Interannual ozone variability at the NCO-P WMO/GAW global station: influence of stratosphere-to-troposphere exchange

Tropospheric ozone (O3) is an atmospheric key compound. It is recognised as a powerful short-lived greenhouse gas, it influences the oxidative capacity of the troposphere, and it affects population health as well as the ecosystem integrity and crop yields. Levels of tropospheric O3 at regional scale are strongly affected by anthropogenic emissions but production from lightning and Stratosphere Troposphere Exchange (STE) still represent not-negligible natural sources. South Asia and the Himalayas are hot-spot regions in terms of current surface O3 levels. For this reason, since March 2006, continuous surface O3 observations are carried out at the Nepal Climate Observatory - Pyramid (NCO-P; 27.95 N, 86.82 E), located at 5079 m a.s.l. in the Nepali Himalayas. As reported in previous investigations dealing with NCO-P observations, surface O3 variability in the Himalayas is strongly affected by upward transport of air-masses from the Indo-Gangetic Plain and Himalayan foothills, as well as stratospheric intrusion events. In this work, episodes of stratospheric air-mass intrusions at NCO-P are studied over the period 2006 - 2013, when continuous O3 measurements are available at the measurement site. The aim is to assess the influence of STE on the surface O3 variability. The analysis will be based on the global STE climatology produced at ETH Zurich for the ERA-Interim reanalysis data set from the European Centre for Medium-Range Weather Forecasts (ECMWF). This dataset, relying on a Lagrangian methodology to diagnose STE, will be used to understand the interannual O3 variations at NCO-P in relation to the corresponding STE variability.