Health concerns about particulate matter (PM) require information on the size and constituents of aerosol, there is evidence that the lowest size fraction may deposit deeper into the lung. Daily mass measurements of PM10 and 3-daily samples of size differential aerosol mass and number concentrations were measured near to a busy toll road in Lak Si, Bangkok during three seasons in 2018. Mass fractions were found by weighing, and samples were analysed for elemental composition. Ca was the metal with the highest concentration, with Fe, Mg, Al and Zn all with near to 1 μg m-3 concentrations. Mn, Cu, Ba and Pb had concentrations nearer 100 ng m-3, V, Cr, Ni, As, Se and Sb ~ 10 ng m-3 and Co, Mo and Cd ~ 1 ng m-3. Most metals were of highest concentration within the PM2.5 fraction, Fe and Ba had the highest concentration in the PM10 fraction, while V, Cr and Ni had the highest concentrations in the submicron fraction.