| Cloud is a complex system consisting of gas phase and liquid phase.The research of cloud formation process and cloud formation mechanism can reveal the transformation of atmospheric components and understand the formation and reaction of the pollutants.The Mountain Tai Cloud Observation Experiment was performed from Juln to August,2015,during w-hich 20 cloud events were observed and 17 sets of cloud water samples were collected.A three-stage collector was used to collect size-resolved cloud samples at the summit of Mt.Tai.Subsequently,analyses of acidity,water-soluble ions,and trace metals were performed and bacterial community were conducted using MiSeq amplicon sequencing.Some analyses started immediately after the sampling that a fraction of the collected volume was divided for the pH value and conductivity measurement.The analyses of H2O2.HCHO,and S(IV)were performed within 1h after the sampling to avoid the compound loss.For water-soluble ions,trace metals,and bacterial community analyses,collected samples were stored freeze and measured uniformly after the whole observation experiment.The pH values of the samples decreased as droplet sizes decreased.Sulfate(SO42-),nitrate(NO3-)and ammonium(NH4+)were the main secondary inorganic ions which their concentration distributed significantly different from size-resolved cloud water.NH4+ had a higher concentration in smaller(stage 2 and stage 3)droplets in almost all samples(16 of 17 sets of samples).In most cases.SO42-had the highest concentrations in stage 2(12 of 17 sets of-samples)but the lowest concentration in stage 3(13 of 17 sets of samples).NO3-had a higher concentration in larger(stage 1 and stage 2)droplets.The Ca2+ concentration increased as droplet size increased.Small droplet samples tended to be more acidic,mainly because of the mure acidic(SO42-and NO3-)and less acid-neutralizing(NH4+ and Ca2+)components.The bacterial community in size-resolved cloud samples were firstly recognized and dominated by the genera of Lactococcus(average abundance 34.9%)and Bacillus(average abundance 34.0%).LEfSe analysis revealed differences of the bacterial community in size-resolved cloud water samples.The genus Bacteroides,which was the only genus from the family Bacteroidaceae in our samples.was the characteristic group in stage 1,the large droplet size fraction.The genus Pedobacter was distinctive feature of the stage 2.And the phylum Actinobacteria was the characteristic group in stage 3.These results were probably caused by the bacterial size.Redundancy analysis suggested that the H2O2,NO2-concentrations,and trace metals exerted complex effects on the bacterial community. |
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