北京地区降水化学组分来源及气溶胶各组分湿清除效率研究

Wet removal of aerosol is not only one of the important processes of the demise of haze, but also is an important factor to affect the acidity of precipitation. Recent studies imply the acid rain in Beijing is becoming the latent problem with the acidity of precipitation increased year by year. Evidences in observation show that concentration of SO2 in Beijing (one of the precursors of acid rain) is decreased obviously under the control of government strategy in desulfurization, while the sulfate in aerosol and precipitation have no significant change and even increasing trend is observed in nitrate. To prevent the deteriorate of acid rain more seriously, the priority step is to answer the sources of inorganic secondary formation pollutants, such as sulfate and nitrate. This study is focused on the characteristics and sources of the wet deposition in Beijing. Firstly, the wet removal ratios of aerosol compositions will be calculated through the measurements of the concentration in both aerosol and precipitation simultaneously. Seasonal characteristics and the differences between the other studies will be captured to study its uniqueness in Beijing and North China Plane (NCP). Secondary, the average coefficients of wet removal of certain aerosol composition under the cloud will be estimated from the height of the bottom of cloud observed by radar. This coefficient is very important to the parameters of wet deposition in models and will be used to construct a new acid transport model available to NCP by replacing the calculated coefficient. Finally, the sources of the wet deposition in Beijing will be traced through the model, multi-step sampling and the isotope of inorganic composition in precipitation.

气溶胶湿清除不仅是大气灰霾消亡的重要过程之一，同时也是影响降水酸度的重要因素。近年来的研究表明，北京市降水酸度逐年增强，酸雨问题日益突出。大量监测数据显示北京大气酸性前体物二氧化硫浓度大幅减小，但降水和大气中的硫酸盐浓度下降并不明显，同时降水中的硝酸盐浓度却在逐年递增,降水酸度逐年递增。为阐明北京及北方酸雨加重的成因，急需探究降水中硫酸盐和硝酸盐的来源和清除机制。本研究以北京地区降水化学组分为研究对象，基于两种观测手段，即同时观测降水和气溶胶中相应组分和对北京325米高塔分层观测，探明不同季节不同气溶胶组分的云下湿清除机制，获得不同雨强的云下清除系数并实现模式参数化，构建一套适合华北地区酸沉降输送模型。以该模型为基础，结合分段采样以及同位素谱测定，综合分析北京降水化学组分的地区及行业来源，并阐明各组分对降水酸度的贡献。

通过分析三年北京分段采样的降水样品发现，降水样品中NH4+，SO42-，NO3-和Ca2+含量最高，与早期研究相比均有所下降。SO42–/NO3–的比值为1.5，NO3–和NH4+比重显著增加。综合利用相关性分析、主因子分析以及后向轨迹等方法，探讨了北京地区降水中各离子组分的来源，结果表明南-东南气团的主要受到人为源污染的影响SO42–以及NH4+浓度较高，主要受人为活动排放的影响。总体上各离子的云下冲刷作用略大于云内清除，其中Ca2+以及NO3–云下清除的占比大于SO42-和NH4+。与其他城市相比北京地区云内清除的贡献略大，表明长距离输送的影响不可忽视。NO3−，SO42−和NH4+三种水溶性无机盐离子平均云下湿清除系数分别是：2.63×10-4 s-1, 6.35×10-4 s-1和2.37×10-4 s-1，彼此之间的云下湿清除系数的变化很大，相对于SO42−和NH4+而言，NO3−的云下湿清除效率更强。针对北风的清除作用和大气气溶胶垂直方向上的分布对湿清除系数的影响，对影响因子做了不确定分析。考虑了北风清除作用的结果降低了风速对云下湿清除系数的不确定性。而考虑了垂直方向上的分布则使得结果的不确定性增加，这是由于缺少气溶胶离子垂直观测的缘故。