Balloon observations of nitric acid aerosol formation in the Arctic stratosphere: I. Gaseous nitric acid

Measurements of the altitude distributions of gaseous nitric acid and aerosols were made in the winter arctic stratosphere using balloon‐borne mass spectrometer‐ and aerosol counter techniques. The measurements took place under conditions of very low stratospheric temperatures, with minimum values of 186 K around 24 km altitude being about 6 degrees below the existence temperature (T(NAT)) for NAT (Nitric Acid Trihydrate) aerosols. At these heights, a local depletion of gaseous nitric acid by up to about 60% was observed suggesting the occurrence of nitric acid incorporation into aerosols, which were in fact observed and whose inferred nitric acid content is roughly consistent with the missing gaseous nitric acid. Our data suggest that rapid formation of NAT‐aerosols associated with substantial condensational depletion of gaseous nitric acid occurred only at temperatures of about 3 to 4 degrees below T(NAT) corresponding to large HNO 3 ‐supersaturations, which are probably required for activation of a major fraction of preexisting condensation nuclei.