Filter-based measurement of light absorption by brown carbon in PM_2.5 in a megacity in South China

Carbonaceous aerosols represent an important nexus between air pollution and climate change. Here we collected filter-based PM_2.5 samples during summer and autumn in 2015 at one urban and two rural sites in Guangzhou, a megacity in southern China, and got the light absorption by black carbon (BC) and brown carbon (BrC) resolved with a DRI Model 2015 multi-wavelength thermal/optical carbon analyzer apart from determining the organic carbon (OC) and elemental carbon (EC) contents. On average BrC contributed 12-15% of the measured absorption at 405nm (LA₄₀₅) during summer and 15-19% during autumn with significant increase in the LA₄₀₅ by BrC at the rural sites. Carbonaceous aerosols, identified as total carbon (TC), yielded average mass absorption efficiency at 405nm (MAE₄₀₅) that were approximately 45% higher in autumn than in summer, an 83% increase was noted in the average MAE₄₀₅ for OC, compared with an increase of only 14% in the average MAE₄₀₅ for EC. The LA₄₀₅ by BrC showed a good correlation (p<0.001) with the ratios of secondary OC to PM_2.5 in summer. However, this correlation was poor (p>0.1) in autumn, implying greater secondary formation of BrC in summer. The correlations between levoglucosan (a marker of biomass burning) and the LA₄₀₅ by BrC were significant during autumn but insignificant during summer, suggesting that the observed increase in the LA₄₀₅ by BrC during autumn in rural areas was largely related to biomass burning. The measurements of light absorption at 550nm presented in this study indicated that the use of the IMPROVE algorithm with an MAE value of 10m²/g for EC to approximate light absorption may be appropriate in areas not strongly affected by fossil fuel combustion; however, this practice would underestimate the absorption of light by PM_2.5 in areas heavily affected by vehicle exhausts and coal burning.