Where Urban and Marine Air Masses Converge: Water-Soluble Gas-Phase Carbon and Nitrogen in the NYC Region

Gas-phase polar compounds, including water-soluble nitrogen (WSN_g), and water-soluble organic carbon (WSOC_g) contribute to ambient fine particulate matter through partitioning into atmospheric waters and forming aqueous secondary organic aerosol (aqSOA), a substantial contributor to fine particulate matter (PM2.5). In this work, we make continuous gas-phase measurements of WSN_g, WSOC_g, and ammonia (NH₃) at the Flax Pond Marine Laboratory, a Photochemical Air Monitoring Station (PAMS) on Long Island, during the Greater New York Oxidant Trace Gas Halogen and Aerosol Airborne Mission (GOTHAAM) field campaign from 14 July to 20 August, 2025. To the best of our knowledge, these are the first continuous atmospheric measurements of WSN_g. We pair measurements with predictions from the U.S. EPA’s Community Multiscale Air Quality (CMAQ). Measured WSN_g concentrations vary ranging from below detection limits to 33.4 ppb, averaging 5.71 ppb (n = 757, ±5.80 𝜎), and exhibit a distinct diurnal pattern with afternoon enhancements out of phase with oxides of nitrogen. The WSOC_g average diurnal profile exhibit afternoon maxima consistent with secondary photochemistry from volatile organic compound (VOC) oxidation. CMAQ accurately reproduces average diurnal profiles of criteria pollutants O₃ (r=0.97) and NO₂ (r=0.86) but is out of phase for both WSN_g or WSOC_g. These findings suggest that CMAQ cannot accurately describe key aqSOA precursors.