HNO 3 partitioning in cirrus clouds

During the 1997 POLSTAR‐1 winter campaign in northern Sweden a flight was performed across a cold trough of air (≃ 196 K) in the tropopause region. Measurements of total water vapour, nitric acid, particles and reactive nitrogen (NO y ) were taken. The particle measurements indicate that about 3% of the particles in the moist tropospheric air were ice particles. Forward and backward facing NO y inlets were used simultaneously to determine condensed phase HNO 3 . The combined NO y and particle measurements reveal that less than 1% of a monolayer of NO y could have resided on the ice particles. This casts doubt on the hypothesis that sedimenting cirrus particles generally lead to a strong downward flux of NO y . In addition to the NO y measurements, independent HNO 3 measurements were used to determine total HNO 3 . Although quantitative uncertainties do not allow to completely rule out that the NO y uptake on ice was limited by total HNO 3 , the combined NO y and HNO 3 data suggest that there was low uptake of NO y on ice despite abundant HNO 3 in the gas phase. Model studies indicate, that the most likely explanation of the measured nitric acid partitioning is given by HNO 3 in ternary solution droplets coexisting with almost HNO 3 free ice in the same air mass.