Oxidized nitrogen and ozone interaction with forests. I: Experimental observations and analysis of exchange with Douglas fir

Abstract

AbstractMeasurements of NOx and O3 exchange to and from a forest canopy are described with reference to canopy and ecosystem processes leading to emission of NO2. The soil below the forested measurement location is shown to be a net source of reactive oxidized nitrogen (as NO). It is shown that a large fraction (around 58% on average) of the emitted NO escapes out of the trunk space to react within and above the canopy.Fluxes of NO2 above the canopy are bi‐directional, with large fluxes and most emission events taking place during the day. Deposition is shown to take place without significant changes in the vertical flux; however there appears to be rapid chemical reaction leading to significant flux divergence taking place during emission events. Above the top of the canopy (22 m), changes of 200% in the NO2 flux between 25 and 35 m are not uncommon.Ozone deposition to the forest system has been investigated. The flux is often related to stomatal deposition, but there are periods when large non‐stomatal fluxes are observed. At night there is a large vertical divergence in the ozone flux above the canopy, probably related to reaction with NO from the forest soil. The reaction of O3 with volatile organic compounds is found, in general, to be negligible with respect to ozone concentrations and fluxes.Ozone is found to deposit mainly to leaf stomata during the day, with a small but not insignificant contribution from NO reaction. However, the nocturnal ozone budget for the forest changes significantly, with most of the measured deposition accounted for by soil uptake and reaction with NO. The resultant problems related to inadequate treatment of air chemistry when interpreting O3 fluxes in high NO emission environments are identified. Copyright © 2004 Royal Meteorological Society.