This award is for support for a two year program to study the origin of nitrate in the atmosphere and surface snow at Summit, Greenland. Preliminary results from unfunded pilot studies suggest that nitric acid (HNO3) is not the only, and may not even be the major nitrogen species contributing to nitrate (NO3-) in summer-time snow at Summit. Peroxy acetyl nitrate and one or more alkyl nitrate species seem to be the most likely candidates, and they may also be contributing to the elevated concentrations of organic acids observed just above the snow surface at Summit. Gradient measurements of soluble acidic gases will be made with 2 mist chamber samplers at different heights in the 1995 and 1996 seasons at Summit. These measurements will be complemented with intensive experiments comparing mist chamber samples to nylon filter samples and the use of nylon filters on the mist chamber inlet, to quantify soluble nitrogen species besides nitric acid. The primary goal of this study will be to enable improved interpretation of the nitrate records in polar ice, but additional insights into reactive nitrogen chemistry over snow covered surfaces are also anticipated.