Atmospheric Environment 34 (2000) 4499}4509 Selection of the scenarios of ozone pollution at southern Taiwan area utilizing principal component analysis Tai-Yi Yu*, Len-Fu W. Chang Graduate Institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan, Taipei, Taiwan, Peoples Republic of China Received 2 July 1999; received in revised form 27 December 1999; accepted 13 January 2000 Abstract The monitoring data analysis performed in this investigation focuses mainly on selecting statistically representative scenarios of ozone pollution in southern Taiwan. Measured data of ozone employed in this analysis were obtained from the monitoring stations of Taiwan Environmental Protection Administration (TEPA). The multivariate statistical technique, principal component analysis (PCA), was proposed to screen the ozone scenarios. Kaiser's varimax rotation method was also employed to separate the study area into several homogenous in#uence regimes. The analysis of spatial average ozone pro"les, backward trajectories and statistical analysis could reveal features of selected ozone scenarios. Analysis results indicated that the "rst four unrotated components and rotated components accounted for more than 70 and 70.8% of the total variance, respectively. The score of the "rst unrotated principal component above 7 is the screening threshold for ozone scenarios. Such ozone scenarios accounted for 14.2% of ozone stations number days with a low occurrence rate of 1.37%. Varimax rotation method successfully separated southern Taiwan into four homogenous ozone subregions, contributing 33.9, 17.3, 12.1 and 2.5% of total the variances, respectively. Evaluating the backward trajectories and spatial ozone pro"les revealed that weak westerly sea breeze is the dominant factor a!ecting the production of the high ozone event for most stations. The backward trajectories also indicated similar meteorological patterns of the stations in the same subregion.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Ozone pollution scenario; Principal component analysis; Varimax rotation method; Backward trajectory 1. Introduction The ozone problem in a large metropolitan area is extremely complex. The emission characteristics and meteorological nature signi"cantly contribute to the formation of a severe ozone episode. Thus, selection of representative scenarios of ozone pollution for a speci"c region and its use in designing the mitigation measures must be based on a sound-scienti"c procedure. The US National Research Council (1991) detailed state-of-art knowledge on the troposphere ozone problem. Selecting ozone scenarios involves consideration of the following factors. First, such scenarios should represent the most * Corresponding author. E-mail address: d2507005@ms31.hinet.net (T.-Yi. Yu). likely meteorological conditions related to previous ozone events and are expected to occur in the future if not controlled. Second, the scenarios must be selected to provide further insight into the causal and consequential relationship between emission sources and a!ected air quality. Doing so allows us to formulate an e!ective mitigation strategy. Southern Taiwan, known for its severe ozone prob- lems, consists of "ve juridical regions: Kaoshuing City, Kaoshuing County, Tainan City, Tainan County and Pingtung County. Air-quality monitoring results ob- tained from Taiwan Environmental Protection Adminis- tration (TEPA), 1998 indicate that ozone episodes occur most frequently during autumn. Chang et al. (1996) at- tributed this situation to profound combinations of me- teorological and emission characteristics of that region. Despite the su$cient solar irradiation, high concentra- tions of ozone seldom occur in southern Taiwan during 1352-2310/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 1 1 2 - 6