Abstract
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 NOx 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.
Highlights
Gaseous nitrous acid (HONO) may contribute up to ∼ 80 % to the primary formation of hydroxyl radicals (OH), which play a key role in the degradation of most air pollutants (Kleffmann et al, 2005, Kleffmann, 2007; Volkamer et al, 2010)
By explicitly calculating HONO formation based on available kinetic data and simple parameterizations, we showed that (a) for low nitrogen oxides (NOx) 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
The radiation and photolysis frequencies at the forest floor are a factor of 10 to 40 lower than above the canopy depending on the time of day and canopy structure (Sörgel et al, 2011b)
Summary
Gaseous nitrous acid (HONO) may contribute up to ∼ 80 % to the primary formation of hydroxyl radicals (OH), which play a key role in the degradation of most air pollutants (Kleffmann et al, 2005, Kleffmann, 2007; Volkamer et al, 2010). In contrast to HONO formation observed on natural surfaces (Zhou et al, 2003, 2011), HONO has not been detected as a primary reaction product of HNO3 photolysis in laboratory studies up to now (Zhu et al, 2010, Schuttlefield et al, 2008, Rubasinghege and Grassian, 2009; Abida et al, 2012). Our setup can be used to derive estimates of the surface fluxes of HONO by the aerodynamic gradient method These fluxes are compared to best estimates of HONO source strengths of three proposed mechanisms derived from measured quantities: (a) soil HONO emissions, (b) photosensitized NO2 conversion, and (c) HNO3 photolysis
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