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 range, 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.