Environmental context. Nitrous acid (HNO₂) is an important source of the hydroxyl radical (OH^•), the most important daytime oxidising species that contributes to the formation of ozone as well as of other secondary pollutants in the troposphere. Understanding the sources and sinks of HNO₂ is of crucial interest for accurately modelling the chemical composition of the troposphere and predicting future trace gas concentrations.

Abstract. Nitrous acid and several other atmospheric components and variables were continuously measured during complex field experiments at seven different suburban and rural sites in Europe. HNO₂ is mainly formed by heterogeneous processes and is often accumulated in the nighttime boundary layer. Our results confirm that the photolysis of HNO₂ is an important source of the hydroxyl radical, not only in the early morning hours but also throughout the entire day, and is often comparable with the contribution of ozone and formaldehyde photolysis. At all research sites unexpectedly high HNO₂ mixing ratios were observed during the daytime (up to several hundred ppt, or pmol mol^–1). Moreover, surprisingly, the HNO₂ mixing ratio at the three mountain sites often showed a broad maximum or several distinct peaks at midday and lower mixing ratios during the night. Assuming a quickly established photo-equilibrium between the known significant gas phase reactions, only a few ppt HNO₂ should be present around noon. The ratio of known sources to sinks indicates a missing daytime HNO₂ source of 160–2600 ppt h^–1 to make up the balance. Based on these values and on production mechanisms proposed in the literature we hypothesise that the daytime mixing ratio levels may only be explained by a fast electron transfer onto adsorbed NO₂.