Affiliation:
1. Interdisciplinary Program in Climate Studies Indian Institute of Technology Bombay Mumbai India
2. Environmental Science and Engineering Department Indian Institute of Technology Bombay Mumbai India
3. Center for Aerosol Science and Engineering Department of Energy, Environmental and Chemical Engineering Washington University in St. Louis St. Louis MO USA
4. Department of Civil Engineering Indian Institute of Technology Delhi New Delhi India
5. Department of Environmental Sciences Maharshi Dayanand University Rohtak Rohtak India
6. Department of Chemical Engineering Indian Institute of Technology Bombay Mumbai India
Abstract
AbstractEstimation of aerosol radiative forcing continues to suffer from large uncertainties, partially from a lack of observations of aerosol optical properties. Limited measurements of the atmospheric aerosol imaginary refractive index (iRI) have been made, especially in some of the world's most polluted regions. In this study, we measured aerosol optical and micro‐physical properties at a regional site, Rohtak, India, representative of polluted cities in the Indo‐Gangetic plains in northern India. The average PM2.5 measured during the campaign was 163 μg/m3 with a single‐scatter albedo of 0.7, indicating the presence of strongly absorbing aerosol components. Measurements of aerosol absorption, scattering, and particle number size distributions were used to estimate the effective refractive index using an established Mie inversion technique. The calculated iRI was spectrally invariant in the visible region with values ranging between 0.076 and 0.145. Brown carbon absorption, estimated using an existing Mie optimization method, ranged 34–88 Mm−1, with strongly absorbing mass absorption cross‐sections (∼1.9 m2/g). Higher iRI were observed during periods with higher brown carbon absorption, which are likely directly emitted from combustion sources. Low volatility organic carbon fractions dominated during these periods, with likely persistence of atmospheric absorption. The iRI values are at the upper end of previously reported ranges of urban aerosol iRI. In a sensitivity analysis to measured parameters, the absorption had the dominant effect on estimated iRI. Measured single scatter albedos, were lower than those from climate model simulations over the region, demonstrating the need for intrinsic property measurements to evaluate and constrain climate models.
Funder
Ministry of Environment, Forest and Climate Change
Publisher
American Geophysical Union (AGU)
Subject
Space and Planetary Science,Earth and Planetary Sciences (miscellaneous),Atmospheric Science,Geophysics