Atmospheric Environment 40 (2006) 3898–3908 The trend of surface ozone in Taipei, Taiwan, and its causes: Implications for ozone control strategies Charles C.-K. Chou a,Ã , Shaw C. Liu a , Chuan-Yao Lin a , Chein-Jung Shiu b , Ken-Hui Chang c a Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan b Department of Atmospheric Sciences, National Taiwan University, Taiwan c Department of Safety Health and Environmental Engineering, National Yunlin University of Science and Technology, Taiwan Received 14 November 2005; received in revised form 21 February 2006; accepted 22 February 2006 Abstract In recent years, the high levels of surface ozone have become a major air pollutant in Taiwan. Over the western plain of Taiwan, where most of the 23 million inhabitants live, the concentration of ozone increased substantially during the last ten years (1994–2003). This increase is puzzling as the emissions of ozone precursors, oxides of nitrogen (NO x ) and volatile organic compounds (VOCs) or non-methane hydrocarbon (NMHC) decreased significantly in the same period. An in- depth analysis focusing on ‘‘total oxidant’’ (estimated by O 3 +NO 2 ) showed that its concentration remained essentially constant in Taipei, Taiwan, during that same ten year time period. Given that the regional tropospheric ozone is increasing, the constant oxidant level suggests that the oxidant production from photochemical reactions in Taipei should have been reduced to some extent, due to the reduction in emissions of ozone precursors. Consequently, the increasing trend in ozone was a result of the reduced titration by NO. These results are important to the formulation of ozone control strategies, especially for newly developed countries like Taiwan. r 2006 Elsevier Ltd. All rights reserved. Keywords: Atmospheric oxidants; Titration effect; Surface ozone trend; Ozone control strategies 1. Introduction The increase in surface ozone (O 3 ) concentrations has been observed in various areas over the north- ern hemisphere (e.g. Vingarzan, 2004, and refer- ences therein). In Taiwan, in recent years, the high levels of surface ozone have increasingly become a major air pollution problem. Since 2001, ozone has replaced particulate matter (PM10) as the major air pollutant responsible for most violations of the ambient air quality standards. In fact, efforts to control air pollutant emissions have significantly reduced ambient levels of nearly all air pollutants in Taiwan during the past decade. The only exception to this improving trend is the increase in ambient ozone concentration. For instance, in Taipei, the largest metropolitan area in Taiwan, the annual average of daily maximum 1 h ozone level increased steadily from 41.6728.1 to 52.4724.3 parts-per- billion by volume (ppbv), or an increase of 26% for the period of 1994–2003. ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.02.018 Ã Corresponding author. E-mail address: ckchou@rcec.sinica.edu.tw (C.C.-K. Chou).