Size-segregated characterization of atmospheric aerosols in Taipei during Asian outflow episodes

Abstract

To investigate the size resolved physico-chemical properties of aerosols transported from the eastern Asian Continent following the winter monsoons, an intensive field experiment was conducted in Taipei, Taiwan in January 2003. Mass concentration and the abundance of major functional groups in size-segregated aerosol samples were measured. The size distribution parameters of the aerosol mass and the associated functional groups were analyzed. The results of the infrared spectroscopic analysis showed that the contents of sulfate and ammonium in Taipei's sub-micron aerosols were significantly increased due to continental pollution. In addition, the Asian outflow aerosols contained higher levels of silicate and nitrate in the super-micron size range. The size-resolved functionalities of the aerosol samples collected during Asian outflow episodes were compared with those of local pollution samples. This comparison revealed that ammonium and sulfate generally coexisted in sub-micron aerosols, suggesting that there are substantial emissions of ammonia to neutralize the acidity of sulfate in the region from eastern China to northern Taiwan. The nitrate had a bimodal distribution over the size range between 0.1 and 10 μm. A significant enhancement was found in coarse nitrate during the Asian outflow episodes, implying heterogeneous reactions of nitric acid on the surface of dust and sea salt particles during the transport processes. High concentrations of sub-micron ammonium nitrate aerosols were observed prior to the frontal passage and served as an important indicator of local photochemical pollution. The aliphatic and carbonyl carbon in the aerosols were suggested as being mostly due to local pollution in Taipei. Nevertheless, our analysis found that the aerosols of Asian outflows had a higher infrared absorbance ratio of carbonyl to aliphatic carbon than those of the local pollution. The enrichment of carbonyl carbon in the Asian outflow aerosols is most likely due to the emissions of biomass burning aerosols in the eastern China.