Sources of atmospheric light-absorbing fine aerosols: Insights from optical source apportionment utilizing measurements made during COVID-19 lockdowns at a COALESCE network site - Bhopal, India

Abstract

The source-related optical characteristics of carbonaceous aerosols which absorb light are not well-known as they can be generated by a variety of sources. In this study, specific source-related optical properties of carbonaceous aerosols and mineral dust were investigated using a two-year dataset of absorption at multiple wavelengths and chemical species measured at Bhopal, India. A receptor model (Positive Matrix Factorization PMF5) was applied to the combined dataset to achieve optical source apportionment of carbonaceous aerosols and mineral dust. PMF resolved six factors accounting for carbonaceous aerosols and dust particulate matter (CDPM), which were identified as coal combustion, traffic exhaust, biomass burning, mineral dust, industry, and secondary organic aerosol (SOA) factors. The average contributions of these factors to the total CDPM mass were 29 % (coal combustion), 20 % (SOA), 16 % (traffic exhaust), 15 % (industry), 14 % (biomass burning), and 6 % (mineral dust (MD)) during the study period. The dominant contributors (at 405 nm) to aerosol light absorption were coal combustion (33 %), traffic exhaust (30 %), and biomass burning (29 %) factors. Specific source-related absorption Ångström exponent (AAE) values indicated that black carbon (BC) was the dominant light absorber in the coal and traffic emission factors. However, brown carbon (BrC) was present in the biomass-burning emissions. The specific source-related mass absorption cross section (MAC) for BC from coal combustion was higher at 635 nm than for MACBC from biomass burning, industry, and traffic exhaust factors. Overall, this study helped in the identification of additional light-absorbing sources of CDPM such as MD and SOA, and differentiate between optical parameters (AAE and MAC) of light-absorbing species in these sources.