Heterogeneous NO2 conversion processes on acid surfaces: possible atmospheric implications

The heterogeneous conversion of NO2 on water/sulphuric acid surfaces was studied in a quartz reactor and a bubbler system. The NO2 decay and the HONO formation are first order in [NO2] and are limited by an uptake coefficient γ≈10-6. It was observed that HONO formation on acid/water surfaces of moderate acidity only occurs via the reaction 2NO2+H2O→HONO+HNO3. The involvement of NO on the HONO formation is of minor importance. The HONO formation rates calculated from the results of the present study at high aerosol load are of the same order of magnitude as the observed HONO formation rates in the troposphere. Possible HONO formation on stratospheric aerosol by the parallel reaction of NO2 with sulphuric acid (cH2SO4>60 wt%) could explain, at least in part, the observed stratospheric OH radical formation in the morning shortly after sunrise.