| In recent years,eddy covariance methods have been widely used in the flux measurement of aerosols.However,due to the characteristics of aerosol particles,there are still many problems in the actual measurement.In this paper,the TSI3788 water-based aerosol particle counter and ultrasonic anemometer were used to measure the aerosol concentration flux on the tower of the University of Science and Technology of China,and the factors affecting the aerosol flux measurement such as Turbulence characteristics,aerosol concentration with meteorological elements and aerosol turbulence characteristics were analyzed.The three-dimensional wind speed measured by ultrasonic anemometer is used to analyze the isotropic characteristics of turbulent flow,and the statistical characteristics of the largest isotropic scale are obtained.Combined with aerosol concentration data and meteorological data,the influence of meteorological elements at the measurement site on aerosol concentration was obtained.By analyzing the high-frequency aerosol number concentration data,the same and different between the aerosol particles and the scalar gas are obtained.The main research contents of this paper are as follows:A dimensionless anisotropic tensor was introduced and the turbulent tensor anisotropic coefficient,defined as C,where C=3d3+1(d3 is the minimum eigenvalue of the tensor)is used to characterize the turbulence anisotropy or isotropy.Turbulence is isotropic when C is close to 1,and turbulence is anisotropic when C is far less than 1.Three-dimensional velocity data collected using a sonic anemometer were analyzed to obtain the anisotropic characteristics of atmospheric turbulence in the urban surface layer.The tensor anisotropic coefficient of turbulent eddies at different scales were calculated.The analysis shows that C is strongly dependent on atmospheric stability(z-zd)/LMO,where z is the measurement height,zd is the displacement height,and Lmo is the Obukhov length.The turbulence at a specific scale under unstable conditions(i.e.,(z-zd)/LMo<0)is closer to isotropic than the turbulence at the same scale under stable conditions.The maximum isotropic scale of turbulence is determined based on the characteristics of the power spectrum in three directions.Turbulence does not behave isotropically when the eddy scale is greater than the maximum isotropic scale whereas it is horizontally isotropic at relatively large scales.The maximum isotropic scale of turbulence is compared to the outer scale of temperature,which is obtained by fitting the temperature fluctuation spectrum using the von Karman turbulent model.The results show that the outer scale of temperature is greater than the maximum isotropic scale of turbulence.A Combined with aerosol particle concentration,particle size distribution and meteorological data such as temperature,relative humidity,wind speed,wind direction and short-wave radiation,the concentration of coarse mode(d>2.5?m,d is particle diameter),accumulation mode(0.25?m<d<2.5?m)and ultrafine(Aitken and nucleation mode,d<0.25?m)particles under different meteorological conditions were presented.The solar downward shortwave radiation decreases(rises),after about 10-20minutes,the ultrafine particles concentration decreases(rises).Diurnal change in radiation causes daily change in the concentration of ultrafine particles,which are subject to the relative humidity at that time.Because of the inhomogeneity of urban pollution sources,particle concentrations were highly dependent on wind speed and direction.For the observation sites in this paper,ultrafine particles concentration was higher when the wind direction was south while accumulation mode particles concentration was higher when the wind direction was north.The concentration of accumulation mode particles varies differently during heavy precipitation and slight precipitation.Because of the combined action of wet deposition and hygroscopic growth,the concentration of accumulation mode particles increases firstly and then decreases.And during the slight precipitation,the concentration only increases with the relative humidity increases.The distribution characteristics of normalized standard deviation and power spectrum were obtained by processing the 10Hz aerosol number concentration data.The normalized standard deviation of the aerosol particle concentration is larger during the day and smaller at night,and the normalized standard deviation of aerosol concentration on sunny days is not much different from that on cloudy days.Comparing the data of different heights,it is found that the standard deviation of the normalization of the roof is larger than that of the lawn.The aerosol particle concentration power spectrum has a certain noise signal,which is derived from the intake duct,instrumentation and observation platform.As the variance increases,the frequency at which noise points appear increases,and the proportion of noise signals decreases.The lower part of the inertia sub-region basically satisfies the-5/3 law.As the frequency increases,the slope gradually deviates from-5/3,which is mainly caused by laminar flow in the intake pipe.Under the unstable layer,the normalized variance of the three-dimensional wind speed,temperature and water vapor varies with the stability parameter z/L are similar to those obtained by the previous researcher formula.The normalized standard deviation of the aerosol particle concentration does not change substantially with stability.The eddy covariance method was used to observe the concentration flux of aerosol particles from January 19 to 27,2019.Since the underlying surface is a typical urban underlying surface,the three-dimensional wind velocity in the calculation of aerosol flux is rotated by the planar-fit method.By analyzing the CO2 flux data for one year,it was shown that the difference of the CO2flux obtained by the three-rotation method and the planar-fit method is about 9%.Through the analysis of long-term CO2flux data and the integration of different frequencies of the cross-spectrum,30 minutes was used as the average time in this experiment.At the same time,the maximum delay covariance is used to determine the delay time of the aerosol particle counter and the ultrasonic anemometer,so that more accurate flux data is obtained.The results of the flux measurement show that the flux during the observation period is basically positive and has obvious characteristics of daily variation,which is larger during the day and smaller at night.Foot-print analysis shows that the aerosol flux at the observation point is mainly contributed by tens of meters to hundreds of meters.By assuming uniform distribution of particles of different particle sizes,the vertical flux of PM 10 was obtained,and the variation of aerosol flux in different weather backgrounds during the measurement period was analyzed.During heavy pollution,the decrease in aerosol flux leads to an increase in PM 10.In continuous sunny days,the PM 10 flux exhibited significant diurnal variations.PM 10 and its flux decreased significantly during the cooling process.The results of eddy correlation method and large aperture scintillation show that the variation of PM 10 flux obtained by the two is consistent,but the specific values are slightly different,which may be caused by the difference of flux source area. |
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