The role of nitrous acid, HONO, as a precursor for hydroxyl radicals in polluted urban air has been recognized for many years. The chemical processes leading to the formation of HONO are, however, still not well understood. Laboratory studies show that HONO formation occurs primarily on surfaces and is first order in NO2. Water also plays an important role in the conversion process. While the latter has been shown in the laboratory, little information is available regarding the influence of water on the NO2 and HONO chemistry in the real atmosphere. Here we present nocturnal DOAS measurements of HONO and NO2 from three field experiments. The observations show that /2> ratios between 10 and 30% relative humidity (RH) do not exceed 0.04, while values of up to 0.09 were observed at higher RH. These results are analyzed by interpreting the observed maximum /2> ratio at a given relative humidity as the pseudo steady state (PSS) between the heterogeneous NO2 to HONO conversion and the HONO loss on surfaces. Theoretical considerations show that the /2> ratio at the PSS is equal to the ratio of the heterogeneous NO2 to HONO conversion coefficient and the reactive HONO uptake coefficient, thus showing that these processes depend on the RH. This result implies that RH has to be considered in the parameterization of HONO formation in air pollution models.