Nitrous acid is a significant photochemical precursor of the hydroxyl radical^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}, the key oxidant in the degradation of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atmospheric measurements^{7, 10, 11, 12, 13, 14, 15, 16, 17} revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that reduction of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces containing humic acid exhibit an organic surface photochemistry that produces reductive surface species, which react selectively with nitrogen dioxide. The observed rate of nitrous acid formation could explain the recently observed high daytime concentrations of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60 per cent of the integrated hydroxyl radical source strengths^{3, 6, 7, 8, 9, 10, 11, 12, 13}. We suggest that this photo-induced nitrous acid production on humic acid could have a potentially significant impact on the chemistry of the lowermost troposphere.

1. Paul Scherrer Institut, Laboratory of Radio- and Environmental Chemistry, CH-5232 Villigen, Switzerland
   
2. Bergische Universität Wuppertal, Physikalische Chemie/FB C, D-42097 Wuppertal, Germany
   
3. Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern, Switzerland
   
4. Laboratoire d'Application de la Chimie à l'Environnement (UCBL-CNRS), F-69622 Villeurbanne, France

Correspondence to: Konrad Stemmler¹ Correspondence and requests for materials should be addressed to K.S. (Email: konrad.stemmler@psi.ch).