A Two-Pronged Attack on the Problem of Aerosol Climate Forcing

基本上，所有认为人为气溶胶影响气候的机制都是通过改变地球的反照率（即地球系统对太阳辐射的反射率）来运作的。卫星的高分辨率反照率产品正在变得可用，与此同时，气溶胶传感卫星正在提供更好的能力来描绘人为气溶胶的全球分布。这表明，更好地理解气溶胶气候强迫的双管齐下的方法可能是有益的——一种侧重于原因（人为气溶胶的全球分布和性质），另一种侧重于效果（反照率的空间/时间变异性）。主要调查人员（pi）正在追求和整合这两个目标。关于第一个目标，pi正在对亚微米气溶胶颗粒大小分数对气溶胶光学深度的贡献进行全球重新评估，以他们小组先前的研究为基础。这个大小部分代表了大部分人为气溶胶。精确的卫星反演这一大小比例是对人为气溶胶全球分布进行经验评估的关键。早些时候，他们与美国国家航空航天局（NASA）的科学家合作，将卫星和现场估计的气溶胶颗粒载荷在这个尺寸范围内进行了比较。当前项目的主要活动包括：(1)对多个卫星仪器和地面太阳光度计遥感反演的气溶胶特征进行相互比较；(2)利用机载和星载激光雷达的垂直分布数据诊断差异；(3)基于光学深度波长依赖性分析的亚微米级气溶胶负荷的全球重新评估；（iv）与美国国家科学基金会资助的一个单独的研究项目协调，将气溶胶观测同化为化学输送模式，以改进对人为气溶胶全球分布的估计。关于第二个目标，pi正在使用一种新的观测工具在各种尺度上检查反照率变率。他们正在使用向下看的激光雷达（机载和星载）来诊断反照率变化的大气原因。他们正在研究介于传统定义的“云”和“气溶胶”之间的状态的普遍性，并试图探测和量化人为气溶胶对区域平均反照率的直接和间接（通过对雾霾和云特性的影响）影响。这项研究的更广泛影响包括它与华盛顿大学大气科学系的教学和推广活动相结合。它包括培养一名博士后科学家和一名研究生。这项研究与几个国家实验室密切协调，例如：美国国家海洋和大气管理局太平洋海洋环境实验室、美国国家海洋和大气管理局地球系统研究实验室、美国国家航空航天局兰利研究中心、美国国家航空航天局戈达德空间飞行中心和美国国家航空航天局艾姆斯研究中心。从这个项目中获得的对气候强迫的改进认识，对于改进对气候变率和变化的了解至关重要。

Essentially all mechanisms by which anthropogenic aerosol is thought to affect climate operate by changing the Earth's albedo (i.e. earth system's reflectivity for solar radiation). High resolution albedo products from satellites are becoming available at the same time that aerosol-sensing satellites are offering an improved ability to delineate the global distribution of anthropogenic aerosol. This suggests that a two-pronged approach to better understanding of aerosol climate forcing may be fruitful - one focused on cause (the global distribution and properties of anthropogenic aerosol), the other on effect (the space/time variability of albedo.) The principle investigators (PIs) are pursuing and integrating these two objectives.Regarding the first objective, the PIs are performing a global reassessment of the sub-micrometer aerosol particle size fraction contribution to aerosol optical depth, building on previous studies by their group. This size fraction represents most of the anthropogenic aerosol. Accurate satellite retrieval of this size fraction is key to empirical assessment of the global distribution of anthropogenic aerosol. Earlier, in collaboration with National Aeronautics and Space Administration (NASA) scientists, they compared satellite to in-situ estimates of aerosol particle loading in this size range. Key activities in the current project include (i) inter-comparing remote retrievals of aerosol characteristics from multiple satellite instruments and ground-based sun photometers, (ii) diagnosis of discrepancies using vertical distribution data from airborne and spaceborne lidar, (iii) global reassessment of submicrometer-size aerosol loading based on analysis of the wavelength dependence of optical depth, and (iv) assimilation of aerosol observations into a chemical transport model in coordination with a separate, NSF-funded, research project for improved estimates of the global distribution of anthropogenic aerosol. Regarding the second objective, the PIs are examining albedo variability at a variety of scales using a new observational tool. They are using downward-looking lidar (airborne and spaceborne) that allows the atmospheric causes of albedo variation to be diagnosed. They are examining the prevalence of states intermediate between traditionally-defined "cloud" and "aerosol" and seeking to detect and quantify the direct and indirect (through impact on haze and cloud properties) influence of anthropogenic aerosol on regional-mean albedo.Broader impacts of the research include its integration with teaching and outreach activities through the University of Washington Department of Atmospheric Sciences. It involves training a post-doctoral scientist and a graduate student. The research is closely coordinated with that of several national laboratories - e.g., National Oceanographic and Atmospheric Administration (NOAA) Pacific Marine Environment Laboratory, NOAA Earth Systems Research Laboratory, NASA Langley Research Center, NASA Goddard Space Flight Center, and NASA Ames Research Center. Improved knowledge of climate forcings derived from this project is essential to improved understanding of climate variability and change.