The interaction between water vapor and atmos-pheric aerosol leads to enhancement in aerosol water content,which facilitates haze development, but its concentrations, sources,and impacts remain largely unknown in polluted urban environ-ments. Here, we show that the Indian capital, Delhi, which tops thelist of polluted capital cities, also experiences the highest aerosolwater yet reported worldwide. This high aerosol water promotessecondary formation of aerosols and worsens air pollution. Wereport that severe pollution events are commonly associated withhigh aerosol water which enhances light scattering and reducesvisibility by 70%. Strong light scattering also suppresses theboundary layer height on winter mornings in Delhi, inhibitingdispersal of pollutants and further exacerbating morning pollutionpeaks. We provide evidence that ammonium chloride is the largestcontributor to aerosol water in Delhi, making up 40% on average, and we highlight that regulation of chlorine-containing precursorsshould be considered in mitigation strategies.