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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 absorbing/imaginary part of the refractive index is in the higher range of previously reported values. The aerosol at the site are dominated by emissions from fossil fuel and biomass burning sources. On comparing these absorbing properties with those simulated by climate models, we find that the climate models are not able to simulate the absorbing nature of these aerosol well. Hence, there is a need for more such measurements of the aerosol refractive index to improve climate models and ultimately improve our understanding of the effect of aerosol on our climate. Key Points: Estimate the aerosol effective refractive index (RI); imaginary RI ranges 0.076–0.145, higher than previously reported valuesA single scatter albedo of 0.7 reveals strongly absorbing components, with brown carbon absorption of 34–88 Mm−1Imaginary RI correlates well with near‐UV absorption by brown carbon, which has low volatility and is likely emitted from combustion sources [ABSTRACT FROM AUTHOR]

Agricultural irrigation has significant potential for altering local climate by reducing soil albedo, increasing evapotranspiration, and enabling greater leaf area. Numerous studies using regional or global climate models have demonstrated the cooling effects of irrigation on mean and extreme temperature, especially over regions where irrigation is extensive. However, these model‐based results have not been well validated due to the limitations of observational data sets. In this study, multiple satellite‐based products, including the Moderate Resolution Imaging Spectroradiometer (MODIS) and Soil Moisture Active Passive (SMAP) data sets, are used to isolate and quantify the local impacts of irrigation on surface climate over irrigated regions, which are derived from the Global Map of Irrigation Areas (GMIA). The relationships among soil moisture, albedo, evapotranspiration, and surface temperature are explored. Strong evaporative cooling by irrigation lowers daytime surface temperature over arid and semi‐arid regions, such as California's Central Valley, the Great Plains, central Asia, and northwestern India. However, the cooling effects are less evident in areas of eastern China and the Lower Mississippi River Basin despite extensive irrigation over these regions. Results are also compared with irrigation experiments using the Community Earth System Model (CESM) to assess the model's ability to represent land–atmosphere interactions in regards to irrigation. CESM greatly underestimates the surface temperature response to irrigation. The comparison between the offline and coupled simulations suggests that the irrigation‐induced cooling can be regulated by the interactions between land surface and atmosphere, and amplified signals are found over the "hot spot" regions. Meanwhile, model resolution can also influence the magnitude of the local cooling by irrigation. This study, for the first time, thoroughly investigated the observed local impacts of irrigation on surface climate based on multiple satellite‐based products at the global scale. Strong evaporative cooling of irrigation on daytime surface temperature is found over arid and semi‐arid regions, such as California's Central Valley, the Great Plains, central Asia, and northwestern India. However, the cooling effects are less evident in areas of eastern China and the Lower Mississippi River Basin despite extensive irrigation over these regions. [ABSTRACT FROM AUTHOR]

Using data from Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer (MODIS) instruments, we have retrieved regional distribution of aerosol column single scattering albedo (parameter indicative of the relative dominance of aerosol absorption and scattering effects), a most important, but least understood aerosol property in assessing its climate impact. Consequently we provide improved assessment of short wave aerosol radiative forcing (ARF) (on both regional and seasonal scales) estimates over this region. Large gradients in north-south ARF were observed as a consequence of gradients in single scattering albedo as well as aerosol optical depth. The highest ARF (-37Wm-2 at the surface) was observed over the northern Arabian Sea during June to August period (JJA). In general, ARF was higher over northern Bay of Bengal (NBoB) during winter and premonsoon period, whereas the ARF was higher over northern Arabian Sea (NAS) during the monsoon and postmonsoon period. The largest forcing observed over NAS during JJA is the consequence of large amounts of desert dust transported from the west Asian dust sources. High as well as seasonally invariant aerosol single scattering albedos (0.98) were observed over the southern Indian Ocean region far from continents. The ARF estimates based on direct measurements made at a remote island location,Minicoy (8.3°?N, 73°?E) in the southern Arabian Sea are in good agreement with the estimatesmade followingmultisatellite analysis. [ABSTRACT FROM AUTHOR]