A new ground-based differential absorption sunphotometer for measuring atmospheric columnar CO_2 and preliminary applications

摘要

Carbon dioxide is commonly considered as the most important greenhouse gas. Ground-based remote sensing technology of acquiring CO_2 columnar concentration is needed to provide validation for spaceborne CO_2 products. A new ground-based sunphotometer prototype for remotely measuring atmospheric CO_2 is introduced in this paper, which is designed to be robust, portable, automatic and suitable for field observation. A simple quantity, Differential Absorption Index (DAI) related to CO_2 optical depth, is proposed to derive the columnar CO_2 information based on the differential absorption principle around 1.57 micron. Another sun/sky radiometer CE318, is used to provide correction parameters of aerosol extinction and water vapor absorption. A cloud screening method based on the measurement stability is developed. A systematic error assessment of the prototype and DAI is also performed. We collect two-year DAI observation from 2010 to 2012 in Beijing, analyze the DAI seasonal variation and find that the daily average DAI decreases in growing season and reaches to a minimum on August, while increases after that until January of the next year, when DAI reaches its highest peak, showing generally the seasonal cycle of CO_2. We also investigate the seasonal differences of DAI variation and attribute the tendencies of high in the morning and evening while low in the noon to photosynthesis efficiency variation of vegetation and anthropogenic emissions. Preliminary comparison between DAI and model simulated XCO_2 (Carbon Tracker 2011) is conducted, showing that DAI roughly reveals some temporal characteristics of CO_2 when using the average of multiple measurements.