Quantification and transformation of water soluble organic nitrogen in a coastal urban airshed
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Atmospheric organic nitrogen (ON) is poorly quantified due to its complexity, measurement difficulty and historical consensus that it was less significant than inorganic nitrogen. Studies that have measured ON are not evenly spread across various environments or geographic regions further contributing to a general lack of knowledge about global atmospheric ON deposition and transformations. In this study, PM2.5 and PM10 samples were collected in an urban coastal airshed, Corpus Christi, TX, USA between November 2019, and October 2020. The organic and inorganic forms of nitrogen were analyzed and the factors controlling their concentrations were examined. The mean concentrations of water soluble organic nitrogen (WSON) in PM2.5 and PM10 were 0.081 ± 0.12 and 0.12 ± 0.11 µg m-3, respectively. The observed WSON concentrations were similar to those observed in remote and marine sites. The WSON concentrations showed significant correlation with NO3⁻ and NH4⁺ during spring season in PM10 indicating that marine source (e.g., sea salt) and biogenic emission are contributing factor for NO3⁻, while agricultural sources are contributing factor for NH4⁺. On average, ON contributed 15% and 10% to total nitrogen in PM2.5 and PM10 at the coastal urban airshed. The average annual dry deposition fluxes for PM2.5 NO3⁻, NH4⁺, and WSON were 0.217, 0.027, and 0.68 kg ha−1 y −1, respectively. The annual dry deposition fluxes for PM10 NO3⁻, NH4⁺, and WSON were 0.55, 0.031, and 2.48 kg ha−1 y −1. The EPA CASTNET network’s closet monitoring site reports annual dry N deposition flux of 3.67 kg ha−1 y −1 , but does not account for ON thus potentially overlooking ~40% of N deposition. Furthermore, the photochemical transformation of ON into IN was investigated and results from these experiments revealed alternating photoproduction and degradation of IN (NO3⁻, NH4⁺). These WSON photochemical transformation will contribute to the increase or decrease of nitrogen bioavailability after deposition in this coastal region. This study highlights the significance of the ON component of atmospheric deposition in this northern Gulf of Mexico region and the need to for ON inclusion when determining coastal N loading and N mitigation strategies.