Gas-particle interactions above a Dutch heathland: I. Surface exchange fluxes of NH3, SO2, HNO3 and HCl

[摘要] A field measurement campaign was carried out over a Dutch heathland toinvestigate the effect of gas-to-particle conversion and ammonium aerosolevaporation on surface/atmosphere fluxes of ammonia and related species.Continuous micrometeorological measurements of the surface exchange ofNH₃, SO₂, HNO₃ and HCl were made and are analyzed here withregard to average fluxes, deposition velocities (V_d), canopy resistances(R_c) and canopy compensation point for NH₃. Gradients of SO₂,HNO₃ and HCl were measured with a novel wet-denuder system with onlineanion chromatography. Measurements of HNO₃ and HCl indicate anR_c of 100 to 200 s m^-1 during warm daytime periods, probably atleast partly due to non-zero acid partial pressures above NH₄NO₃ and NH₄Cl on the leaf surfaces. Although it is likely that thisobservation is exacerbated by the effect of the evaporation of airborneNH₄⁺ on the gradient measurements, the findings nevertheless addto the growing evidence that HNO₃ and HCl are not always deposited atthe maximum rate. Ammonia (NH₃) fluxes show mainly deposition, withsome periods of significant daytime emission. The net exchange could bereproduced both with an R_c model (deposition fluxes only) usingresistance parameterizations from former measurements, as well as with thecanopy compensation point model, using parameterizations derived from themeasurements. The apoplastic ratio of ammonium and hydrogen concentration(Γ_s=[NH₄⁺]/[H⁺]) of 1200 estimated from themeasurements is large for semi-natural vegetation, but smaller thanindicated by previous measurements at this site.

[学科分类] 大气科学