Measurement of the ~(17)O-excess (δ~(17)O) of tropospheric ozone using a nitrite-coated filter

摘要

The ~(17)O-excess (δ ~(17)O) of tropospheric ozone (O_3) serves as a useful marker in studies of atmospheric oxidation pathways; however, due to the complexity and expense of currently available analytical techniques, no systematic sampling campaign has yet been undertaken and natural variations in δ ~(17)O(O_3) are therefore not well constrained. METHODS The nitrite-coated filter method is a new technique for O_3 isotope analysis that employs the aqueous phase NO_2 - + O_3 → NO_3 - + O_2 reaction to obtain quantitative information on O_3 via the oxygen atom transfer to nitrate (NO_3~-). The triple-oxygen isotope analysis of the NO_3~- produced during this reaction, achieved in this study using the bacterial denitrifier method followed by isotope-ratio mass spectrometry (IRMS), directly yields the δ~(17)O value transferred from O_3. This isotope transfer process was investigated in a series of vacuum-line experiments, which were conducted by exposing coated filters to O 3 of various known δ~(17)O values and then determining the isotopic composition of the NO_3~- produced on the filter. RESULTS The isotope transfer experiments revealed a strong linear correlation between the δ ~(17)O of the O_3 produced and that of the oxygen atom transferred to NO_3~-, with a slope of 1.55 for samples with bulk δ ~(17)O(O_3) values in the atmospheric range (20-40%). This finding is in agreement with theoretical postulates that place the 17O-excess on only the terminal oxygen atoms of ozone. Ambient measurements yield average δ ~(17)O(O_3) bulk values in agreement with previous studies (22.9 ± 1.9%). CONCLUSIONS The nitrite-coated filter technique is a sufficiently robust, field-deployable method for the determination of the triple-oxygen isotopic composition of tropospheric O_3. Further ambient measurements will undoubtedly lead to an improved quantitative view of natural δ ~(17)O(O_3) variation and transfer in the atmosphere.