A study of gas-aerosol equilibrium and aerosol pH in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

A thermodynamic equilibrium model was applied to study the interactions of gas-phase NH3, HNO3, and HCl with size-resolved aerosols and estimate aeros ol pH in the remote marine boundary layer during the First Aerosol Characte rization Experiment (ACE 1). Analysis of model results and field measuremen ts indicates that accumulation-mode aerosols were probably in equilibrium w ith NH3, HNO3, and HCl simultaneously. The largest coarse-mode aerosols did not appear to be in equilibrium with HNO3, but may have been in equilibriu m with NH3 and HCl. The estimated pH of accumulation-mode aerosols was 0-2, a function primarily of the amount of sulfate relative to sea salt present in that mode. By contrast, the estimated equilibrium pH of coarse-mode aer osols was 2-5, a function primarily of relative humidity and gas-phase HCl. Prior to exposure to HCl, the estimated pH of fresh sea spray aerosols was 7-9, a function primarily of relative humidity. Sensitivity tests showed t hat the drying of aerosols during the sampling process may have volatilized up to 30% of NH4+.