Filter-based measurements of UV–vis mass absorption cross sections of organic carbon aerosol from residential biomass combustion: Preliminary findings and sources of uncertainty

Abstract

Combustion of solid biomass fuels is a major source of household energy in developing nations. Black (BC) and organic carbon (OC) aerosols are the major PM2.5 (particulate matter with aerodynamic diameter smaller than 2.5μm) pollutants co-emitted during burning of these fuels. While the optical nature of BC is well characterized, very little is known about the properties of light-absorbing OC (LAOC). Here, we report our preliminary findings on the mass-based optical properties of LAOC emitted from the combustion of four commonly used solid biomass fuels – fuel-wood, agricultural residue, dung-cake, and mixed – in traditional Indian cookstoves. As part of a pilot field study conducted in central India, PM2.5 samples were collected on Teflon filters and analyzed for their absorbance spectra in the 300–900nm wavelengths at 1nm resolution using a UV-Visible spectrophotometer equipped with an integrating sphere. The mean mass absorption cross-sections (MAC) of the emitted PM2.5 and OC, at 550nm, were 0.8 and 0.2m2g−1, respectively, each with a factor of ~2.3 uncertainty. The mean absorption Ångström exponent (AǺE) values for PM2.5 were 3±1 between 350 and 550nm, and 1.2±0.1 between 550 and 880nm. In the 350–550nm range, OC had an AǺE of 6.3±1.8. The emitted OC mass, which was on average 25 times of the BC mass, contributed over 50% of the aerosol absorbance at wavelengths smaller than 450nm. The overall OC contribution to visible solar light (300–900nm) absorption by the emitted particles was 26–45%. Our results highlight the need to comprehensively and accurately address: (i) the climatic impacts of light absorption by OC from cookstove emissions, and (ii) the uncertainties and biases associated with variability in biomass fuel types and combustion conditions, and filter-based measurement artifacts during determination of MAC values.