S. R. Kawa

Simulation studies for a space-based CO2 lidar mission

We report results of initial space mission simulation studies for a laser-based, atmospheric CO2 sounder, which are based on real-time carbon cycle process modelling and data analysis. The mission concept corresponds to the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS) recommended by the US National Academy of Sciences’ Decadal Survey. As a pre-requisite for meaningful quantitative evaluation, we employ a CO2 model that has representative spatial and temporal gradients across a wide range of scales. In addition, a relatively complete description of the atmospheric and surface state is obtained from meteorological data assimilation and satellite measurements. We use radiative transfer calculations, an instrument model with representative errors and a simple retrieval approach to quantify errors in ‘measured’ CO2 distributions, which are a function of mission and instrument design specifications along with the atmospheric/surface state. Uncertainty estimates based on the current instrument design point indicate that a CO2 laser sounder can provide data consistent with ASCENDS requirements and will significantly enhance our ability to address carbon cycle science questions. Test of a dawn/dusk orbit deployment, however, shows that diurnal differences in CO2 column abundance, indicative of plant photosynthesis and respiration fluxes, will be difficult to detect.

Using continental observations in global atmospheric inversions of CO2: North American carbon sources and sinks

We evaluate North American carbon fluxes using a monthly global Bayesian synthesis inversion that includes wellcalibrated carbon dioxide concentrations measured at continental flux towers. We employ the NASA Parametrized Chemistry Tracer Model (PCTM) for atmospheric transport and a TransCom-style inversion with subcontinental resolution. We subsample carbon dioxide time series at four North American flux tower sites for mid-day hours to ensure sampling of a deep, well-mixed atmospheric boundary layer. The addition of these flux tower sites to a global network reduces North America mean annual flux uncertainty for 2001–2003 by 20% to 0.4 Pg C yr-1 compared to a network without the tower sites. North American flux is estimated to be a net sink of 1.2 ± 0.4 Pg C yr-1 which is within the uncertainty bounds of the result without the towers. Uncertainty reduction is found to be local to the regions within North America where the flux towers are located, and including the towers reduces covariances between regions within NorthAmerica. Mid-day carbon dioxide observations from flux towers provide a viable means of increasing continental observation density and reducing the uncertainty of regional carbon flux estimates in atmospheric inversions.

A laser sounder for measuring atmospheric trace gases from space

Mounting concern regarding global warming and the increasing carbon dioxide (CO2) concentration has stimulated interest in the feasibility of measuring CO2 mixing ratios from space. Precise satellite observations with adequate spatial and temporal resolution would substantially increase our knowledge of the atmospheric CO2distribution and allow improved modeling of the CO2 cycle. Current estimates indicate that a measurement precision of better than 1 part per million (1 ppm) will be needed in order to improve estimates of carbon uptake by land and ocean reservoirs. A 1-ppm CO2 measurement corresponds to approximately 1 in 380 or 0.26% long-term measurement precision. This requirement imposes stringent long-term precision (stability) requirements on the instrument In this paper we discuss methods and techniques to achieve the 1-ppm precision for a space-borne lidar.

Oxygen Spectroscopy Laser Sounding Instrument for Remote Sensing of Atmospheric Pressure

We report on the progress of an oxygen spectroscopy laser sounding instrument designed as a calibration channel for a carbon dioxide (CO2) laser sounding instrument. We have developed a pulsed, frequency-doubled, fiber laser transmitter for use in an oxygen instrument. The instrument concept uses the pressure broadening of spectroscopic lines of the diatomic oxygen A-band to deduce atmospheric pressure. There are many uses for this measurement but we are developing it primarily to make a measurement of the dry mixing ratio of CO2. The CO2 measurement can be affected by changes in atmospheric properties such as humidity, temperature and pressure. To remove these variances requires measuring a stable, well-mixed gas like oxygen. We will report on the basic theory behind the instrument, measurements made at a test site at Goddard, review the current state of the instrument technologies and the necessary steps to bring them to space readiness, and review the current state of the instrument development.

Lidar technology for measuring trace gases on Mars and Earth

Many fundamental questions about planetary evolution require monitoring of the planets atmosphere with unprecedented accuracy at both high and low latitudes, over both day and night and all seasons. Each planetary atmosphere presents its own unique challenges. For the planets/moons that have relatively low surface pressure and low trace gas concentrations, such as Mars or Europa, the challenge is to have enough sensitivity to measure the trace gas of interest. For Earth, the challenge is to measure trace gases with very high precision and accuracy in the presence of other interfering species. An orbiting laser remote sensing instrument is capable of measuring trace gases on a global scale with unprecedented accuracy, and higher spatial resolution that can be obtained by passive instruments. For Mars, our proposed measurement uses Optical Parametric Amplifiers (OPA) and Integrated Path Differential Absorption (IDPA) in the 3-4 um spectral range to map various trace gas concentrations from orbit on a global scale. For earth, we propose to use Erbium Doped Fiber Amplifier technology (EDFA) and IDPA at 1.57 and OPA at 1.65 μm to measure carbon dioxide and methane concentrations respectively.