Change in global aerosol composition since preindustrial times

[摘要] To elucidate human induced changes of aerosol load and composition in theatmosphere, a coupled aerosol and gas-phase chemistry transport model of thetroposphere and lower stratosphere has been used. The present 3-D modelingstudy focuses on aerosol chemical composition change since preindustrialtimes considering the secondary organic aerosol formation together with allother main aerosol components including nitrate. In particular, we evaluatenon-sea-salt sulfate (nss-SO₄⁼), ammonium (NH₄⁺),nitrate (NO₃⁻), black carbon (BC), sea-salt, dust, primary andsecondary organics (POA and SOA) with a focus on the importance of secondaryorganic aerosols. Our calculations show that the aerosol optical depth (AOD)has increased by about 21% since preindustrial times. This enhancement ofAOD is attributed to a rise in the atmospheric load of BC,nss-SO₄⁼, NO₃, POA and SOA by factors of 3.3, 2.6, 2.7,2.3 and 1.2, respectively, whereas we assumed that the natural dust andsea-salt sources remained constant. The nowadays increase in carbonaceousaerosol loading is dampened by a 34–42% faster conversion of hydrophobicto hydrophilic carbonaceous aerosol leading to higher removal rates. Thesechanges between the various aerosol components resulted in significantmodifications of the aerosol chemical composition. The relative importanceof the various aerosol components is critical for the aerosol climaticeffect, since atmospheric aerosols behave differently when their chemicalcomposition changes. According to this study, the aerosol compositionchanged significantly over the different continents and with height sincepreindustrial times. The presence of anthropogenically emitted primaryparticles in the atmosphere facilitates the condensation of thesemi-volatile species that form SOA onto the aerosol phase, particularly inthe boundary layer. The SOA burden that is dominated by the naturalcomponent has increased by 24% while its contribution to the AOD hasincreased by 11%. The increase in oxidant levels and preexisting aerosolmass since preindustrial times is the reason of the burden change, sinceemissions have not changed significantly. The computed aerosol compositionchanges translate into about 2.5 times more water associated with nonsea-salt aerosol. Additionally, aerosols contain 2.7 times more inorganiccomponents nowadays than during the preindustrial times. We find that theincrease in emissions of inorganic aerosol precursors is much larger thanthe corresponding aerosol increase, reflecting a non-linear atmosphericresponse.

[学科分类] 大气科学