Tropospheric ozone variations at the Nepal Climate Observatory-Pyramid (Himalayas, 5079 m a.s.l.) and influence of deep stratospheric intrusion events

The paper presents the first 2 years of continuous surface ozone (O-3) observations and systematic assessment of the influence of stratospheric intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P; 27 degrees 57'N, 86 degrees 48'E), located in the southern Himalayas at 5079 m a.s.l.. Continuous O-3 monitoring has been carried out at this GAW-WMO station in the framework of the Ev-K2-CNR SHARE and UNEP ABC projects since March 2006. Over the period March 2006-February 2008, an average O-3 value of 49 +/- 12 ppbv (+/- 1 delta) was recorded, with a large annual cycle characterized by a maximum during the pre-monsoon (61 +/- 9 ppbv) and a minimum during the monsoon (39 +/- 10 ppbv). In general, the average O-3 diurnal cycles had different shapes in the different seasons, suggesting an important interaction between the synoptic-scale circulation and the local mountain wind regime.