HNO3 partitioning in cirrus clouds

During the 1997 POLSTAR-1 winter campaign in northern Sweden a flight was p erformed across a cold trough of air (similar or equal to 196 K) in the tro popause region. Measurements of total water vapour, nitric acid, particles and reactive nitrogen (NOy) were taken. The particle measurements indicate that about 3% of the particles in the moist tropospheric air were ice parti cles. Forward and backward facing NOy inlets were used simultaneously to de termine condensed phase HNO3. The combined NOy and particle measurements re veal that less than 1% of a monolayer of NOy could have resided on the ice particles. This casts doubt on the hypothesis that sedimenting cirrus parti cles generally lead to a strong downward flux of NOy. In addition to the NO y measurements, independent HNO3 measurements were used to determine total HNO3. Although quantitative uncertainties do not allow to completely rule o ut that the NOy uptake on ice was limited by total HNO3, the combined NOy a nd KNO3 data suggest that there was low uptake of NOy on ice despite abunda nt HNO3 in the gas phase. Model studies indicate, that the most likely expl anation of the measured nitric acid partitioning is given by HNO3 in ternar y solution droplets coexisting with almost HNO3 free ice in the same air ma ss.