Bu Ting-ting, Wang Xian-hua, Ye Han-han, Jiang Xin-hua
Guang Pu Xue Yu Guang Pu Fen Xi. 2016 Jan;36(1):186-90.
High precision retrieval of atmospheric CH4 is influenced by a variety of factors. The uncertainties of ground properties and atmospheric conditions are important factors, such as surface reflectance, temperature profile, humidity profile and pressure profile. Surface reflectance is affected by many factors so that it is difficult to get the precise value. The uncertainty of surface reflectance will cause large error to retrieval result. The uncertainties of temperature profile, humidity profile and pressure profile are also important sources of retrieval error and they will cause unavoidable systematic error. This error is hard to eliminate only using CH4 band. In this paper, ratio spectrometry method and CO2 band correction method are proposed to reduce the error caused by these factors. Ratio spectrometry method can decrease the effect of surface reflectance in CH4 retrieval by converting absolute radiance spectrometry into ratio spectrometry. CO2 band correction method converts column amounts of CH4 into column averaged mixing ratio by using CO2 1.61 μm band and it can correct the systematic error caused by temperature profile, humidity profile and pressure profile. The combination of these two correction methods will decrease the effect caused by surface reflectance, temperature profile, humidity profile and pressure profile at the same time and reduce the retrieval error. GOSAT data were used to retrieve atmospheric CH4 to test and validate the two correction methods. The results showed that CH4 column averaged mixing ratio retrieved after correction was close to GOSAT Level2 product and the retrieval precision was up to -0.24%. The studies suggest that the error of CH4 retrieval caused by the uncertainties of ground properties and atmospheric conditions can be significantly reduced and the retrieval precision can be highly improved by using ratio spectrometry method and CO2 hand correction method.
大气CH4的高精度反演受多种因素影响。地面特性和大气条件的不确定性是重要因素，如地表反射率、温度廓线、湿度廓线和气压廓线。地表反射率受多种因素影响，难以获得精确值。地表反射率的不确定性会给反演结果带来较大误差。温度廓线、湿度廓线和气压廓线的不确定性也是反演误差的重要来源，会导致不可避免的系统误差。仅利用CH4波段难以消除这种误差。本文提出了比值光谱法和CO2波段校正法来减少这些因素引起的误差。比值光谱法通过将绝对辐射光谱转换为比值光谱，可降低地表反射率在CH4反演中的影响。CO2波段校正法利用CO2的1.61μm波段将CH4柱总量转换为柱平均混合比，可校正由温度廓线、湿度廓线和气压廓线引起的系统误差。这两种校正方法的结合将同时降低地表反射率、温度廓线、湿度廓线和气压廓线引起的影响，减少反演误差。利用GOSAT数据反演大气CH4来测试和验证这两种校正方法。结果表明，校正后反演得到的CH4柱平均混合比接近GOSAT二级产品，反演精度达到-0.24%。研究表明，利用比值光谱法和CO2波段校正法可显著降低由地面特性和大气条件不确定性引起的CH4反演误差，大幅提高反演精度。
