Sources of particulate matter in the northeastern United States in summer: 2. Evolution of chemical and microphysical properties

Measurements of aerosol particle size distributions and composition and of trace and reactive gas mixing ratios were made on the NOAA WP‐3D aircraft downwind of mixed urban/industrial sources in the northeastern United States (U.S.). These measurements were made in noncloudy air during July and August 2004, under conditions where cloud processing was not likely to play an important role in oxidation chemistry. Under these conditions, particulate sulfate was found to be produced with an exponential time constant of ∼3.5 d from the gas‐phase oxidation of SO2, which was ubiquitous but inhomogeneously distributed in the pollution plumes. When submicron particle mass concentrations exceeded 15 μg m−3, sulfate and associated ammonium dominated the composition; at lower mass concentrations particulate organic matter (OM) dominated. Since most of the urban plumes sampled contained substantial SO2 from nearby industrial sources, the apportionment of aerosol mass between OM and sulfate compounds under noncloudy conditions was governed largely by the differences between the oxidation timescales of SO2 and those of precursor volatile organic compounds (VOCs) relative to their transport time. These differences in oxidation timescales may explain much of the variability in previously published OM/sulfate ratios for this region. These observations indicate that even with higher‐than‐expected secondary OM formation, in the northeastern U.S. the potential inorganic particulate mass from SO2 emissions significantly exceeds the potential secondary OM from anthropogenic VOC emissions.