Abstract
Abstract. The paper presents the first 2 years of continuous surface ozone (O3) observations and systematic assessment of the influence of stratospheric intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P; 27°57' N, 86°48' E), located in the southern Himalayas at 5079 m a.s.l.. Continuous O3 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 O3 value of 49±12 ppbv (±1δ) 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 O3 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 O3 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 O3, 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 O3 due to SI at the NCO-P. This analysis led to the conclusion that during SI O3 significantly increased by 27.1% (+13 ppbv) with respect to periods not affected by such events. Moreover, the integral contribution of SI (O3S) to O3 at the NCO-P was also calculated, showing that up to 13.7% of O3 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 O3 levels could affect NCO-P during SI, thus influencing the variability of tropospheric O3 over the southern Himalayas.
Highlights
Even if about 90% of atmospheric O3 molecules resides in the stratosphere, tropospheric O3 strongly influences the radiative budget of the atmosphere (Forster et al, 2007) and the oxidation capacity of the troposphere (Gauss et al, 2003)
Due to the increasing emissions of anthropogenic O3 precursors (Ohara et al, 2007), the issue appears especially urgent with regard to South Asia, where a vast region extending from the Indian Ocean to the Himalayas is characterised by the presence of copious amounts of aerosol and pollutant gases, with severe implications on regional climate, air-quality and food safety (Ramanathan et al, 2008)
With the aim of contributing to clarifying the role played by transport processes in determining the O3 levels and variability in one of the world’s most critical high-altitude regions, a systematic assessment was performed of the influence of stratospheric intrusions (SI) on O3 concentrations recorded at this GAW-WMO station, which is considered representative of the Southern Himalayas remote troposphere
Summary
Even if about 90% of atmospheric O3 molecules resides in the stratosphere, tropospheric O3 strongly influences the radiative budget of the atmosphere (Forster et al, 2007) and the oxidation capacity of the troposphere (Gauss et al, 2003). Due to the increasing emissions of anthropogenic O3 precursors (Ohara et al, 2007), the issue appears especially urgent with regard to South Asia, where a vast region extending from the Indian Ocean to the Himalayas is characterised by the presence of copious amounts of aerosol and pollutant gases (the so-called Atmospheric Brown Cloud), with severe implications on regional climate, air-quality and food safety (Ramanathan et al, 2008). There is still a gap in knowledge concerning the typical levels and variations of tropospheric O3 over the high Himalayas where, besides influencing the mountain ecosystem air quality, it can strongly contribute to the atmospheric heating of absorbing aerosols in the Brown Cloud (Ramanathan et al, 2008)
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