| Abstract | Depositions of atmospheric pollutants and associated adverse effects have been widely reported by many studies conducted in the temperate climate. However, such information in the tropical region is still scarce. This study aimed at providing data on dry deposition fluxes of selected airborne pollutants and their deposition velocities (Vd). The study was carried during dry season (November 2005 - February 2006) at a suburb site in the Bangkok Metropolitan Region, Thailand. Dry deposition fluxes of selected air pollutants were directly measured using Noll's deposition plates, which use greased Mylar trips that held on smooth knife-edge acrylic plates. Samples were collected at 2 levels, 1 m and 3nm. At each level samples were collected on both upper and lower sides of the deposition plates. Totally, seven sets of weekly dry deposition samples were collected and analyzed for particulate matter (PM) mass, and particulate content of 13 ionic species (Na+, NH4-', Lie, h+, Mgt+, Ca 2% C1-, F-, Br-, NO,-, N03-, S042-, P043-); and semi-volatile compounds, including twelve PCBs and fifteen chlorinated pesticides. Simultaneously, size segregated airborne PM concentration was collected by a Cascade Inmpactor. About two 72 h samples were taken during a week, which were then matched with the weekly deposition samples. The lower and tipper side fluxes of total PCBs values ranged between 159-189 ng.m-2. day-1 on 1m level and 169-279 ng.m-2.day-1 on 3m level of the plates. The average particulate chlorinated pesticides fluxes observed in the range between 96-144 ng.m-2.day-1 on 1m level and 124-128 ng.m-2.day-1 on 3m level of the plates. Dry deposition fluxes of cations vary from 0.06 to 1.6 mg.m-2.day-1 while those of anions vary from 0.04 to 2.6 mg.m-2.day-1. Deposition fluxes of S042- and N03 ions, the main factors causing acidic deposition, vary in the range of 0.7 - 2.4 mg.m-2.day-1, 0.3 - 2.6 mg.m-2.day-1, respectively. Dry deposition velocities (Vd) were calculated by dividing deposition fluxes with the corresponding average ambient concentrations of the considered pollutants. Vd of PCBs at both levels is in the range of 0.23- 0.34 cm s-1 at the upper sides of plate and about 0.19 - 0.21 cm. s-1 at the lower sides. For chlorinated pesticides, dry deposition velocities at the upper sides ranged between 0.32 -0.35 cm s-1 and at lower sides about 0.26 - 0.28 cm. s-1. Vd of ion is ranging from 0.06 -- 2.4 cm/s on upper sides and 0.02 - 1.3 cm/s on the lower sides. To determine the Vd of PM10 for different size ranges, mass percentage of the collected particles onto dry deposition plates were estimated based on number of particles counted under the Scanning Electron Microscopy (SEM). The values then were used to estimate Vd for PM of different size ranges together with the airborne mass concentration in each range. The deposition velocity of the larger particles of PM10 is higher than that of fine particles on both upper and lower sides. Particles with the minimum deposition velocity were of the diameter range of 0.7-1.1 pm on upper sides of the plates and in the range of 0.4 - 0.7 tm on the lower sides. Relationships between deposition fluxes and meteorological variables (wind speed, wind direction and temperature) were studied through the correlation matrix and PCA using SPSS. Possibility of long-range transported pollutants to the study site was studied using the backward trajectories obtained by the HYSPLIT model |