Ammonium Chloride Associated Aerosol Liquid Water Enhances Haze in Delhi, India

The interaction between
water vapor and atmospheric aerosol leads
to enhancement in aerosol water content, which facilitates haze development,
but its concentrations, sources, and impacts remain largely unknown
in polluted urban environments. Here, we show that the Indian capital,
Delhi, which tops the list of polluted capital cities, also experiences
the highest aerosol water yet reported worldwide. This high aerosol
water promotes secondary formation of aerosols and worsens air pollution.
We report that severe pollution events are commonly associated with
high aerosol water which enhances light scattering and reduces visibility
by 70%. Strong light scattering also suppresses the boundary layer
height on winter mornings in Delhi, inhibiting dispersal of pollutants
and further exacerbating morning pollution peaks. We provide evidence
that ammonium chloride is the largest contributor to aerosol water
in Delhi, making up 40% on average, and we highlight that regulation
of chlorine-containing precursors should be considered in mitigation
strategies.