A new method for using 18O to trace ozone deposition

Isotopically labelled ozone (O-18(3)) is an ideal tool to study the deposition Of O-3 to plants and soil, but no studies have made use of it due to the technical difficulties in producing isotopically enriched ozone. For O-18(3) to be used in fumigation experiments, it has to be purified and stored safely prior to fumigations, to ensure that the label is present predominantly in the form Of O-3, and to make efficient use of isotopically highly enriched oxygen. We present a simple apparatus that allows for the safe generation, purification, storage, and release Of O-18(3). Following the purification and release Of O-3, about half (by volume) of the O-18 is present in the form of O-3. This means that for a given release of O-18(3) into the fumigation system, a roughly identical volume of O-18(2) is released. However, the small volume of this concurrent O-18(2) release (100 nmol mol(-1) in our experiment) results in only a minor shift of the much larger atmospheric oxygen pool, with no detectable consequence for the isotopic enrichment of either soil or plant materials. We demonstrate here the feasibility of using O-18 as an isotopic tracer in O-3 fumigations by exposing dry soil to 100 nmol mol-1 O-18(3) for periods ranging from 1 to 11 h. The O-18 tracer accumulation in soil samples is measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and the results show a linear increase in O-18/O-16 isotope ratio over time, with significant differences detectable after 1h of exposure. The apparatus is adapted for use with fumigation chambers sustaining flow rates of 1m(3) min(-1) for up to 12h, but simple modifications now allow larger quantities of O-3 to be stored and continuously released (e.g. for use with open-top chambers or FACE facilities). Copyright (C) 2009 John Wiley & Sons, Ltd.