| The atmospheric aerosols play an important role in air quality deterioration and climatechange. The particle size, size distribution, concentration and chemical component will helpto evaluate the aerosol radiative forcing and provide information for taking air controlregulation as well. Therefore, the study of the aerosol characteristics is a key point in thisfield. Mt. Tai (the summit of1534m a.s.l) located in Eastern China, which was slightlyaffected by the local pollution and could be representative for the regional background. Themeasurement was conducted at Mt. Tai from July2010to Dec2011. All the instrumentsconnected to splitter with a PM2.5inlet and autocycled drier system to keep the relativehumidity of aerosol samples lower than40%. The particle number size distribution (PNSD),together with the aerosol scattering and absorption information were derived by differentinstruments. Chemical component measurement and size segregated sampling wereperformed during the intensive period.The average number concentration of nucleation mode (3-25nm), Aitken mode (25-100nm), accumulation mode (100-1000nm) and particle number concentration of PM2.5was1950,5075,3715and10740cm-3. The number concentration was quite close to those atShangdianzi regional background station in North China Plain, but higher than those in theglobal regional station in China, Waliguan and background stations abroad, like the borealforest and mountain site. The particle number concentration had noticeable seasonal anddiurnal variation, which could be contributed to the gas to particle reactions in the daytime,the development of boundary layer, meteorological factors, as well as the long rangetransportation. The air mass originating from south resulted in the pollutants accumulatingand northwesterly air mass favored the increase of the nucleation mode particles.NPF event were observed on40%of the measurement days, with the frequency as highas that in Beijing urban and rural environment. It occurred most frequently in spring and leastin summer. The mean formation rate was4.0±3.7cm-3s-1, with the net increase rate ofnucleation mode particles,2.6±2.2cm-3s-1and the loss due to coagulation,1.4±1.8cm-3s-1.The mean growth rate was6.1±2.5nm h-1. The case study showed classical NPF eventsenhanced the aerosol mass concentration, scattering coefficient, cloud condensation nucleinumber concentration and hygroscopic parameter.The mean scattering coefficient at different wavelength was180.0(450nm),142.9(550nm) and101.2(700nm) Mm-1and the backscattering coefficient was20.3(450nm),16.7 (550nm) and14.7(700nm) Mm-1, respectively. The mean value of absorption coefficientwas13.0Mm-1. The other information including the single scattering albedo,hemispheric-back scattering fraction and ngstr m exponent were also reported. They were0.88,0.14and1.40, respectively. The optical properties were affected by the surface wind,long range transportation and the development of boundary layer. They also had clearlyseasonal and diurnal variations. Mie model was used to evaluate the effect of chemicalcomponent on the aerosol optical properties. The result showed that the sulfate contributedmost to the extinction and scattering and then came to the particle organic matter and nitrate.Based on the two-component optical equilibrium aerosol mode (absorption andnon-absorption component) and MIE simulations, the optical closure experiment wasperformed. Two extremely mixing states, internal and external, were considered in the model.The result showed that the simulated scattering and backscattering coefficient were quiteclose to those measured, with the difference smaller than10%and3%, respectively. MonteCarlo uncertainty simulation had been done for the optical closure and gave the uncertaintyof MIE model and it was8%. Considering all the possible uncertainties, the simulatedparameters at two extreme states covered78%scattering data and80%backscattering data.The case studies showed that there was more BC in external mixing state for the freshparticles, but more in internal mixing state for the aged particle.
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