Dynamics of Air Pollution Transport in Late Wintertime over Kathmandu Valley, Nepal: As Revealed with Numerical Simulation

Abstract

Air pollution characteristics over the Kathmandu Valley in wintertime were numerically investigated by using a comprehensive transport?chemistry?deposition model of air pollutants together with the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5). In Kathmandu, Nepal, it is known that double-layered local flows, that is, the southwesterly and northwesterly winds, formed as combined valley wind and plain-to-plateau wind, develop every day in the afternoon. In this study, the effect of local flows on air pollution in Kathmandu has been clarified. Detailed analysis of diurnal variation of air pollution transport elucidated the basic nature of the air pollution: 1) the vertical spreading of the pollutants, accumulated during the nighttime, by mixing-layer activity in the late morning, before the intrusion of the two local flows; 2) the late afternoon redevelopment of a shallow polluted layer and the pollutant transport toward the eastern neighboring valley, caused by the double-layered local flows; and 3) the nighttime partial comeback of the pollutants from the eastern mountain pass to the Kathmandu Valley and the gradual spreading of the pollutants over the valley caused by intermittent but organized horizontal flow caused by mountain winds, which also suppresses excess accumulation of fresh pollutants in the source area. Mass balance of sulfur compounds in the Kathmandu Valley is also discussed.