Study On Black Carbon Aerosol And Its Climatic Effect Over China

Black carbon aerosol (BC) is one of the most important aerosol components in atmosphere. It is emitted directly at the source from imcomplete combustion processes such as fossil fuel and biomass burning. It could affect climate through different ways, resulting in global and regional climate changing. It absorbs solar radiation and alters cloud albedo, cloud precipitation, cloud life time and single-scattering albedo (SSA) by acting as cloud condensation nuclei (CCN), hence affecting atmospheric circulation and hydrologic cycle. Additionally, BC in snow through deposition processes may reduce snow albedo hence leading to surface warming and changing local or regional circulation. It is meaningful and necessary to investigate the climate effects of BC aerosol over China for China emitting high levels of BC and locating at typical Asian monsoon region. Here we first present numerical simulations on accessing the temporal and spatial variations of BC surface concentration, column burden, and optical depth (AOD); then investigated BC induced radiative forcing (including direct, first indirect and BC-cloud SSA induced radiative forcing) and its climate responses using a regional climate model coupled with a troposphere atmospheric chemistry model (RegCCMS).Regional climate and chemistry modeling system was further developed in this study. Several schemes (including aerosols affecting cloud number concentration, effective radius, auto conversion rate, BC affecting cloud droplet SSA and snow albedo and BC mixing with scattering aerosols) were added to the system to investigate BC induced climate effects. Simulated results show that high levels of BC loading concentrate in Sichuan, Chongqing, Guizhou, north China, center China and Yangtze River Basin with significant seasonal variation. Local annual mean BC surface concentration, column burden and AOD reached14.0μg/m3,8.0mg/m2and 0.11, respectively. Linear correlations between simulations and observations (from ground-base and satellite products) in this study indicate that the coefficients are0.81for BC surface concentration,0.52for BC column burden and0.71for BC optical depth.First, BC absorption exerts a positive/negative radiative forcing at the top of the atmosphere (TOA)/surface(SRF). Similar to its loading, strong BC direct radiative forcing appears in southwest, center and east of China (Max:>7.0in clear sky and>6.0W/m2in cloudy sky), yielding regional TOA BC direct radiative forcing of1.16W/m2and0.81W/m2for clear and cloudy skies, respectively. Regional mean SRF radiative forcing in clear-sky or all-sky is-3.74W/m2or-2.42W/m2. Due to BC absorption, cloud amount and cloud liquid water path decrease significantly in east China while the total absorbed solar radiation increase over the most parts of China, hence surface getting warmer and dryer. Regional changes of annual mean cloud amount, cloud water, absorbed solar radiative forcing, surface air temperature and precipitation are-0.017%,-0.104g/m2,+1.22W/m2,0.042K and-0.003mm/d. Additionally, summer rainfall over China increase in the south and drought in the north and temperature gradient between land and ocean would get smaller in cold season and larger in summer due to BC absorbing solar radiation over China mainland.Second, sensitive studies to direct effects with different mixing states (external, internal and partially internal mixture) of BC with other scattering aerosols (including sulfate, nitrate and organic carbon aerosols) were performed in this study. BC was assumed to be the core of the mixed aerosol in internal mixing scheme. Results indicated that the smallest SSAs of aerosols over center to east China are0.94,0.63and0.92for external, internal and partially internal mixture, respectively. The partially internal mixture SSA is more consistent with that of satellite observation. Annual mean cloudy sky TOA direct radiative forcing over the region are-1.1,+0.5and-0.6W/m2for external, internal and partially internal mixing schemes. Cloud amount, liquid water path, surface air temperature and precipitation decreased while total absorbed solar radiation increased in all these three schemes. Aerosols had the most serious absorption (+1.75W/m2) in internal mixing scheme, leading to the surface being less cool (-0.03K) than other two schemes.Third, BC indirect effect was studied. CCN-BC results in higher cloud droplet number concentration (+70.9/cm3), smaller cloud droplet size (-0.15μm) under the condition with fixed cloud water and low autoconversion rate (-7.51×10-6g/kg/s) from cloud water to rain water. Simulated BC first indirect radiative forcing at TOA is-0.95W/m2, stronger than its direct radiative forcing. Due to BC total indirect effects, annual cloud mount and water increase while absorbed solar radiation, surface air temperature and precipitation decrease, yielding regional mean of+0.114%,+1.42g/m2,-2.02W/m2,-0.135K and-0.06mm/d. Surface cooling and drying leads to less water vapor exchange between land and atmosphere. Additionally, indirect effects lead to summer rainfall drought, downdraft enhanced and atmosphere cooling over China mainland. BC second indirect effect is more significant than its first indirect effect. Combined BC direct effect with its indirect effect (short for combined effect), the results show that cloud amount and total absorbed solar radiation decrease (by-0.025%and-0.733W/m2) while cloud liquid water path increase (+1.28g/m2). Thus, surface getting cooler and dryer. In this experiment, BC direct effect had much more contribution to cloud amount change and BC indirect effects had more significant influence on the rest meteorological factors changes.Fourth, study on BC affecting cloud SSA hence changing regional climate was performed. BC within cloud droplet reduce the SSA of mixture drop, leading to a positive/negative radiative forcing at TOA/SRF. National averaged TOA radiative forcing ranged from0.04W/m2in January to0.096W/m2in April, with significant seasonal variation. SRF forcing is about1.25to1.46times to that at TOA, which is one order magnitude smaller to BC direct and indirect radiative forcing. Cloud amount decreased and air temperature increased owing to this kind of radiative forcing. However, the climate responses are much weaker than BC direct and indirect effects.Finally, study on BC affecting snow albedo hence leading to regional climate change was performed. The dry/wet depositions of BC on snow can reduce snow albedo and affect snowmelt. Simulated results show that there are high BC concentrations in snow over center, east, northeast and southwest China in winter, which exceed200.0ng/g. Surface albedo reduced significantly in these region (Max:>5%; Regional mean:0.25%). Owing to the smaller snow surface albedo, surface air temperature (>0.2K) and total absorbed solar radiation increase (>5.0W/m2) in most regions, leading to snowmelt (<-5cm) and local atmospheric circulation change. In the condition with snowfall, BC concentration would get lower while the albedo would get higher.