Gas-particle interactions above a Dutch heathland: I. Surface exchange fluxes of NH<sub>3</sub>, SO<sub>2</sub>, HNO<sub>3</sub> and HCl

Abstract

Abstract. A field measurement campaign was carried out over a Dutch heathland to investigate the effect of gas-toparticle conversion and ammonium aerosol evaporation on surface/atmosphere fluxes of ammonia and related species. Continuous micrometeorological measurements of the surface exchange of NH 3 , SO 2 , HNO 3 and HCl were made and are analyzed here with regard to average fluxes, deposition velocities (V d ), canopy resistances (R c ) and canopy compensation point for NH 3 . Gradients of SO 2 , HNO 3 and HCl were measured with a novel wet-denuder system with online anion chromatography. Measurements of HNO 3 and HCl indicate an R c of 100 to 200 s m −1 during warm daytime periods, probably at least partly due to non-zero acid partial pressures above NH 4 NO 3 and NH 4 Cl on the leaf surfaces. Although it is likely that this observation is exacerbated by the effect of the evaporation of airborne NH + 4 on the gradient measurements, the findings nevertheless add to the growing evidence that HNO 3 and HCl are not always deposited at the maximum rate. Ammonia (NH 3 ) fluxes show mainly deposition, with some periods of significant daytime emission. The net exchange could be reproduced both with an R c model (deposition fluxes only) using resistance parameterizations from former measurements, as well as with the canopy compensation point model, using parameterizations derived from the measurements. The apoplastic ratio of ammonium and hydrogen concentration ( s =[NH + 4 ]/[H + ]) of 1200 estimated from the measurements is large for semi-natural vegetation, but smaller than indicated by previous measurements at this site.

Exchange of reactive nitrogen compounds: concentrations and fluxes of total ammonium and total nitrate above a spruce canopy

“…For the determination of total ammonium and total nitrate fluxes this is not of relevance, but it does affect flux determination of individual gaseous and/or particulate compounds (NH 3 , HNO 3 , NH + 4 , NO − 3 ) as phase changes may lead to flux divergence (e.g. Nemitz et al, 2004a;Brost et al, 1988;Huebert et al, 1988). However, as long as the characteristic time scale of chemical transformation is large in comparison to the turbulent timescale, fluxes of compounds that underlie rapid chemical transformation may be determined with sufficient accuracy when treating them as not-reactive (De Arellano and Duynkerke, 1992;Nemitz et al, 2004b).…”

“…Farquhar et al, 1980) and may not always be applied for other ecosystems. The dry deposition of particles, such as particulate NH + 4 and NO − 3 is typically very different to the dry deposition of gaseous species (Nemitz et al, 2004a). Generally, deposition velocities depend on aerosol particle size and roughness of the underlying surface and they are about one order of magnitude smaller than those of gaseous compounds (Gallagher et al, , 2002.…”

“…While R a and R b can be calculated from measured micrometeorological quantities, surface related parameters like R c or C 0 are derived using empirical parameterizations or are deduced from other studies (cf. Nemitz et al, 2004a;Trebs et al, 2006;Sutton et al, 2000). However, parameterisations of the individual resistances are mostly derived for flat homogeneous terrain and low vegetation and may not be adequate for complex sites, like highly structured, hilly, or forested sites (Hertel et al, 2006;Wesely and Hicks, 2000;Andersen and Hovmand, 1999).…”

“…The application of the flux-gradient-similarity theory (see below) presumes that vertical fluxes of the measured compounds are constant with height within the atmospheric surface layer (Dyer and Hicks, 1970), which implies that they are considered chemically non-reactive tracers (e.g., Trebs et al, 2006 Sutton, 2004;Nemitz et al, 2004a). The sum of both phases, total ammonium (tot-NH + 4 ) and total nitrate (tot-NO − 3 ), are, however, conservative quantities in this respect Brost et al, 1988).…”

Review and parameterisation of bi-directional ammonia exchange between vegetation and the atmosphere

“…The cuticles are mainly a sink for NH 3 (van Hove et al, 1989). As ammonia is soluble, it can deposit rapidly to leaf cuticles (Sutton et al, 1993a;Duyzer, 1994), though cuticular uptake tends to saturate at high atmospheric NH 3 concentrations (Jones et al, 2007b), and can be enhanced in the presence of atmospheric acids (van Hove et al, 1989;Erisman and Wyers, 1993;Nemitz et al, 2001). The consequence is that the resistance for NH 3 deposition to plant surfaces are controlled by many factors including the availability of moisture, leaf area and atmospheric chemistry.…”