Numerical Simulation Of The Factors Affecting The Formation Of The Elevated Layer Of Black Carbon Aerosol And Its Interaction With Turbulence

Studies on the detection of layers with elevated black carbon aerosol(BC)concentrations and the formation conditions of these layers help understand the vertical distribution of BC concentrations,which will provide a basis for the assessment of climate effects and early BC pollution warnings.By using the Weather Research and Forecasting with Chemistry(WRF-Chem)numerical model,we performed a numerical simulation analysis on the authenticity of strong elevated BC concentration layers that were detected by an aircraft in the mixing layer over Harbin,China,which was a high-emission area,on a clear sunny afternoon in the early heating period of 2016.We then discussed possible problems and solutions when non-vertical paths were used to detect the vertical distribution of BC concentrations.Finally,we discussed the favorable conditions for the formation of elevated BC concentration layers by weak vertical flow.The results showed that the horizontal variability of BC concentration in the mixing layer in the observation area in Harbin was sufficiently large during the measurement.This produced a false elevated layer,as detected by the aircraft during one round of spiral flight in the mixing layer.The root mean square of the horizontal distribution of BC concentration did not change with height in the mixing layer during the daytime,but it decreased with the thickness of the mixing layer and was higher in the mixing layer than in the free atmosphere.Therefore,the thinner the mixing layer,in which the vertical distribution of the BC concentration was detected in an inclined path,the stronger interference of the horizontal variability on the detected results.When a spiral flight detection path was used,the aircraft should fly at least two rounds in the mixing layer.In the daytime,due to strong turbulence in the mixing layer,weak vertical uplift was not favorable for the occurrence of elevated BC concentration layers in the mixing layer.In the nighttime,if weak vertical uplift was well matched with the high value of the BC concentration or its vertical gradient,elevated BC concentration layers could be formed in the atmosphere.Specifically,if the BC concentration in the convergence zone of weak vertical updraft was large enough,or if the BC concentration in the center of weak vertical updraft was large enough,it was easy to form a high value layer of BC concentration.Compared with upper layers far from the ground,nighttime elevated layers were easier to form in lower layers near the ground because high BC concentrations or large vertical gradients were more likely to occur in the lower layers.Both cases facilitated the occurrence of large vertical upward transport rates of BC.In the simulation of WRF-Chem SCM model,it was found that under the background of weak vertical updraft,the higher the concentration of black carbon aerosol layer,the greater the gradient,the faster the turbulence diffusion and the faster the mixing.After sunrise,the solar radiation increased,and turbulence developed rapidly.The turbulence kinetic energy in the range of 0.3-0.4 m~2/s~2 developed to the height where the black carbon aerosol layer was located.The black carbon aerosol layer rapidly mixed downward.The black carbon aerosol dome effect strengthened the turbulence kinetic energy in the upper layer and weakened the turbulence kinetic energy in the lower layer,which inhibited the development of the boundary layer and aggravated the pollution.In the middle layer of the boundary layer,the dome effect was the most obvious,and the more obviously the turbulence kinetic energy was weakened.The dome effect is stronger in clean areas than in polluted ones with the same BC loading.