INTRODUCTION
In many countries in Asia, open burning for agricultural land clearing is commonly practiced. In particular, open burning of rice straw after harvesting in Southeast Asia is intensive during the dry season, which may contribute significantly to the ambient air pollution. In Thailand, where rice is the major agricultural product, the area of annual harvested rice paddies, including both the major crop (growth season: May-October) and minor crop (November-April), is about 9.6 million hectare1. Based on our questionnaire survey, over 90% of the rice paddies during the high harvesting season (November-December) are burned. The intensive burning period of rice straw from both crops lasts from November to April of the following year2.