Spatiotemporal variability of submicrometer particle number size distributions in an air quality management district

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First measurements of ambient 10–1000nm particle number concentrations (NTOT) and size distributions were made at an urban, coastal, mountain and downwind site within the Central Taiwan Air Quality Management District during a cold and a warm period. The primary objectives were to characterize the spatial and temporal variability of the size-fractionated submicrometer particles and their relationships with copollutants and meteorological parameters. The results show that the ultrafine particles (<100nm) are the major contributor to the NTOT. The mean NTOT was highest at the urban site, whereas lower and comparable at the three other sites. Although the mean NTOT at each site showed insignificant differences between study periods, their diurnal patterns and size distribution modal characteristics were modestly to substantially different between study sites. Correlation analyses of time-resolved collocated aerosol, copollutants and meteorological data suggest that the observed variability is largely attributable to the local traffic and to a lesser extent photochemistry and SO2 possibly from combustion sources or regional transport. Despite sharing a common traffic source, the ultrafine particles were poorly correlated with the accumulation particles (100–1000nm), between which the latter showed strong positive correlation with the PM2.5 and PM10. Overall, the NTOT and size distributions show modest spatial heterogeneity and strong diurnal variability. In addition, the ultrafine particles have variable sources or meteorology-dependent formation processes within the study area. The results imply that single-site measurements of PM2.5, PM10 or NTOT alone and without discriminating particle sizes would be inadequate for exposure and impact assessment of submicrometer particle numbers in a region of diverse environments.