A criterion for identifying Asian dust events based on Al concentration data collected from northern Taiwan between 2002 and early 2007

We attempted to identify Asian dust (AD) events between February 2002 and February 2007 in northern Taiwan using aerosol Al measurements. We subsequently used the results to propose a criterion for defining AD events. A total of 30 AD events were detected based on Al spiking concentrations, and these occurred frequently in 2002 and 2006. The dust plumes that were identified occurred between November and April, which were concentrated in February, March, and April. There were 35 dusty days, which accounted for 1.89% of the whole study period, and for 3.75% when considering only November through April. The seasonality of atmospheric Al is quite evident, reaching a maximum in spring and winter and a minimum in summer. This pattern is related to the long-range transport of AD in the northeasterly monsoon and the cease of AD contributions, the efficient removal by heavy rain, and effective mixing in summer. According to air-mass trajectory analysis, the primary dust regions are located in and around the Gobi Desert and Loess Plateau. The geometric mean concentration of the data set excluding the AD cases is 502 ng m(-3) with a geometric standard deviation (GSD) of 2.8. On the basis of our success in identifying the major AD events, the "geometric mean x2 GSD'' (i.e., 2800 ng m(-3) in our study area) is proposed as a straightforward criterion to define the significant AD event. As such, the statistic-based criterion could be applicable for other areas.