Modeling study of three consecutive high ozone episodes over Taiwan in spring 2007

A three-dimensional photochemistry model is applied to analyze three unusual high O 3 episodes that occurred continuously in Taiwan from May 1 to 11, 2007. During this period, the first high ozone episode was observed in northern and central western Taiwan on May 4. Following this episode, the second episode was observed throughout Taiwan on May 7, and continued for several days until May 10, when the third episode was observed in central western and southwestern Taiwan. This was the worst O 3 period in Taiwan over the past two decades. During this period, 70% of the local air quality station reported an O 3 concentration of over 120 ppb, and a maximum O 3 concentration of 175 ppb was observed in background Taiwan. Based on model analyses, the sources of the three high O 3 episodes differ. The high O 3 concentration observed during the first episode is mainly attributed to the northeastward transport of O 3 precursors and concentrations from northern and central western Taiwan under southwesterlies prior to frontal passage, chemically producing O 3 over 30 ppb h −1 in northern Taiwan. During the second episode, horizontal advection of Asian outflow during the passage of an anticyclone and front contributes a maximum of 25 ppb h −1 in both northern and southern tip of Taiwan, respectively, increasing to more than 75 ppb h −1 in southwestern Taiwan because of the combination with the local source. During the third episode when the prevailing easterlies associated with a departing anticyclone to the east of Taiwan is blocked by the high central mountain, local O 3 chemical production and horizontal transport contribute a maximum of 25 ppb h −1 and 35 ppb h −1 in central and southwestern Taiwan, respectively, under a calm and high background O 3 condition. Source from Taiwan contributes more than half of the high O 3 concentration over northwestern Taiwan prior to frontal passage during the first episode, but decreases to mostly below 40% over western Taiwan during the second episode when Asian outflow occurs, and increases to more than 60% over central and southwestern Taiwan due to pollution accumulation under an anticyclonic departure.