Use of a detailed model to study the exchange of NO(x) and O3 above and below a deciduous canopy

A 1D deposition model derived from work by Baldocchi, has been used to interpret the exchange of NO, NO2 and O3 measured over an orchard. Where possible, direct measurements from the orchard were used in favour of existing model parameterisations. The model incorporates NO(x)-O3 chemistry and an emission of NO at the air-soil interface. Model results showed good agreement with measured NO2 and O3 fluxes above the canopy in terms of trends and directions. Similar to Duyzer et al., the model showed a net upward flux of NO2 was observed above the canopy at low ambient NO2 concentrations. This was related to the below canopy chemical production of NO2 (via the NO + O3 reaction) rather than a plant physiological effect. The concentration below which emission from within the canopy takes place (the equilibrium concentration or the bulk canopy compensation point - CCP(NO2) is related to the NO soil emission (F(NOsoil) and the canopy resistance (R(C)). This offers an explanation of the bi-directional nature of NO2 flux measurements above forest canopies where no biogenic emission occurs. The CCP(NO2) can be approximated by αF(NOsoil) R(CO3), where α is a dimensionless constant relating to the production of NO2 beneath the canopy (= 0.5 for the Norwood Park orchard). This gives a daytime CCP(NO2) of 2 ppbv with a nocturnal value of around 4 ppbv. It was concluded that the orchard was not a strong source of NO(x) as the NO2 concentrations were often measured as being higher than the calculated CCP(NO2). However, the emission of NO2 from forest canopies may be significant in less polluted areas of Europe