Nitric acid phase partitioning and cycling in the New England coastal atmosphere

Abstract

During summer 2004, HNO3 and size‐resolved aerosols were measured in parallel, and corresponding dry‐deposition fluxes were modeled at Appledore Island, Maine, as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field program. HNO3 concentrations varied widely on the timescale of hours; however, all days were characterized by a minimum near sunrise. Mixing ratios normally peaked in the early afternoon; maximum and median concentrations of HNO3 during the campaign were 337 and 22.8 nmol m−3, respectively. Aerosol NO3− exhibited a bimodal size distribution with a primary peak associated with sea salt at ∼4 μm and a secondary sub‐μm peak. The median NO3− concentrations in sub‐ and super‐μm diameter size fractions were 3.3 and 7.7 nmol m−3, respectively. Peak HNO3 and super‐μm NO3− concentrations were associated with westerly and southwesterly flow regimes respectively. The multiphase cycling of HNO3 was evaluated as a function of transport sector. Although median total nitrate (HNO3 + NO3−) concentrations were higher under westerly flow, higher median dry‐deposition rates of total nitrate were associated with southwesterly flow. Sea‐salt concentrations were ∼3 times greater during southwesterly flow, which shifted the phase partitioning toward particulate NO3−. Consequently, under westerly flow, HNO3 deposited more quickly than aerosol NO3−, while for southwesterly flow, the fluxes from the two phases were comparable. On the basis of all data, the median dry‐deposition fluxes for HNO3 and aerosol NO3− were 8.2 and 5.6 μmol m−2 d−1; super‐μm size fractions dominated the NO3− flux.