Observations of condensation nuclei in the Airborne Antarctic Ozone Experiment: Implications for new particle formation and polar stratospheric cloud formation

Abstract

The ER‐2 Condensation Nucleus Counter (ER‐2 CNC) was operated in the Airborne Antarctic Ozone Experiment in August, September, and October 1987. The ER‐2 CNC measures the mixing ratio of particles, CN, with diameters from approximately 0.02 μm to approximately 1 μm. Comparisons of CN and other aerosol measurements with N2O show that the vertical profile of the sulfate aerosol was probably displaced downward by the subsidence and mixing processes that determined the inclination of the N2O isopleths. At altitudes above the minimum in the CN mixing ratio profile, CN mixing ratios correlated negatively with that of N2O, demonstrating new particle production. The region of particle production was above and south of the 160 ppbv N2O isopleth. The relationship between CN and N2O did not change noticeably when moving from air containing normal levels of reactive nitrogen compounds (NOy) to air depleted in those compounds. This suggests that the removal of NOy was accompanied by the removal of a small minority of the CN. Concentrations of CN are compared with those of larger particles to study cloud formation mechanisms. In some cases condensation of water or nitric acid trihydrate following rapid cooling caused most of the CN to grow to diameters larger than 0.81 μm. Models published elsewhere predict that these conditions should result in a large fraction of particles growing to these sizes. Episodes of this type probably did not remove reactive nitrogen compounds through particle sedimentation from the chemically perturbed region. Small numbers of large particles were observed in other instances. The composition of the particles and their origins are unclear.