Determination of the triple oxygen isotopic composition of tropospheric ozone in terminal positions using a multistep nitrite-coated filter-pack system.

Rationale: The triple oxygen isotopic composition (Δ O) of tropospheric ozone (O ) is a useful tracer for identifying the source and is essential for clarifying the atmospheric chemistry of oxidants. However, the single nitrite-coated filter method is inaccurate owing to the nitrate blank produced through the reaction of nitrite and oxygen compounds other than O .

Methods: A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O to nitrite on each filter to determine the Δ O of O in terminal positions (denoted as Δ O(O ) ). The NO produced by this reaction is chemically converted into N O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions.

Results: The reciprocal of the NO quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ O of NO . Using the linear relation, we corrected the changes in Δ O of NO on the filters. We verified the accuracy of the new method through the measurement of artificial O with known Δ O(O ) value that had been determined from the changes in Δ O of O . The Δ O(O ) of tropospheric O was in agreement with previous studies.

Conclusions: We accurately determined the δ O and Δ O values of tropospheric O by blank correction using our new method. Measurements of Δ O(O ) of the ambient troposphere showed 1.1 ± 0.7‰ diurnal variations between daytime (higher) and nighttime (lower) due likely to the formation of the temperature inversion layer at night.