Trends in Sulfate Dry Deposition over Mixed Dipterocarp Forest in Thailand using Relaxed Eddy Accumulation Method

The relaxed eddy accumulation method (REA) was applied for direct measurement of sulfate fluxes over mixed dipterocarp forest. The sampling system using a four-stage filter pack was designed to take updraft or downdraft air samples in the direction of the vertical wind velocity. The deadband of the velocity profile was set at ±0.5 σw in which an air sample with velocity within the set value is rejected. The forest site was located at a latitude of 13° 35’ 13.3’’N and longitude of 99° 30’ 3.9’’E Ratchaburi province in Thailand. The sampling period was set in 4 time intervals: 6-12, 12-18, 18-24 and 24-6 h for 3 consecutive days/month (n=144) throughout one year. All the micrometeorological parameters for flux estimation were measured in real time. The concentration and flux of sulfate were found to reach a peak value of 1.32 μg m^-3 and 8.35 μg m^-2 h^-1, respectively, at noon time which indicated the effect of atmospheric instability caused by a high heat transfer during the day. The one-year average deposition velocity was observed to be 0.24 cm s^-1.The coefficient β obtained under high ambient temperature and a humid environment in this tropical climate was 0.49. It has also been observed that βis relatively insensitive to atmospheric stability.