Deposition of atmospheric nitrous acid on alkaline snow surfaces

The photolysis of atmospheric nitrous acid (HONO) is a significant source of OH radicals in remote and Polar Regions. HONO is produced in/on snow surfaces in a photochemical reaction from nitrate ions. In an attempt to quantify the production of HONO at a snow covered mid-latitude location we made measurements of HONO fluxes for a 10-day period at the Mt. Cimone (MTC) research station in the Italian northern Apennines (2165 m asl) during March 2004. Production fluxes under normal background conditions were small, and reached maximum values of 20 nmol m-2 h-1 only two occasions. However, during a transport event of Saharan dust to MTC we observed deposition fluxes of up to 120 nmol m-2 h-1of HONO on to the snow surface. The deposited Sahara dust had rendered the surface snow alkaline, so that large amounts of acids could be absorbed from the atmosphere.