Dynamics of PM2.5 concentrations in Kathmandu Valley, Nepal

This study analyzed daily patterns and dynamics of PM(2.5) concentrations in the Kathmandu Valley during three winters. The PM(2.5) data showed a daily repetitive cycle which represents influence of local air flow and dispersion and accumulation of air pollutants in the valley. Two concentration peaks were observed in the morning and in the evening periods, and they fell down during the daytime and the nighttime periods. This indicates local emission sources as major contributors in the valley. The more pronounced morning peak compared to the evening peak showed that the upslope wind in the morning helped to move the polluted inversion layer downward, subsequently adding to freshly emitted pollutants and causing a sharp pollutant concentration rise in the morning. Katabatic wind and rise of temperature in the basin during the day helped the pollutant upflow and dilution, resulting in a sharp PM(2.5) concentration decline. Through the afternoon, the decrease in air temperature followed by decrease in wind speed caused to lower PM(2.5) peaks in the evening. Also, higher morning peaks of PM(2.5) concentrations compared to the evening indicated pollution from the previous day is added to the fresh emission. The valley had increased PM(2.5) from the beginning of October which continued till the first week of February. The increase in PM(2.5) peak fit the logistic equation y=[k/(1+exp (p-qx)]+asin(bx) where k, p, q, a, and b are constants.