Atmospheric turbidity over Kathmandu valley

The atmosphere of Kathmandu Valley has been investigated by using Sunphotometer and Nephelometer during the pre-monsoon period of 1999. The atmospheric turbidity parameters (extinction coefficient for 500 nm wavelength τ_AG and Angstrom coefficient β) are found high in the morning and show decreasing trends from morning to late afternoon on average. Vertical dispersion of pollutants and increasing pollutant flushing rate by increasing wind speed from morning to late afternoon is the cause for this decreasing trend of turbidity over the valley. Being surrounded by high hills all around the valley, horizontal exit of pollutants without vertical dispersion is not possible. The scattering coefficient b_scat of aerosols in ground level troposphere is also found high in the morning, which decreases and becomes minimum during afternoon. During late afternoon, b_scat again shows a slightly increasing trend. The reason is the increasing vehicular emission during late afternoon rush period. The average values of Angstrom exponent α, β, τ_AG and b_scat are found to be 0.624±0.023, 0.299±0.009, 0.602±0.022 and 0.353±0.014 km⁻¹, respectively. About 76.8% of the observed values of β lie above 0.2 indicating heavy particulate pollution in the valley. A comparison of observed values of turbidity parameters with other major cities of the world shows that Kathmandu is as polluted as cities like Jakarta, Kansas, Beijing, Vienna, etc.