A climatological view of HNO3 partitioning in cirrus clouds

A new in situ climatology of cirrus ice water content (IWC) is used, together with observed molar ratios of HNO3/H2O in cirrus ice particles, to estimate the range of HNO3 content in cirrus ice in the temperature interval 185-240 K. We find that nearly over the complete temperature range HNO3 percentages in ice between 0.01 and 100% are possible in cirrus clouds and that IWC is a major parameter determining the content of HNO3 in ice at given temperatures. Considering average conditions, the HNO3 content increases with decreasing temperature from 1% to about 10% in the range 240-200 K. For colder ice clouds, the average HNO3 content again decreases down to 6%. At higher temperatures, less efficient HNO3 uptake limits the HNO3 content in cirrus ice, while at low temperatures small IWCs permit only little HNO3 in ice, thus causing the convex-shaped average HNO3 content curve. The highest HNO3 content is expected in tropical ice clouds with very large IWCs, especially at temperatures between 190 and 210 K. Thus, tropical cirrus clouds show the highest potential to vertically redistribute HNO3. Copyright (C) 2008 Royal Meteorological Society