Source apportionment and light absorption properties of black and brown carbon aerosols in the Brahmaputra River valley region

Abstract

Abstract The light absorption due to winter black carbon (BC) and brown carbon (BrC) in a complex-terrain region of northeast India was studied. The BC concentrations were measured by using a seven-wavelength Aethalometer. The hourly average BC concentration at 880\u202fnm was 10.02\u202f±\u202f3.17\u202fμg\u202fm−3, with the highest of 30.48\u202fμg\u202fm−3 on most foggy days. The mean Angstrom Absorption Exponent (AAE370-880) for the spectrum 370–880\u202fnm was 1.25, and the highest was 1.28 during the winter nighttime. The light absorption of BC dominated at all wavelengths from 370 to 950\u202fnm. Source apportionment using the site-specific spectral properties showed that the average contribution from the fossil fuel (road traffic emissions) was around 70% and 30% from biomass burning. The primary brown carbon (BrCpri) contributed a much higher (17.5%) to the total aerosol light absorption (babs) than secondary brown carbon (BrCsec) (7.8%) at 370\u202fnm. The BrCpri and BrCsec absorption and contribution to babs peaking to 24.9% and 12.4% during winter nights at 370\u202fnm, indicating open biomass burning a dominant source of BrC. Further, the exposure to BC in winter might cause a higher health risk equivalent to 25 passive cigarette smoking per day.