Tropical tropospheric ozone observed in Thailand

The mixing ratios of surface ozone at two rural/remote sites in Thailand, Inthanon and Srinakarin, have been measured continuously for the first time. Almost identical seasonal variations of O-3 With dry season maximum and a wet season minimum with a large seasonal amplitude are observed at both sites during 1996-1998. At Inthanon, the monthly averaged O-3 mixing ratios range 9-55 ppb, with the annual average of 27 ppb. The ozone mixing ratios at Srinakarin are in the similar ranger 9-45 ppb with annual average of 28 ppb. Based on trajectory analysis of O-3 data at Inthanon, the long-range transport of O-3 under Asian monsoon regime could primarily explain the low O-3 mixing ratios of 13 ppb in clean marine air mass From Indian Ocean during Wet season but only partly explain the relatively low O-3 mixing ratios, 26 ppb or less, in continental air mass From northeast Asia either in wet or dry season. The highest O-3 mixing ratios are found in air masses transported within southeast Asia, averaged 46 ppb in dry season. The high O-3 mixing ratios during the dry season are suggested to be significantly due to the local/sub-regional scale O-3 production triggered by biomass burning in southeast Asia rather than long-range transport effect. (C) 2001 Elsevier Science Ltd. All rights reserved.