Strongly Absorbing Aerosol Refractive Indices in the Highly Polluted Indo-Gangetic Plains
TAVEEN KAPOOR, Harish C Phuleria, Benjamin Sumlin, Nishit Shetty, Gupta Anurag, Mahak Bansal, Sandeep Duhan, Shahzar Khan, Jitender Laura, Pooja Manwani, Rajan K. Chakrabarty, Chandra Venkataraman, Washington University in St. Louis
Abstract Number: 396
Working Group: Remote and Regional Atmospheric Aerosol
Abstract
The Indo-Gangetic plains in India experience prolonged periods of poor air quality and haze during the post-monsoon and winter months. Large aerosol emissions and unfavorable meteorological conditions combine to lead to this phenomenon. However, little is understood about the nature of aerosol optical properties and their relation to chemical composition over the region, which ultimately determines their climate impacts. In this study, we measured the wintertime aerosol optical properties at Rohtak, a regionally representative site in the Indo-Gangetic plains. We measured the real-time aerosol absorption coefficients, scattering coefficients and particle number size distribution. These measurements were used to calculate the aerosol effective refractive index (inverted using the PyMieScatt Python package), the first report of this property in the region. Aerosol particles were collected on filter substrates and used to estimate the PM2.5 and thermo-optically fractionated total carbon concentrations. PM2.5 concentrations of 163 μg/m3 with a relatively high EC/OC ratio revealed that the aerosol particles are likely strongly absorbing in nature, corroborated by the measured small single-scatter albedos (0.7) and large imaginary refractive index (~0.1). This imaginary (absorbing) refractive index (~0.1) was at the higher end of previous measurements of the same property in the ambient atmosphere. Further, strong absorption strengths in the mid-visible wavelengths, implied the presence of BrC constituents, estimated to contribute to ~32% of the near-UV aerosol absorption. The imaginary refractive index positively correlated with brown carbon (BrC) absorption contribution, revealing BrC dominance on aerosol absorption. Strong correlations between the organic and elemental carbon fractions and the predominance of thermal fractions of organic carbon (OC3 and OC4), reveals that BrC constituents likely originated from primary combustion sources and had substantial contributions from low-volatility compounds. Through a brief comparison with climate model simulated properties, we show that the strongly absorbing nature of the aerosol is not simulated well by the climate models. The present study's findings could help improve the accuracy of climate modelling simulations that underestimate aerosol absorption and extinction in the region.