Surface/atmosphere exchange and chemical interaction of gases and aerosols over oilseed rape

Measurements of NH3, HCl, HNO3 and HNO2 gas as well as NH4+, NO3−, Cl− and SO42− aerosol are used to investigate their surface exchange fluxes and the potential for gas–particle interactions at a clean coastal Scottish site. Mean concentrations of HNO3 and HCl were small at 0.68 and 0.32 μg m−3, respectively. At relative humidities (h)<85% measured gas concentration products (Km) were smaller than the predicted dissociation constants (Ke), suggesting potential for aerosol evaporation, but at high h, Ke of NH4Cl was exceeded at the mean canopy height. Above the canopy, small aerosol concentrations resulted in estimated chemical time-scales of >3 min. Thus, chemical reactions should not have affected NH3 flux measurements by aerodynamic gradient methods (AGMs), except for very low turbulence when AGM is not applicable. Within the canopy, however, the diffusive transport provided enough time for NH4Cl to be generated. This was substantiated by measurements of NH4+ emission and high Cl− aerosol concentrations within the canopy. Micrometeorological measurements above the canopy indicated that gaseous Cl compounds were emitted for most of the time, and this was supported by the source/sink distributions of gaseous and aerosol Cl compounds calculated from in-canopy profiles as well as high apoplastic Cl− concentrations. Although emission of CH3Cl has been reported for other Brassica species, an unrealistically large emission would be necessary to cause the observed above-canopy gradients. Emission of HCl liberated from unidentified water pools of high Cl− or leaf surface reactions is a more likely source of gaseous Cl compounds.