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, contributes to total aerosol mass burden,
modulating atmospheric chemistry through aerosol surface reactions and
reducing 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 in the Indian region. The fac-tors 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 loses its significance as RH is lowered. ALWC, depending
upon the particle hygroscopicity, displays a sharp non-linear rise
beyond a critical value of ambient RH. Further analysis coupling
WRF-Chem simulation with ISORROPIA2.1 revealed significant spatial
heterogeneity in ALWC over India, strongly associating with regions of
high aerosol 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 the region. Our findings
re-emphasize that high aerosol mass resulting from intense pollution is
vital in modulating aerosol–climate interaction under favorable
meteorological conditions. They suggest the need for pollution control
strategies to be directed at the reduction in emissions of specific
species like NH3 and NOx, which were observed to contribute to the
enhancement of PM and ALWC during wintertime in the region.