"Most research on stratosphere and troposphere exchange (STE) is carried out using model simulations, but here, we directly reveal the phenomenon through in-situ measured high-precision profiles. Such height coverage and high-precision in-situ measurement profiles of trace gases are very rare in China", says Prof. LIU Yi, a researcher at the Institute of Atmospheric Physics (IAP). "Also, the profiles are valuable in many areas, such as the retrieval of local surface flux, model validation, and atmospheric transport research."

The method used for measuring these profiles, called aircore, was applied for the first time in China in this study. Specifically, the aircore campaign was conducted at Xilinhot, Inner Mongolia, on 11 and 15 June 2018, by a collaborative team comprising the research groups of LIU Yi and WANG Yong from the IAP, as well as that of FANG Shuangxi from the China Meteorological Administration.

Before the start of the campaign, a very large sandstorm hit Xilinhot. "We were worried because the launch of aircore requires good weather condition." recalled LIU. "Fortunately the sky was clear when the campaign started." During the campaign, two experiments were conducted, on 13 and 14 June, respectively, in which CO and CO2 profiles from the surface to around 25 km were successfully measured.

The structure of the profiles on these two days were totally different from the boundary layer to the middle stratosphere, and the inter-relationship between the two trace gases changed from positive to negative. The positive relationship between CO and CO2 was intelligible, but the negative one is rarely seen.

Based on meteorological analyses and XCO2 (the total column mixing ratio of CO2), the negative relationship was caused by the link between the low CO concentration and high CO2 concentration, and vice versa. From the vertical perspective, the laminar structure in which the high CO concentration and low CO concentration were intertwined was caused by the process of STE. From the horizontal perspective, the CO2 gradient was large. The 3-D atmospheric structure was well considered.

The findings are published in Advances in Atmospheric Sciences.

Successful launch (left) and return (right) of Aircore in Inner Mongolia on 13 June 2018 (photographs taken by YI You and FANG Shuangxi).

"This first aircore campaign was really successful because the STE process that was our main focus was well observed and illustrated, which is rare", concludes LIU. "We also conducted two other aircore campaigns, in Inner Mongolia in November 2018 and on the Tibetan Plateau in August 2019, respectively. There are more trace gases from the Tibet campaign to be measured, such as CO2, CO, CH4, N2O and O3. We want to investigate the pathways through which pollutants released from the surface are transported to the lower stratosphere under the control of the South Asian high, and this research is underway."

Citation: Yi, Y., and Coauthors, 2020: Direct observations of atmospheric transport and stratosphere–troposphere exchange from high-precision carbon dioxide and carbon monoxide profile measurements. Adv. Atmos. Sci., 37(7), https://doi.org/10.1007/s00376-020-9227-2 .

Media contact: Ms.LIN Zheng, jennylin@mail.iap.ac.cn