The globally averaged combined l and and oceansurface temperature exhibited a warming of 0.85 [0.65–1.06]℃ over 1880–2012, as reported by the IntergovernmentalPanel on Climate Change(IPCC)WorkingGroup I Fifth Assessment Report(AR5)(IPCC,2013). Human activities contributed to changes intemperature through changing concentrations of bothwell-mixed greenhouse gases(WMGHGs, includingcarbon dioxide(CO₂), methane(CH₄), nitrous oxide(N₂O), and halocarbons) and short-lived species(nitrogen oxides(NO_x), carbon moNO_xide(CO), nonmethanevolatile organic compounds(NMVOCs), troposphericO₃, aerosols, and aerosol precursors). Theradiative forcing(RF)values over 1750–2011 estimatedby the IPCC AR5 are 3.00 W m⁻² by emissionsof WMGHGs and –0.64 W m⁻² by emissions ofshort-lived species(Fig. 1), indicating that short-livedspecies also play important roles in climate change.Among the short-lived species, tropospheric O₃ isestimated to have a global mean RF of 0.40 W m⁻²(Table 8.6 in IPCC(2013)).

Fig. 1 Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climatechange. Positive(negative)radiative forcing indicates a warming(cooling)effect on climate.(Source: Figure SPM.5 ofIPCC WGI AR5 Summary for Policymakers(IPCC,2013))

Figure options

Short-lived species influence climate change inseveral ways depending on their chemical and physicalproperties. Firstly, the short-lived gases can influenceconcentrations of WMGHGs and aerosols bychemical reactions. For example, CO, NMVOCs, and NO_x are precursors of tropospheric O₃. Emissions ofCO and NMVOCs lead to CO₂ formation in the atmosphere.Emissions of NO_x in the atmosphere influenceconcentrations of CH₄ by altering OH concentrations, and also contribute to nitrate aerosol formation.Secondly, some of the short-lived species, such astropospheric O₃ and aerosols, exert radiative forcingto the energy balance of the earth’s climate system.Tropopspheric O₃ is a greenhouse gas that leads toglobal warming(Fig. 1). Aerosols influence climateby interactions with radiation through scattering orabsorbing of solar or longwave radiation, and by interactionswith clouds through altering cloud properties.Major anthropogenic aerosol species in the atmosphereinclude sulfate, nitrate, ammonium, organiccarbon(OC), and black carbon(BC), all of which havea cooling effect on climate except that BC has a warmingeffect. Thirdly, short-lived species participate incomplex biogeochemical processes that can influenceconcentrations of WMGHGs. For example, the depositionof nitrogen and O₃ can influence carbon cycle bychanging l and cover(Lamarque et al., 2005; Janssens et al., 2010).

Due to the unidentified sinks for atmosphericCO₂,Jacobson(2005)assumed that CO₂ has a lifetimeof 30–95 yr. Prather et al.(2012)derived that thepresent-day atmospheric lifetime is 9.1±0.9 yr for CH₄ and 131±10 yr for N₂O. Tropospheric O₃ was reportedto have a lifetime of about 3 weeks based on modelingstudies(Liao and Seinfeld, 2005; Stevenson et al., 2006). Aerosols have even shorter lifetimes of severaldays(Kaufman et al., 2000). Jacobson(2004)comparedthe time-dependent changes in globally averagednear-surface temperature due to eliminating anthro-pogenic emissions of each of CO₂, CH₄, as well as BC and OC from fossil fuel and biofuel sources. Consideringthat BC and OC were co-emitted species fromanthropogenic and biomass burning sources, Jacobson(2004)suggested that the control of BC+OC may bethe most effective method of slowing global warmingfor a specific period(about 10 yr), although OC itselfhas a cooling effect. The assessment report of UnitedNations Environment Programme(UNEP,2011)proposedtwo approaches to mitigate global warming:one is to control the peak temperature by reducingthe concentrations of short-lived species, such as BC, CH₄, and tropospheric O₃; the other is to control thelong-term climate warming by reducing CO₂ concentrations.UNEP(2011)demonstrated by simulationsof climate models that reductions in BC and troposphericO₃ can be an effective method to slow the rateof climate change within the first half of this century.Climate benefits from reduced O₃ are achieved by reducingemissions of some of its precursors, especiallymethane that is also a powerful greenhouse gas.

With the rapid economic development, concentrationsof short-lived species are relatively high inChina; hence quantifying the role of short-lived speciesin regional and global climate change is especiallyimportant. Despite the complex interactions amongWMGHGs and short-lived gases, this review is focusedon BC and tropospheric O₃ due to their warming effects(Fig. 1). Ground and satellite measurements ofconcentrations or optical properties of BC and troposphericO₃ over China, as well as the model estimatesof radiative forcing by these two species are summarized.We also review regional and global modelingstudies which have investigated climate change drivenby BC and tropospheric O₃ in China. Based on thereview of previous studies, the key priorities for futureresearch on climatic effects of BC and troposphericO₃ are highlighted. This review is mainly focused onthe studies of BC and tropospheric O₃ by Chinese scientistsover the past several years, and the climaticimpacts are confined to the impacts of BC and O₃in the atmosphere within the Chinese territory. Notethat China’s climate may be affected by BC and O₃outside China due to complex feedbacks in the climatesystem.

BC(alternatively referred to as elementary carbon and soot)is released into the atmosphere duringthe incomplete combustion of fossil fuel, biofuel, and biomass. Emissions of BC in China are large due toits large population, substantial fuel consumption, and often-inefficient combustion conditions, which were reportedto be responsible for about 19% of the globalBC emission(Qin and Xie, 2012).

Figure 2 shows the spatial distributions of BCemissions. BC emissions were much higher in easternChina than in western China. Western China istypically low in emissions, due to the relatively underdevelopedeconomy and the small consumption of fossilfuels. Western regions, including Tibet, Xinjiang, Qinghai, Gansu, and Ningxia, cover 42.4% of territory, made up only 3.8%,4.0%,4.7% and 5.7% of nationalBC emissions in 1985,1995,2005, and 2009(Qin and Xie, 2012), respectively. Table 1 summarizes BCemissions in China from different studies. The annualtotal emission of BC in China after 2006 exceeded 1.79Tg yr⁻¹(Zhang Q. et al., 2009; Qin and Xie, 2012;Wang et al., 2012). According to Lu et al.(2011), the percentage contributions from sectors of power, industry, residential, transport, and biomass burning to thetotal BC emission in China were 1.1%,27.1%,50.6%,15.3%, and 5.9%, respectively, in 2010(Table 1).

Fig. 2 Spatial distributions of BC emissions(GgC grid−1)at a resolution of 0.1°× 0.1°in 2010. Theemissions were compiled by the Emissions Databasefor Global Atmospheric Research(EDGAR)team and are downloaded from http://edgar.jrc.ec.europa.eu/htap−v2/index.php?SECURE=123.

Figure options

Tab. 1 Summary of BC emissions from different emission sectors in China(units: Gg yr⁻¹)

Table options

Due to the different instruments used to measureconcentrations, BC was also reported as elementarycarbon or soot in previous studies. From a measurementst and point, BC mostly refers to aerosol measuredwith photo-absorption techniques, whereas elementarycarbon mostly refers to aerosol measured withthermal/optical techniques. The mass concentrationsdetermined with these two techniques can sometimesbe significantly different. For the purpose of this review, we consider BC and elementary carbon to beequivalent in our review and refer the readers to Table 2 for the BC measurement methods used in the studiesreviewed below.

Tab. 2 Summary of BC measurement methods used in the referred studies

Table options

Since 1999, carbonaceous aerosols have been measuredat numerous locations in China at remote sites(Qu et al., 2006), regionally representative rural sites(Zhang et al., 2005; Gao et al., 2008), and urban sites in large and mega cities(Xu et al., 2002; Cao et al., 2007; Zhang X. et al., 2008; Cao et al., 2009). Figure 3 presents the measured BC concentrations in 2006 at18 stations in China(Zhang X. et al., 2008). The averageannual mean concentration of BC was 0.35±0.01μg m⁻³ at the remote background sites of Shangri-la, Zhuzhang, and Akdala,3.6±0.93 μg m⁻³ at the regionalsites of Taiyangshan, Longfengshan, Dunhuang, Lin’an, Jinsha, Lhasa, Nanning, Dalian, Gaolanshan, Shangdianzi, and Zhenbeitai, and 11.2±2.0 μg m⁻³at the urban sites of Panyu, Zhengzhou, Chengdu, Gucheng, and Xi’an. The highest annual mean BCconcentration of 14.2 μg m⁻³ was found at Xi’an in2006(Zhang X. et al., 2008). Similar magnitudes ofmeasured BC concentrations were also reported in thestudy of Cao et al.(2007).

Fig. 3 Locations and annual averaged concentrations ofobserved BC in 2006. The concentrations at Shangdianzi, Zhenbeitai, Akdala, Shangri-la, and Zhuzhang were from2004 to 2005.(Source: Fig. 1 of Zhang X. et al.(2008))

Figure options

Measured concentrations of BC in China showedstrong seasonal variations, with maximum concentrationsin winter and minimum values in summer(Cao et al., 2007,Cao 2009; Zhang X. et al., 2008). Cao et al.(2007)showed that the observed BC concentrationsaveraged over the 14 cities in China were 9.9 μg m⁻³in winter(6–20 January) and 3.6 μg m⁻³ in summer(3 June–30 July)of 2003. Such seasonal variationscan be explained by the largest coal combustion inwinter and the largest precipitation associated withthe East Asian summer monsoon that washes out BCin summer.

Historical changes in BC are important for under-standing the historical changes in climate. Changesin deposition of BC in China were reconstructed byusing ice core records(Xu et al., 2009; Cong et al., 2013)or lake sediments(Han et al., 2011). extracted ice cores from five sites in theTibetan Plateau and found that the average BC concentrationsin ice cores increased from 4.57 ng g⁻¹ in1956 to 12.5 ng g⁻¹ in 2006. Han et al.(2011)used a150-yr(1850–2000)sediment record of Lake Chaohu inAnhui Province and showed that BC concentrations inthe sediment exhibited stable low values(below 150000ng g⁻¹)prior to the late 1970s, and a sharp increase to450000 ng g⁻¹ in the last three decades, correspondingwell with the rapid industrialization of China. Conget al.(2013)reported the variation of BC in sedimentof Nam Co Lake in the Tibetan Plateau. From the1850s to the early 1900s, deposition fluxes of BC toNam Co Lake were generally constant, which can beconsidered as background level without significant disturbancefrom human activities. After the 1900s, BCfluxes showed a gradual and continuous increase, indicatingthat the influence from anthropogenic sourcesbegan in the interior of the Tibetan Plateau. Fromthe 1960s to the early 2000s, the increasing trend ofBC deposition flux accelerated significantly. Note thatBC in ice core or lake sediment is dependent on BCconcentrations in the atmosphere as well as wet and dry deposition of atmospheric BC.

As reviewed above, previous measurements of BCwere quite limited to BC at the surface. The observationsof vertical distribution of BC are needed forthe studies of climatic effect of BC, considering thata large fraction of BC particles from biomass burningin South Asia can be transported to the middle troposphereover China in spring of every year(Zhang et al., 2010b). The measurements of size distribution arealso essential, because recent observational study indicatedthat quite a large fraction of BC in China canstay in coarse mode(Wang et al., 2014). Note that thestudies of climatic effect of aerosols require long-termmeasurements, since climate represents multi-year averagesof meteorological parameters.

Aerosol optical depth(AOD)represents light at tenuation by aerosols, which is an important parameterthat determines the climatic effect of aerosols. Figure 4 shows the annual mean AOD retrieved by ModerateResolution Imaging Spectrometer(MODIS)thatis averaged over 2001–2010. The high AOD values of0.5–0.9 occurred over a large fraction of eastern China.The AOD values exceeded 0.5 over the heavily pollutednorthern China, the Sichuan basin, the Yangtze RiverDelta, and the Pearl River Delta. Such features ofAOD were captured by many modeling studies(e.g.,Ma et al., 2007; Cui et al., 2009; Zhang et al., 2010a;Lou et al., 2014).

For BC, its unique strong absorption of solarradiation is represented by aerosol absorption opticaldepth(AAOD), or the non-scattering fraction ofAOD. Figure 5 shows the AErosol RObotic NETwork(AERONET)sunphotometer and Ozone MonitoringInstrument(OMI)satellite retrievals of clearskyAAOD. Measurements from AERONET showedthat the AAOD values were in the range of 0.015–0.07, with high values over northern China where concentrationsof both BC and mineral dust aerosols werehigh. On an annual mean basis, AERONET AAODover Asia(30°–70°N,100°–160°E)was 0.036, whichwas much higher than the AAOD of 0.007 over NorthAmerica(20°–55°N,130°–70°W) and of 0.015 overEurope(30°–70°N,15°W–45°E)(Koch et al., 2009).The AAOD values from OMI exhibited high values ofexceeding 0.05 over northern China, the Sichuan basin, and the south border of China. Note that both BC and mineral dust aerosols absorb radiation, so AAOD is a useful measure of BC in regions where mineral dust isnot a dominant species.

Fig. 4 Annual mean AOD retrieved by Moderate ResolutionImaging Spectrometer(MODIS)that is averaged over2001–2010. MODIS datasets are level 3 products downloadedfrom http://ladsweb.nascom.nasa.gov.

Figure options

Fig. 5 Aerosol absorption optical depth(AAOD,×100)from AERONET(at 676 nm, color circles) and OMI(at 500 nm, color shades). AERONET measurementswere carried out in different years; the AAODfor a specific site is the average over years with measurementsavailable. The OMI datasets downloadedfrom http://disc.sci.gsfc.nasa.gov/giovanni are averagedover 2005–2007. The AERONET datasets are downloadedfrom http://aeronet.gsfc.nasa.gov/cgi-bin/climo−menu−v2−new.

Figure options

The single scattering albedo(SSA)represents theratio of aerosol scattering coefficient to extinction coefficient.It is an important optical parameter that determineswhether aerosols have a cooling or a warmingeffect. Over a specific location, SSA is dependent onthe mixing of scattering and absorbing aerosol species.Ramanathan et al.(2001)showed that SSA exceeding0.95 led to a negative aerosol forcing at the topof atmosphere(TOA) and SSA less than 0.85 led toa positive forcing. Bergin et al.(2001)reported thatthe values of SSA were about 0.81 in polluted northernChina. Lee et al.(2007)found that the averageof SSA values over China was 0.89±0.04 at 500 nmfor 2005. Table 3 summarizes ground-based measurementsof SSA in China from the literature. The largedifferences in SSA can be explained by the differencesin shape, size, chemical composition, and hydroscopicgrowth of aerosol particles.

Tab. 3 Summary of ground-based measurements of aerosol single scattering albedo in China. The measurementsare taken from Liao and Chang(2014)

Table options

Qiu and Yang(2008)showed that measured SSAvalues were the smallest in winter among all seasons.The increase in BC emission might have led to the decreasein SSA in Beijing during 1993–2001(Qiu et al., 2004). Lyapustin et al.(2011)examined AOD valuesin Beijing from both the AERONET measurements and the MODIS retrievals, and reported an increasingtrend in SSA in Beijing during 2007–2010 relativeto the previous 5 years. Particularly, as dust particleswere mixed with anthropogenic aerosols duringthe transport, SSA values generally showed increases, because the retrieved SSA of dust ranged from 0.92(at 0.44 μm)to 0.97(at 1.02 μm) and the SSA of anthropogenicaerosols ranged from 0.89±0.04(at 0.44μm)to 0.83±0.05(at 1.02 μm)(Xia et al., 2005). Caoet al.(2014)also reported that SSA increased duringdust events in Beijing in 2005.

Table 4 summarizes the estimated radiative forcing(RF)of BC over China, including direct radiativeforcing(DRF), first indirect radiative forcing(FIRF), and semi-direct radiative forcing. The DRF reviewedin this work refers to an instantaneous change in net(downward minus upward)radiative flux(shortwaveplus longwave, in W m⁻²)due to an imposed changein concentration of a chemical species(i.e., BC in thissection and O₃ in Section 3.3). The semi-direct effectof BC is the difference in cloud forcing with and withoutthe impact of BC on SSA of cloud droplet(Zhuang et al., 2010a), and the FIRF of BC is the change incloud forcing with and without the impacts of BC oneffective radius of cloud droplets(Zhuang et al., 2013).The RF of BC is usually defined in terms of change innet radiative flux at the TOA or at the surface.

The DRF of BC is always positive at the TOA and negative at the surface, with the spatial distributionof DRF at the TOA similar to that of DRF at thesurface. Chung and Seinfeld(2005), by using a globalclimate model with online simulation of BC, reportedthat the present-day maximum TOA all-sky BC DRFover eastern China was about 5–6 W m⁻² on an annualmean basis. Zhang H. et al.(2008), by using theglobal aerosol dataset in a radiative transfer model, showed that the TOA clear-sky BC DRF over easternChina reached 3.2 and 4.0 W m⁻² in winter and summer, respectively. Wu et al.(2008)simulated by usingthe Regional Climate Chemistry Modeling System(RegCCMs)found that BC DRF values were strongerin southern China than in northern China during thisperiod; the all-sky BC DRF was 0.64 W m⁻² at theTOA and –1.69 W m⁻² at the surface over northernChina(32°–50°N,105°–120°E), while it was 1.55 Wm⁻² at the TOA and –3.10 Wm⁻² at the surface oversouthern China(22°–30°N,100°–120°E). Zhang H. etal.(2009) and Wang et al.(2009)estimated the DRFof BC by using the global Community AtmosphereModel Version 3(CAM3). Under all-sky conditions, the present-day annual mean BC DRF over easternChina was simulated to be 2.5Wm⁻² at the TOA and –3.2W m⁻² at the surface. Zhuang et al.(2010b)estimatedthat the annual mean BC DRF values at TOAwere 1.02 W m⁻² for clear skies and 0.75 W m⁻² forall skies over China(25°–45°N,100°–130°E)in 2006.Zhuang et al.(2013)showed that the annual mean BCDRF over China(20°–50°N,100°–130°E)was 0.81 Wm−2 at the TOA in 2006, and the strongest TOA BCDRF was 6.0 W m⁻² over the Sichuan basin. Amongthe BC DRF estimates for China, the values obtainedby Zhang et al.(2012)were relatively small; the simulatedannual mean TOA BC DRF exhibited maximumvalue of 1.0 W m⁻² under all-sky and of 0.8 W m⁻²under clear-sky conditions. The low bias in simulatedBC column burden might have led to the small BCforcing in Zhang et al.(2012).

BC can act as cloud condensation nuclei(CCN) and influence cloud albedo, exerting a negative FIRFat the TOA. Zhuang et al.(2009)reported that theregional average(20°–50°N,90°–120°E)of TOA BCFIRF was –0.39 W m⁻² in January and –1.18 W m⁻² in July 2003. Zhuang et al.(2013)estimated that theTOA FIRF of BC was –0.95 W m⁻² over China(20°–50°N,100°–130°E), which was larger than its DRF, leading to a net RF of BC of –0.15 W m⁻² at theTOA for 2006.

BC can burn clouds through heating the ambientair around and clouds(semi-direct effect of BC), which reduces cloud cover and allows more shortwaveradiative fluxes to reach the surface. Zhuang et al.(2010a)showed that the average semi-direct RF valuesover eastern China(20°–50°N,100°–130°E)were0.04,0.10,0.09, and 0.06 W m⁻² at the TOA in January, April, July, and October 2003, respectively.

As shown in Table 4, the simulated regional meanDRF values are in the range of 0.75–2.5 Wm⁻², whichare significant as compared to the global mean radia tive forcing values of BC, tropospheric O₃, and CO₂.IPCC(2013)reported that between 1750 and 2011, the global and annual mean RF values of BC, troposphericO₃, and CO₂ were 0.40(0.05–0.80)W m⁻²,0.40(0.20–0.60)W m⁻², and 1.82(1.63–2.01)W m⁻², respectively. Recently,Bond et al.(2013)reported theannual mean BC DRF exceeded 5 W m⁻² over a largefraction of eastern China, which further underscoresthe importance of BC in regional warming of climate.However, few studies have examined semi-direct and indirect effect RF of BC; the RF values of semi-direct and indirect effect of BC have large uncertainties and are subject to further studies.

Tab. 4 Summary of estimates of radiative forcing of BC in China

Table options

Increases in BC concentrations in the atmospherewere simulated to lead to a warming over China.Chang et al.(2009)found by using a coupled globalaerosol-climate model that the direct effect of BCincreased the surface air temperature averaged overeastern China(20°–50°N,100°–130°E)by 0.62℃ over1950–2000. Jiang Y. et al.(2013)used the NCARCommunity Atmospheric Model version5(CAM5) and found that during 1850–2000, the direct and indirecteffects of BC led to changes in surface air temperatureby –0.6 to 0.3 K over East China(20°–45°N,105°–122.5°E). Guo et al.(2013)reported by usingthe United Kingdom High-Resolution Global EnvironmentModel(HiGAm)that BC direct effect changedthe monthly and regional mean surface air temperatureover East Asia(20°–45°N,100°–122°E)by–0.4 to 0.1 K during April–September. The simulatednegative changes in temperature by BC indicated thecomplex BC-cloud feedbacks.

BC affects the large-scale circulation and hydrologicalcycle by heating the air and hence altering theregional atmospheric stability. Menon et al.(2002)considered the direct effect of BC in the Goddard Institutefor Space Studies(GISS)global climate model and reported that the radiative effect of BC in China and India led to the observed “northern droughts and southern floods” in China over the past severaldecades. Chang et al.(2009)used a coupled globalaerosol-climate model and found that the direct effectof BC increased precipitation over 1950–2000 by0.07 mm day⁻¹ as precipitation was averaged overeastern China(20°–50°N,100°–130°E). Zhuang et al.(2013)showed by using the RegCCMs that the combinedeffect of BC(semi-direct plus indirect effect)led to regional mean change in precipitation by –0.09mm day⁻¹(or –7.4%)over eastern China(20°–50°N,100°–130°E)for 2006. Jiang Y. et al.(2013)consideredboth direct and indirect effects of BC in theNCAR CAM 5 and reported that BC led to summertimechange in precipitation by –0.08 mm day⁻¹ from1850 to 2000 as precipitation was averaged over EastChina(20°–45°N,105°–122.5°E).

BC also contributes to the retreat of the glaciersover the Himalayas(Ming et al., 2008; Xu et al., 2009;Menon et al., 2010; Kopacz et al., 2011; Wang Z. et al., 2011; Wang X. et al., 2014). As BC is depositedover snow and sea ice, it significantly enhances solarabsorption by snow and ice. Ming et al.(2008)estimatedradiative forcing by using BC retrieved from a40-m shallow ice core from the East Rongbuk Glacierin the high Himalayas. They reported a local radiativeforcing of as large as 5.0 W m⁻² by BC deposited inthe glacier, suggesting that BC in the atmosphere overthe Himalayas and consequently in the glaciers cannotbe neglected when assessing the dual warming effectson glacier melting. Menon et al.(2010)simulated byusing the NASA GISS climate model(ModelE)withon-line aerosol chemistry that about 0.9% of snow/icecover decreases over the Himalayas during 1990–2000was caused by direct effect, indirect effect, and depositionof BC aerosol. Wang Y. et al.(2011)simulated byusing the BCC−AGCM that the regional mean radiativeforcing due to BC deposited on snow/ice reached2.8 W m⁻² over the Tibetan Plateau and led to increasesin annual mean temperature by 1.6 K in thatregion.

Modeling studies reviewed above show large uncertainties, either in simulated radiative forcing or insimulated climate responses, although the regional and global climate models start to have the capabilitiesto simulate BC-radiation and BC-cloud interactions.The uncertainties arise in part from emissions inventories, representation of concentrations, vertical profiles, mixing states, and optical properties of BC. The representationof the aging of BC and the role of BC ascloud condensation nuclei remains to be the most difficultchallenges in simulation of climatic effect of BC(IPCC,2013). Nationwide long-term measurements ofaerosol concentrations, aerosol optical properties, and cloud properties in China are called for to constrainmodel simulations.

The most important O₃ precursors in the atmosphereinclude NO_x, CO, and volatile organic compounds(VOCs). Motor vehicle exhaust, industrialemissions, and chemical solvents are the major anthropogenicsources of these chemicals. As an example,Table 5 summarizes the annual emissions of NO_x, CO, and NMVOCs in eastern China(20°–50°N,110°–126°E), which are based on David Streets’ 2006 emissioninventory(http://mic.greenresource.cn/intex-b2006)(Zhang Q. et al., 2009). Note that troposphericO₃ has a lifetime of about 3 weeks(Liao and Seinfeld, 2005; Stevenson et al., 2006) and therefore a large fractionof O₃ in China can be attributed to backgroundO₃ and anthropogenic emissions from foreign countries(Wang Y. et al.(2011)).

Tab. 5 Annual emissions of ozone precursors ineastern China(20°–50°N,110°–126°E)

Table options

Figure 6 shows the distributions of seasonal meansurface-layer concentrations of O₃ simulated for 2006in the study of Lou et al.(2014)by using a globalchemical transport model. Simulated concentrationsof O₃ in eastern China are the lowest in DJF because of the weak photochemistry as a result of weak solarradiation in this season. Concentrations of O₃ in easternChina in MAM and SON are in the ranges of 40–55 and 35–50 ppbv, respectively. The maximum O₃concentrations over eastern China are simulated to be60–75 ppbv in JJA. Note that the high O₃ concentrationsof exceeding 70 ppbv over the Tibetan Plateau inMAM result from the transport of O₃ from the stratosphereto troposphere(Wild and Akimoto, 2001).

Fig. 6 Simulated seasonal mean surface-layer concentrations(ppbv)of O3 for 2006.(a)DJF(December–January–February),(b)MAM(March–April–May),(c)JJA(June–July–August), and (d)SON(September–October–November), respectively.(Remade from Fig. 2 of Lou et al.(2014))

Figure options

Table 6 compiles the ground-based measurementsof O₃ in China from the literature. DuringJuly–September of 2001–2006, the average measuredO₃ concentrations at 6 urban sites in Beijingwas 26.6±2.8 ppbv(Tang et al., 2009). At remotesites(Fenghuanshan(40.5◦N,124◦E), Waliguanin the Qinghai-Tibetan Plateau(36.3◦N,100.9◦E), Lin’an(31.3◦N,120.4◦E), and Longfengshan Mountain(44.7◦N,127.6◦E)), O₃ concentrations were generallyin the range of 20–60 ppbv(Yan et al., 1997,2003; Wang et al., 2004; Takami et al., 2006; Wang et al., 2006). The background O₃ concentrations arerelatively higher in western China as a result of thetransport of O₃ from the stratosphere. For example, concentrations at Waliguan(36.3◦N,100.9◦E)were inthe range of 40–60 ppbv, which were higher than themeasured values at other background stations. Withrespect to the long-term trend of O₃, surface concentrationsmeasured in Lin’an, a background station ineastern China, showed that the monthly highest 5%O₃ concentrations increased over 1991–2006(Xu et al., 2008).

Tab. 6 Summary of ground measurements of ozone in China

Table options

Estimating of the climatic effect of troposphericO₃ requires knowledge of not only surface concentrationsbut also vertical distributions and column burdens.Ozonesonde datasets were available at only anumber of sites in China. Based on 810 vertical profilesof O₃ measured by aircraft in different seasons of1995–2005,Ding et al.(2008)showed that the averagemixing ratio of O₃ in Beijing increased from about40 ppbv at the ground to about 50 ppbv at 2-km altitude.Satellite measurements are useful for analysesof the distributions and seasonal variations of troposphericcolumn O₃ concentration over China becauseof the excellent spatial and temporal coverage. Liu etal.(2006)presented the first directly retrieved globaldistribution of tropospheric column O₃ from GlobalOzone Monitoring Experiment(GOME)ultravioletmeasurements from December 1996 to November 1997.The retrieved columns clearly showed changes owingto convection, biomass burning, stratospheric influence, pollution, and transport. By using measurementsfrom the Infrared Atmospheric Sounding Interferometer(IASI)instrument aboard the EuropeanMetop-A satellite(launched in October 2006), Dufouret al.(2010)showed that the maximum O₃ occursin late spring and early summer(May–June)in Beijing.Wang Y. et al.(2011)examined the month tomonth variation of mean tropospheric O₃ column fromTropospheric Emission Spectrometer(TES)for eastern(east of 110◦E) and western(west of 110◦E)China.TES retrievals suggest that column burden of troposphericozone over both eastern and western Chinahas a maximum in late spring/early summer and minimumin winter. Although biogenic emissions, temperature, and radiation are the highest in southeastern and southwestern China in July, O₃ concentrations inthose regions are generally low in summer because ofthe summer monsoon circulation that brings clean airfrom the oceans. Zhang et al.(2014)reported thatthe retrieved tropospheric O₃ column concentrationfrom OMI averaged over China and from 2010–2013was 34.0 DU.

Tropospheric O₃ exerts radiative forcing at bothlongwave and shortwave spectral b and s. Few studieshave examined the RF by tropospheric O₃ over China.By using a coupled regional chemistry-climate model(RegCM₂),Wang et al.(2005)showed that troposphericO₃ had a shortwave forcing of 0.19 W m⁻² and a longwave forcing of 0.46 W m⁻² at the tropopause.They reported that the normalized net radiative forcingover China was 0.02 W m⁻² DU−1, lower thanthe global mean values of 0.03–0.05 W m⁻² DU−1reported in the literature. By using the IPCC AR5emissions inventories,Chang et al.(2009)reportedthat the anthropogenic radiative forcing by troposphericO₃ averaged over eastern China(18◦–45◦N,95◦–125◦E)was 0.53 W m⁻² at the tropopause.

Several studies examined climate responses to RFof tropospheric O₃ in China. Wang et al.(2004)useda coupled regional chemistry-climate model based onRegCM₂ to simulate the concentrations and climaticeffect of tropospheric O₃ in China. They reported thatthe monthly mean column burden of O₃ was about30 DU and led to changes in surface air temperatureby –0.8 to 0.8 K over eastern China. The negativechanges in temperature were mainly associated withthe feedback of clouds. Based on the transient climatesimulations of the GISS ModelE,Hansen et al.(2007)predicted that the increases in surface air temperatureby tropospheric O₃ were up to 0.5 K overeastern China from 1900 to 2003. On the basis of aglobal coupled chemistry-climate simulation, Chang etal.(2009)reported that the warming by troposphericO₃ was 0.43 K in eastern China over 1950–2000.

The large changes in simulated surface air temperaturereviewed above underscore the importance ofconsidering O₃ in policies for mitigating global warming.It should be noted that many previous studies onregional climate changes in China did not account forthe role of tropospheric O₃, which would lead to lowbiases in simulated warming in China.

Due to the warming effect of BC and troposphericO₃ as reviewed above, reductions in short-lived BC and tropospheric O₃ have been proposed as a complementarystrategy to reductions in greenhouse gases.Recent studies started to identify approaches to mitigateboth air pollution and global warming by reducingconcentrations of short-lived species such as troposphericO₃ and BC(UNEP,2011; Shindell et al., 2012;Bond et al., 2013; Liao and Chang, 2014; Zhang et al., 2014). BC and OC are always co-emitted, but in differentproportions from different emission sources. BCis also co-emitted with other scattering aerosols suchas sulfate from fossil fuel burning. As a result, theeffect of BC mitigation depends on how BC and coemittedaerosol species affect cloud properties(cloudalbedo, cloud amount, and cloud lifetime) and hencecloud radiative forcing. If such a perturbation wereto result in a reduction in TOA cloud cooling, theamount of BC reduction would oppose the amount bywhich the TOA direct BC heating is also reduced. Forexample,Chen et al.(2010)considered two presentdaymitigation scenarios: 50% reduction of primaryBC/OC mass and number emissions from fossil fuelcombustion(referred to as HF), and 50% reductionof primary BC/OC mass and number emissions fromall primary carbonaceous sources(fossil fuel, domesticbiofuel, and biomass burning)(referred to as HC). Theglobal mean TOA changes in radiative forcing for thetwo scenarios, relative to present day, were calculatedto be 0.13±0.33 W m⁻²(HF) and 0.31±0.33 W m⁻²(HC), indicating the large uncertainty in the net effectof some BC control measures on global warming.

Few studies have examined the effect of reductionsin O₃ on global warming. As a short-lived specieswith lifetime of about 3 weeks(Liao and Seinfeld, 2005; Stevenson et al., 2006), a large fraction of O₃in China can be attributed to background O₃ and anthropogenicemissions from foreign countries. Wang Y. et al.(2011)used the global chemical transportmodel GEOS-Chem to identify contributions of emissionsfrom various source types(natural and anthropogenic) and regions(domestic and foreign)to the spatialdistribution and seasonality of tropospheric O₃ inChina. Assuming that total O₃ is the sum of totalbackground O₃(TBO; simulated with global naturalemissions as well as anthropogenic emissions outsideChina) and China pollution O₃(CPO; O₃ formationfrom anthropogenic emissions in China), the annualmean TBO over China was calculated to be 44.1 ppbv, with maximum value of 50.7 ppbv in MAM and minimumvalue of 40.9 ppbv in JJA, accounting for 93% and 81% of total surface O₃ in these seasons, respectively.Annual mean CPO was calculated to be 5.4ppbv, ranging from 1.4 ppbv in DJF to 9.9 ppbv inJJA. Average over China, CPO contributed about 20%of total O₃ in JJA. These model results indicate thatdomestic reductions in O₃ through controlling O₃ preNO.4 LIAO Hong and SHANG Jingjing 539cursors(such as NO_x, CO, and NMVOCs)may not behelpful for regional climate; it is important to establishemission control collaboration to achieve mutualbenefits among different countries/regions.

Noted that while BC and O₃ affect the regionalclimate, climate change can affect their distributions and concentrations by altering natural emissions, chemical reactions, transport, and deposition(Liao et al., 2006; Jiang H. et al., 2013; Qu et al., 2013; Wang et al., 2013). Such chemistry-aerosol-climate feedbacksneed to be accounted for in policies for future emissionreductions.

As presented in sections above, important advanceshave been made during the past decade inunderst and ing concentrations and distributions of BC and tropospheric O₃ and their roles in climate changein China. Emissions inventories of tropospheric O₃ and aerosol precursors as well as aerosols have becomeavailable for China domain, which allow one tosimulate concentrations of BC and tropospheric O₃ and estimate their climatic effects by using numericalmodels. Increasing availability of ground-based measurements and satellite measurements of short-livedspecies and their optical properties as well as cloudproperties has provided datasets to constrain the simulatedclimatic effects of aerosols and tropospheric O₃.However, estimates of the net effect of BC and O₃control measures on global warming are still subjectto large uncertainties. The fundamental science needspertaining to reductions in short-lived species to mitigateglobal warming are summarized in Fig. 7 and described below.

Fig. 7 Fundamental science needs pertaining to reductions in short-lived species to mitigate global warming.

Figure options

(1)Quantification of emissions of BC and O₃precursors from different sectors in China, which arerequired for making plans of control measures. It isalso important to quantify all the chemical species thatare co-emitted with BC or O₃ precursors can influenceconcentrations of both long-lived greenhouse gases and short-lived species. Such information will be helpfulfor estimating the net radiative forcing of a specificBC or O₃ control measure.

(2)Nationwide long-term measurements of size-resolved mass concentrations of BC and other aerosolspecies as well as number concentrations of aerosols.Since previous measurements(measurements at urbansites for short time periods)were mostly designedfor air quality studies, nationwide long-term measurementsare especially important for climate studies and need coordinated funding support. With fairly goodfunding support, satellite measurements have excellentspatial and temporal coverage, which are also usefulfor analyses of the physical/chemical/optical characteristicsof O₃ and aerosols. These measurements arenecessary for evaluation of emission inventories, developmentof chemistry-aerosol-climate models, and theassessment of the effect of emission reduction measures(Wang et al., 2012; Xu et al., 2013). Better underst and ingof BC and tropospheric O₃ also requiressatellite measurements of fires. One of the largest uncertaintieswith the fire observations is in estimatingthe actual amount of BC and O₃ precursors such asNO_x that are emitted.

(3)Improving the underst and ing of aerosol-cloudinteractions. How chemical species co-emitted withBC influence clouds is one of the most difficult challenges, because the microphysical processes involvedare very complex. Although the direct radiative effectof BC has been shown to be a strong warming, thesemi-direct and indirect effects of BC have large uncertaintieseither with sign and magnitude. Consideringthat aerosol-cloud interactions have the largest uncertaintiesamong the drivers of climate change(IPCC,2013), quantification of aerosol-cloud interactions iscritical for assessing the benefits of emission controlmeasures.

(4)Continuous development of fully coupledchemistry-aerosol-climate models. As reported byIPCC(2013), a large fraction of climate models cansimulate chemistry and transport of tropospheric O₃ and aerosols, but few of them have fully coupledmeteorology-aerosol-cloud-radiative forcing feedbacks, especially with the consideration of all major anthropogenic and natural aerosol species and the radiativeeffects of aerosols on distributions and concentrationsof chemical species.

(5)Through atmospheric transport, pollutantscan be carried from one location to another. Therefore, how different countries/regions are accountablefor the emissions and how they can establish emissioncontrol collaboration to achieve mutual benefits arevery important questions. These issues are relativelyless studied in current literature and remain an importantbottleneck affecting the effectiveness of emissioncontrol.