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

Measurements of NH 3, HCl, HNO 3 and HNO 2 gas as well as NH 4 +, NO 3 −, Cl − and SO 4 2− 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 HNO 3 and HCl were small at 0.68 and 0.32 μg m −3, respectively. At relative humidities ( h)3 min. Thus, chemical reactions should not have affected NH 3 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 NH 4Cl to be generated. This was substantiated by measurements of NH 4 + 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 CH 3Cl 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.