Mountain valley circulations and the observed diurnal cycle of aerosol number concentration over central Nepal

Aerosol size distribution was measured using SMPS with sizes in the range 14 nm – 340 nm during the pre-monsoon season over Central Nepal (Dhulikhel and Besisahar) as part of the JAMEX09 campaign. Meteorological variables such as wind speed, direction, temperature, pressure and rainfall were also measured at the surface during the sampling period. The diurnal cycle of wind, pressure and temperature suggests consistent mountain-valley wind circulations over Besisahar, which is located on a narrow NS oriented reach of the Marsyangdi river. Data analysis of the aerosol size distribution also exhibited a strong diurnal cycle with aerosol peaks during the transition period of boundary layer growth and decay, while the minimum aerosol concentration were observed during the afternoon and late night. Here, results from an idealized numerical study over the topography of Marsyangdi river Valley (Besisahar) using radiosonde soundings [from MOHPREX campaign, Barros and Lang (2002)], are presented to explain the physical mechanism of observed diurnal cycle of aerosol number concentration.