USE OF A DETAILED MODEL TO STUDY THE EXCHANGE OF NOX AND O-3 ABOVE AND BELOW A DECIDUOUS CANOPY

A 1D deposition model derived from work by Baldocchi (1988, Atmospheri c Environment 22, 869-884), has been used to interpret the exchange of NO, NO2 and O-3 measured over an orchard (Walton et al., accepted Atm ospheric Environment). Where possible, direct measurements from the or chard were used in favour of existing model parameterisations. The mod el incorporates NOx-O-3 chemistry and an emission of NO at the air-soi l interface. Model results showed good agreement with measured NO2 and O-3 fluxes above the canopy in terms of trends and directions. Simila r to Duyzer et al. (1995, TNO Report, TNO-MWR 951148), the model showe d a net upward flux of NO2 was observed above the canopy at low ambien t NO2 concentrations. This was related to the below canopy chemical pr oduction of NO2 (via the NO + O-3 reaction) rather than a plant physio logical effect. The concentration below which emission from within the canopy takes place (the equilibrium concentration or the bulk canopy compensation point-CCPNO2) is related to the NO soil emission (F-NOsoi l) and the canopy resistance (R-c). This offers an explanation of the bi-directional nature of NO2 flux measurements above forest canopies w here no biogenic emission occurs. The CCPNO2 can be approximated by al pha FNOsoilRCO3 where alpha is a dimensionless constant relating to th e production of NO2 beneath the canopy (=0.5 for the Norwood Park orch ard). 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 sour ce of NOx as the NO2 concentrations were often measured as being highe r than the calculated CCPNO2. However, the emission of NO2 from forest canopies may be significant in less polluted areas of Europe. (C) 199 7 Elsevier Science Ltd.