The oval shaped tectonic basin, Kathmandu valley, is located in the middle sector of Himalayan range
which is surrounded by tiers upon tier of green mountains ranging from 500m to 3000m above the sea
level. The valley has two narrow river gorges in the southwest and northwest edges and short hills in
the southwest edge connecting the neighboring valley Banepa. The wind of the valley is usually
exchanged by entering from the southwest and the northwest gorges and exit from the southeast short
hills to the neighboring valley. Being surrounded by high hills and mountains the horizontal exist of the
air pollutants produced from the valley without vertical dispersion is less possible. This has led sharp
increase of particulate matter (PM) concentration in Kathmandu valley with rapid urbanization. PM
measurement data from six different locations in the Kathmandu Valley recorded from October 2002 to
March 2007 were used to investigate PM10 and PM2.5 concentration dynamics in the valley. Monthly
average data of the urban areas, which have much higher concentrations than the rural areas, even
exceeded the daily standard level of PM10, in Nepal, 120 μm/m3. The concentration of PM increased
with the beginning of winter (October) and reached to the highest level in February, the end of winter
showing continuous accumulation in the atmosphere.