A regional CO<sub>2</sub> observing system simulation experiment for the ASCENDS satellite mission

Abstract

Abstract. Top-down estimates of the spatiotemporal variations in emissions and uptake of CO 2 will benefit from the increasing measurement density brought by recent and future additions to the suite of in situ and remote CO 2 measurement platforms. In particular, the planned NASA Active Sensing of CO 2 Emissions over Nights, Days, and Seasons (ASCENDS) satellite mission will provide greater coverage in cloudy regions, at high latitudes, and at night than passive satellite systems, as well as high precision and accuracy. In a novel approach to quantifying the ability of satellite column measurements to constrain CO 2 fluxes, we use a portable library of footprints (surface influence functions) generated by the Stochastic Time-Inverted Lagrangian Transport (STILT) model in combination with the Weather Research and Forecasting (WRF) model in a regional Bayesian synthesis inversion. The regional Lagrangian particle dispersion model framework is well suited to make use of ASCENDS observations to constrain weekly fluxes in North America at a high resolution, in this case at 1 • latitude × 1 • longitude. We consider random measurement errors only, modeled as a function of the mission and instrument design specifications along with realistic atmospheric and surface conditions. We find that the ASCENDS observations could potentially reduce flux uncertainties substantially at biome and finer scales. At the grid scale and weekly resolution, the largest uncertainty reductions, on the order of 50 %, occur where and when there is good coverage by observations with low measurement errors and the a priori uncertainties are large.Uncertainty reductions are smaller for a 1.57 µm candidate wavelength than for a 2.05 µm wavelength, and are smaller for the higher of the two measurement error levels that we consider (1.0 ppm vs. 0.5 ppm clear-sky error at Railroad Valley, Nevada). Uncertainty reductions at the annual biome scale range from ∼ 40 % to ∼ 75% across our four instrument design cases and from ∼ 65% to ∼ 85 % for the continent as a whole. Tests suggest that the quantitative results are moderately sensitive to assumptions regarding a priori uncertainties and boundary conditions. The a posteriori flux uncertainties we obtain, ranging from 0.01 to 0.06 Pg C yr −1 across the biomes, would meet requirements for improved understanding of long-term carbon sinks suggested by a previous study.

The impact of transport model differences on CO<sub>2</sub> surface flux estimates from OCO-2 retrievals of column average CO<sub>2</sub>

“…It should be noted here that the low sensitivity of column measurements to PBL CO 2 variations is often considered a weakness, since surface flux signals are the largest in the PBL. Efforts are currently underway to construct active remote-sensing instruments that are preferentially sensitive to the lower troposphere (Wang et al, 2014). Our OSSEs suggest that, were such an instrument to be deployed, the uncertainty of surface flux estimates derived from that instrument might very well be larger than from an OCO-2like column CO 2 instrument due to transport model uncertainty near the surface.…”

On the Ability of Space‐Based Passive and Active Remote Sensing Observations of CO2 to Detect Flux Perturbations to the Carbon Cycle

“…The authors concluded that it was unlikely that a dawn-dusk orbit would constrain the diurnal cycle, which is linked to separating sinks from photosynthesis during the day and sources from respiration at night. Wang et al (2014) employed a synthesis inversion framework and demonstrated the capability of ASCENDS observations to provide significant uncertainty reductions on regional scale fluxes. This work included the first relative comparison of CO 2 measurements from different ASCENDS instrument concepts to constrain fluxes and found that under the same assumed measurement precisions and in the absence of transport error, the SCO2 instrument concept provided a somewhat stronger flux constraint due to its greater sensitivity nearer the surface (Figure 1).…”

Role of space station instruments for improving tropical carbon flux estimates using atmospheric data

“…Active remote sensing systems that use a laser tuned to the centre (and to one side) of a CO 2 absorption line would also provide measurement coverage during the daytime and nighttime and, more importantly, observe through broken clouds and to cloud tops. There are variety of lidar mission concepts for CO 2 and methane that are in different stages of development 135 , 136 , which would also help improve understanding of regional, and potentially local, carbon budgets 137 , 138 . Demonstrating these concepts on the ISS would be an efficient way to mature the technologies while delivering urgently needed observation capabilities on anthropogenic emissions in the locations where population growth is fastest, although we acknowledge active systems will be more sensitive to platform stability.…”