Study On Properties Of Atmospheric Particles And Wet Scavenging Over Semi-Aid Region In North-west China

Particulate matter have an important role in climate change, environmental issues, human health and other aspects. The behavior of particulate matter in the atmospheric environment depends on its particle size and its distribution, and the distribution of the particles is the basis of the study of the pollution of particles. In the research of physical properties of particulate matter, the research of number concentration and spectral distribution is the most direct, which can directly reflect the physical properties of particles. The particle size distributions have a certain regularity, significantly affected by meteorological factors, discussed the impact of meteorological factors on the particle concentration and particle size distribution, which is the basis of particle physics research, optical properties, is helpful for reveal the formation mechanism and source of particulate matter. The wet removal of precipitation is an important sink of particulate matter, and the scavenging coefficient is an important parameter of precipitation removal. By analyzing the changes in the number concentration of different particle size segments before and after the precipitation, Calculating scavenging coefficient, to further exploring the relationship between the scavenging coefficient and precipitation intensity and rainfall duration, has some reference value for understanding wet removal mechanism in-depth, assessing the effect of precipitation on removing particles, improving air quality models forecast accuracy, and controlling particulate pollution.From June,2007 to December,2008, using the data of APS-3321 particle size spectrometer of semi-arid climate and Environment Observatory (SACOL) of Lanzhou University, analyzing characteristics of particle concentration with time and concentration distribution, studing on the correlation between particulate matter and meteorological elements using the meteorological data of meteorological observation tower of the same period, investigating the effect of temperature, relative humidity and wind speed on the particle number concentration. In the paper, use precipitation data of observation period to to calculate scavenging coefficient,analyze the removal effect of precipitation on the number concentration of different particle size segment,discuss the relationship between the scavenging coefficient and precipitation intensity, precipitation duration, precipitation. The main conclusions are as follows:(1) In 2008,the maximum value of the particulate number concentration appeared in January, the minimum value occurred in June, the main reason is the heating and negative diffusion conditions in January increased the number of fine particles, wet deposition and wind speed and other favorable diffusion conditions in June decreased particle number concentration. The annual change of the concentration of mass was bimodal type, the maximum appeared in February, and the second peak value occurred in May, the peak of May was related to the number of coarse particles during the passage of dust weather.(2) Particle size distribution characteristics in background weather conditions showed that atmospheric particle number concentration spectrum is unirnodal type, and peak mainly occurred around 0.673 μm, mainly in the accumulation mode. Surface concentration distribution is bimodal, with a peak at around 0.723μm and 3.051μm. Mass concentration distribution is bimodal, with peak at about 0.777μm and 5.829μm.(3) Dust weather conditions, the particle number concentration spectrum is unimodal type and peak occurred at 0.673μm, particle number concentration of more than 1μm is 3 times of particle number concentration in background weather conditions. The mass concentration spectra in the dust weather is unimodal type, and the peak value occurred at 7.324 m.(4) The distribution of the particle size distribution is fitted by the normal distribution and the J spectrum, and the relative error of fitting is 0.357.(5) The concentration of particulate matter was negatively correlated with temperature, and was positively correlated with humidity. The effect of wind speed on the particle concentration exists a critical value, when the wind speed is less than 4m/s, and the concentration of the particles decreases with the increase of wind speed. When the wind speed is greater than 4 m/s, as the wind speed increases, the particle number concentration increases, it may be associated with the threshold wind velocity of surface dust.(6) Scavenging coefficient change in the range of 3.38·10-5 to 3.15·10-4-1. The precipitation process has different effects on the removal of the number of different particle size segment, the study found that number concentration increases after precipitation in particle size of more than 2.5μm and less than lOμm. which may be related to the increase of the particle moisture absorption. The scavenging of precipitation of the number concentration of particle size segment of less than 1μm and particle size segment more than 10μm is obvious, but the minimum value of the particle in particle size of more than 2.5μm and less than 10μm is the lowest.(7) The precipitation intensity had obvious effect on the particle number concentration, and the correlation coefficient reached 0.715.