Seasonal Variability of PM2.5, BC, and PM2.5. Chemical Characteristics at Busy Roadways in Kathmandu Valley, Nepal

Kathmandu valley located in the foothills of Himalaya in Nepal suffers from serious air pollution problems. Near-roadway PM2.5 and BC were measured at six sites in the Kathmandu valley using a portable scattering nephelometer (pDR-1500, Thermo Inc., US) and a microaethalometer (Aeth Labs, US), respectively. 37 mm polytetrafluoroethylene filter samples were analyzed by a laboratory-based X-ray fluorescence (XRF) spectrometer (QUANT’X, Thermo Inc., US) for elements, with subsequent filter extraction in deionized water followed by ion chromatography (ICS-1100, Thermo Inc., US) for water-soluble ions. PM2.5 concentrations at six different locations in the Kathmandu Valley, Nepal showed distinct seasonal variability. There was a reduction of 57-74% in PM2.5 levels during the monsoonal period (July 20 – August 22, 2014) compared to the drier winter season (February 16 – April 4, 2014). Daily means of PM2.5 were 124.76 and 45.92 μg/m3 during winter and monsoon, respectively. BC concentrations, however, were marginally reduced during the monsoon (13.46 μgC/m3) compared to that in winter (16.74 μgC/m3). Four sites located along a busy commercial ring road had higher PM2.5 levels than the two sites located inside the ring road. Chemical analysis of 24 hour PM2.5 filter samples shows dust and traffic sources as the most important PM emission source at these locations. Silica, calcium, aluminum, and iron were the most abundant elements during both winter and monsoon, with the total concentrations of 12.13 and 8.85 μg/m3, respectively. Coefficient of divergence calculated from the four main sites resulted in more heterogeneity for chemical species compared to PM2.5 and BC. This suggests though PM2.5 and BC levels might be similar in the valley, their emission sources and production might differ across the valley. Our findings provide important insights on physical and chemical characteristics of particulate matter and its sources, which will be useful in designing appropriate mitigation measures, which the valley desperately needs.