Design of inversion procedure for the airborne CO2 -IPDA LIDAR: a preliminary study

China will launch the atmospheric environment monitoring satellite, which is equipped with a CO2-integrated path differential absorption (IPDA) LIDAR, in the coming years. The space-borne IPDA LIDAR is believed to supplement current passive remote sensing techniques in terms of effective observations at nights, as well as in high-latitude regions and heavily polluted areas. Currently, no LIDAR-based satellite is operational for CO2 detection in orbit despite the fact that Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) and Advanced Space Carbon and Climate Observation of Planet Earth (A-SCOPE) are dedicated to fill this gap. However, the ESA mission proposal A-SCOPE dedicated for CO2 measurement was not selected, and ASCENDS from NASA remains in the loop but on low priority. Therefore, it is of great significance to explore the feasibility and effectiveness of this novel technique and to identify potential differences among its CO2 concentration products and the passive remote sensing technique. In this article, we developed an initial data-processing procedure for an airborne CO2-IPDA LIDAR, which is the minified prototype of the forthcoming space-borne LIDAR. We tested the effectiveness of this procedure and evaluated the performance of the minified prototype in a flight test over ocean, urban, and mountainous terrain. The column-weighted xCO(2) (XCO2) retrievals obtained by the airborne IPDA LIDAR were considerably more sensitive to the gradients of the dry-air mixing ratio of CO2 (xCO(2)) than the XCO2 products of OCO-2 and in situ measurements of point xCO(2). The mean XCO2 values over the ocean, urban, and mountainous area were 411.07, 425.71, and 417.87 ppm with STDs of 1.93, 0.85, and 0.96 ppm, respectively. We used altitude-dependent xCO(2) obtained by a decline-climb flight to calculate a reference for XCO2 over the ocean. The difference between XCO2 obtained using two means was less than 0.5 ppm. Moreover, the actual random error coincided well with the simulated random error, suggesting that our previous performance-evaluation model was reliable. This model predicted that a relative random error of less than 0.3% would be very likely for the forthcoming satellite mission over land. However, measuring CO2 concentrations precisely over oceans was identified as a very challenging work. Improvements in hardware technology are unlikely to narrow this gap largely. Thus, developing dedicated algorithms to address CO2 measurement over oceans by using IPDA LIDAR is necessary.