Tropospheric ozone variations at the Nepal Climate Observatory-Pyramid (Himalayas, 5079 m a.s.l.) and influence of deep stratospheric intrusion events 1Institute for Atmospheric Science and Climate, National Research Council, Bologna, Italy
2ETHZ, Zurich, Switzerland
3Laboratoire de Glaciologie et Géophysique de l'Environnement, St Martin d'Hères Cedex, France
4Laboratoire de Meteorologie Physique, CNRS, Université Blaise Pascal, Aubiere cedex, France
5EV-K2-CNR Committee, 24126 Bergamo, Italy
Received: 21 December 2009 – Published in Atmos. Chem. Phys. Discuss.: 20 January 2010 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 (±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 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.
Revised: 06 July 2010 – Accepted: 08 July 2010 – Published: 16 July 2010
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.
Citation: Cristofanelli, P., Bracci, A., Sprenger, M., Marinoni, A., Bonafè, U., Calzolari, F., Duchi, R., Laj, P., Pichon, J.M., Roccato, F., Venzac, H., Vuillermoz, E., and Bonasoni, P.: Tropospheric ozone variations at the Nepal Climate Observatory-Pyramid (Himalayas, 5079 m a.s.l.) and influence of deep stratospheric intrusion events, Atmos. Chem. Phys., 10, 6537-6549, doi:10.5194/acp-10-6537-2010, 2010.