气溶胶对大气CO_2短波红外遥感探测影响的模拟分析

气溶胶引起的光学路径长度改变是影响高分辨率近红外光谱反演大气CO_2浓度的重要误差源.本文利用高精度大气辐射传输模式模拟中国碳卫星观测,结合CALIPSO(Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)卫星的气溶胶廓线产品研究了不同特性的气溶胶对卫星观测光谱的影响.模拟结果显示:气溶胶散射引起的光学路径长度改变与气溶胶类型、模态以及垂直分布密切相关;城市型和海洋型气溶胶对观测光谱影响很大;多层分布的积聚模态大陆型和海洋型气溶胶在光学厚度小于0.3时,会引起5%以内的负辐射变化,随光学厚度不断增加会引起正的辐射变化;主要以粗粒子模态存在的气溶胶在不同的垂直分布情况下均会引起辐射的负变化,从而造成CO_2浓度的高估;另外,随气溶胶分布高度变高,负的辐射变化程度会逐渐减小.

Simulation analysis of aerosol effect on shortwave infrared remote sensing detection of atmospheric CO2

The research of carbon dioxide (CO₂) sources and sinks within the carbon cycle is significant for enhancing our understanding of global climate change. Space based measurement of CO₂ concentration in lower atmosphere by reflected sunlight in near infrared (NIR) band has become a hot research topic at present. The global characteristic of atmospheric CO₂ retrieval from NIR is studied using the expected measurement performance of Tansat (Tan Satellite) mission. With the development of CO₂ retrieval algorithms, the light-path modification due to multiple scattering by aerosol is identified as a major source of error when retrieving CO₂ from high resolution near-infrared spectrum. The present study focuses on atmospheric CO₂ retrieval sensitivity to aerosol properties such as aerosol types, aerosol modes, and profiles aiming at the demands for retrieval accuracy of CO₂ no larger than 0.3%-0.5% on a regional scale. Here, we carry out the aerosol scattering effects analysis on retrieving atmospheric CO₂ near 1610 nm using the simulated nadir observation for Tansat based on CALIPSO aerosol profile products and SCIATRAN radiative transfer model. The results show that light path modification due to aerosol scattering is closely related to their types, modes and vertical distributions. For aerosol types, on the one hand, urban aerosol has the most significant influence on the measured radiance, followed by maritime aerosols, and has a much smaller influence for rural aerosol, which will lead to overestimated CO₂ concentration for the typical surface albedo. On the other hand, the measured radiance will decrease with the increase of aerosol optical thickness (AOT) for urban and rural aerosols, but exactly the opposite to maritime aerosols. For aerosol modes and vertical distributions, aerosols in accumulation mode, the continental aerosols with multilayer aerosol vertical distribution and maritime aerosols with AOT less than 0.3 will bring about less than 5% of negative radiance changes, and will cause positive changes with the increase of AOT. However, aerosols in coarse mode will always cause negative changes of radiance regardless of aerosol vertical distribution, and thus resulting in an overestimation of CO₂. In addition, the higher the aerosol layer distributed, the smaller the negative radiance change is. If aerosol profiles can be successfully retrieved as a state vector, then it can be expected that satellite measurement can lead to tremendous improvement in CO₂ retrieval precision. This study provides important information about estimations of the influence of aerosol property on CO₂ retrieval algorithm. All these results can contribute to improving the accuracy of CO₂ retrieval.