The episode was aggravated by the nighttime strong surface inversion when the Mae Moh Valley was under the influence of high-pressure system in the winter season. Sulfur dioxide emitted from the Mae Moh power plant over the night was trapped and accumulated in the stable surface inversion layer above the Mae Moh Valley developed after sunset. Heat from the sun during morning hours of the following day gradually breaks up the surface inversion layer to the layer where SO2 is trapped and accumulated. Consequently, the trapped SO2 fumigates down to the ground level resulting in sudden rise of ground level SO2 concentration to a very high level between 10:00 a.m. to 14:00 p.m. as shown in Figure 1. However, daily average SO2 concentration was normally below the standard of 300 g/m3.