Conversion of sea salt aerosol to NaNO3 and the production of HCl: Analysis of temporal behavior of aerosol chloride/nitrate and gaseous HCl/HNO3 concentrations with AIM

Equilibrium modeling predicts that atmospheric sea salt can partition gas-phase HNO3 to solid- or aqueous-phase NaNO3. One month of semi-continuous and simultaneous measurements of particulate chloride and nitrate and gaseous HCl and HNO3 concentrations were made in the Tampa, Florida, as part of the Bay Region Atmospheric Chemistry Experiment (BRACE). Tampa's proximity to coastal and bay waters enriches its atmosphere with sea salt. To help explain and interpret the observed time-dependent concentration and gas-to-particle phase partitioning behavior for the NaCl–HNO3 reaction, we applied the Aerosol Inorganics Model III (AIM) to the measurement data. Good agreement between model predictions and observations was found. Measurement and modeling results suggested that coarse-mode sea salt particles from the Atlantic Ocean arrived in the morning at the monitoring site when relative humidity (RH) was high and the nature of the equilibrium least favored the outgassing of HCl from the particles. As the RH dropped in the afternoon, the equilibrium favored outgassing of HCl and the particulate nitrate concentration increased even as the concentration of coarse particles decreased. This effect was tied to the change in the ratio of nitrate to chloride activity coefficients γNO3-/γCl- with RH. AIM simulations indicated that this ratio approached unity at high RH but could take on small values (∼0.05) at the lowest RH observed here. Thus, the particle phase slightly favored nitrate over chloride at high RH and greatly favored it at lower RH. Modeling revealed how diurnal changes in RH can rapidly shift HNO3 concentrations from gas- to particle-phase and thus affect the distance over which nitrogen is transported.