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

Abstract

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.