关中地区气溶胶辐射效应对大气温度层结和边界层高度影响的观测研究

Aerosols alter surface temperature and atmospheric adiabatic heating via direct radiative effect (DRE) consisting of both absorption-induced atmospheric heating and scattering- and absorption-induced surface cooling, which suppresses dispersion of pollutants and leads to further increases in aerosol concentration in the lower planetary boundary layer (PBL). Although the Aerosol-PBL interactions play an important role in enhancing the haze pollution in China, some aspects of the feedback mechanism, for instance the relative contributions of scattering and absorbing aerosols, are not well understood. Few studies have been conducted to quantify the separate roles of the radiative effects of absorbing and scattering aerosols in perturbing the vertical profiles of temperature, and they showed large discrepancies in the responses of air temperature to the aerosol DRE in the PBL. Moreover, the single scattering albedo (SSA), as a primary determinant of whether aerosol has warming or cooling effects in the earth system, was not well-considered in those studies. It is thus necessary to understand the impact of DRE on the atmospheric temperature structures and the PBL height (PBLH) by taking both AOD and SSA into account. .The complex terrain of Guanzhong basin (GZB), which is nestled between the Qinling Mountains in the south and the Loess Plateau in the north, facilitates the occurrence of air stagnations and the accumulation of air pollutants, and along with the fast-growing industries and city expansions, heavy air pollution frequently engulf the basin, causing the basin to be one of the most polluted areas in China, and thereby providing a unique opportunity to study the impacts of aerosol DRE on the atmospheric vertical temperature structure and PBLH. This study will take advantage of extensive measurements made from “Qinling aerosol and cloud microphysical field observation” and “Megacities meteorological observation experiment at Xi’an” to investigate how aerosol DRE acts on the atmospheric vertical temperature structure and the PBLH, with special emphasis on the influence of SSA on the relative contributions of scattering and absorbing aerosols to the changes in atmospheric vertical temperature structure and PBLH. The extent and mechanism of the impact of aerosol on surface air quality will be summarized based on these observation results. The data and knowledge obtained from this project will play a role in providing scientific support for making sound policies for controlling air pollution in GZB.

气溶胶对边界层（PBL）的反馈作用对重污染形成有重要影响，但其散射、吸收性组分在其中的相对贡献仍存在巨大不确定性，这与模式对化学组分，或者说单次散射反照率（SSA）的模拟误差密切相关。受观测资料限制，相关模拟研究很少对SSA进行充分的讨论，基于系统性的观测资料对这一问题开展研究很有必要。.受地形和人为活动影响，关中静稳天气多发、大气污染严重，为开展相关观测研究提供了理想场所。本项目将基于“中国气象局秦岭气溶胶与云微物理野外科学试验基地”和“西安超大城市观测试验”获取的气溶胶物理光学特性和边界层气象综合观测资料，开展晴空条件下气溶胶直接辐射效应对大气温度层结和PBL高度（PBLH）的影响研究，并重点分析SSA在其中的作用，以期揭示散射性和吸收性气溶胶组分对大气热力结构的影响幅度、贡献比例及其对SSA的依赖关系，获取气溶胶改变大气温度层结和PBLH的直接观测证据和关中气溶胶-PBL反馈机制。

气溶胶通过直接辐射效应改变地表温度和大气热力结构，进而改变大气稳定度、边界层高度和近地面空气质量，这一过程对大气重污染的形成具有重要影响，但其中部分机理细节还不明晰，气溶胶散射性组分和吸收性组分对反馈作用的相对贡献仍不清楚。为此，本项目综合利用气溶胶和边界层气象观测资料，在关中地区开展不同类型气溶胶直接辐射效应对大气温度层结和边界层高度的影响研究。.1) 关中大气污染严重且气溶胶吸收性极强，550nm气溶胶光学厚度(AOD)、400-800nm气溶胶波长指数(AWE)和500nm气溶胶单次散射反照率(SSA)的年均值分别为0.54±0.35、1.01±0.41和0.834±0.06。AOD和AWE季节变化明显，其中，AOD以冬季最高，夏季最低；AWE以夏季最高，春季最低。SSA具有强烈的波长依赖性，在沙尘气溶胶含量高的春季，SSA随波长增加而上升，其他三季SSA随波长增加而下降，导致SSA季节变化相对复杂。关中地区各个季节沙尘事件发生时期的AOD均小于非沙尘期AOD，两者比例在0.42-0.63。.2)关中地区气溶胶对整个地气系统以冷却作用为主，同时具有强烈的大气加热作用，其在地表、大气顶和大气内部引起的直接辐射强迫年均值分别为-43.99、-13.75和30.24 W/m2。由于垂直分布模态和光学性质的差异，沙尘气溶胶在大气顶引起的辐射强迫效率为+3.70 W/m2，散射性和吸收性气溶胶在大气顶引起的辐射强迫效率分别为-37.75和-18.11W/m2，前者对地气系统表现为净加热作用，后两者表现为净冷却作用。沙尘气溶胶在高空引起的辐射加热率远大于非沙尘期间的散射性气溶胶和吸收性气溶胶，但其对近地面大气温度层结的影响相对较弱。.3) 气溶胶直接辐射效应对地面气温和大气热力结构具有显著影响。每增加1个单位AOD，将使地面气温下降-3.11 ℃。通过分析不同高度气温垂直递减率与AOD的关系，发现气溶胶含量增加将增强边界层内部大气稳定性和上部大气不稳定性。就全年平均而言，强吸收性气溶胶所引起的近地面大气稳定度的增强幅度是弱吸收性气溶胶的两倍，并且，PBL上方大气不稳定性的变化与吸收性气溶胶含量显著相关，而与散射性气溶胶无关。在地面污染程度相当的情况下，吸收性气溶胶对应的有效边界层高度显著低于散射性气溶胶，表明吸收性气溶胶在关中地区气溶胶—PBLH互馈作用中起主导作用。