Complex Interplay Between Organic and Secondary Inorganic Aerosols With Ambient Relative Humidity Implicates the Aerosol Liquid Water Content Over India During Wintertime

Abstract

Aerosol Liquid Water Content (ALWC), a ubiquitous component of atmospheric aerosols, modulates atmospheric chemistry through aerosol surface reactions and reduces the atmospheric visibility. However, the complex dependency of ALWC on aerosol chemistry and relative humidity (RH) in the Indian region remains poorly characterized. Here, we combine available measurements of aerosol chemical composition with thermodynamic model, ISORROPIA2.1, to reveal a comprehensive picture of ALWC in fine mode aerosols during the winter season over the Indian continental region. The factors modulating ALWC are primarily dependent on the RH, such that the effect of aerosol dry mass and hygroscopicity are significant at high RH while the effect of hygroscopicity significantly reduces with decreasing RH. ALWC is observed to display a sharp non‐linear rise, beyond a critical value of ambient RH dependent on the particle hygroscopicity. Further analysis by coupling Weather Research Forecasting‐Chem simulation with ISORROPIA2.1 revealed significant spatial heterogeneity in ALWC over India, strongly associating with regions of high aerosol mass loading and RH. The Indo‐Gangetic Plain is consequently observed to be a hotspot of higher ALWC, which explains the prevalent conditions of haze and smog during winter in this region. Our findings re‐emphasize that high aerosol mass resulting from intense pollution is vital in modulating aerosol–climate interaction under favorable meteorological conditions. Observations suggest the need for localized pollution control strategies, directed at the reduction in aerosol emissions of specific chemical composition observed to contribute to the enhancement in PM through an increase in ALWC during wintertime in the region.

Thermodynamical framework for effective mitigation of high aerosol loading in the Indo-Gangetic Plain during winter

“…( S7 – S9 ). As many previous studies in India considered fixed concentrations of HCl + Cl − (25 µg m −3 ) as model input due to a lack of gas-phase HCl observations in their studies 24 , 25 , we also fixed the input concentrations to 25 µg m −3 . To make the analysis more robust, we fixed the concentrations of all three gas + aerosol constituents, i.e., HCl + Cl − , HNO 3 + NO 3 − , and NH 3 + NH 4 + .…”

“…The average ε(NH 4 + ) for PM 1 and PM 2.5 was 0.42 ± 0.17 and 0.55 ± 0.15 respectively, attributed to the substantial excess ammonia left in the gas-phase. Due to this, several studies have highlighted that pollution control strategies in India should be directed at reducing high NH 3 concentrations 25 , 35 . Here, in this study, we did not investigate the effect of pH and ALWC on the phase partitioning of sulfate ε(SO 4 2− ).…”