Vertical Distribution Of Physical And Chemical Characteristics Of PM_2.5 At Mt.Hua And The New Particle Formation

In this study,two sampling sites at the mountainside and the foot side of Mt.Hua,?vertical elevation difference is about 700 m?were chosen for investigation on the vertical distribution of physical and chemical properties of atmospheric aerosols in Guangzhong Basin,inland China.PM_2.5 aerosols had been collected at the two sites simultaneously from August 27 to September 17,2016,along with the online measurements on the concentrations of SO₂,NO_x and O₃ gases,particle number concentrations and cloud condensation nuclei?CCN?concentrations by using SO₂/NO_x/O₃ gas monitors,scanning mobility particle sizer?SMPS?and CCN Counter.PM_2.5 aerosols at the two sites were collected by using a high-volume sampler with a4 hr interval and analyzed for inorganic ions,organic carbon?OC?,elemental carbon?EC?,water-soluble organic carbon?WSOC?,and water-soluble organic nitrogen?WSON?.The vertical profiles of concentration,composition,and diurnal variation of the fine particles and gaseous pollutants were characterized.New particle formation events and CCN concentrations at the mountains are compared with those in Xi'an,a nearby city with a distance of 80 km southwest to the two Mt.Hua sites.Our results showed that PM2.5 concentrations at the mountainside and the foot side of Mt.Hua during the summer campaign were 46.9±38.2?g·m^-3 and 76.0±44.3?g·m^-3,respectively.The concentration at the foot side was 60%higher than that at the mountainside,which was consistent with the difference in visibility at the two sites that is on average 22.2 km at the mountainside and 14.1 km at the foot side,respectively.Such differences in PM_2.5 loadings and visibilities could be explained by the significant industrial emissions,motor vehicle exhausts,and residential emissions,which resulted in serious air pollution at the foot side than at the mountainside.According to the second grade 24-hr average value of China National Ambient Air Quality Standard(75?g·m^-3),we identified 2016.9.6-2016.9.11 as the clean period,during which the daily PM_2.5 was less than 75?g·m^-3.In contrast,we identified 2016.9.12-2016.9.17 as the haze period,during which the daily PM_2.5 was higher than 75?g·m^-3.Our results showed that the content of organic matter?OM?at both sites was higher in the clean period than in the haze period.In contrast,the concentration of SNA?sulfate,nitrate,and ammonium?in the haze period was much higher than that in the clean period,which was in part due to the higher relative humidity?RH?in the haze period that is favorable for the heterogeneous formation of sulfate and nitrate.Concentration(24.0±15.0?g·m^-3)of inorganic ions in the PM_2.5.5 samples was higher at the foot side than that(16.6±15.7?g·m^-3)at the mountainside with an order of mass concentration as SO₄^2->NO₃^->NH₄⁺>Ca²⁺.Among all the inorganic ions,SO₄^2-,NO₃^-and NH₄⁺are the dominant species,accounting for 89%and 85%of the total at the mountainside and the foot side,respectively.The fine particulate NH₄⁺mostly existed in the forms of?NH₄?₂SO₄ and NH₄NO₃ at the mountainside and in the forms of NH₄HSO₄ and NH₄NO₃ at the foot side.Concentrations of ions in PM_2.5 at the mountainside showed a“unimodal”pattern,peaking at the noontime due to the development of the boundary layer and the influence of the valley wind,which transported more pollutants onto the mountainside from the lowland regions.In contrast,those at the foot side showed a“bimodal”pattern with two peaks occurring in the morning and night,respectively,mainly due to the vehicle emission influence.Acidity of the fine particles was estimated by using the equivalent ratio of anions to cations and the thermodynamic equilibrium model of ISORROPIA-II,respectively.Both methods showed that the acidity of PM_2.5 at the foot side?pH=2.9±2.0?is stronger than that at the mountainside?pH=3.2±2.3?in the Mt.Hua region.The concentrations of OC,EC,WSOC,and WSON of PM_2.5 at the mountainside were lower than those at the foot side.Mass ratio of WSOC/OC at the mountainside was 0.66,which was about 20%higher than that at the foot side?0.53?,indicating an enhanced production of secondary organic aerosols?SOA?during the aerosol transport process either from the foot side to the mountainside on the daytime or from a long-range transport at night.OC/EC and WSOC/OC ratios were higher during the daytime than those during the nighttime at both sites,which can be explained by the daytime photochemical oxidation of aerosols.Moreover,higher concentration of O₃ at the mountainside also strengthened the aerosol aging effect.Correlation between WSON and NH₄⁺was more significant than that between WSON and NO₃^-,indicating that WSON in the Mt.Hua region was mainly derived from agricultural sources.Due to the high temperature,low humidity,and high concentrations of precursor gases in the summer,more new particle formation?NPF?events happened in Mt.Hua and Xi'an during summer,in comparison with that during winter.We found that in the NPF periods the precursor gas?e.g.,O₃ and SO₂?concentration was higher,the total number concentration of particles was also higher,but PM_2.5 concentration was lower and geometric mean diameter?GMD?of ambient particles was also relatively smaller,compared to those in the non-NPF periods.In addition,CCN concentrations at the mountainside and the foot side of Mt.Hua and the urban site of Xi'an showed that the CCN concentration was the highest at the foot side,followed by Xi'an and the mountainside,because the total number concentrations of aerosols at the mountainside was the lowest.The diurnal variation of CCN concentration showed a“unimodal”characteristic at the mountainside,and a“bimodal”characteristic at the foot side,being similar to those of PM_2.5 and the major ions.