Particle size distribution at major on-road, roadside, and university-ground sites in Lebanon were studied in summer 2011. In a predominant old traffic fleet, it is shown that calculated PM2.5 mass emission factors (EFs) conform to those of heavy duty vehicles. When compared to roads in California, higher PM2.5 mass but similar particle number EFs are obtained for the average fleet of the on-road sites. This confirms the observed particle size distribution pattern, rich in particles in the accumulation range mainly between 0.425 and 0.675 microm with a prevalent peak at 0.475 microm. Corresponding total particle counts (TC) measured on the roadside are as high as 14,050 particles/cm3 and are up to 67% higher than particle counts measured at the university-ground site. In a hot, dry and humid summer weather with consistent temperature oscillations, particle dispersion is shown to be a function of meteorological factors, mainly the effect of the boundary-layer thickness, with particle counts measured during the morning being around 40% higher than particle counts measured during the afternoon.
Implications: In a hot and humid Mediterranean summer, high emission factors are associated with an old car fleet. The observed diurnal variation in the particle count is attributed to the change in the thickness boundary layer in summer. In comparison to road sites, the particle size distribution shows the prevalence of larger size particles. Particle counts measured at the roadside sites are at least 20% higher than those of the road sites. The findings call for the reinforcement of local regulations on car age. Furthermore, the high number of particles can cause or aggravate a number of health and ecosystem problems.