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Brown Carbon Light Absorption over India: Research Status and Need for Discerning Climate Impacts
Published in the journal Environmental Science and Technology Air as a perspective article in 2025.
Link to full article: https://doi.org/10.1021/acsestair.5c00010
Abstract: Brown carbon (BrC) absorbs solar radiation and influences the regional and global climate. A major pollutant in India, BrC absorption has received reasonable research attention, with studies focusing on spatial-temporal variability and emission sources. By synthesizing the available literature, this paper presents the current research status on the topic and identifies gaps that need to be addressed to better understand the climate impacts of BrC in India. In terms of the measurement of BrC light absorption, we find that the solvent extraction technique is the most commonly used, followed by the Ångström exponent extrapolation and the component subtraction techniques. Measurement sites are concentrated in the Indo-Gangetic plains, which show significantly higher absorption levels than other regions. Seasonal variability shows increased absorption during postmonsoon and winter periods, particularly in peninsular India. Though limited in number, studies on emission sources identify biomass burning and vehicular emissions as key contributors. In terms of radiative impact, assessments from the Indo-Gangetic plains suggest that BrC contributes substantially to direct radiative effects, potentially accounting for a net 31–48% warming. Global climate models assign a net forcing of 0.5–2 W m–2 over India; however, they rely on parametrizations derived from non-Indian locations and sources, which may not accurately represent BrC over India. Overall, expanded spatial coverage, diverse source characterization, and improved radiative effect estimations are key to decoding BrC’s climate impact in India and globally.
Emissions from agricultural fires in India: field measurements of climate relevant aerosol chemical and optical properties
Emissions from agricultural fires in India: field measurements of climate relevant aerosol chemical and optical properties
Published in the journal Environmental Science: Atmospheres in 2025.
Link to full article: https://doi.org/10.1039/D4EA00104D
Abstract: Carbonaceous aerosol particles are associated with large uncertainties in their climate impacts because of incomplete knowledge of their optical properties and emission magnitudes. Biomass-burning sources significantly contribute to carbonaceous aerosol emissions in India, with crop residue burning being crucial during post-harvest months. Here, for the first time, we study the chemical and optical properties of emission aerosols using in situ real-time and filter-based measurements from significantly contributing crop residue straws, stalks, and stems in India. Emitted particles exhibited optical behaviour characteristic of the brown-black carbon absorption continuum, with large mass absorption cross-sections (MAC520: 8.2 ± 9.6 m2 g−1) and small absorption Angström exponents (AAE370/660: 1.97 ± 0.81). They contain significant amounts of lower volatility organic (OCLV) and elemental carbon fractions. The relative abundances of OCLV correlate positively with MAC520 and negatively with AAE370/660, implying significant absorption exerted by OCLV, with likely atmospheric persistence. Additionally, we measured emission factors for a complete list of particulate chemical constituents. Emission factors of elemental carbon are larger than those in earlier studies, indicating a 1.6–3.8 times increase in the climate warming potential of the emitted particles from crop residue burning. The intrinsic property measurements and the emissions estimates made here can aid climate modelling efforts that underestimate aerosol absorption in the region.
Spatial Distribution in Surface Aerosol Light Absorption Across India
Published in the journal Geophysical Research Letters in 2024.
Link to full article: https://doi.org/10.1029/2024GL110089
Plain Language Summary: Particulate pollution in the atmosphere scatter and absorb incoming solar energy, thus cooling or warming Earth's atmosphere. In developing countries and especially in India, one of the most polluted regions of the world, the extent to which particles can absorb solar energy and warm the atmosphere is not well understood. Here, for the first time, we measure particle absorption simultaneously at nine ground sites across India, in diverse geographical regions with different levels and types of particulate pollution. We find that organic carbon particles exert large absorption at near-ultraviolet wavelengths, which contain significant solar energy. These light absorbing organic carbon particles, called brown carbon, are emitted in large quantities from biomass burning (e.g., burning crop residue and cooking on wood-fired stoves). Comparing ground measurements of absorption with satellite-retrieved measurements that are representative of the entire atmospheric column, we find that near-surface atmospheric particles can exert significant warming. This study highlights the need to improve climate model simulations of particulate pollution's impact on the climate by incorporating spatiotemporal surface-level absorption measurements, including absorption by brown carbon particles.
Abstract: Light-absorbing carbonaceous aerosols that dominate atmospheric aerosol warming over India remain poorly characterized. Here, we delve into UV-visible-IR spectral aerosol absorption properties at nine PAN-India COALESCE network sites (Venkataraman et al., 2020, https://doi.org/10.1175/bams-d-19-0030.1). Absorption properties were estimated from aerosol-laden polytetrafluoroethylene filters using a well-constrained technique incorporating filter-to-particle correction factors. The measurements revealed spatiotemporal heterogeneity in spectral intrinsic and extrinsic absorption properties. Absorption analysis at near-UV wavelengths from carbonaceous aerosols at these regional sites revealed large near-ultraviolet brown carbon absorption contributions from 21% to 68%—emphasizing the need to include these particles in climate models. Further, satellite-retrieved column-integrated absorption was dominated by surface absorption, which opens possibilities of using satellite measurements to model surface-layer optical properties (limited to specific sites) at a higher spatial resolution. Both the satellite-modeled and direct in-situ absorption measurements can aid in validating and constraining climate modeling efforts that suffer from absorption underestimations and high uncertainties in radiative forcing estimates.
Reassessing the availability of crop residue as a bioenergy resource in India: a field-survey based assessment
Published in the Journal of Environmental Management in 2023.
Link to full article: https://doi.org/10.1016/j.jenvman.2023.118055
Abstract: Second-generation bioenergy, a carbon neutral or negative renewable resource, is crucial to achieving India's net-zero emission targets. Crop residues are being targeted as a bioenergy resource as they are otherwise burned on-field, leading to significant pollutant emissions. But estimating their bioenergy potential is problematic because of broad assumptions about their surplus fractions. Here, we use comprehensive surveys and multivariate regression models to estimate the bioenergy potential of surplus crop residues in India. These are with high sub-national and crop disaggregation that can facilitate the development of efficient supply chain mechanisms for its widespread usage. The estimated potential for 2019 of 1313 PJ can increase the present bioenergy installed capacity by 82% but is likely insufficient alone to meet India's bioenergy targets. The shortage of crop residue for bioenergy, combined with the sustainability concerns raised by previous studies, imply a need to reassess the strategy for the use of this resource.
Optical properties and refractive index of wintertime aerosol at a highly polluted north‐Indian site
Published in the Journal of Geophysical Research: Atmospheres in 2023.
Link to full article: https://doi.org/10.1029/2022JD038272
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.
Abstract: 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.
Estimation of real-time brown carbon absorption: An observationally constrained Mie theory-based optimization method
Published in the Journal of Aerosol Science in 2022.
Link to full article: https://doi.org/10.1016/j.jaerosci.2022.106047
The paper also featured on the journal cover page (shown below)!
Abstract: Methods to estimate absorption of brown carbon (BrC), a significant fraction of atmospheric absorption, rely on estimating the difference between total measured absorption at near-UV, and that of black carbon (BC). Extrapolation of absorption measured at near-IR wavelengths (assumed exerted by BC alone) use different assumptions of the wavelength dependence of absorption Ångström exponent (AAEBC). Here, we develop an improved method exploiting real-time multi-wavelength absorption and particle count measurements in a Mie based optimization framework, incorporating spectral observational constraints (measured absorption at 880 nm and AAE880-660). An optimization approach, using a Mie model with core-shell and core-gray shell mixing schemes, is used to derive BC size distribution parameters (absorbing core diameter and scattering shell thickness). Goodness of fit (Mie optimization model vs. measurement) was R = 0.77–0.94 (near-IR absorption) and within 4%–30% for BrC estimation. A sensitivity analysis of input parameters (BC geometric standard deviation and refractive index) bounded estimated BrC of 32%. Application to a polluted urban site (Delhi) and a regional background site (Darjeeling) estimated BrC absorption (% contribution) at 370 nm as 18–117 Mm−1 (15%–29%) and 2–12 Mm−1 (5%–21%), respectively. Estimated BrC absorption was larger at the regional background site (Darjeeling) but smaller at the polluted site (Delhi) when compared to constant AAE and two-component approaches. Method efficacy is reinforced through larger estimated BrC absorption at Delhi coinciding with agricultural stubble burning periods in North India. The developed method uses multi-wavelength absorption observational constraints to improve the robustness of BrC estimation.
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