Long-range transport of Asian dust and air pollutants to Taiwan: observed evidence and model simulation

Abstract. Long-range transport of Asian dust and air pollutants are major environmental concerns of Taiwan during the winter monsoon season when northeasterly winds prevail following passages of cold fronts. Based on hourly measurements of Taiwan Environmental Protection Administration (TEPA) air quality monitoring stations, Lidar and in-situ IC, a significant long-range transport dust and air pollutants event on 18 March 2005 has been identified. During this episode, drastically elevated concentrations of PM₁₀, CO and SO₂ along with the strong northeasterly on 18 March were observed over background Wanli station, with peaks of about 170 μgm⁻³, 1.0 ppm and 14 ppb, respectively. We have found that air masses of air pollutants and Asian dust are transported separately. Although the mixing takes place on the way to Taiwan, it mixes slightly when they arrived in Taiwan. The major component of the first PM₁₀ peak were air pollutants, evidenced by the consistent peaks of SO₄²⁻ and NO₃ measured by in-situ IC, while no significant depolarization was measured by Lidar. In contrast, the evident non-spherical particles and hourly PM₁₀ concentration consistently varied with Ca²⁺ indicating that mineral dust was the major component of the second peak. Trajectory analysis showed that these two peaks come from quite different sources areas. The air masses of the first peak mainly come from anthropogenic area and transport in the low boundary layer (<1500 m) while the masses of the second peak originate from high altitude (>4000 m) of desert areas. Numerical results showed significant agreement of temporal and vertical variation of aerosol concentration with observations. The phenomena of split air parcels between air pollutants and Asian dust transported to Taiwan are strongly associated with the transport paths and stable and dry atmospheric boundary conditions.