Dry deposition modeling of nitrate and sulfate by using particle size distribution data

Dry deposition and air sampling were undertaken simultaneously in the ambient air of Ping Tung area by using several dry deposition plates, two MOUDIs (Micro-orifice Uniform Deposit Impactors) and one NRI (Noll Rotary Impactor) from December 1995 to May 1996 in Southern Taiwan. The NRI and MOUDI were used to collect ambient coarse and fine particulate, respectively. Dry deposition plate was applied to collect particle deposition flux. An ion chromatography (Diones 2000i/SP) equipped with 4 mm AG4A-SC and AS4A-SC column was employed to analyze the anion species (NO3- and SO42-). The eluent solution is 1.8 mM sodium carbonate/ 1.7mM sodium bicarbonate. The calculated dry deposition flux for each particle size range was obtained by using measured mass-size distribution data between 0.18 and 100 mu m diameter and a dry deposition model. The mean modeled/measured (Md/ Ma) ratio of dry deposition flux varied between 1.11 and 1.28, between 1.28 and 1.42 and between 0.88 and 0.97 for nitrate, sulfate and total particle mass, respectively. The results indicated that nitrate and sulfate were slightly overestimated by a dry deposition model, while total particle mass was slightly underestimated by this model. In general, by using the particle size distribution data, this dry deposition model can provide a good predication for the dry deposition flux of nitrate, sulfate and total particle mass. More than 87.5 % of nitrate and more than 92.9 % of sulfate dry deposition flux are contributed by particle diameters larger than 10 mu m. This is due to the fact that particle size larger than 10 pm have higher dry deposition velocities (>4.65 cm/ sec) and control the majority of the dry deposition flux.