The bowl-shaped valley is formed by a ring of hills with only one narrow winding river outlet and five low mountains passes connecting neighboring valleys, which are at substantially lower elevations. The unique topographic features coupled with high emissions make the valley particularly vulnerable to air pollution. Especially in winter (Nov-Feb), when dry weather is prevalent, temperature inversions occur in the valley at night and early morning, which limit the vertical
dispersion, and pollutants are trapped close to the ground for extended periods.
Major air pollution emission sources in the valley are traffic, industry including brick kilns, and unmanaged solid waste (which leads to frequent open burning of the solid waste). The fast growing road traffic fleet is also an important source of air pollution in the valley, especially with the congested traffic conditions and the high elevation of the valley. Vehicle emissions contain a range of toxic air pollutants and climate-forcers, including greenhouse gases (GHGs), black carbon
(BC), and ozone precursors.
The emissions from vehicles contain many different pollutants and some of them have both direct and indirect impacts on the climate through chemical and physical processes in the atmosphere.
For instance, SO2 and NOx have cooling effects through the formation of sulfate and nitrate aerosol, respectively, in the atmosphere.
The DoTM (Department of Transport Management) has stated that, at present, around 19,500 automobiles are registered on an average every month. Registration of private four-wheelers has also doubled in one year across the country numbering 21,647, not just in Bagmati Zone. According to the data, the number doubled in 2015/016, with the registration of 28,361 new ones. The number of registered motorcycles in Bagmati Zone is 55,844 and 64,927 in the fiscal years 2014/015 and 2015/016 respectively. Likewise, the registration of two-wheelers nationwide was counted at 196,383 and 267,439 in 2014/015 and 2015/016 respectively.
The daily consumption of petrol and diesel within the Kathmandu valley is around 350,000 and 450,000 liters respectively which combines to release 1.6 million kg of CO2 in the environment. So does the brick kiln present in the Kathmandu valley. The pollution is not only affecting the air but the water, soil and noise too. The direct drainage of sewage from the household to the rivers has degraded the quality. Also, the solid in-degradable waste in large amount has affected the quality of soil.