Multisite study of ambient sub-micrometer and ultrafine particles in the central Taiwan airshed during cold months

Yi-Ting Wang, Hung-Chieh Hsu, LI-HAO YOUNG

China Medical University, Taichung, Taiwan

     Abstract Number: 112
     Last modified: November 4, 2009

Abstract
The objective of this study is to characterize the spatial and temporal variations of ambient sub-micrometer and ultrafine particles (UFPs) in the central Taiwan airshed in the autumn/winter. Four Taiwan EPA air quality sites were selected for field sampling during October, 2007 to January, 2009. The sampling sites are 20-40 km spatially apart and nominally represent urban, urban-downwind, industrial, and rural areas, respectively. We used a sequential mobility particle spectrometer and condensation particle counter (SMPS/CPC) to measure the number size distributions of 0.01 – 1 um particles. The aerosol data were analyzed along with on-site air pollutant and meteorological data acquired from the Taiwan EPA air quality monitoring sites. The measured average UFP (< 0.1 um) number concentration was 17500 cm$^-3, which is comparable to that observed in many urban cities worldwide. The urban site, in particular, showed substantially higher UFP number concentrations than the other sites, of which the concentrations were similar. Temporally, peak UFP number concentrations consistently coincided with the morning and evening rush-hour periods and, as expected, showed the strongest correlations with CO and NOx. This suggests an important contribution from traffic emissions. However, we occasionally observed bursts of nucleation mode particles (< 0.03 um) during midday, indicating an origin not directly associated with traffic emissions but photochemistry. Such new particle formation events are rather unexpected in polluted air, where the average PM2.5 consistently exceeds 25 ug m-3. The UFPs, on average, accounted for 76% of measured total particle number concentration, with a geometric mean diameter of 0.05 um and a geometric standard deviation of 2.30. The mass concentrations of PM10 and PM2.5, on the other hand, showed poor correlations with the UFP concentrations, confirming a widely-accepted consensus that mass concentrations are poor surrogates for number concentrations.