We measured HONO concentrations during a winter field campaign in a suburban site. We compared calculations with measurements and found missing source of HONO. Much higher HONO missing source is found when snow was present at ground. This evidences the existence of a large HONO formation in presence of snow. These findings may significantly affect the photochemistry in urban winter atmosphere. a b s t r a c t Nitrous acid measurements were carried out during the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) winter field campaign at the SIRTA observatory in Paris surroundings from the 20th of January to the 15th of February 2010. At the end of the campaign, significant snow events occurred leading to snow accumulation at the sampling site during the last days. These specific conditions gave the opportunity to examine the HONO budget with and without the presence of snow at ground. Much higher HONO sources were found for the days when the site was covered by snow. This provides evidence for the existence of a large snowpack source of HONO in mid-latitude polluted regions that needs to be investigated for a better understanding of wintertime photochemistry.