| Aerosols can be activated to form cloud condensation nuclei(CCN)under certain conditions,participate in the process of cloud formation and rain,and then affect the global water cycle and radiation balance.Recognizing and understanding the temporal and spatial distribution,spectral distribution,and physicochemical properties of aerosols are still scientific issues that need to be continuously improved in the research of cloud precipitation and regional climate.In order to investigate the aerosol particle size distribution,new particle formation(NPF),CCN distribution characteristics and their relationship at a regional background mountainous site.The number concentration of aerosol and CCN were measured by a wide-range particle spectrometer(WPS)and a cloud condensation nuclei counter(CCNC)at Mt.Tian from July 31 to September 9,2019.With meteorological data and HYSPLIT model,distribution characteristics and sources of aerosol and CCN at Mt.Tian in summer were analyzed.The specific conclusions are as follows:(1)During the observation period,the average aerosol number concentration at Mt.Tian is 5475.6 cm-3,which is dominated by the Aitken mode,accounting for 78.4%.At 0.1%,0.2%,0.4%,0.6%and 0.8%supersaturation,the mean values of CCN number concentration are 187.7cm-3,747.8 cm-3,1671.6 cm-3,2213.1 cm-3 and 2641.4 cm-3,respectively.Using the 3-parameter formula NCCN=N0(1-exp(-BSk))to fit the CCN activation spectrum is better than the 2-parameter formula NCCN=CSk.The daily variation of aerosol number concentration appears a unimodal distribution with a peak at 17:00,and the CCN number concentration appears a bimodal distribution with peaks at 18:00 and 21:00,which were affected by the height of the boundary layer and valley winds.The particle size and supersaturation have a greater influence on the activation of aerosol to CCN.The number concentration of aerosol and CCN in the northerly wind is higher,and the lower relative humidity is conducive to the occurrence of NPF events at Mt.Tian.(2)A total of 19 NPF events occurred during the observation period,with a frequency of46%,which is related to the solar radiation intensity and the diurnal variation of the boundary layer.The average formation rate and growth rate of NPF events are 0.64 cm-3·s-1 and 4.05nm·h-1.NPF events often occur in the weather with higher temperature,lower air pressure,lower relative humidity and higher wind speed.NPF events influenced the diurnal variation of aerosol size distribution greatly,which usually occurs between 13:00 and 17:00.At this time,the aerosol number concentration of nuclear mode and Aitken mode increases sharply,while the increase of Accumulation mode is relatively slow.The NPF event at Mt.Tian is one of the important sources of CCN number concentration.The average growth rate of CCN number concentration is 20-40%after the NPF event.(3)Using the HYSPLIT model,the air masses during the observation period are divided into 4 groups:northeast air mass,the southwest air mass,western air mass,and northwest air mass.Influenced by the source of the air mass and the specific transport path,the concentrations of aerosol and CCN in the northeast and northwest air masses from the north of Tianshan Mountains are relatively high.9 and 7 times NPF events occurred under these two influences,and the average growth rates are 4.44 nm·h-1 and 4.38 nm·h-1,respectively.The other three NPF events happened in the southwest air mass,and the average growth rate was only 2.10 nm·h-1.The results of PSCF and CWT show that there are three main source paths of aerosol at Mt.Tian:Changxi Hui Autonomous Prefecture-Wujiaqu-Urumqi in the northeast,Xinhe County-Luntai County-Hejing County in the southwest,and Shawan County-Shihezi-Hutubi County in the northwest.The main source area of CCN is the urban area of Urumqi in the northeast. |
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