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Artifacts due to inter-particle and gas-particle interactions during PM2.5 sampling were quantified by comparingthe measurement results between the annular denuder-filterpack system and the filterpack system without denuder.Measurements were carried in Seoul for 10days in each season; Nov. 2004, Jan. 2005, Mar. 2005, and Jul. 2005,respectively. In each day, two 12-h samples were obtained. The concentrations of nitrate and chloride showedseasonal variations mainly due to the availability of ammonium to neutralize nitrate or chloride. Nitrates andchloride losses were prominent in summer. Since most of ammonia was used to neutralize sulfuric acid and formedammonium sulfate in summer, nitrate and chloride could not exist in particles and ammonium loss was smallerthan other seasons.