Characteristics Of Water Soluble Ions In PM_2.5 And CCN In North China:A Simultaneous Measurement At The Summit And Foot Of Mt.Tai

To better understand the characteristics and sources of water soluble ions（WSI）in North China（NC）,fine particles(PM_2.5)were simultaneously sampled at the summit（SM）and foot（FT）of Mount Tai from May 12^th to June 24^th,2017,and the specific conclusions are as follows:The concentration of WSI decreased in recent years,especially,SO₄^2-decreased both in mass concentration and proportion.NO₃^-concentration changed little,but the proportion of NO₃^-increased,indicating that although the stationary sources was still the main sources of WSI in the NC,the contribution of mobile sources has become evident.The concentration of WSI was significantly lower at SM(22.26±16.53μg·m^-3)than that at FT(31.02±21.92μg·m^-3),of which secondary ions(SO₄^2-,NO₃^-and NH₄⁺)accounted for the main part of WSI,respectively.The difference of WSI concentration was not obvious at SM in daytime and night,but varied greatly at FT,which was affected by BLH.Affected by the temperature difference between day and night,the concentration of NO₃^-was lower at night at FT.The mass concentration of SO₄^2-was almost the same day and night,but the proportion of SO₄^2-in WSI was higher in daytime.The analyses also indicated that NH₄⁺existed mainly in the form of NH₄HSO₄ and NH₄NO₃,but（NH₄）₂SO₄ could also exist,especially when emissions of NH₄⁺and NH₃ were increased during daytime at FT.The analysis results of[NO₃^-]/[nss-SO₄^2-]and[NH₄⁺]/[nss-SO₄^2-]can be inferred that NO₃^-was mainly formed by the combination of gaseous ammonia and nitric acid.The results PCA illustrated that secondary aerosols,a mixture of biomass burnings and sea-salts,and crustal/soil dusts were the main sources at Mount Tai.Backward air-mass trajectories showed that air masses with the highest frequency and WSI concentrations were originated from the southwest.A severe pollution event occurred during June 14^th–June 16^thwas documented at both FT and SM,which was affected by the regional pollution in the NC and the passage of an anti-cyclonic high system.Southerly and southwesterly airflows carried pollutants from Shanxi and Henan province to Mount Tai,and pollutants were topography-forced uplifting along the southern slope of Mount Tai,resulting in a simultaneously high concentration of WSI at SM and FT.The number concentration of CCN at Mount Tai increased with the increase of super-saturation（SS）during the observation period,and presented obvious continental features according the fitting of CCN spectrum.The diurnal variation of CCN showed a bimodal distribution,with the highest concentration occurring at 16:00–17:00 PM.A lower concentration peak appeared at 6:00 AM,which was speculated to be related to the tourists watching sunrise at SM,the joss sticks burning of temples and morning cooking process of hotels.And the diurnal variation of CCN under different SS was essentially consistent.In addition,the number concentration of CCN presented significant positive correlation with the mass concentration of WSI,and the correlation coefficient decreased with the increase of SS.