气溶胶云下雨洗过程中的动力学机制研究

It has been known that atmospheric aerosols have strong impacts on climatic and environmental issues. Through radiative forcing, aerosols have direct (scattering or absorption of solar radiation) and indirect (increasing cloud condensation nuclei (CCN)) effects on climate. The deposition of aerosols delivers atmospheric loads of many compounds to ecosystems. Atmospheric aerosols are also responsible for many urban-air-pollution-related problems..Aerosol removal processes remain an important source of uncertainty in many modeling studies, such as the global aerosol transport models, global chemical transport models and air quality models. Obviously, Precipitation scavenging of atmospheric aerosol particles is an important removal process that should be included in the models mentioned above. A parameter known as the scavenging coefficient has been used in the aerosol mass continuity equation in those models to represent below-cloud particle scavenging. To understand thoroughly these problems, aerosol emission, transport, transformation, and removal mechanisms should be quantified at multiple scales. Either the earlier or some of the current models only dealt with bulk aerosol mass without the complexity of the relationships between aerosols and rain drops such as size-resolved number and mass concentrations, the background flow fields, and other parameters affecting the scavenging of aerosols by rain droplets. Unfortunately, recent aerosol model intercomparisons show significant differences in modeled atmospheric aerosol concentrations that might be due to differences in the model representations of wet removal of aerosols..Considering the requirement for the information about the precipitation of aerosols during wet removal process in the modeling programs, especially in below-cloud scavenging process, the aerosol scavenging coefficients would be obtained or improved by employing the combination of fluid dynamic and aerocolloidal dynamic theories. The mechanisms, i.e. diffusion deposition, interception, inertial impaction and phoretic forces, which affecting the collection of aerosols on raindroplets for full range of size distributions, would be investigated under the same flow background of viscous flow and potential flow, respectively. The results would be used to improve the expressions of collection efficiencies for fine and large particles, respectively. Based on these results, the bulk scavenging process would be estimated by numerical simulations under turbulent flow fields with the considerations of multi-mechanisms. The enhancement of the scavenging coefficients, which were obtained in the previous studies with one to two order of magnitude under-prediction of the value for the theoretical calculations relative to the majority of field measurements, would be corrected and improved by using the results of laminar and turbulent analyses of present study.

研究表明，大气气溶胶通过辐射强迫对气候产生直接或间接影响，因而具有强烈的气候和环境效应。气溶胶沉积会向地表生态系统增加环境负荷，并与许多城市大气污染引起的问题有关。在许多模式研究中，对气溶胶清除过程的估计上存在着很大的不确定性。这些不确定性主要是由于对湿清除的表达不同而产生的。. 本研究将考虑有关模式研究中对云下气溶胶雨洗过程估计的要求，根据流体和气溶胶动力学基本理论，考虑全区间气溶胶粒径分布，在粘性流和势流模型下，对粒子在雨滴表面的捕集问题，采用统一背景流场，严格分析在扩散、拦截、惯性碰撞和泳力场联合作用时，气溶胶在雨滴的表面捕集效率，以解决或改进已有研究中，对粒子捕集过程描述的背景流场不一致问题、及由此导致的捕集效率理论值过低问题。在这一基础上，针对湍流背景场，对雨洗气溶胶过程进行整体模拟，考察湍流对实际捕集过程的增益效应。通过对层流和湍流情况的求解，修正捕集效率偏低的理论表述。

云下雨洗是大气气溶胶清除的重要过程。在全球和区域大气气溶胶输送模式和空气质量评估模式等研究与应用中，对气溶胶湿清除效率计算的准确性将影响到这些模式对气溶胶输送过程估计的准确性，但通常采用Slinn关系得到的云下雨洗理论结果与很多实际观测存在偏差，利用不同的雨滴捕集气溶胶效率公式得到的清洗系数与观测值可相差高达2个数量级。.本项目根据流体动力学和气溶胶动力学理论，分别在粘性流和势流条件下，通过严格理论求解和数值模拟，分析了在扩散、拦截和惯性等捕集机理联合作用时的捕集效率，证明了无论雨滴附近的流场是势流还是粘性流，大、小雨滴(尺度为100um到1000um)对亚微米气溶胶的扩散捕集效率都与Peclet数的-0.50次方到-0.54次方有关，而非Slinn关系式表示的与Peclet数的-1/3和-1/2次方均有关。.针对湍流背景场，对雨洗气溶胶过程进行了细致模拟，讨论了湍流扩散对粒子捕集的增益效应，通过引入“碰撞概率”，重新定义了适用于湍流捕集效率计算方法。在此基础上，分析了湍流效应对捕集效率的影响。结果发现，所有粒径粒子的碰撞概率分布特征都会受到背景湍流强度的影响，尤其是对dp < 5um的粒子的影响明显，捕集效率因湍流效应可能增加至少1个数量级。.通过数值求解，给出了一个新的适用于气溶胶云下雨洗过程中雨滴对粒子的惯性与拦截耦合捕集效率的估计。研究还表明，对小雨滴，若同时考虑重力及雨滴表面滑移效应，则集聚模粒子的捕集效率将高于以往研究中采用雨滴硬球模型得到的结果。如对1um左右粒子捕集效率有近1个数量级增加。若粒子密度增大，重力和滑移效应对惯性捕集效率的影响将进一步加强。.考虑气溶胶粒子的疏水性，数值求解了多分散气溶胶多机理联合作用的动力学方程，获得了捕集过程中雨滴-粒子系统三维形态图谱。发现在粗模粒子数浓度较高时，小雨滴表面将出现粒子凝并形态，并对累积模粒子捕集有明显增益效果，可能使效率提高1到2个数量级。因此，经典雨滴捕集效率计算方法可能低估了实际降雨过程中对气溶胶的清除能力。

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