Air pollution transport in the Kathmandu valley/basin has been investigated by numerical simulation of local flows and the observation of NO 2 and SO 2 . The observation was performed at 22 sites with passive samplers from February to April 2001, and the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) was utilized for the flow simulation. The calculation reproduced reasonably well the surface wind and temperature at the Tribhuvan International Airport (TIA) as well as the vertical wind profile taken at the center of the valley by sodar observation. The calculation showed that two characteristic local flows tend to intrude into the valley/basin in the afternoon through the mountain gaps surrounding Kathmandu, that is, the southwesterly from the Indian Plain and the northwesterly from the valley west to Kathmandu. These cool wind layers meet at the center of the Kathmandu basin and form a double-layering structure there. The lower layer is shallow with a depth of about 250 m, being composed of the cooler southwesterly air mass from the Indian Plain. It was concluded that this local flow structure suppresses vertical mixing and leads to high air pollution by decreasing the daytime ventilation of air mass over the valley. The observations performed during the period confirmed it.

Kathmandu Valley is one of the largest and most polluted metropolitan regions in the Himalayan foothills. Rapidly expanding urban sprawl and a growing fleet of vehicles, and industrial facilities such as brick factories across the valley have led to conditions where ambient concentrations of key gaseous air pollutants are expected to exceed Nepal's National Ambient Air Quality Standards (NAAQS) and World Health Organization (WHO) guidelines. In order to understand the spatial variation of the trace gases in the Kathmandu Valley, passive samples of SO 2 , NO x , NO 2 , NH 3, and O 3 were collected simultaneously from fifteen locations between March and May 2013. A follow-up study during two separate campaigns in 2014 sampled these gases, except ammonia, one site at a time from thirteen urban, suburban and rural stationary sites. In 2013, urban sites were observed to have higher weekly averaged NO 2 and SO 2 (22.4 ± 8.1 µg m -3 and 14.5 ± 11.1 µg m -3 , respectively) than sub-urban sites (9.2 ± 3.9 µg m -3 and 7.6 ± 2.8 µg m -3 , respectively). Regions located within 3 km of brick factories had higher SO 2 concentrations (22.3 ± 14.7 µg m -3 ) than distant sites (5.8 ± 1.1 µg m -3 ). Higher O 3 (108.5 ± 31.4 µg m -3 ) was observed in rural locations compared to urban sites (87.1 ± 9.2 µg m -3 ), emphasizing the importance of meteorological factors and precursor species for ozone production and titration. Parallel to previous studies, these results suggest that ground-level O 3 , as its levels frequently exceeded guidelines throughout the sampling periods, is an important concern throughout the valley. NH 3 near polluted rivers and SO 2 around brick factories are also important pollutants that need more intensive monitoring, primarily due to their importance in particulate matter formation chemistry.

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Meteorological effects on wintertime air pollution in the Kathmandu Valley were investigated using SOnic Detection and Ranging soundings, Automatic Weather Station measurements, numerical simulation using Weather 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. The Weather 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.

Jomsom Airport of Nepal is considered to be one of the world's most extreme airports. The frequency of aircraft crashing along the Kali Gandaki River Valley and over the Jomsom Airport is very high. Pre-monsoon time local flow characteristics over the Kali Gandaki Valley has been studied to understand the meteorological hazards for aviation activities along the valley and over the Jomsom Airport with the application of the WRF Modeling System initialized with NCEP meteorological, USGS 24 categories land use, and 30 second terrain data. Four days long simulation was carried out for the period of 15 May 0000 UTC to 19 May 0000 UTC, 2012 to capture the 16 May 2012 incidence of light aircraft crashing in the late morning time of the day. The study revealed that there are enough grounds to believe that the crashing of the plane resulted due to the adverse local flow system prevailing over the Jomsom Airport area during the period. The significant subsidence from the relatively strong boundary layer wind aloft, the ground level opposite wind and the low-level turbulence might have leaded the airplane to crash just after its takeoff from the runway. The local flow system over the Kali Gandaki Valley makes afternoon time flights over the Jomsom Airport almost impossible and the narrow slot of time during morning time is also not free from difficult situation.