Nitrogen dry deposition causes pH modification of ecosystems, promotes eutrophication in some water bodies, interferes with the nutrient geochemical cycle on land, and has a deteriorating effect on buildings. In this study, a water surface sampler (WSS) and knife-leading-edge surrogate surface (KSS) covered with both a Nylasorb filter and a greased disk were used to directly measure nitrate dry deposition in Chicago between May and October 1997. Concurrently, the KSS covered with both a citricacid-coated paper filter and a greased disk and the WSS were used to measure ammonia dry deposition. The average measured dry deposition flux for HNO{sub}3 was 3.78±1.24 mg m{sup}-2 day{sup}-1; for particulate nitrate, it was 1.46±0.3 mg m{sup}-2day{sup}-1; and for ammonia gas, it was 2.64±1.15 mg m{sup}-2 day{sup}-1. Nitrate fluxes to the WSS and Nylasorb filter on the KSS were statistically equal, as were the total ammonia fluxes to the WSS and the citric acid-impregnated filter on the KSS.The experimental measurements indicated that HNO{sub}3 and particulate nitrate were the major species responsible for the nitrate flux to the WSS and that ammonia gas was the malor source of deposited ammonia. The average mass transfer coefficients (MTCs) of HNO{sub}3 and NH{sub}3 to the WSS were 1.5±0.22 and 2.46±1 cm/s, respectively. SO{sub}2 and HNO{sub}3 MTCs were statistically the same. After adjusting forthe differences in molecular weights, the HNO{sub}3 and NH{sub}3 mass transfer coefficientswere statistically equal to the SO{sub}2 MTC.
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