Laboratory studies reported here and in previous work show that the reaction of NO(g) with surface adsorbed HNO3 may be a significant source of HONO in polluted urban atmospheres. If these laboratory studies can be extrapolated to ambient conditions, this heterogeneous reaction may generate HONO to about the same extent as the hydrolysis of NO2 on surfaces, which is greater than the heterogeneous reaction of NO, NO2 and water. It may also be involved in generating HONO in snowpacks, and important in reconciling the discrepancy between measured and modeled HNO3/NOx ratios in the troposphere. ¿ 2000 American Geophysical Union