methods to determine cloud droplet concentrations from aerosol concentrations and other relevant parameters should be used in climate simulations.Furthermore, since precipitation during Asian summer monsoon is associated with strong convective clouds,the interactions between aerosols and strong convective clouds,as proposed by Rosenfeld et al.(2008),are expected to be important and should be the subject of future work.

Acknowledgements.This work was supported by the National Basic Research Program of China(Grant No.2014CB441202),the Chinese Academy of Sciences Strategic Priority Research Program (Grant No.XDA05100503),the National Natural Science Foundation of China(Grant Nos.41475137 and 41321064),and Special Funding in Atmospheric Science(Grant No.GYHY200906020).

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(Received 1 May 2014;revised 2 July 2014;accepted 26 July 2014)

?Corresponding author:LIAO Hong

Email:hongliao@mail.iap.ac.cn

estimates of radiative forcing by tropospheric O3 and aerosols are summarized.We also review regional and global modeling studies that have investigated climate change driven by tropospheric O3and/or aerosols in China;the predicted sign and magnitude of the responses in temperature and precipitation to O3/aerosol forcings are presented.Based on this review,key priorities for future research on the climatic effects of air pollutants in China are highlighted.