Observations of vertical air mass exchange and new particles formation at Everest-Pyramid GAW Station

In the framework of the Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Enviroment) and UNEP ABC (Atmospheric Brown Clouds) projects, the Everest-Pyramid monitoring station is active since March 2006 in the Khumbu valley (Nepal), at 5079 m asl in order to make up for a lack of information at high altitude in Himalayan region performing continuous measurements of chemical, physical and optical properties of aerosol and tropospheric ozone, as well as non-continuous measurements of halocarbons and other greenhouse gases. First two years of measurements describing the behaviours of aerosol, ozone and GHGs showed that this remote area can be affected by local, regional or long range transports of air masses, as well as frequent vertical air exchanges. Air mass intrusions from stratosphere or upper troposphere are characterized by simultaneous rise of ozone content and diminution of relative humidity. During several events of Statosphere (Upper troposphere) ­ Troposphere Exchange (STE), a sudden increase of ultrafine particles number was observed at the end of STE event. The interface between very clean air and the more polluted air has been identified as an efficient mechanism for new particles formation. We found that aerosol concentrations are driven by intense nucleation events; over a one year period, new particles formation events took place on more than 35% of the days, predominantly during sunny conditions, the highest frequency reported so far. Correlation between STE and new particles formation, as well as quantification of STE influence on ozone budget at the station will be performed in this study.