Aqueous, multi-phase chemistry represents an important transformation process for many atmospheric compounds.  In aqueous systems, including atmospheric waters, pH is a master variable that controls the speciation, phase and partitioning of many species.  Since no direct measurement exists, the pH of aqueous aerosol particles is difficult to characterize and knowledge of its spatial and temporal distribution is not well understood.  The subsequent influence of pH on multi-phase atmospheric processes is also not well constrained.  In this study, we compare aqueous aerosol conditions between urban and rural environments that are located in close proximity to each other.  Aerosol composition and gas-phase measurements are combined with meteorological parameters as inputs into the ISORROPIA thermodynamic equilibrium model.  We find that the urban heat island (UHI) has a dramatic effect on the aerosol liquid water content and the aerosol pH over spatial scales of only a few km.  For similar chemical composition and loading, particles in urban air contain systematically less water – up to a 3x decrease – and are systematically more acidic – up to 1 pH unit lower – than particles in rural air.  The differences vary diurnally and seasonally, although the nature of the UHI is distinct for each urban area.  We explore some of the most important implications for multi-phase chemistry associated with these observations.

Contrasting multi-phase chemistry in urban and rural environments

254th National Meeting (2017)

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