Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the Himalayas may lead to substantial radiative forcing in South Asia with potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat. An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. Here, the first evidence of very frequent new particle formation events occurring up to high altitudes is presented. A 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid (Nepal, 5,079 m above sea level), the highest atmospheric research station, is shown. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys. During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.

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 postmonsoon 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 theyshow 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. Nighttime measurements were subsequently selected to study the FT composition according to different air masses, and the effect of long range transport to the station.

Aerosol mass and the absorbing fraction are important variables, needed to constrain the role of atmospheric particles in the Earth radiation budget, both directly and indirectly through CCN activation. In particular, their monitoring in remote areas and mountain sites is essential for determining source regions, elucidating the mechanisms of long range transport of anthropogenic pollutants, and validating regional and global models. Since March 2006, aerosol mass and black carbon concentration have been monitored at the Nepal Climate Observatory-Pyramid, a permanent high-altitude research station located in the Khumbu valley at 5079 m a.s.l. below Mt. Everest. The first two year averages of PM1 and PM(1-10) mass were 1.94 µg m-3 and 1.88 µg m-3 , with standard deviations of 3.90 µg m-3 and 4.45 µg m-3, respectively, while the black carbon concentration average is 160.5 ng m-3, with a standard deviation of 296.1 ng m-3.Both aerosol mass and black carbon show well defined annual cycles, with a maximum during the premonsoon season and a minimum during the monsoon. They also display a typical diurnal cycle during all the seasons, with the lowest particle concentration recorded during the night, and a considerable increase during the afternoon, revealing the major role played by thermal winds in influencing the behaviour of atmospheric compounds over the high Himalayas. The aerosol concentration is subject to high variability: in fact, as well as frequent “background conditions” (55% of the time) when BC concentrations are mainly below 100 ng m-3 , concentrations up to 5 µg m-3 are reached during some episodes (a few days every year) in the premonsoon seasons.The variability of PM and BC is the result of both short-term changes due to thermal wind development in the valley, and long-range transport/synoptic circulation. At NCO-P, higher concentrations of PM1 and BC are mostly associated with regional circulation and westerly air masses from the Middle East, while the strongest contributions of mineral dust arrive from the Middle East and regional circulation, with a special contribution from North Africa and South-West Arabian Peninsula in post-monsoon and winter season.

In this work we present the new ABC-Pyramid Atmospheric Research Observatory (Nepal, 27.95 N, 86.82 E) located in the Himalayas, specifically in the Khumbu valley at 5079 m a.s.l. This measurement station has been set-up with the aim of investigating natural and human-induced environmental changes at different scales (local, regional and global). After an accurate instrumental set-up at ISAC-CNR in Bologna (Italy) in autumn 2005, the ABC-Pyramid Observatory for aerosol (physical, chemical and optical properties) and trace gas measurements (ozone and climate altering halocarbons) was installed in the high Khumbu valley in February 2006. Since March 2006, continuous measurements of aerosol particles (optical and physical properties), ozone (O3) and meteorological parameters as well as weekly samplings of particulate matter (for chemical analyses) and grab air samples for the determination of 27 halocarbons, have been carried out. These measurements provide data on the typical atmospheric composition of the Himalayan area between India and China and make investigations of the principal differences and similarities between the monsoon and pre-monsoon seasons possible. The study is carried out within the framework of the Ev-K2-CNR “SHARE-Asia” (Stations at High Altitude for Research on the Environment in Asia) and UNEP—“ABC” (Atmospheric Brown Clouds) projects. With the name of “Nepal Climate Observatory—Pyramid” the station is now part of the Observatory program of the ABC project.

The Himalayan–Karakoram range is located in one of the most densely populated and very rapidly developing world areas. Monitoring of atmospheric composition in this area can play a relevant role in evaluating the background conditions of the free troposphere and quantifying the pollution present at high altitudes, as well as in studying regional and long-range transport phenomena. Due to technical and logistic difficulties in carrying out measurements at high altitude in the Himalaya, no systematic observations of atmospheric constituents are available for this area. Thus, a new measurement station in such a region represents a unique source of data, able to make up for the prior lack of this information. For these reasons, in the framework of the SHARE-Asia and ABC projects, a remote monitoring station, the ABC-Pyramid Laboratory, will be installed in the Khumbu valley near Mt. Everst at 5079 m a.s.l. Continuous in situ measurements of chemical, physical and optical properties of aerosol, surface ozone concentration, as well as non-continuous measurements of halocarbons and other greenhouse-gas concentrations will be carried out. This monitoring station was projected, realised and tested in Bologna at CNR-ISAC Institute during autumn 2005. It was designed to be controlled by remote login and to operate for the long-term in extremely adverse weather conditions. This station represents an ideal place for studying regional and long-range air mass transport, due to natural and human processes. Precious 5-day forecast information about air-masses circulation at the ABC-Pyramid site will be supplied daily by Lagrangian backward trajectories, including suitable forecasts of stratosphere-troposphere exchange phenomena.