Aerosol characteristics of different types of episode

Daily and hourly average data from nine air-quality monitoring stations distributed across central Taiwan, which include ten items (i.e., PM₁₀, PM_2.5, wind direction, wind speed, temperature, relative humidity, SO₂, NO₂, NO, and CO), were collected from 2005 to 2009. Four episode types: long-range transport with dust storms (DS), long-range transport with frontal pollution (FP), river dust (RD), and stagnant weather (SW), and one mixed type of episode were identified. Of these four episode types, the SW was the dominant type, averaging about 70 %. The mean ratio of PM_2.5/PM₁₀ was the lowest during the RD episodes (0.42), while the mean ratio of PM_2.5/PM₁₀ was the highest during the SW episodes (0.64). Fine aerosol (PM_2.5) and coarse aerosol (PM_10–2.5) samples were collected by high-volume samplers for chemical composition analysis, from only three stations (Douliou, Lunbei, and Siansi) during the days of SW, RD, DS, and FP. The concentrations of PM_2.5 and three ionic species (NH₄ ⁺, NO₃ ^?, and SO₄ ^2?) all showed significant differences among the four episode types. The highest levels of NO₃ ^? (12.1 μg/m³) and SO₄ ^2? (20.5 μg/m³) were found during the SW and FP episodes, respectively. A comparison on the spatial similarity of aerosol compositions among the episodes and/or non-episodes (control) was characterized by the coefficient of divergence (CD). The results showed higher CD values in PM_10–2.5 than in PM_2.5, and the CD values between RD episodes and the other three episodes were higher than those between two types of episode for the other three episodes. The ratios of SOR (sulfur oxidation ratio), SO₄ ^2?/EC (elemental carbon), NOR (nitrogen oxidation ratio), and NO₃ ^?/EC showed that sulfate formation was most rapid during the FP, while nitrate formation was most rapid during the SW.