To investigate the acidity variation characteristics of fine particulate matter (PM_(2.5)) in the areas down-wind of Jing-Jin-Ji region in winter, the Qimu Island Environmental Monitoring Station, situated in Shandong peninsula, conducted a PM_(2.5) sampling project for the period from 3 January-11 February, 2014 according to the sample collection schedule: one sampling every 12 hours. The analytic results of the samples showed that the average mass concentrations of PM_(2.5) during the sampling period were (77.1±58.7) μg/m~3; the total content of water soluble ions including SO_4~(2-), NO_3~-, Cl~-, NH_4~+, Ca~(2+), K~+, Na~+ and Mg~(2+) accounted for (37.3±16.3)% of PM_(2.5) mass, among others, the share of SO_4~(2-), NO_3~-and NH_4~+ were the most, and no obvious variation of the contents of PM_(2.5) and the ions was noted day and night, but the variation was comparatively remark- able according to the trajectory classification of clustered air mass. On the basis of the assessment by the ion balance method and modeling of ISORROPIAⅡ, total acidity and in-situ acidity were (309±544) nmol/m~3and (193±292) nmol/m~3, respective- ly, and no notable variation of total acidity of PM_(2.5) was observed for the day and night but the change was significant according to classified clustered air mass. The M-K test indicated that acidity decreased when the 2nd class clustered air mass oc- curred, and the linear regression analysis suggested that it was the in-situ acidity that notably influenced the total acidity and the water content in PM_(2.5) was unlikely the main factor that restricted acidity of PM_(2.5).</p
