EVIDENCE FOR A SURFACE SOURCE OF ATMOSPHERIC NITROUS-ACID

Measurements of surface fluxes of HONO (and for comparison HNO3) made above grassland in eastern England by the gradient technique are reported. The HONO data divide equally between periods with net upward and net downward fluxes. The occurrence of upward fluxes is associated with NO2 concentrations in excess of ca. 10 ppb and the data are interpreted in terms of the net effect of two processes. On the one hand, HONO is dry deposited with considerable efficiency (deposition velocities 0.2-1.7 cm s-1) whilst on the other, HONO is formed at the surface by heterogeneous reactions of NO2. At low NO2 concentrations the deposition process dominates, whilst with high NO2 there is a net release of HONO to the atmosphere. The magnitudes of upward fluxes of HONO are consistent with production rates of HONO measured in the atmosphere in this and other studies, and it appears that heterogeneous production at the surface might account for the hitherto unidentified major source of atmospheric HONO.