In spite of being located at the heart of the highest mountain range in the world, the Himalayan Nepal Climate Observatory (5079 m a.s.l.) at the Ev-K2-CNR Pyramid is shown to be affected by the advection of pollution aerosols from the populated regions of southern Nepal and the Indo-Gangetic plains. Such an impact is observed along most of the period April 2006–March 2007 addressed here, with a minimum in the monsoon season. Backtrajectoryanalysis indicates long-range transport episodes occurring in this year to originate mainly in the west Asian deserts. At this high altitude site, the measured aerosol optical depth is observed to be about one order of magnitude lower than the one measured at Ghandi College (60 m a.s.l.), in the IndoGangetic basin. As for Ghandi College, and in agreement with the in situ ground observations at the Pyramid, the fine mode aerosol optical depth maximizes during winter and minimizes in the monsoon season. Conversely, total optical depth maximizes during the monsoon due to the occurrence of elevated, coarse particle layers. Possible origins of these particles are wind erosion from the surrounding peaks and hydrated/cloud-processed aerosols. Assessment of the aerosol radiative forcing is then expected to be hampered by the presence of these high altitude particle layers, which impede an effective, continuous measurement of anthropogenic aerosol radiative properties from sky radiance inversions and/or ground measurements alone. Even though the retrieved absorption coefficients of pollution aerosols were rather large (single scattering albedo of the order of 0.6–0.9 were observed in the month of April 2006), the corresponding low optical depths (0.03 at 500 nm) are expected to limit the relevant radiative forcing. Still, the high specific forcing of this aerosol and its capability of altering snow surface albedo provide good reasons for continuous monitoring.

A Cimel sunphotometer operating in the framework of the AERONET project has been installed at the Himalayan Ev-K2-CNR Pyramid (5079 m a.s.l.) in the year 2006, as site Ev-K2-CNR. The observational activity will provide a characterization of the optical and microphysical properties of atmospheric aerosols, in particular of the atmospheric brown cloud (ABC) in the Himalayan region. This paper will describe the Cimel sunphotometer measurement technique, will introduce to the AERONET programme and will evaluate the contribution of the proposed Ev-K2-CNR AERONET site to the study of the ABC.

At the end of March 2006 a Cimel CE-318 sunphotometer was installed. It provides a characterization of aerosol optical and microphysical properties of the air column above the station. These measurements allow to follow the time evolution of the aerosol optical depth (AOD) at eight wavelengths, plus other properties as single scattering albedo (SSA, fundamental in computing the aerosol effects on solar radiation), particle size distribution (SD, fundamental in assessing the ABC effects on formation and lifetime of water vapor clouds) and refractive index.

Sun photometer ( Cimel CE-318) measurements of the direct (collimated) solar radiation provide information to calculate the columnar aerosol optical depth (AOD). AOD can be used to compute columnar water vapor (Precipitable Water) and estimate the aerosol size using the Angstrom parameter relationship. Sunphotometer from March 2006 provides a characterization of aerosol optical and microphysical properties of the air column above the station. It is an automatic sun-tracking and sky radiometer for measuring the aerosol optical depth at 8 wavelengths between 340 and 1020 nm.

At the end of March 2006 a Cimel CE-318 sunphotometer was installed at The Nepal Climate Observatory - Pyramid (NCO-P) within the framework of the Aerosol Robotic Network, AERONET (http://aeronet.gsfc.nasa.gov, EvK2-CNR site). It provides a characterization of aerosol optical and microphysical properties of the air column above the station.