Abstract
Organic aerosol (OA) constitutes a substantial fraction of fine particles and affects both human health and climate. It is becoming clear that OA absorbs light substantially (hence termed Brown Carbon, BrC), adding uncertainties to global aerosol radiative forcing estimations. The few current radiative-transfer and chemical-transport models that include BrC primarily consider sources from biogenic and biomass combustion. However, radiocarbon fingerprinting here clearly indicates that light-absorbing organic carbon in winter Beijing, the capital of China, is mainly due to fossil sources, which contribute the largest part to organic carbon (OC, 67 ± 3%) and its sub-constituents (water-soluble OC, WSOC: 54 ± 4%, and water-insoluble OC, WIOC: 73 ± 3%). The dual-isotope (Δ14C/δ13C) signatures, organic molecular tracers and Beijing-tailored emission inventory identify that this fossil source is primarily from coal combustion activities in winter, especially from the residential sector. Source testing on Chinese residential coal combustion provides direct evidence that intensive coal combustion could contribute to increased light-absorptivity of ambient BrC in Beijing winter. Coal combustion is an important source to BrC in regions such as northern China, especially during the winter season. Future modeling of OA radiative forcing should consider the importance of both biomass and fossil sources.
Highlights
Organic aerosol (OA) is a major component of fine and ultrafine particles with significant impacts on air quality, human health, and climate[1,2]
Studies on sources and source-specific properties of ambient BrC are rare in key regions such as China, which is associated with high carbonaceous aerosol emissions[33], especially from anthropogenic sources including primary organic aerosol (POA) and secondary organic aerosol (SOA) contributions[34,35]
During two field campaigns conducted in the summer and winter of 2013 in Beijing, 24-h average atmospheric organic carbon (OC) loading and its two sub-constituents (WSOC and water-insoluble organic carbon (WIOC), calculated as the difference between OC and water-soluble organic carbon (WSOC), with propagated uncertainty of 9%) were a factor of 3, 1.5 and 5 times greater in winter than summer, respectively
Summary
Seasonal variation of OA component and its light absorption. Atmospheric OA levels in Beijing showed a clear seasonal variation. Water and methanol extracts of wintertime fine particle (PM2.5) samples showed higher light-absorbance compared to summer, with stronger light-absorption by methanol extracts than water extracts at all wavelengths, especially at shorter wavelength (
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