Advancing Carbonaceous Aerosol Characterization in India to Improve Regional Climate Risk Assessment

Aerosol-induced changes to the surface and atmospheric energy balance are crucial for understanding regional climate change, particularly in the Indian subcontinent, where carbonaceous aerosols contribute to atmospheric warming. However, our understanding of aerosol impacts lags that of greenhouse gases, partially due to a lack of primary observations regarding aerosol optical properties. This presentation synthesizes regional-scale and localized measurements of strongly absorbing carbonaceous aerosols to help constrain these gaps and identify key directions for future research. Analysis from the PAN-India COALESCE network, spanning nine regional sites, revealed significant spatiotemporal heterogeneity in aerosol absorption. Spectral measurements showed that brown carbon (BrC) contributes between 21% and 68% to near-UV absorption nationally. Despite this significant contribution, absorption by BrC particles is not routinely integrated into most climate models. While these national trends highlight a widespread underestimation of absorption, they also underscore the need for a more granular understanding of optical properties. Further intensive measurements were conducted at Rohtak, India, a representative urban-regional site in the highly polluted Indo-Gangetic Plain. Measurements revealed extreme aerosol loading (PM_2.5 ~ 163 µ/m3) and strong absorption, with a single-scatter albedo (at 550 nm) of 0.7. Using a Mie inversion technique, we estimated the imaginary refractive index (a measure of aerosol absorption strength) to be between 0.076 and 0.145—values residing at the upper end of reported urban ranges globally. This high imaginary refractive index is attributed to black carbon and strongly absorbing BrC (mass absorption cross-sections at 550 nm of 1.9 m² g^-1) from primary combustion sources. Notably, the persistence of this absorption was linked to a dominance of low-volatility organic carbon fractions—termed dark brown carbon—that resists photo-bleaching. These particles exhibit absorption extending to longer, near-infrared wavelengths, warranting further investigation and inclusion in climate models. A systematic review of existing literature suggests that the detection method for BrC absorption significantly influences the reported magnitude and may potentially bias spectral signals, thereby complicating current model constraints. 

These findings have direct implications for regional climate risk. The measured single-scatter albedo values are lower than those utilized in current climate simulations over South Asia. This systematic underestimation of absorption likely leads to biased projections of regional radiative forcing, surface dimming, and atmospheric heating rates. Such discrepancies could result in significant uncertainties regarding downstream meteorological extremes and climate risks. These risks can only be mitigated through improved measurements with more extensive spatiotemporal coverage to provide the constraints necessary for robust climate projections.