DIURNAL CYCLE OF O-3 AND MONOTERPENES IN A CONIFEROUS FOREST - IMPORTANCE OF ATMOSPHERIC STABILITY, SURFACE EXCHANGE, AND CHEMISTRY

A one-dimensional model of turbulent diffusion and chemistry has been applied to evaluate measurements of the diurnal variation Of 03 and monoterpenes in a coniferous forest in Sweden. The role of photochemical, meteorological, and surface exchange processes was investigated by studying days characteristic of neutral and stable/unstable conditions, respectively. Atmospheric stability, mixing height, and eddy diffusivities were estimated from acoustic soundings (SODAR) measurements. The atmospheric concentrations and surface exchange rates Of O3, monoterpenes and NO(x) were taken from measurements at the site, using chamber techniques for the flux measurements. The chemical scheme applied in the model included 45 reactions representative of a clean air situation with rather small impact of anthropogenic hydrocarbon emissions. The model calculations showed the predominant role of turbulent mixing in the observed diurnal variation Of 03 and monoterpenes. The reaction with OH accounted for 50-60% of the terpene loss over a 24-hour period with a well-mixed boundary layer and about 40% when nighttime conditions were stable. Increasing the background 03 concentration to 80-90 ppbv increased the relative importance of the terpene + 03 reaction to equal that of the OH reaction. The terpene chemical lifetime was found to be shortest, less than 1 hour, under neutral nighttime conditions when NO3 concentrations were high. Under stable nighttime conditions, the lower boundary layer became chemically less active due to the depletion Of 03 and NO3. Terpene reactions were found to be significant nighttime sinks for 03 and N03 under stable conditions and during periods or at sites of nocturnal terpene emission rates greater than 150 mug/M2 h. An increase in NO(x) concentrations from 0.5 ppbv to 1 and then 5 ppbv, changed 03 concentrations first up and then down, by not more than 1 ppbv. At terpene emission rates of 50-100 mug (M2 h) -1, the terpene + OH reaction caused less than 5% of the OH loss near the surface, while at a rate of 1500 mug (m2 h)-1, it accounted for up to 30-40% of OH loss. The impact decreased rapidly with height. The daytime terpene+NO3 reaction was found to be a minor pathway for NO(x) in comparison to the OH + NO2 reaction.