衛星からの晴天域の近赤外太陽散乱光観測による二酸化炭素気柱量推定手法の検討 : 誤差評価と鉛直気圧グリッドの最適化 Study of Retrieving Column Amount of Carbon Dioxide from Satellite-based Near-infrared Observation of Solar Scattered Light in Clear Sky Condition : Error Estimation and Optimization of Vertical Pressure Grid

A method of retrieving CO2 column amount has been developed for near infrared solar radiation measurement in clear sky condition. It is based on the maximum a posteriori retrieval method in which a vertical pressure grid is optimized in terms of uniform data density using the normalized Jacobian matrix. CO2 retrieval error due to instrumental noise, aerosol, and surface property has been evaluated for the CO2 1.6μm absorption band. The instrumental noise yields about 0.11% retrieval error in CO2 column amount in the case of surface albedo of 0.3. Although retrieval error due to aerosol depends on a composition and size of aerosol, it rises with increase in the aerosol optical thickness and tends to overestimate with increase in the ground surface albedo. When the aerosol distributes near surface and the optical thickness is smaller than 0.2 (at 0.55μm), CO2 retrieval error is almost within 1%. However, the aerosol floating in the upper troposphere and the stratosphere such as the yellow sand and volcanic ash significantly affects CO2 column amount in which retrieval error results in much larger than 1%. The wavenumber dependency of surface albedo is also an error factor that can not be neglected. But the linear approximation at wavenumber intervals of 25cm-1 can reduce the retrieval error due to the wavenumber dependency.