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. Short-term O-3 behaviour in the middle/lower troposphere (e. g. at the altitude level of NCO-P) can be significantly affected by deep SI which, representing one of the most important natural input for tropospheric O-3, can also influence the regional atmosphere radiative forcing. To identify days possibly influenced by SI at the NCO-P, a specially designed statistical methodology was applied to the time series of observed and modelled stratospheric tracers. On this basis, during the 2-year investigation, 14.1% of analysed days were found to be affected by SI. The SI frequency showed a clear seasonal cycle, with minimum during the summer monsoon (1.2%) and higher values during the rest of the year (21.5%). As suggested by back-trajectory analysis, the position of the subtropical jet stream could play an important role in determining the occurrence of deep SI transport on the southern Himalayas. We estimated the fraction of O-3 due to SI at the NCO-P. This analysis led to the conclusion that during SI O-3 significantly increased by 27.1% (+13 ppbv) with respect to periods not affected by such events. Moreover, the integral contribution of SI (O-3S) to O-3 at the NCO-P was also calculated, showing that up to 13.7% of O-3 recorded at the measurement site could be possibly attributed to SI. On a seasonal basis, the lowest SI contributions were found during the summer monsoon (less than 0.1%), while the highest were found during the winter period (up to 24.2%). Even considering the rather large uncertainty associated with these estimates, the obtained results indicated that, during non-monsoon periods, high O-3 levels could affect NCO-P during SI, thus influencing the variability of tropospheric O-3 over the southern Himalayas.