Seasonal Trends in Snow Photochemistry Impacts on Soluble Acidic Gases at Summit, Greenland

Previous work at Summit has established that mixing ratios of HONO, HCOOH and CH₃COOH are greatly enhanced in firn air, and that snow to air fluxes support enhanced mixing ratios in the bottom of the atmosphere above the ice sheet. Experimental manipulations have shown that the processes producing the firn air enhancements of these acidic gases respond quickly to changes in light and temperature, but it has been difficult to separate photolytic production from thermal desorption. In March and April, 2004, an intensive sampling campaign was conducted during the spring season when solar elevation and temperatures were both naturally increasing dramatically. Comparison between the 2004 campaign and results from summer 2003 indicate that production of HONO is already active by the end of March and steadily increased through May, but had not reached peak summer intensity by early May. In contrast, the mixing ratios of the carboxylic acids were greatly suppressed through the third week of April, but then increased rapidly to levels characteristic of full summer. We are exploring relationships between temperature and actinic fluxes, in both the air and snow, and the mixing ratios of these gases. This natural experiment yielded non-synchronous changes in light and temperature that may yield greater insight into the relative importance of photochemical production and thermal desorption as sources of these acids in sunlit snow.