Effect of sea salt and calcium carbonate interactions with nitric acid on the direct dry deposition of nitrogen to Tampa Bay, Florida

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Abstract Equilibrium modeling predicts that atmospheric sea salt and CaCO3 can partition gas-phase HNO3 to solid or aqueous-phase NaNO3 and Ca(NO3)2. We hypothesized that this partitioning reduces the direct dry deposition of nitrogen to Tampa Bay, as the average deposition velocities of the aerosol species are typically less than that of the gas. As a corollary to this, we investigated whether a 15-μm diameter particle could remain suspended in the atmosphere long enough for the gas-to-particle nitrogen transfer to represent a significant fraction of the nitrogen flux. To test these hypotheses, we applied to local inorganic aerosol concentration and size distribution measurements a published nitrate accumulation model with literature values of uptake coefficients. For the time-dependent NaNO3 and Ca(NO3)2 formation computations, we adopted a closed-system approach and assumed externally mixed particles, and considered relative humidity averages of 60%, 78% and 90%. An integrated NOAA Buoy gas deposition and Williams particle deposition model was used with 1 year of local meteorology and water temperature data to obtain average over water deposition velocities, segregated by the 25th, 50th and 75th percentile wind speeds. The calculated HNO3 dry deposition rates in the absence of aerosols were 0.61, 1.5 and 3.3 kg-N ha−1 yr−1 at the low, medium and high wind speeds, respectively. Modeled reductions in the dry nitrogen flux ranged from 15% at low wind speed and high relative humidity conditions to 76% at high wind speed and high relative humidity conditions. The modeled flux divergence indicated that sea salt and mineral dust diminished the direct HNO3 nitrogen deposition to Tampa Bay. Modeled nitrate nitrogen deposition rates, however, were dominated by nitrogen transferred to a 15-μm diameter particle. These results highlight the need to both monitor and model the coarse particle nitrogen for estimating nitrogen deposition in a coastal environment.