大气气溶胶干沉降研究进展

An overview of atmospheric aerosol dry deposition

从干沉降速度定义出发,综述了过去几十年来国内外在气溶胶干沉降实验技术和理论方面的主要进展.气溶胶粒子从大气向地表沉降的过程决定于颗粒物的粒径、密度和空气粘性系数,同时受空气动力学阻力、粘滞层阻力和表面收集阻力的影响,这些阻力分别与大气层温度、风速、相对湿度等微气象条件密切相关.获取干沉降速度的方法主要有示踪法、梯度法和涡流相关法等.风速的三维瞬时量、动能、摩擦速度、温度和涡旋扩散系数可由超声风速温度仪测定.由于气溶胶具有很宽的粒径谱分布,通常使用几台仪器(如串联式多级采样器、空气动力学粒径谱分布仪和扫描电迁移率颗粒物粒径谱仪)进行联合测量.干沉降参数化方案中,基于斯托克斯定律的物理模型着重于平衡重力、浮力和阻力的作用,半经验方案则进一步考虑了大气湍流、分子运动以及表面捕获机制包括布朗扩散、碰撞、截留、反弹、热泳和扩散泳.然而,参数化方案预测结果在某些粒径段与外场测量数据仍存在显著差异.结合目前干沉降研究存在的问题,对今后气溶胶干沉降的研究方向和技术方法进行了展望.

This paper presents a review of main research progresses on the aerosol dry deposition experiments and theoretical models over the past few decades,started with the definition of dry deposition velocity.Dry deposition of particles from the atmosphere to the earth is not only dependent on the aerosol size,the aerosol density and the air viscosity,but also affected by the aerodynamic resistance,the viscous resistance and the surface collection resistance,which are related to the micro-meteorological factors such as the atmosphere temperature,the wind speed and the relative humidity.The typical methods for the determination of dry deposition velocity include tracking technique,concentration gradient method and eddy correlation method.The instantaneous three velocity components,turbulent kinetic energy,friction velocity,temperature and eddy diffusion coefficient can be measured by the Ultrasonic Anemometer Thermometer.Aerosol size distribution is commonly obtained by the combined measuring devices(Serial multistage sampler,Aerodynamic Particle Sizer and Scanning Mobility Particle Sizer)due to the wide particle size range.In the parameterization of the particle dry deposition,the physical scheme is based on the Stokes’Law which emphasizes the equilibrium of gravitation,buoyancy and drag force,while the semi-empirical formulae introduce the atmospheric turbulence,the molecular motion and the surface collection mechanism which accounts for particle Brownian diffusion,collision,interception,rebound,thermophoresis and diffusiophoresis.However,the discrepancy between model predictions and field measurements is still significant for a certain particle size range.In view of the open problems in the dry deposition,perspectives on the future research directions and techniques are provided.