*4.1. Air Pollution Exposure*

Exposure to PM2.5 was generally higher in the spring season than in summer in the Kathmandu Valley (Table 1), but the elemental species contribution of PM2.5 was found to be enhanced during summer than during spring (Figure 3). It was mainly PM2.5 that saw the reduction in summer. BC levels were not significantly different between two seasons, suggesting only minor differences in traffic-related PM sources during two seasons. BC, emitted from incomplete combustion of fossil fuels and biomass, is used as a tracer of combustion sources and has been used to investigate traffic pollution in urban areas [52–54]. All pollution measurements were taken from roadside at the busiest intersections, where traffic-related activities were the major source of PM2.5. There were also other sources such as dust resuspension, construction-related activities, refuse burning, brick-kiln, and these activities were minimized during summer than in spring. Meteorology might play an important role as well, and summer monsoonal conditions likely had an important determinative effect on PM2.5, a pollutant with a multitude of sources by decreasing concentrations through atmospheric washout, whereas BC was less impacted by washout. Temperature and total precipitation were lower in the spring season compared to in summer. Thus, a reduced atmospheric boundary layer height and reduced wind, lower temperature and low or no precipitation may have impacted the removal of PM2.5 during spring. Among the trace gases, ozone is slightly increased, but not significantly, during summer than in spring as it is formed through a photochemical process.
