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.