| Abstract | The objective of this study was to monitoring wet and dry deposition in Bangkok Metropolitan Regions. The Pollution Control Department and Asian Institute of Technology sites were chosen as to represent urban and suburban areas. This study conducted weekly sampling for 9 months (June 2016 – February 2017) and the results were compiled with the previous study data for the entire monitoring period from (September 2015 to February 2017) The weekly samples of rainwater were collected by an automatic wet-only collector and the sample were then analyzed for water soluble ions (SO42-, NO3-, Cl-, NH4+, Na+, K+, Ca2+and Mg2+) and were used to assess the wet deposition. The gaseous pollutants were collected by the filter pack method and analyzed for SO₂, HNO3, HCl, and NH3. The analyses were conducted using the Ion Chromatograph with establish QA/QC. The gas concentrations together with the acidic components in particulate matters analyzed by a parallel study (Mahawong, 2017) were used in to calculate the dry deposition using the dry deposition velocity obtained by the resistance method. Deposition fluxes, dry and wet, were compared with the critical loads for the Pathumthani province to assess potential impacts on the terrestrial ecosystem. The results shows that the pH in rainwater at PCD and AIT site were 5.24 ± 0.80 and 5.48 ± 0.70while the electrical conductivity were 2.02 ± 1.11and 2.08 ± 1.65 mS/m, respectively. The ion species concentration of rainwater samples at PCD site was: NH4+>Ca2+>NO3->SO42->Cl->Na+>Mg2+>K+ while at AIT site was Ca2+>NH4+>NO3->SO42->Cl-> Na+>Mg2+>K+. The total wet deposition at both PCD and AIT sites, ranked NH4+>Ca2+>NO3->SO42->Cl->Na+>K+>Mg2, had the values of 86.1, 53.7, 39.5, 37.8, 26.0, 17.9, 12.8, and 12.3 mEq/m², respectively, while the values for the species ranking order at AIT were 65.4, 63.5, 39.5, 33.2, 27.2, 17.1, 14.4, and 5.3 mEq/m², respectively. The QC criteria of ion balance (R1) and pAi measured showed that there may be contributions from other acidic species such as organic acids which caused the total cations were higher than the total anion. The criteria for conductivity of R2 however showed that 79.1% was in acceptable range. The gas concentrations ranked was NH3> SO₂> HNO3 > HCl at both sites with the average level at PCD site of 13.5, 2.1, 1.0, and 0.7 ppb, respectively while that at AIT site were 15.0, 1.4, 0.9, and 0.6 ppb, respectively. Ion composition of particulate matters (Mahawong, 2017) showed the rank at both sites of pSO42-> pNH4+>pNO3-.The dry deposition velocity shows higher values of SO₂ and HNO3 over tree cover and building & road, respectively and NH3 over agricultural surface. The dry fluxes of sulfur compounds at PCD and AIT site were 0.44 ± 0.24 and 0.59 ± 0.27 mmol/m².month, respectively, while the dry deposition fluxes of nitrogen compounds were 5.32 ± 5.80 and 6.72 ± 2.22 mmol/m².month, respectively. The total deposition of sulfur (Dry SO₂+ Dry pSO42-+ Wet SO42-) at PCD and AIT site from September 2015 to February 2017 were 37 and 821 kg/km².18 month respectively while the total nitrogen compounds (Dry HNO3 + Dry NH3+ Dry pNO3-+ Dry pNH4++ Wet NO3-+ Wet NH4+) were 3132 and 3043 kg/km2.18 month, respectively. The actual sulfur deposition in 2016 was 586 kg/km².yr that was lower than the critical load values (3,000 –5,000kg/km².yr) while the nitrogen deposition was 2,235 kg/km².yr that was in range the critical load (6,090–9,030kg/km².yr). for the terrestrial ecosystem in Pathumthani hence it was no potential risk in this area. |