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

Abstract

Rationale: The triple oxygen isotopic composition (Δ 17 O) of tropospheric ozone (O 3 ) 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 3 .Methods: A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O 3 to nitrite on each filter to determine the17 O of O 3 in terminal positions (denoted as Δ 17 O(O 3 ) term ). The NO 3 À produced by this reaction is chemically converted into N 2 O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions. Results: The reciprocal of the NO 3 À quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ 17 O of NO 3 À . Using the linear relation, we corrected the changes in Δ 17 O of NO 3 À on the filters. We verified the accuracy of the new method through the measurement of artificial O 3 with known Δ 17 O(O 3 ) term value that had been determined from the changes in Δ 17 O of O 2 . The Δ 17 O(O 3 ) term of tropospheric O 3 was in agreement with previous studies. Conclusions: We accurately determined the δ 18 O and Δ 17 O values of tropospheric O 3 by blank correction using our new method. Measurements of Δ 17 O(O 3 ) term 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.