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Vertical mixing ratio profiles of nitrous acid (HONO) were measured in aclearing and on the forest floor in a rural forest environment. For theforest floor, HONO was found to predominantly deposit, whereas for the clearing, netdeposition dominated only during nighttime and netemissions were observed during daytime. For selected days, net fluxes ofHONO were calculated from the measured profiles using the aerodynamicgradient method. The emission fluxes were in the range of 0.02 to 0.07 nmol m−2 s−1 and thus were in the lower range of previousobservations. These fluxes were compared to the strengths of postulated HONOsources. Laboratory measurements of different soil samples from both sitesrevealed an upper limit for soil biogenic HONO emission fluxes of 0.025 nmol m−2 s−1. HONO formation by light-induced NO2 conversion wascalculated to be below 0.03 nmol m−2 s−1 for the investigateddays, which is comparable to the potential soil fluxes. Due to lightsaturation at low irradiance, this reaction pathway was largely found to beindependent of light intensity, i.e. it was only dependent on ambientNO2. We used three different approaches based on measured leaf nitrate loadingsfor calculating HONO formation from HNO3 photolysis. While the firsttwo approaches based on empirical HONO formation rates yielded values in thesame order of magnitude as the estimated fluxes, the third approach based onavailable kinetic data of the postulated pathway failed to producenoticeable amounts of HONO. Estimates based on reported cross sections ofadsorbed HNO3 indicate that the lifetime of adsorbed HNO3 wasonly about 15 min, which would imply a substantial renoxification. Althoughthe photolysis of HNO3 was significantly enhanced at the surface, thesubsequent light-induced conversion of the photolysis product NO2 didnot produce considerable amounts of HONO. Consequently, this reaction mightoccur via an alternative mechanism. By explicitly calculating HONO formation based on available kinetic dataand simple parameterizations, we showed that (a) for low NOx the light-induced conversion of NO2 on humic acids is already light saturatedby the early morning, (b) HONO formation from photolysis of adsorbed HNO3appears to proceed via an alternative mechanism and (c) estimates of HONOemissions from soil are very sensitive to mass transfer and acidic soils donot necessarily favour HONO emissions

A comparison of measured HONO uptake and release with calculated source strengths in a heterogeneous forest environment