Air quality in Southern Thailand during haze episode in relation to air mass trajectory

Southeast Asia haze with elevated particulate matter (PM10) often blankets Southern Thailand during June-Augustperiod. During the haze period of August 2005, the highest 24h PM10 concentration was 92 μg/m3 in Songkhla and 108 μg/m3in Phuket. Though the levels were still below the 24h PM10 National Ambient Air Quality Standard of 120 μg/m3, they wereconsiderably higher than the annual average levels of 32 μg/m3 in Songkhla and 56 μg/m3 in Phuket. The variation in dailyPM10 levels during this period was largely dependent on the prevailing wind directions and the intensity of fire, expressed asthe number of hotspots on NOAA satellite images, at the upwind regions. The backward trajectories obtained by HYSPLIT4model revealed that on the days when PM10 peaked the air masses arriving at the cities passed over the intensive fire regionin Sumatra Island. The peak PM10 levels were observed in Phuket on 16th and in Songkhla on 13th August 2005, i.e. duringthe same period when a large number of hotspots were seen in Sumatra Island of Indonesia (10th-16th August 2005). The PM10level dropped when the wind changed direction from southwesterly to southeasterly hence the air mass trajectory did notpass the fire region. The transport time of air masses from the source region to Southern Thailand during this period wasaround 2-3 days.