A June 2013 trough pulled stratospheric ozone down across China, adding to surface ozone pollution

Why it matters

Natural stratospheric ozone makes it harder to judge how much surface ozone pollution people can cut. This case shows that multi-site ozonesondes can check reanalysis data. It also suggests that the public EAC4 reanalysis could support multiyear studies of stratospheric intrusion.

What they did

The authors studied a deep upper-level trough event over China from 10 to 13 June 2013. They used nine ozonesonde profiles from Beijing, Changchun and Hong Kong, nationwide surface ozone data, and AIRS, MERRA2 and EAC4 ozone products. They checked each product against the sondes. They then used EAC4’s stratospheric ozone tracer (O3S) to map the intrusion in 3D and to estimate its share of surface ozone.

Key findings

• Beijing sondes showed secondary ozone peaks above 400 ppbv from 11 June, rising to 650 ppbv by 13 June. Changchun showed a layer above 120 ppbv on 12 June and Hong Kong a layer above 80 ppbv on 13 June.
• EAC4 matched the sondes best of the three products, with R = 0.947 and a mean absolute bias of 19.1 ppbv. AIRS missed the upper peaks, and MERRA2 underestimated boundary layer ozone.
• On 10 June the O3-rich tongue hit the Tibetan Plateau. Surface stratospheric ozone there exceeded 30 ppbv, over 30 % of surface ozone.
• On 13 June in eastern China, stratospheric ozone made up 3–15 ppbv of surface ozone, or 2 %–10 %. It was about 10 % in the North China Plain.
• In eastern China the intruded air stayed in the lower free troposphere. It reached the surface only through convective mixing, which fits the clearer weather that raised photochemical ozone production.

Limitations

• The study covers a single event, which the authors say may not be fully representative.
• Only nine sonde profiles at three sites were available, giving a limited snapshot.

Glossary

• Stratospheric intrusion: Ozone-rich air from the stratosphere sinking into the lower atmosphere.
• Ozonesonde: A balloon-borne instrument that measures an ozone profile from the ground up to about 35 km.
• EAC4: ECMWF’s atmospheric composition reanalysis, which combines satellite data with a chemistry model.
• O3S: A model tracer for ozone that came from the stratosphere.