Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 6743-6757, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
08 Jun 2017
Secondary ozone peaks in the troposphere over the Himalayas
Narendra Ojha1, Andrea Pozzer1, Dimitris Akritidis1,2, and Jos Lelieveld1,3 1Atmospheric Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
2Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, Thessaloniki, Greece
3Energy, Environment and Water Research Center, The Cyprus Institute, Nicosia, Cyprus
Abstract. Layers with strongly enhanced ozone concentrations in the middle–upper troposphere, referred to as secondary ozone peaks (SOPs), have been observed in different regions of the world. Here we use the global ECHAM5/MESSy atmospheric chemistry model (EMAC) to (i) investigate the processes causing SOPs, (ii) explore both their frequency of occurrence and seasonality, and (iii) assess their effects on the tropospheric ozone budget over the Himalayas. The vertical profiles of potential vorticity (PV) and a stratospheric ozone tracer (O3s) in EMAC simulations, in conjunction with the structure of SOPs, suggest that SOPs over the Himalayas are formed by stratosphere-to-troposphere transport (STT) of ozone. The spatial distribution of O3s further shows that such effects are in general most pronounced in the northern part of India. Model simulated ozone distributions and backward air trajectories show that ozone rich air masses, associated with STT, originate as far as northern Africa and the North Atlantic Ocean, the Middle East, as well as in nearby regions in Afghanistan and Pakistan, and are rapidly (within 2–3 days) transported to the Himalayas. Analysis of a 15-year (2000–2014) EMAC simulation shows that the frequency of SOPs is highest during the pre-monsoon season (e.g. 11 % of the time in May), while no intense SOP events are found during the July–October period. The SOPs are estimated to enhance the tropospheric column ozone (TCO) over the central Himalayas by up to 21 %.

Citation: Ojha, N., Pozzer, A., Akritidis, D., and Lelieveld, J.: Secondary ozone peaks in the troposphere over the Himalayas, Atmos. Chem. Phys., 17, 6743-6757,, 2017.
Publications Copernicus
Short summary
We investigate the processes, frequency of occurrence and seasonality, and effects of strongly enhanced ozone layers in the middle–upper troposphere (SOPs) over the Himalayas using a global model (EMAC). Rapid transport of stratospheric air masses is found as a key underlying process. Model predicts more frequent SOP events during the pre-monsoon. SOPs are found to significantly enhance the tropospheric ozone column over the Himalayas.
We investigate the processes, frequency of occurrence and seasonality, and effects of strongly...