Use of a detailed model to study the exchange of NOx and O3 above and below a deciduous canopy

A 1D deposition model derived from work by Baldocchi (1988, Atmospheric Environment 22, 869–884), has been used to interpret the exchange of NO, NO2 and O3 measured over an orchard (Walton et al., accepted Atmospheric Environment). Where possible, direct measurements from the orchard were used in favour of existing model parameterisations. The model incorporates NOx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. (1995, TNO Report, TNOMWR 951148), 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-CCPNOBO2) is related to the NO soil emission (FNOsoil) and the canopy resistance (Rc). This offers an explanation of the bi-directional nature of NO2 flux measurements above forest canopies where no biogenic emission occurs. The CCPNO2 can be approximated by αFNOsoil RCO3, where a is a dimensionless constant relating to the production of NO2 beneath the canopy ( = 0.5 for the Norwood Park orchard). This gives a daytime CCPNO2 of 2 ppbv with a nocturnal value of around 4 ppbv. It was concluded that the orchard was not a strong source of NOx as the NO2 concentrations were often measured as being higher than the calculated CCPNO,. However, the emission of NO2 from forest canopies may be significant in less polluted areas of Europe.