The effect of formic acid on the deliquescence of model sea-salt aerosol particles

Abstract

We report measurements of the deliquescence relative humidity (DRH—the relative humidity at which deliquescence occurs) of NaCl aerosol in the presence of varying amounts of formic acid vapour. The observations were carried out on submicron aerosol particles in a room-temperature flow tube. Dry NaCl aerosol particles were produced by nebulizing a sodium chloride solution and drying the droplets in a diffusion dryer. The dried particles were exposed to increasing relative humidity in the presence of varying amounts of formic acid until the deliquescence point was reached, as detected from the FTIR spectrum. The formic acid caused a reduction in the DRH from 75%, the value in the absence of acid, to 68.5% at a pH of 2.7. The reduction of DRH results from processes occurring in an aqueous layer on the surface of the particles and does not occur in the presence of formic acid vapour alone. If ambient sea-salt aerosols scavenge acidic pollutants to the same extent as has been observed for acid fogs, the resulting decrease in DRH could be substantially in excess of that measured in these experiments.