Optical properties of water soluble and organic soluble carbonaceous aerosols at an urban location in India

Abstract

We report measurements of the optical properties of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) in the metropolitan city of Mumbai (19.01° N, 72.92° E), India. The MSOC and WSOC extracts were analysed using UV–visible spectroscopy. The study covered a period of nine months from September 2017 to May 2018. On average, MSOC constituted 30% and WSOC constituted 24% of the PM2.5 mass for the sampling period with peak concentration observed in the winter season. The absorption coefficients of MSOC were on average 1.57 times higher than WSOC for the sampling period. The absorption coefficients of MSOC and WSOC were correlated with the brown carbon absorption coefficients. Mass absorption cross-section (MAC) was calculated by normalizing the absorption coefficients with its concentration, and the absorption angstrom exponent (AAE) was calculated by exponential fitting of the absorption coefficients. The MAC values for WSOC were estimated to be 1.03 ± 0.39 m2 g⁻1, while for MSOC, it was 1.41 ± 0.76 m2 g⁻1. The relative radiative forcing compared to black carbon was estimated at 10.1 ± 5.2% and 6.3 ± 3.8% for MSOC and WSOC, respectively.