[MTT37-P01] Determination on the triple oxygen isotopic compositions of tropospheric ozone
Keywords:ozone, triple oxygen isotopic composition, troposphere
In this study, we determined the oxygen isotopic compositions including the triple oxygen isotopic compositions (Δ17O), by passing air sample through nitrite (NO2-)-coated filters, which allows the reaction of O3 with NO2- to produce NO3-. The oxygen isotopic composition of NO3-was then determined to estimate the oxygen isotopic composition of O3. To remove atmospheric HNO3 prior to the reaction between O3 and NO2-, nylon filters were placed before the nitrite-coated filters. In order to obtain a high collection efficiency for the reaction between O3 and NO2-, the O3 collection was conducted at a flow rate less than 0.5L/min. The NO2- on the filter was removed by adding hydrogen azide (N3H) which selectively converts NO2- to N2O. Then, NO3- was converted to N2O, which is converted to O2 to be injected into an isotope mass spectrometer (MAT252), allowing quantification on the Δ17O value of NO3-. From the values determined for NO3-, the oxygen atoms derived from NO2- was subtracted to determine the oxygen isotopic compositions of O3. Please note that the oxygen isotopic composition determined through this method is not the average isotope composition of oxygen atoms in O3 (Δ17O (O3) bulk), but is the isotope composition of the oxygen atoms in the terminal positions (Δ17O (O3) terminal) of O3.
Observation on the atmospheric O3 was conducted from August to December, 2017, at Nagoya University. The Δ17O values of ozone were between +32‰~+39‰ which coincided well with those previously determined for tropospheric ozone (35‰±4‰). Besides, the Δ17O values of ozone were the lowest in August, and were the highest in November. The seasonal variation in the Δ17O values is most likely due to the stratospheric influence on the tropospheric O3. We also found about 1‰ differences in the Δ17O values between day and night. We concluded that the formation of an inversion layer in night time was responsible for the lower Δ17O values. That is to say, while the 17O-depleted O3 produced at ground level heights under the high pressure condition occupied major portion of O3 in night time due to the inversion layer, the 17O-enriched O3 produced at the upper layers contributed to O3 in day time through the active vertical convection.