A new method for using 18O to trace ozone deposition

Isotopically labelled ozone (¹⁸O₃) is an ideal tool to study the deposition of O₃ to plants and soil, but no studies have made use of it due to the technical difficulties in producing isotopically enriched ozone. For ¹⁸O₃ 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 O₃, 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 ¹⁸O₃. Following the purification and release of O₃, about half (by volume) of the ¹⁸O is present in the form of O₃. This means that for a given release of ¹⁸O₃ into the fumigation system, a roughly identical volume of ¹⁸O₂ is released. However, the small volume of this concurrent ¹⁸O₂ 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 ¹⁸O as an isotopic tracer in O₃ fumigations by exposing dry soil to 100 nmol mol^{?1 18}O₃ for periods ranging from 1 to 11 h. The ¹⁸O tracer accumulation in soil samples is measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and the results show a linear increase in ¹⁸O/¹⁶O isotope ratio over time, with significant differences detectable after 1 h of exposure. The apparatus is adapted for use with fumigation chambers sustaining flow rates of 1 m³ min^?1 for up to 12 h, but simple modifications now allow larger quantities of O₃ to be stored and continuously released (e.g. for use with open‐top chambers or FACE facilities). Copyright © 2009 John Wiley & Sons, Ltd.