Prapat Pentamwa and Nguyen Thi Kim Oanh

pp. 539 - 546

Abstract

Southeast Asia haze with elevated particulate matter (PM₁₀) often blankets Southern Thailand during June-August period. During the haze period of August 2005, the highest 24h PM₁₀ concentration was 92 µg/m³ in Songkhla and 108 µg/m³ in Phuket. Though the levels were still below the 24h PM₁₀ National Ambient Air Quality Standard of 120 µg/m³, they were
considerably higher than the annual average levels of 32 µg/m³ in Songkhla and 56 µg/m³ in Phuket. The variation in daily PM₁₀ levels during this period was largely dependent on the prevailing wind directions and the intensity of fire, expressed as the number of hotspots on NOAA satellite images, at the upwind regions. The backward trajectories obtained by HYSPLIT4 model revealed that on the days when PM₁₀ peaked the air masses arriving at the cities passed over the intensive fire region in Sumatra Island. The peak PM₁₀ levels were observed in Phuket on 16^th and in Songkhla on 13^th August 2005, i.e. during the same period when a large number of hotspots were seen in Sumatra Island of Indonesia (10^th-16^thAugust 2005). The PM₁₀ level dropped when the wind changed direction from southwesterly to southeasterly hence the air mass trajectory did not
pass the fire region. The transport time of air masses from the source region to Southern Thailand during this period was around 2-3 days.