Methodology for direct field measurements of ozone flux to foliage with shoot chambers

Values of ozone (O-3) flux measured on the shoot scale by enclosure techniques can provide detailed information about the partition and seasonality of the removal pathways on the foliage. However, O-3 reactivity makes such measurements potentially inaccurate, particularly in field conditions where ambient concentration is low. Special consideration should be given to the losses on the chamber wall material, which can rival in magnitude and dynamics the measure of shoot removal. We propose a way of taking into account this dynamic behaviour in the context of field monitoring with simple chambers based on dynamic principles. We accommodated an existing field monitoring gas exchange system for CO2 and water vapour to measure simultaneous, but independent, O-3 fluxes. We report here the system's performance based on the analysis of 19-month data. The background noise of the measurements is mainly imposed by the analyser and is around 2 mug m(-3). The resolution of the system is in the order of 10(-2)mug m(-2) s(-1). The disturbance due to wall-losses was corrected thoroughly but it was mostly needed when the shoot activity was low and the mass flow signals produced by the shoots and the walls came close to each other. In spite of the disfavouring combination of chamber material, O-3 reactivity and low concentrations, we have successfully followed diurnal and seasonal trends Of O-3 removal in Scots pine foliage. (C) 2002 Elsevier Science Ltd. All rights reserved.