The source of nitrous acid, HONO, in the troposphere remains uncertain, even after two decades of research. It is currently believed that HONO is formed by heterogeneous conversion of NO2 on either the ground or the aerosol surface. While this conversion has been studied in the laboratory, few atmospheric studies have been reported. Here we present the first simultaneous determination of the vertical gradients and fluxes of HONO, its precursor NO2, and SO2 over a flat grass surface in the polluted atmosphere. The measurements were performed in Milan, Italy, during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono (LOOP/PIPAPO) study in summer 1998, using differential optical absorption spectroscopy. While deposition of NO2 onto the ground was frequently observed, heterogeneous HONO formation was much smaller than expected. We can explain our observation by a mechanism that consists of a combination of NO2 and HONO deposition, and a heterogeneous conversion of NO2 to HONO on the ground. The compensation point for deposition and formation of HONO is characterized by a HONO/NO2 ratio of ~0.03, indicating that only one HONO molecule is released into the gas phase for every 33 NO2 molecules deposited. Our measurements also show that direct emission of HONO is an important source in strongly polluted areas.