Vertical distributions of N2O isotopocules in the equatorial stratosphere
Sakae Toyoda1, Naohiro Yoshida1,2, Shinji Morimoto3, Shuji Aoki3, Takakiyo Nakazawa3, Satoshi Sugawara4, Shigeyuki Ishidoya5, Mitsuo Uematsu6, Yoichi Inai3, Fumio Hasebe7, Chusaku Ikeda8, Hideyuki Honda8, and Kentaro Ishijima91Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan 2Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan 3Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 4Miyagi University of Education, Sendai 980-0845, Japan 5National Institute of Advanced Industrial Scien ce and Technology (AIST), Tsukuba 305-8569, Japan 6Atmosphere and Ocean Research Ins titute (AORI), The University of Tokyo, Kashiwa, 277-8564, Japan 7Division of Earth System Science, Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan 8Institute of Space and Astronautical Sciences (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara 252-5210, Japan 9Project Team for HPC Advanced Predictions Using Big Data, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, 236-0001, Japan
Received: 23 Mar 2017 – Accepted for review: 03 Apr 2017 – Discussion started: 06 Apr 2017
Abstract. Vertical profiles of nitrous oxide (N2O) and its isotopocules, isotopically substituted molecules, were obtained over the equator at altitudes of 16–30 km. Whole air samples were collected using newly developed balloon-borne compact cryogenic samplers over the Eastern Equatorial Pacific in 2012 and Biak Island, Indonesia in 2015. They were examined in the laboratory using gas chromatography and mass spectrometry. The mixing ratio and isotopocule ratios of N2O in the equatorial stratosphere showed a weaker vertical gradient than the previously reported profiles in the subtropical and mid-latitude and high-latitude stratosphere. From the relation between the mixing ratio and isotopocule ratios, further distinct characteristics were found over the equator: (1) Observed isotopocule enrichment factors (ϵ values) in the middle stratosphere (25–30 km) are almost equal to ϵ values reported from broadband photolysis experiments conducted in the laboratory. (2) ϵ values in the lower stratosphere (< ca. 25 km) are about half of the experimentally obtained values, being slightly larger than those observed in the mid-latitude and high-latitude lower stratosphere. These results suggest the following. (1) The time scale of horizontal mixing in the tropical middle stratosphere is sufficiently large for in-situ photolysis of N2O, mainly because of strong upwelling and transport barrier between the tropics and extratropics. (2) The air in the tropical lower stratosphere is exchanged with extratropical air on a time scale that is shorter than that of photochemical decomposition of N2O. Previously observed ϵ values, which are invariably smaller than those of photolysis, can be explained qualitatively using a three-dimensional chemical transport model and using a simple model that assumes mixing of aged tropical air and extratropical air during residual circulation. Results show that isotopocule ratios are useful to examine the stratospheric transport scheme deduced from tracer–tracer relations.
Toyoda, S., Yoshida, N., Morimoto, S., Aoki, S., Nakazawa, T., Sugawara, S., Ishidoya, S., Uematsu, M., Inai, Y., Hasebe, F., Ikeda, C., Honda, H., and Ishijima, K.: Vertical distributions of N2O isotopocules in the equatorial stratosphere, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-272, in review, 2017.