HNO sub(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 ([perspective to] 196 K) in the tropopause region. Measurements of total water vapour, nitric acid, particles and reactive nitrogen (NO sub(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 sub(y) inlets were used simultaneously to determine condensed phase HNO sub(3) . The combined NO sub(y) and particle measurements reveal that less than 1% of a monolayer of NO sub(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 sub(y) . In addition to the NO sub(y) measurements, independent HNO sub(3) measurements were used to determine total HNO sub(3) . Although quantitative uncertainties do not allow to completely rule out that the NO sub(y) uptake on ice was limited by total HNO sub(3) , the combined NO sub(y) and HNO sub(3) data suggest that there was low uptake of NO sub(y) on ice despite abundant HNO sub(3) in the gas phase. Model studies indicate, that the most likely explanation of the measured nitric acid partitioning is given by HNO sub(3) in ternary solution droplets coexisting with almost HNO sub(3) free ice in the same air mass.