Justus Notholt

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.

Global CO2 fluxes inferred from surface air-sample measurements and from TCCON retrievals of the CO2 total column

We present the first estimate of the global distribution of CO_2 surface fluxes from 14 stations of the Total Carbon Column Observing Network (TCCON). The evaluation of this inversion is based on 1) comparison with the fluxes from a classical inversion of surface air-sample-measurements, and 2) comparison of CO_2 mixing ratios calculated from the inverted fluxes with independent aircraft measurements made during the two years analyzed here, 2009 and 2010. The former test shows similar seasonal cycles in the northern hemisphere and consistent regional carbon budgets between inversions from the two datasets, even though the TCCON inversion appears to be less precise than the classical inversion. The latter test confirms that the TCCON inversion has improved the quality (i.e., reduced the uncertainty) of the surface fluxes compared to the assumed or prior fluxes. The consistency between the surface-air-sample-based and the TCCON-based inversions despite remaining flaws in transport models opens the possibility of increased accuracy and robustness of flux inversions based on the combination of both data sources and confirms the usefulness of space-borne monitoring of the CO_2 column.

Latitudinal variations of trace gas concentrations in the free troposphere measured by solar absorption spectroscopy during a ship cruise

The latitudinal variations of atmospheric trace gas column abundances have been measured during a ship cruise between 57°N and 45°S on the central Atlantic. The measurements were performed in October 1996 using high-resolution solar absorption spectroscopy in the infrared. The analysis method employed permits the retrieval of the total column densities of 20 different trace gases and for a few compounds the vertical mixing ratio profiles. For CH4 an interhemispheric difference of 3% was observed. The total columns of the shorter-lived trace gases CO and C2H6, analyzed between 57°N and 45°S, reveal a slight maximum in the tropics and a substantial increase north of 45°N. The total columns of C2H2 and HCN, detectable between 30°N and 30°S, reveal a maximum in the tropics of the Southern Hemisphere. For CH2O, studied between 57°N and 45°S, a well-pronounced maximum is observed in the tropics. The profile retrieval gives high mixing ratios for CO, C2H6, and O3 north of 40°N in the lower troposphere. In the tropics high concentrations are found for all three compounds in the entire troposphere, even above 12 km. The measurements have been used to estimate averaged mixing ratios of the trace gases for the free troposphere between 0 and 12 km. In the tropics the data give high values: for example, more than 200 pptv for HCN, 750 pptv for CH2O, 100 ppbv for CO and 100 pptv for C2H2. These values are comparable to or higher than what has been observed at midlatitudes, indicating the importance of biomass burning emissions on the tropospheric composition.

Ground‐based observations of Arctic O3 loss during spring and summer 1997

Ground-based solar absorption spectra were measured from Fairbanks, Alaska (65°N, 148°W) from March to September 1997 by the Jet Propulsion Laboratory (JPL) MkIV Fourier transform infrared (FTIR) spectrometer. The derived column abundances of 03 declined by 35% over this period (20% in April and May, and 15% during the summer), whereas those of HF, a long-lived tracer, changed by less than 5%. High-latitude, summertime balloon observations reveal similar shapes for the volume mixing ratio profiles of O 3 and HF in the lower stratosphere, where most of their column abundance resides. Vertical transport should therefore have similar effects on the column abundances of O 3 and HF. Data from the Halogen Occultation Experiment (HALOE) show a poleward decrease in the O 3 /HF ratio at all stratospheric altitudes, so that any reductions in column O 3 due to horizontal meridional transport would have been accompanied by even larger reductions in column HF. Therefore the observed column O 3 decrease must be the result of chemical loss processes. Column measurements of other atmospheric gases show a summertime maximum in the NO x /NO y column ratio and little change in the chlorine partitioning. We conclude that most of the reduction in column O 3 over Fairbanks from March to September 1997 was likely driven by NO x chemistry. These conclusions are supported by the similar behavior of column abundances measured by another ground-based FTIR spectrometer based in Ny Alesund, Spitsbergen, (79°N, 12°E).

On the use of HF as a reference for the comparison of stratospheric observations and models

Hydrogen fluoride (HF) is often used as a simple reference for other column observations of chemically active stratospheric species. However, seasonal and shorter timescale variations in column HF make its use as a reference more complicated. In this paper we characterize the expected magnitude of these variations in HF, and variations of ratio quantities involving HF, using a two-dimensional (2-D) photochemical model and two versions of a three-dimensional (3-D) transport model. The 2-D model predicts that the column ratios HNO3/HF and HCl/HF increase from midlatitudes to the tropics, although this is very sensitive to HCl and HNO3 abundances in the tropical upper troposphere. Seasonal variations in vertical motion modifys the predicted ratios; for example, wintertime descent at high latitudes decreases HCl/HF. The ratio HNO3/HF at high latitudes is strongly modified by seasonal variations in the chemical partitioning of the odd nitrogen (NOy) species. We compare these model predictions with ground-based Fourier transform infrared spectroscopy (FTIR) observations of HF along with HCl, ClONO2 and HNO3 obtained at eight northern hemisphere sites between October 1994 and July 1995. We investigate quantitatively how HF can be used as a tracer to follow the evolution of observations at a single station and to intercompare results from different stations or with photochemical models. The magnitude of the 3-D model HF column agrees well with the observations, except on some occasions at high latitudes, giving indirect support for the important role of COF2 in the stratospheric inorganic fluorine budget. The observed day-to-day variability in the column ratios HCl/HF and HNO3/HF is much larger at high latitudes. This variability is reproduced in the 3-D models and is due to horizontal motion. Short timescale vertical displacement of the species profiles is estimated to have a small effect on the column ratios. In particular, we analyze the usefulness of the observed column ratio (ClONO2 + HCl)/HF as an indicator for chlorine activation. Current measurement uncertainties limit the degree of activation which can be unambiguously detected using this observed quantity, but we can determine that chlorine-activated air was observed above Aberdeen (58°N) on 6 days in late January 1995.

An uncertainty budget for ground-based Fourier transform infrared column measurements of HCl, HF, N2O, and HNO3 deduced from results of side-by-side instrument intercomparisons

The results of side-by-side instrument intercomparisons of Fourier transform infrared (FTIR) spectrometers at Ny-Alesund, Spitzbergen (79°N, 12°E), during May/June 1995 and at Harestua, Norway (60°N, 10°E), during September/October 1994 are reported. The spectrometers were operated simultaneously recording atmospheric spectra in the midinfrared using the Sun as a source. The differences in vertical columns of HCl, HF, N2O, and HNO3 measured simultaneously by different instruments were as large as 5.8, 7.7, 2.8, and 4.3%, respectively, having mean absolute values 0.5, 1.6, 1.0, and 1.6%, respectively. These results were used to derive 1σ overall uncertainties of 9.7% for HCl, 7.7% for HF, 6.4% for N2O, and 14.3% for HNO3. Separate uncertainties are quoted for the precision of daily column measurements which exclude the contribution due to uncertainty in the line parameters. These are 7.7% for HCl, 5.7% for HF, 4.9% for N2O, and 6.8% for HNO3. These numbers are estimated for the local conditions at the intercomparison sites but will differ from site to site depending on the knowledge of local atmospheric conditions at the time of measurement.

Ground-based infrared solar spectroscopic measurements of carbon monoxide during 1994 Measurement of Air Pollution From Space flights

Results of the comparison of carbon monoxide ground-based infrared solar spectroscopic measurements with data obtained during 1994 Measurement of Air Pollution From Space (MAPS) flights are presented. Spectroscopic measurements were performed correlatively with April and October MAPS flights by nine research groups from Belgium, Canada, Germany, Japan, New Zealand, Russia, and the United States. Characterization of the techniques and error analysis were performed. The role of the CO a priori profile used in the retrieval was estimated. In most cases an agreement between spectroscopic and MAPS data is within estimated MAPS accuracy of _+ 10%.

stratospheric trace gas concentrations in the Arctic polar night derived by FTIR‐spectroscopy with the Moon as IR light source

In winter 1992/93 we have performed ground-based FTIR measurements in the Arctic (79°N, 12°E) to derive column densities of stratospheric trace gases within the polar vortex. Due to the polar night the moon had to serve as infrared light source instead of the sun. It was possible to perform FTIR-measurements for about a week around full moon. Column densities of N2O, CH4, HF, HCl, O3, NO2, HNO3 and ClONO2 have been obtained. The spectra were recorded at 0.01 cm−1 or 0.02 cm−1 resolution using MCT or InSb detectors. Measurements with an aerosol lidar performed at the same site show that polar stratospheric clouds (PSCs) appeared several times during the polar night. It seems that the concentrations of the stratospheric trace gases are strongly influenced by the occurrence of PSCs.

Heterogeneous conversion of HCl and ClONO2 during the Arctic winter 1992/1993 initiating ozone depletion

Ground-based Fourier transform infrared (FTIR) measurements, performed in the Arctic polar night using the Moon as an infrared light source, reveal already during the first half of December 1992 a considerable reduction in the amount of stratospheric HCl and ClONO2. Balloon sondes yield that in the altitude range 16–22 km an ozone reduction occurred as soon as the circulating air masses were irradiated by sunlight. Within the same altitude range several polar stratospheric cloud (PSC) events are being reported throughout the winter. Assuming that the reduction of HCl and ClONO2 had occurred in the same altitude range, where the ozone depletion and PSCs were observed, our measurements imply that almost all HCl and ClONO2 in that altitude range had disappeared. This means that the stratosphere was primed for ozone depletion very early in winter. The results confirm model studies that in the Arctic, already weak PSC events can lead to a strong conversion of chlorine reservoir compounds into active forms, initiating the ozone depletion, as soon as sunlight is available.

Lidar measurement of planetary boundary layer height and comparison with microwave profiling radiometer observation

The paper is on the determination of the height of the planetary boundary layer (BLH) by means of lidar measurements and application of the continuous wavelet transform method. The retrieved heights are compared to results from numerical models based on the parcel method. The latter allows to determine the entrainment zone; the required information concerning the surface heat flux and the temperature profile are provided from a microwave radiometer and sonic anemometer. The authors retrieve a set of BLHs for Lanzhou and Yuzhong.