Brian J. Connor

The Total Carbon Column Observing Network

A global network of ground-based Fourier transform spectrometers has been founded to remotely measure column abundances of CO2, CO, CH4, N2O and other molecules that absorb in the near-infrared. These measurements are directly comparable with the near-infrared total column measurements from space-based instruments. With stringent requirements on the instrumentation, acquisition procedures, data processing and calibration, the Total Carbon Column Observing Network (TCCON) achieves an accuracy and precision in total column measurements that is unprecedented for remote-sensing observations (better than 0.25% for CO2). This has enabled carbon-cycle science investigations using the TCCON dataset, and allows the TCCON to provide a link between satellite measurements and the extensive ground-based in situ network.

Precision requirements for space-based XCO 2 data

Precision requirements are determined for space-based column-averaged CO_2 dry air mole fraction (X_(CO)_2) data. These requirements result from an assessment of spatial and temporal gradients in (X_(CO)_2) the relationship between (X_(CO)_2) precision and surface CO_2 flux uncertainties inferred from inversions of the (X_(CO)_2) data, and the effects of (X_(CO)_2) biases on the fidelity of CO_2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these (X_(CO)_2) data precision requirements.

Global Characterization of CO2 Column Retrievals from Shortwave-Infrared Satellite Observations of the Orbiting Carbon Observatory-2 Mission

The global characteristics of retrievals of the column-averaged CO2 dry air mole fraction, XCO2, from shortwave infrared observations has been studied using the expected measurement performance of the NASA Orbiting Carbon Observatory-2 (OCO-2) mission. This study focuses on XCO2 retrieval precision and averaging kernels and their sensitivity to key parameters such as solar zenith angle (SZA), surface pressure, surface type and aerosol optical depth (AOD), for both nadir and sunglint observing modes. Realistic simulations have been carried out and the single sounding retrieval errors for XCO2 have been derived from the formal retrieval error covariance matrix under the assumption that the retrieval has converged to the correct answer and that the forward model can adequately describe the measurement. Thus, the retrieval errors presented in this study represent an estimate of the retrieval precision. For nadir observations, we find single-sounding retrieval errors with values typically less than 1 part per million (ppm) over most land surfaces for SZAs less than 70° and up to 2.5 ppm for larger SZAs. Larger errors are found over snow/ice and ocean surfaces due to their low albedo in the spectral regions of the CO2 absorption bands and, for ocean, also in the O2 A band. For sunglint observations, errors over the ocean are significantly smaller than in nadir mode with values in the range of 0.3 to 0.6 ppm for small SZAs which can decrease to values as small as 0.15 for the largest SZAs. The vertical sensitivity of the retrieval that is represented by the column averaging kernel peaks near the surface and exhibits values near unity throughout most of the troposphere for most anticipated scenes. Nadir observations over dark ocean or snow/ice surfaces and observations with large AOD and large SZA show a decreased sensitivity to near-surface CO2. All simulations are carried out for a mid-latitude summer atmospheric profile, a given aerosol type and vertical distribution, a constant windspeed for ocean sunglint and by excluding the presence of thin cirrus clouds. The impact of these parameters on averaging kernels and XCO2 retrieval errors are studied with sensitivity studies. Systematic biases in retrieved XCO2, as can be introduced by uncertainties in the spectroscopic parameters, instrument calibration or deficiencies in the retrieval algorithm itself, are not included in this study. The presented error estimates will therefore only describe the true retrieval errors once systematic biases are eliminated. It is expected that it will be possible to retrieve XCO2 for cloud free observations and for low AOD (here less than 0.3 for the wavelength region of the O2 A band) with sufficient accuracy for improving CO2 surface flux estimates and we find that on average 18% to 21% of all observations are sufficiently cloud-free with only few areas suffering from the presence of persistent clouds or high AOD. This results typically in tens of useful observations per 16 day ground track repeat cycle at a 1° × 1° resolution. Averaging observations acquired along ~1° intervals for individual ground tracks will significantly reduce the random component of the errors of the XCO2 average product for ingestion into data assimilation/inverse models. If biases in the XCO2 retrieval of the order of a few tenth ppm can be successfully removed by validation or by bias-correction in the flux inversion, then it can be expected that OCO-2 XCO2 data can lead to tremendous improvements in estimates of CO2 surface-atmosphere fluxes.

Infrared measurements of the ozone vertical distribution above Kitt Peak

The vertical distribution of the ozone in the troposphere and the lower and middle stratosphere has been retrieved from a series 0.005–0.013 cm−1 resolution infrared solar spectra recorded with the McMath Fourier transform spectrometer at the National Solar Observatory on Kitt Peak. The analysis is based on a multilayer line-by-line forward model and a semiempirical version of the optimal estimation inversion method by Rodgers. The 1002.6–1003.2 cm−1 spectral interval has been selected for the analysis on the basis of synthetic spectrum calculations. The characterization and error analysis of the method have been performed. It was shown that for the Kitt Peak spectral resolution and typical signal-to-noise ratio (≥100) the retrieval is stable, with the vertical resolution of ≈5 km attainable near the surface degrading to ≈10 km in the stratosphere. Spectra recorded from 1980 through 1993 have been analyzed. The retrieved total ozone and vertical profiles have been compared with total ozone mapping spectrometer (TOMS) satellite total columns for the location and dates of the Kitt Peak measurements and about 100 ozone ozonesoundings and Brewer total column measurements from Palestine, Texas, from 1979 to 1985. The total ozone measurements agree to ±2%. The retrieved profiles reproduce the seasonally averaged variations with altitude, including the ozone spring maximum and fall minimum measured by Palestine sondes, but up to 15% differences in the absolute values are obtained.

Space‐based near‐infrared CO2 measurements: Testing the Orbiting Carbon Observatory retrieval algorithm and validation concept using SCIAMACHY observations over Park Falls, Wisconsin

Space-based measurements of reflected sunlight in the near-infrared (NIR) region promise to yield accurate and precise observations of the global distribution of atmospheric CO_2. The Orbiting Carbon Observatory (OCO) is a future NASA mission, which will use this technique to measure the column-averaged dry air mole fraction of CO_2 (X_(CO)_2) with the precision and accuracy needed to quantify CO_2 sources and sinks on regional scales (∼1000 × 1000 km^2) and to characterize their variability on seasonal timescales. Here, we have used the OCO retrieval algorithm to retrieve (X_(CO)_2) and surface pressure from space-based Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) measurements and from coincident ground-based Fourier transform spectrometer (FTS) measurements of the O_2 A band at 0.76 μm and the 1.58 μm CO_2 band for Park Falls, Wisconsin. Even after accounting for a systematic error in our representation of the O_2 absorption cross sections, we still obtained a positive bias between SCIAMACHY and FTS (X_(CO)_2) retrievals of ∼3.5%. Additionally, the retrieved surface pressures from SCIAMACHY systematically underestimate measurements of a calibrated pressure sensor at the FTS site. These findings lead us to speculate about inadequacies in the forward model of our retrieval algorithm. By assuming a 1% intensity offset in the O_2 A band region for the SCIAMACHY (X_(CO)_2) retrieval, we significantly improved the spectral fit and achieved better consistency between SCIAMACHY and FTS (X_(CO)_2) retrievals. We compared the seasonal cycle of (X_(CO)_2)at Park Falls from SCIAMACHY and FTS retrievals with calculations of the Model of Atmospheric Transport and Chemistry/Carnegie-Ames-Stanford Approach (MATCH/CASA) and found a good qualitative agreement but with MATCH/CASA underestimating the measured seasonal amplitude. Furthermore, since SCIAMACHY observations are similar in viewing geometry and spectral range to those of OCO, this study represents an important test of the OCO retrieval algorithm and validation concept using NIR spectra measured from space. Finally, we argue that significant improvements in precision and accuracy could be obtained from a dedicated CO_2 instrument such as OCO, which has much higher spectral and spatial resolutions than SCIAMACHY. These measurements would then provide critical data for improving our understanding of the carbon cycle and carbon sources and sinks.

Ground‐based microwave observations of ozone in the upper stratosphere and mesosphere

A 9-month-long series of mesurements of ozone in the upper stratosphere and mesosphere is reported. The measurements are presented as monthly averages of profiles in blocks of roughly 20 min local time and as night-to-day ratios. An error analysis predicts accuracies of 5-26% for the monthly profiles and 2.5-9% for the ratios. The data are compared to historical data from Solar Mesosphere Explorer (SME) and limb infrared monitor of the stratosphere (LIMS), and it is shown how to remove the effect of different vertical resolution from the comparisons. The microwave data typically agree to better than 10% with SMF and nighttime LIMS ozone at all altitudes below the 0.1-mbar surface. Comparison of the microwave night-to-day ratio with the corresponding ratio from LIMS suggests that nonlocal thermodynamic equilibrium effects in the LIMS daytime data exceed 10% at all pressures less than or equal to 1 mbar.

Validation of the Aura Microwave Limb Sounder ClO measurements

We assess the quality of the version 2.2 (v2.2) ClO measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS v2.2 ClO data are scientifically useful over the range 100 to 1 hPa, with a single- profile precision of similar to 0.1 ppbv throughout most of the vertical domain. Vertical resolution is similar to 3-4 km. Comparisons with climatology and correlative measurements from a variety of different platforms indicate that both the amplitude and the altitude of the peak in the ClO profile in the upper stratosphere are well determined by MLS. The latitudinal and seasonal variations in the ClO distribution in the lower stratosphere are also well determined, but a substantial negative bias is present in both daytime and nighttime mixing ratios at retrieval levels below (i. e., pressures larger than) 22 hPa. Outside of the winter polar vortices, this negative bias can be eliminated by subtracting gridded or zonal mean nighttime values from the individual daytime measurements. In studies for which knowledge of lower stratospheric ClO mixing ratios inside the winter polar vortices to better than a few tenths of a ppbv is needed, however, day - night differences are not recommended and the negative bias must be corrected for by subtracting the estimated value of the bias from the individual measurements at each affected retrieval level.

Increases in middle atmospheric water vapor as observed by the Halogen Occultation Experiment and the ground‐based Water Vapor Millimeter‐Wave Spectrometer from 1991 to 1997

Water vapor measurements made by the Halogen Occultation Experiment (HALOE) from 1991 to 1997 are compared with ground-based observations by the Water Vapor Millimeter-wave Spectrometers (WVMS) taken from 1992 to 1997 at Table Mountain, California (34.4°N, 242.3°E), and at Lauder, New Zealand (45.0°S, 169.7°E). The HALOE measurements show that an upward trend in middle atmospheric water vapor is present at all latitudes. The average trend in the HALOE water vapor retrievals at all latitudes in the 40–60 km range is 0.129 ppmv/yr, while the average trend observed by the WVMS instruments in this altitude range is 0.148 ppmv/yr. This trend is occurring below the altitude where changes in Lyman α associated with solar cycle variations should produce a significant increase in water vapor during this period, and is much larger than the ∼0.02 ppmv/yr trend in water vapor associated with increases in methane entering the stratosphere. In addition to the water vapor increase, HALOE measurements show that there is a temporal decrease in methane at altitudes between 40 and 70 km. This indicates an increase in the conversion of the available methane to water vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of ∼0.1 K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.

Retrieval of atmospheric CO2 with enhanced accuracy and precision from SCIAMACHY: validation with FTS measurements and comparison with model results

The Bremen Optimal Estimation differential optical absorption spectroscopy (DOAS) (BESD) algorithm for satellite based retrievals of XCO_2 (the column-average dry-air mole fraction of atmospheric CO_2) has been applied to Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) data. It uses measurements in the O_2-A absorption band to correct for scattering of undetected clouds and aerosols. Comparisons with precise and accurate ground-based Fourier transform spectrometer (FTS) measurements at four Total Carbon Column Observing Network (TCCON) sites have been used to quantify the quality of the new SCIAMACHY XCO_2 data set. Additionally, the results have been compared to NOAAs assimilation system CarbonTracker. The comparisons show that the new retrieval meets the expectations from earlier theoretical studies. We find no statistically significant regional XCO_2 biases between SCIAMACHY and the FTS instruments. However, the standard error of the systematic differences is in the range of 0.2 ppm and 0.8 ppm. The XCO_2 single-measurement precision of 2.5 ppm is similar to theoretical estimates driven by instrumental noise. There are no significant differences found for the year-to-year increase as well as for the average seasonal amplitude between SCIAMACHY XCO_2 and the collocated FTS measurements. Comparison of the year-to-year increase and also of the seasonal amplitude of CarbonTracker exhibit significant differences with the corresponding FTS values at Darwin. Here the differences between SCIAMACHY and CarbonTracker are larger than the standard error of the SCIAMACHY values. The difference of the seasonal amplitude exceeds the significance level of 2 standard errors. Therefore, our results suggest that SCIAMACHY may provide valuable additional information about XCO_2, at least in regions with a low density of in situ measurements.

Ground‐based microwave monitoring of stratospheric ozone

A microwave instrument developed for operational measurements of ozone for the Network for Detection of Stratospheric Change is discussed. The instrument observes two spectral lines near 3-mm wavelength with a bandwidth of 630 MHz, allowing profile retrieval from 20 to 70 km. The observing technique and calibration procedures are described. The measurement forward model and retrieval algorithm are formulated. Preliminary comparisons with a colocated ground-based lidar and the SAGE II instrument are presented. The measurements are shown to typically agree to within 5 to 10 percent.