Wintertime Boundary Layer Evolution and Air Pollution Potential Over the Kathmandu Valley, Nepal

Meteorological effects on wintertime air pollution in the Kathmandu Valley were investigated using SOnic Detection and Ranging soundings, AutomaticWeather Station measurements, numerical simulation usingWeather Research and Forecasting data, and Chemical Transport Modeling during February 2013. A surrogate for black carbon was used for transport simulation in order to better understand the effects of local meteorological factors on air pollution. In the simulation, the emission strength of the black carbon surrogate was assumed temporally constant and spatially uniform over the Kathmandu Valley floor. TheWeather Research and Forecasting simulation results were well correlated with observed meteorological measurements and demonstrated diurnal periodicity such as intrusion of westerly-northwesterly wind into the Kathmandu Valley and modification of the boundary layer activity due to afternoon wind. The transport simulation suggested long-lasting weak wind, thermally stable stratification, and associated small turbulence during the night and morning caused potentially severe air pollution. We propose a method using wind velocity and turbulent kinetic energy in the surface layer to characterize pollution level in the Kathmandu Valley.