Seasonal variations of aerosol size distributions based on long-term measurements at the high altitude Himalayan site of Nepal Climate Observatory-Pyramid (5079 m), Nepal K. Sellegri1, P. Laj2, H. Venzac1, J. Boulon1, D. Picard1, P. Villani2, P. Bonasoni3, A. Marinoni3, P. Cristofanelli3, and E. Vuillermoz4 1Laboratoire de Météorologie Physique, UMR 6016, CNRS/University of Clermont-Ferrand, Clermont-Ferrand, France 2Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), CNRS/University of Grenoble, Grenoble, France 3Institute for Atmospheric Science and Climate (ISAC), CNR, Bologna, Italy 4Ev-K2-CNR Committee, Bergamo, Italy
Abstract. The present paper investigates the diurnal and seasonal variability of the
aerosol total number concentration, number and volume size distribution
between 10 nm and 10 μm, from a combination of a scanning mobility
particle sizer (SMPS) and an optical counter (OPC), performed over a
two-year period (January 2006–February 2008) at the Nepal Climate Observatory-Pyramid
(NCO-P) research station, (5079 m a.s.l.). The annual average number
concentration measured over the two-year period at the NCO-P is 860 cm−3.
Total concentrations show a strong seasonality with maxima during
pre-monsoon and post-monsoon seasons and minima during the dry and monsoon
seasons. A diurnal variation is also clearly observed, with maxima between
09:00 and 12:00 UTC. The aerosol concentration maxima are mainly due to nucleation
processes during the post-monsoon season, as witnessed by high nucleation
mode integrated number concentrations, and to transport of high levels of
pollution from the plains by valley breezes during the pre-monsoon season,
as demonstrated by high accumulation mode integrated number concentrations.
Night-time number concentration of particles (from 03:00 to 08:00 NST) are
relatively low throughout the year (from 450 cm−3 during the monsoon
season to 675 cm−3 during the pre-monsoon season), indicating the of
high altitudes background level, as a result of downslope winds during this
part of the day. However, it was found that these background concentrations
are strongly influenced by the daytime concentrations, as they show the same
seasonal variability. If nighttime concentrations were presumed to be
representative of free troposphere (FT)/residual layer concentrations, they
would be found to be two times higher than at other lower altitudes European
sites, such as the Jungfraujoch. However, BL intrusions might contaminate
the free troposphere/residual layer even at this altitude, especially during
regional air masses influence. Night-time measurements were subsequently
selected to study the FT composition according to different air masses, and
the effect of long range transport to the station.
Citation: Sellegri, K., Laj, P., Venzac, H., Boulon, J., Picard, D., Villani, P., Bonasoni, P., Marinoni, A., Cristofanelli, P., and Vuillermoz, E.: Seasonal variations of aerosol size distributions based on long-term measurements at the high altitude Himalayan site of Nepal Climate Observatory-Pyramid (5079 m), Nepal, Atmos. Chem. Phys., 10, 10679-10690, doi:10.5194/acp-10-10679-2010, 2010.