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The gaseous exchange of ozone at terrestrial surfaces: Non-stomatal deposition to grassland

By M. Coyle

Abstract

Tropospheric ozone has detrimental effects on the health of vegetation and humans. It is also a direct greenhouse gas and plays a role in the chemistry of many other gases. Current measurements and model results show concentrations have increased and this is likely to continue. Thus it is important to understand how ozone is generated and processed in the atmosphere. New critical levels that use accumulated stomatal uptake rather than atmospheric concentration have recently been set by the UNECE to quantify the effect of ozone on vegetation. There are models available that can estimate the stornatal flux but the non-stomatal component is not well described. The total flux of ozone was measured over grassland in central Scotland during 2001 to 2004 using micrometeorological methods and the uptake by stomata estimated by similarity with the water-vapour flux. The non-stomata1 component is estimated using Rns = (Rc-l - Rs-l)-1 where Rns = non-stomatal surface resistance to ozone deposition, Rc = total canopy resistance, Rs = stomatal resistance. The dataset is used to examine the processes controlling deposition at this site and a new parameterisation for the non-stornatal component is described. There are two forms of model commonly used to estimate stomatal resistance: Iarvis type multiplicative models, and Ball-Berry photosynthesis based approaches. Both methods are employed to estimate bulk-canopy stomatal conductance and so gap-fill the measured time series. The resulting estimates of Rns were found to decrease with increasing surface temperature, solar radiation, humidity and friction velocity. The response of Rns to these variables was also dependant on surface wetness with deposition being enhanced over wet surfaces compared to dry, upto temperatures of ~20°C. New parameterisations based on these results are developed and implemented in a total-deposition model. The results are compared to the measured data and estimates from both a standard type of model and one utilising an existing parameterisation for Rns based on humidity. The parameterisation was found to improve estimates of total canopy resistance for wet surfaces compared to both the other models, although for dry surfaces it did not perform as well as the humidity based model

Topics: Ecology and Environment, Atmospheric Sciences
Year: 2006
OAI identifier: oai:nora.nerc.ac.uk:4016
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