Characterization of nitrate particulate dry deposition by vacuum-deposited thin film reaction method

Air samples were collected at a highway intersection, coastal location and suburban area in Taichung, Taiwan for the characterization of nitrate containing particulates (NCPs) in dry deposition from October 2002 to February 2003. Sampling was accomplished with a surrogate surface technique in non-raining days. Collecting tapes were precoated with nitron (C20H16N4) in a vacuum evaporator controlled below 10−6 mm Hg. Immediately after collection, samples were placed in a desiccator with a relative humidity of 85% for 2 h to form distinctive products of NCPs. These products were then examined with a scanning electron microscopy. It was found that the average number frequencies of NCPs in total particulate were 41.9%, 35.7%, and 36.8% at the highway intersection, coastal and suburban areas, respectively. Most collected particulates were non-nitrate containing at these sampling sites. The average volume shape factor value of NCPs at the highway intersection was higher than those at the other sampling sites, suggesting that NCPs at the highway intersections were more irregular in shape. Average NCP dry deposition fluxes were 3.9, 2.2 and 2.1 μg m–2 s–1 and non-nitrate containing particulate dry deposition fluxes were 5.4, 3.9 and 3.2 μg m–2 s–1 at the highway intersection, coastal and suburban areas. NCPs <10 μm in size contributed 3.4%, 9.6% and 6.4% by weight to the total dry deposition at the highway intersection, coastal, and suburban areas, respectively, showing that over 90% of dry deposition was particulates larger than 10 μm in diameter. The number median diameters (NMDs) of NCP were 0.47, 0.39 and 0.57 μm and mass median diameters (MMDs) of NCP were 24.2, 26.3 and 24.6 μm at the highway intersection, coastal and suburban sites, respectively. The coastal area had the lowest NMD and highest MMD. The average values of dynamic shape factor (FD) were calculated as 1.21 for fine particles and 1.38 for coarse particles.