Spectral Light Absorption in Aerosol Emissions from Anthropogenic Biomass Burning in India

Chimurkar Navinya, Taveen Kapoor, CHANDRA VENKATARAMAN, Gupta Anurag, Harish C Phuleria, Rajan K. Chakrabarty, Indian Institute of Technology Bombay

     Abstract Number: 35
     Working Group: Aerosol Physics

Abstract
Short lived climate forcers include carbonaceous aerosols (black carbon-BC and organic aerosol-OA) which perturb Earth’s radiation balance and drive climate change. There is growing evidence supporting the relevance of light absorbing organic carbon, termed brown carbon (BrC), to climate change, but gaps in understanding optical properties of BrC hinder its representation in climate models. In India, atmospheric measurements have established significant BrC absorption, possibly related to the prevalence of biomass burning for energy and waste disposal, pointing to the need to better understand emission aerosols.

Here we studied spectral optical properties of emission aerosol samples from different types of anthropogenic biomass burning activities in India, including residential cooking and heating, brick manufacture and agricultural residue burning, using field measurements made during the COALESCE (Venkataraman et al. 2020) campaign. Spectral absorption of methanol extracts of the organic carbon fraction of filter collected particles, was estimated from spectrophotometer measurements, made between 300 nm to 900 nm, with absorption normalization to limit signal drift. Thermally resolved carbon fractions were concurrently measured using thermal optical reflectance. The spectral absorption of BrC was used to derive the mass absorption cross-section (MAC_BrC), imaginary refractive index (k_BrC,550) and its spectral dependence (w).

We find significantly large values of BrC absorption and the imaginary refractive index in the extracts. A regression-based relationship to the BC/OA revealed k_BrC,550 values increasing from ~0.006 to 0.74, and w decreasing from 5.24 to ~0 across a range in BC/OA ratio of 0 to 20, showing behavior consistent across the black-brown carbon light absorption continuum. An imaginary refractive index ~2-3 times larger than that in previous studies, indicates strong absorption. Combining this with an emissions inventory over India, the paper will discuss spatial patterns in BrC absorption over India and implications for its influence on regional climate.

[1] Venkataraman, C., M. Bhushan, S. Dey, D. Ganguly, T. Gupta, G. Habib, A. Kesarkar, H. Phuleria, R. Sunder Raman (2020) Indian network project on Carbonaceous Aerosol Emissions, Source Apportionment and Climate Impacts (COALESCE), Bull. Am. Met. Soc., https://doi.org/10.1175/BAMS-D-19-0030.1