Reply on RC1

Abstract

Using a multistep nitrite-coated filter-pack system for sampling, we determined the seasonal variations in the triple oxygen isotopic composition (Δ17O) of tropospheric ozone (O3) in the terminal positions (Δ17Oterm(O3)) in the cities Nagoya and Niigata (Japan) in the eastern Asia region to quantify the mixing ratio of stratospheric O3 within the total tropospheric O3 supplied by stratosphere–troposphere transport (STT). In Nagoya, diurnal variations have also been studied. Both the average Δ17Oterm(O3) and their 1σ variation ranges agreed well with previous studies, (37.5 ± 1.4) ‰ in Nagoya and (37.0 ± 1.7) ‰ in Niigata. The average difference in Δ17Oterm(O3) between daytime (higher) and nighttime (lower) was (1.4 ± 0.7) ‰ (1σ) in Nagoya, which was responsible for the formation of a stable boundary layer at night, reducing mixing with high Δ17Oterm(O3) from the free troposphere. We also found a significant correlation between 7Be activity concentrations and the Δ17Oterm(O3), implying that STT was responsible for the elevated Δ17Oterm of O3 in the troposphere. By using the relationship between the reciprocal of concentrations and Δ17Oterm of tropospheric O3, we estimated the Δ17O of stratospheric O3 supplied through the STT (Δ17OSTT), together with that produced through photochemical reactions at surface altitude (Δ17Osur). Moreover, using Δ17OSTT and Δ17Osur, we estimated the mixing ratios of stratospheric O3 (i.e., O3 produced in the stratosphere and supplied to the troposphere through STT) in each tropospheric O3 (fSTT), as well as the absolute concentrations of stratospheric O3 supplied through STT in the troposphere (CSTT(O3)). The CSTT(O3) exhibited minimum values in summer ((5.3 ± 1.0) ppb) and maximum values in late winter to spring ((15.9 ± 2.1) ppb). Although the fSTT values were higher than those estimated using the chemistry climate models from past studies, the trends of the seasonal variations were consistent with them. We concluded that Δ17O successfully provided observational constraints on the STT of O3.