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

Abstract. Vertical mixing ratio profiles of nitrous acid (HONO) were measured in a clearing and on the forest floor in a rural forest environment. For the forest floor, HONO was found to predominantly deposit, whereas for the clearing, net deposition dominated only during nighttime and net emissions were observed during daytime. For selected days, net fluxes of HONO were calculated from the measured profiles using the aerodynamic gradient 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 previous observations. These fluxes were compared to the strengths of postulated HONO sources. Laboratory measurements of different soil samples from both sites revealed an upper limit for soil biogenic HONO emission fluxes of 0.025 nmol m⁻² s⁻¹. HONO formation by light-induced NO₂ conversion was calculated to be below 0.03 nmol m⁻² s⁻¹ for the investigated days, which is comparable to the potential soil fluxes. Due to light saturation at low irradiance, this reaction pathway was largely found to be independent of light intensity, i.e. it was only dependent on ambient NO₂.

We used three different approaches based on measured leaf nitrate loadings for calculating HONO formation from HNO₃ photolysis. While the first two approaches based on empirical HONO formation rates yielded values in the same order of magnitude as the estimated fluxes, the third approach based on available kinetic data of the postulated pathway failed to produce noticeable amounts of HONO. Estimates based on reported cross sections of adsorbed HNO₃ indicate that the lifetime of adsorbed HNO₃ was only about 15 min, which would imply a substantial renoxification. Although the photolysis of HNO₃ was significantly enhanced at the surface, the subsequent light-induced conversion of the photolysis product NO₂ did not produce considerable amounts of HONO. Consequently, this reaction might occur via an alternative mechanism.

By explicitly calculating HONO formation based on available kinetic data and simple parameterizations, we showed that (a) for low NO_x the light-induced conversion of NO₂ on humic acids is already light saturated by the early morning, (b) HONO formation from photolysis of adsorbed HNO₃ appears to proceed via an alternative mechanism and (c) estimates of HONO emissions from soil are very sensitive to mass transfer and acidic soils do not necessarily favour HONO emissions.