Sources and characteristics of light-absorbing fine particulates over Delhi through the synergy of real-time optical and chemical measurements

Abstract

This study examines the light absorption characteristics of fine atmospheric particulates using the co-located real-time measurements of chemical and optical aerosol characteristics at a site located in Delhi during winter. The contribution of absorption by black carbon and brown carbon (BC and BrC) to the total absorption (babs) is computed using the absorption Ångström exponent (AAE) method. The period average contribution of BrC absorption (babs_BrC) to babs is found to be highest at 370 nm (23%) that decreased exponentially with increasing wavelengths, i.e.,18, 12, 10, and 4% at 470, 520, 590, and 660 nm, respectively. The absorption spectrum of BrC is used to study the bulk composition of BrC, which indicates that primary BrC was dominant during morning and night time, whereas secondary BrC was significant during the rest of the time. Further, organic aerosols (OA) were divided into different factors using positive-matrix factorization (PMF) analysis, and the mass absorption efficiency (Eabs) of each factor was assessed through multivariate linear regression of babs_BrC with OA factors. The biomass burning OA (BBOA) exhibited the highest Eabs at 370 nm (0.86 m2 g−1), followed by semi-volatile oxygenated OA (SVOOA; 0.67 m2 g−1) and hydrocarbon-like OA (HOA; 0.42 m2 g−1). Further, although the composition of OA was dominated by LVOOA (32%) and BBOA (32%), followed by SVOOA (22%), and HOA (14%), their contribution to bBrC followed different order due to differences in their mass absorption efficiencies. The BBOA contributed almost half (48%) of babs_BrC followed by SVOOA (26%), and HOA (10%). This study provides quantitative information on the sources of BrC and their relative contribution to BrC absorption over a heavily polluted region, which is still lacking in the literature. These results have implications in understanding the source-specific BrC absorption.