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 NH 3 , HNO 3 , and HCl with size‐resolved aerosols and estimate aerosol pH in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1). Analysis of model results and field measurements indicates that accumulation‐mode aerosols were probably in equilibrium with NH 3 , HNO 3 , and HCl simultaneously. The largest coarse‐mode aerosols did not appear to be in equilibrium with HNO 3 , but may have been in equilibrium with NH 3 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 aerosols 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 that the drying of aerosols during the sampling process may have volatilized up to 30% of NH + 4 .