Heterogeneous production of nitrous acid on soot in polluted air masses

Abstract

Polluted air masses are characterized by high concentrations of oxidized nitrogen compounds which are involved in photochemical smog and ozone formation. The OH radical is a key species in these oxidation processes. The photolysis of nitrous acid (HNO₂), in the morning, leads to the direct formation of the OH radical and may therefore contribute significantly to the initiation of the daytime photochemistry in the polluted planetary boundary layer. But the formation of nitrous acid remains poorly understood: experimental studies imply that a suggested heterogeneous formation process involving NO₂ is not efficient enough to explain the observed night-time build-up of HNO₂ in polluted air masses¹. Here we describe kinetic investigations which indicate that the heterogeneous production of HNO₂ from NO₂ on suspended soot particles proceeds 10⁵ to 10⁷ times faster than on previously studied surfaces. We therefore propose that the interaction between NO₂ and soot particles may account for the high concentrations of HNO₂ in air masses where combustion sources contribute to air pollution by soot and NO_x emissions. We believe that the observed HNO₂ formation results from the reduction of NO₂ in the presence of water by C–O and C–H groups in the soot. Although prolonged exposure to oxidizing agents in the atmosphere is likely to affect the chemical activity of these groups, our observations nevertheless suggest that fresh soot may have a considerable effect on the chemical reactions occurring in polluted air.