Optical Properties and Refractive Index of Wintertime Aerosol at a Highly Polluted North‐Indian Site

Estimation 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. Plain Language Summary Particles in the atmosphere, called aerosol, can absorb or reflect/scatter sunlight to heat or cool the atmosphere, depending upon their physical and chemical properties. Climate models try to simulate these properties to understand their effect on the climate. The strength of the absorption is determined by the size of the particle (generally the diameter) and its refractive index (a complex number) which is a material property. In this study, we measure the refractive index of atmospheric aerosol at Rohtak, which lies in the highly polluted Indo‐Gangetic plains in India. We find that the aerosol are very absorbing in nature, that is, they are warming the atmosphere, and that the absor