Secondary ozone peaks in the troposphere over the Himalayas

Abstract

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 (O 3 s) in EMAC simulations, in conjunction with the structure of SOPs, suggest that SOPs over the Himalayas are formed by stratosphereto-troposphere transport (STT) of ozone. The spatial distribution of O 3 s 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 %.

On the impact of future climate change on tropopause folds and tropospheric ozone

“…2 there is a distinct upward and poleward shift in the Southern Hemisphere (SH) mid-latitude jet during all seasons, which is also identified during DJF and SON in the Northern Hemisphere (NH), yet less pronounced. A poleward-upward shift of the westerly jet in response to greenhouse warming was reported by several previous studies using individual models (Butler et al, 2010;Orbe et al, 2015;Doherty et al, 2017) or ensembles of models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (Lorenz and DeWeaver, 2007), and the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5) (Swart and Fyfe, 2012;Delcambre et al, 2013;Yim et al, 2016). Moreover, a rise of the tropopause is seen during all seasons in both the NH and SH extratropics, which on an annual basis is estimated at about 8.8 and 5.8 hPa, respectively.…”

Surface ozone at Nam Co in the inland Tibetan Plateau: variation, synthesis comparison and regional representativeness

“…With tropopause folding further north in the summer, the air masses from the northern Tibetan Plateau may contribute more to the surface ozone levels at Nam Co Station than the air masses from the southern Tibetan Plateau. Ojha et al (2017) found that the potential vorticity layer in the summer was weaker than during the late winter and in the spring in the central Himalayan region which is to the south of Nam Co Station. In the autumn (September, October and November) and the winter (December, January and February), the mixing heights at Nam Co Station were much lower than in the spring and the summer.…”

Very high stratospheric influence observed in the free troposphere over the northern Alps – just a local phenomenon?

“…However, it is not only the persistence that matters: high ozone values have also been reported in subtropical intrusions. For example, very strong ozone signatures in the troposphere exceeding 200 ppb have been detected over northern India (Ojha et al, 2014(Ojha et al, , 2017. High ozone values in the middle and upper troposphere from regions next to the STJ have even been observed above Garmisch-Partenkirchen (Germany) after transport almost all the way around the Northern Hemisphere (six cases: Trickl et al, 2011; see also Langford, 1999).…”