Abstract
We have previously demonstrated a pulsed direct detection IPDA lidar to measure range and the column concentration of atmospheric CO2. The lidar measures the atmospheric backscatter profiles and samples the shape of the 1,572.33 nm CO2 absorption line. We participated in the ASCENDS science flights on the NASA DC-8 aircraft during August 2011 and report here lidar measurements made on four flights over a variety of surface and cloud conditions near the US. These included over a stratus cloud deck over the Pacific Ocean, to a dry lake bed surrounded by mountains in Nevada, to a desert area with a coal-fired power plant, and from the Rocky Mountains to Iowa, with segments with both cumulus and cirrus clouds. Most flights were to altitudes >12 km and had 5–6 altitude steps. Analyses show the retrievals of lidar range, CO2 column absorption, and CO2 mixing ratio worked well when measuring over topography with rapidly changing height and reflectivity, through thin clouds, between cumulus clouds, and to stratus cloud tops. The retrievals shows the decrease in column CO2 due to growing vegetation when flying over Iowa cropland as well as a sudden increase in CO2 concentration near a coal-fired power plant. For regions where the CO2 concentration was relatively constant, the measured CO2 absorption lineshape (averaged for 50 s) matched the predicted shapes to better than 1% RMS error. For 10 s averaging, the scatter in the retrievals was typically 2–3 ppm and was limited by the received signal photon count. Retrievals were made using atmospheric parameters from both an atmospheric model and from in situ temperature and pressure from the aircraft. The retrievals had no free parameters and did not use empirical adjustments, and >70% of the measurements passed screening and were used in analysis. The differences between the lidar-measured retrievals and in situ measured average CO2 column concentrations were <1.4 ppm for flight measurement altitudes >6 km.
Highlights
Increasing atmospheric CO2 is widely accepted as the largest anthropogenic factor causing climate change, there is considerable uncertainty about its global budget
During this flight the lidar measurements were made to the top of the marine stratus cloud deck from aircraft altitudes that varied from 1.5 to 13 km
The plot shows lidar retrievals using the atmospheric profiles from the Modern-Era Retrospective Analysis For Research and Applications (MERRA) model, and those using the atmosphere sampled by the DC-8 during the spiral down, and the column average CO2 values from the AVOCET in situ sensor
Summary
Increasing atmospheric CO2 is widely accepted as the largest anthropogenic factor causing climate change, there is considerable uncertainty about its global budget. An inherent error source with space-based passive spectrometers is optical scattering from aerosols and thin clouds, cirrus, in the illumination or observation paths [10,11]. CO2 absorption measurements made at slant angles using km-sized footprints are susceptible to errors caused by changes in the optical path length caused by variability of the surface reflectivity within the footprint. To overcome these limitations, the US National Research Council’s 2007 Decadal Survey recommended a new space-based CO2 measuring mission called ASCENDS [12] using the laser absorption spectroscopy approach. ESA has conducted mission definition studies for a similar space mission called A-SCOPE [3,13], and their lidar sensitivity and spectroscopic analyses have been published [14,15]
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