Diurnal fluxes of HONO above a crop rotation

Abstract. Nitrous acid (HONO) fluxes were measured above an agricultural field site near Paris during different seasons. Above bare soil, different crops were measured using the aerodynamic gradient (AG) method. Two LOPAPs (LOng Path Absorption Photometer) were used to determine the HONO gradients between two heights. During daytime mainly positive HONO fluxes were observed, which showed strong correlation with the product of the NO₂ concentration and the long wavelength UV light intensity, expressed by the photolysis frequency J(NO₂). These results are consistent with HONO formation by photosensitized heterogeneous conversion of NO₂ on soil surfaces as observed in recent laboratory studies. An additional influence of the soil temperature on the HONO flux can be explained by the temperature-dependent HONO adsorption on the soil surface. A parameterization of the HONO flux at this location with NO₂ concentration, J(NO₂), soil temperature and humidity fits reasonably well all flux observations at this location.