UV-induced NO_y emissions in gas-exchange chambers enclosing Scots pine shoots: an analysis on their origin and significance

Abstract

3 Raivonen, M. 2008. UV-induced NO y emissions in gas-exchange chambers enclosing Scots pine shoots: an analysis on their origin and significance. Dissertationes Forestales 71. 50 p. Available at http://www.metla.fi/dissertationes/df71.htm ABSTRACTIt is essential to have a thorough understanding of the sources and sinks of oxidized nitrogen (NO y ) in the atmosphere, since it has a strong influence on the tropospheric chemistry and the eutrophication of ecosystems. One unknown component in the balance of gaseous oxidized nitrogen is vegetation. Plants absorb nitrogenous species from the air via the stomata, but it is not clear whether plants can also emit them at low ambient concentrations. The possible emissions are small and difficult to measure.The aim of this thesis was to analyse an observation made in southern Finland at the SMEAR II station: solar ultraviolet radiation (UV) induced NO y emissions in chambers measuring the gas exchange of Scots pine (Pinus sylvestris L.) shoots. Both measuring and modelling approaches were used in the study. The measurements were performed under noncontrolled field conditions at low ambient NO y concentrations.The chamber blank i.e. artefact NO y emissions from the chamber walls, was dependent on the UV irradiance and increased with time after renewing the Teflon film on chamber surfaces. The contribution of each pine shoot to the total NO y emissions in the chambers was determined by testing whether the emissions decrease when the shoots are removed from their chambers. Emissions did decrease, but only when the chamber interior was exposed to UV radiation. It was concluded that also the pine shoots emit NO y . The possible effects of transpiration on the chamber blank are discussed in the summary part of the thesis, based on previously unpublished data.The possible processes underlying the UV-induced NO y emissions were reviewed. Surface reactions were more likely than metabolic processes. Photolysis of nitrate deposited on the needles may have generated the NO y emissions; the measurements supported this hypothesis. In that case, the emissions apparently would consist mainly of nitrogen dioxide (NO 2 ), nitric oxide (NO) and nitrous acid (HONO). Within studies on NO y exchange of plants, the gases most frequently studied are NO 2 and NO (=NO x ). In the present work, the implications of the emissions for the NO x exchange of pine were analysed with a model including both NO y emissions and NO y absorption. The model suggested that if the emissions exist, pines can act as an NO x source rather than a sink, even under relatively high ambient concentrations.