Rationale: The triple oxygen isotopic composition (Δ17 O) of tropospheric ozone (O3 ) 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 O3 .
Methods: A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O3 to nitrite on each filter to determine the Δ17 O of O3 in terminal positions (denoted as Δ17 O(O3 )term ). The NO3 - produced by this reaction is chemically converted into N2 O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions.
Results: The reciprocal of the NO3 - quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ17 O of NO3 - . Using the linear relation, we corrected the changes in Δ17 O of NO3 - on the filters. We verified the accuracy of the new method through the measurement of artificial O3 with known Δ17 O(O3 )term value that had been determined from the changes in Δ17 O of O2 . The Δ17 O(O3 )term of tropospheric O3 was in agreement with previous studies.
Conclusions: We accurately determined the δ18 O and Δ17 O values of tropospheric O3 by blank correction using our new method. Measurements of Δ17 O(O3 )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.
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