Reducing the impact of source brightness fluctuations on spectra obtained by Fourier-transform spectrometry

We present a method to reduce the impact of source brightness fluctuations (SBFs) on spectra recorded by Fourier-transform spectrometry (FTS). Interferograms are recorded without AC coupling of the detector signal (DC mode). The SBF are determined by low-pass filtering of the DC interferograms, which are then reweighted by the low-pass, smoothed signal. Atmospheric solar absorption interferograms recorded in DC mode have been processed with and without this technique, and we demonstrate its efficacy in producing more consistent retrievals of atmospheric composition. We show that the reweighting algorithm improves retrievals from interferograms subject to both gray and nongray intensity fluctuations, making the algorithm applicable to atmospheric data contaminated by significant amounts of aerosol or cloud cover

Spaceborne measurements of atmospheric CO_2 by high-resolution NIR spectrometry of reflected sunlight: An introductory study

We introduce a strategy for measuring the column-averaged CO_2 dry air volume mixing ratio X_(CO_2) from space. It employs high resolution spectra of reflected sunlight taken simultaneously in near-infrared (NIR) CO_2 (1.58-mm and 2.06-mm) and O_2 (0.76-mm) bands. Simulation experiments, show that precisions of ~0.3–2.5 ppmv for X_(CO_2) can be achieved from individual clear sky soundings for a range of atmospheric/surface conditions when the scattering optical depth t_s is less than ~0.3. When averaged over many clear sky soundings, random errors become negligible. This high precision facilitates the identification and correction of systematic errors, which are recognized as the most serious impediment for the satellite X_(CO_2) measurements. We briefly discuss potential sources of systematic errors, and show that some of them may result in geographically varying biases in the measured X_(CO_2). This highlights the importance of careful calibration and validation measurements, designed to identify and eliminate sources of these biases. We conclude that the 3-band, spectrometric approach using NIR reflected sunlight has the potential for highly accurate X_(CO_2) measurements

Measured HDO/H_2O ratios across the tropical tropopause

We present the first simultaneous measurements of HDO and H_2O in the tropical upper troposphere (UT) and lower stratosphere (LS) as derived from infrared solar absorption spectra acquired by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. We find, surprisingly, that the observed HDO/H_2O ratio does not decrease with altitude in this region despite a factor of 4–5 decrease in the water vapor mixing ratio. This observation is inconsistent with the view that dehydration in the tropical UT/LS is by gradual processes, and suggests a major role by convective processes

Atmospheric CO_2 retrieved from ground-based near IR solar spectra

The column-averaged volume mixing ratio (VMR) of CO_2 over Kitt Peak, Arizona, has been retrieved from high-resolution solar absorption spectra obtained with the Fourier transform spectrometer on the McMath telescope. Simultaneous column measurements of CO_2 at ∼6300 cm^−1 and O_2 at ∼7900 cm^−1 were ratioed to minimize systematic errors. These column ratios were then scaled by the mean O_2 VMR (0.2095) to yield column-averaged vmrs of CO_2. These display similar behavior to the Mauna Loa in situ surface measurements. During the period 1977–1995, the column-averaged mixing ratio of CO_2 increased at an average rate of 1.49 ± 0.04 ppmv/yr with seasonal variations of ∼7 ppmv peak-to-peak. Our retrievals demonstrate that this remote technique is capable of precisions better than 0.5%

Simulation of denitrification and ozone loss for the Arctic winter 2002/2003

We present simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the Arctic winter 2002/2003. We integrated a Lagrangian denitrification scheme into the three-dimensional version of CLaMS that calculates the growth and sedimentation of nitric acid trihydrate (NAT) particles along individual particle trajectories. From those, we derive the HNO3 downward flux resulting from different particle nucleation assumptions. The simulation results show a clear vertical redistribution of total inorganic nitrogen (NOy), with a maximum vortex average permanent NOy removal of over 5 ppb in late December between 500 and 550 K and a corresponding increase of NOy of over 2 ppb below about 450 K. The simulated vertical redistribution of NOy is compared with balloon observations by MkIV and in-situ observations from the high altitude aircraft Geophysica. Assuming a globally uniform NAT particle nucleation rate of 3.4·10&#8722;6 cm&#8722;3 h&#8722;1 in the model, the observed denitrification is well reproduced. In the investigated winter 2002/2003, the denitrification has only moderate impact (<=10%) on the simulated vortex average ozone loss of about 1.1 ppm near the 460 K level. At higher altitudes, above 600 K potential temperature, the simulations show significant ozone depletion through NOx-catalytic cycles due to the unusual early exposure of vortex air to sunlight

Isotopic fractionation of carbonyl sulfide in the atmosphere: Implications for the source of background stratospheric sulfate aerosol

In order to assess the contribution of carbonyl sulfide to stratospheric sulfate aerosol (SSA), we examined the stratospheric OC^(34)S and OC^(32)S concentration profiles in the infrared limb-transmittance spectra acquired by the JPL MkIV instrument. We found that OC^(34)S is preferentially depleted by solar photolysis. The derived ^(34)S enrichment factor: ε = +73.8 ± 8.6‰, in conjunction with literature values of δ^(34)S ∼ +11‰ for tropospheric OCS, and a ∼ 10% net processing of the OCS transported upwardly into the stratosphere, suggests that aerosol sulfate proceeding from OCS should be highly enriched in δ^(34)S ∼ 80‰, comparing our prediction with previous reports of δ^(34)S ∼ +2.6‰ for background SSA, we infer either that OCS is a minor contributor to SSA or that current views about its ^(34)S-abundance and atmospheric circulation are seriously flawed

H2O and δD profiles remotely-sensed from ground in different spectral infrared regions

We present ground-based FTIR (Fourier Transform Infrared) water vapour analyses performed in four different spectral regions: 790–880, 1090–1330, 2650–3180, and 4560–4710 cm−1. All four regions allow the retrieval of lower, middle, and upper tropospheric water vapour amounts with a vertical resolution of about 3, 6, and 10 km, respectively. In addition the analyses at 1090–1330 and 2650–3180 cm−1 allow the retrieval of lower and middle/upper tropospheric δD values with vertical resolutions of 3 and 10 km, respectively. A theoretical and empirical error assessment – taking coincident Vaisala RS92 radiosonde measurements as a reference – suggests that the H2O data retrieved at high wavenumbers are slightly more precise than those retrieved at low wavenumbers. We deduce an H2O profile precision and accuracy of generally better than 20% except for the low wavenumber retrieval at 790–880 cm−1, where the assessed upper precision limit of middle/upper tropospheric H2O is 35%. The scatter between the H2O profiles produced by the four different retrievals is generally below 20% and the bias below 10%, except for the boundary layer, where it can reach 24%. These values well confirm the theoretical and empirical error assessment and are rather small compared to the huge tropospheric H2O variability of about one order of magnitude thereby demonstrating the large consistency between the different H2O profile retrievals. By comparing the two δD profile versions we deduce a precision of about 8 and 17‰ for the lower and middle/upper troposphere, respectively. However, at the same time we observe a systematic difference between the two retrievals of up to 40‰ in the middle/upper troposphere which is a large value compared to the typical tropospheric δD variability of only 80‰.M. Schneider has been supported by the Deutsche Forschungsgemeinschaft via the project RISOTO (Geschaftszeichen SCHN 1126/1-1 and 1-2)

Quantifying the loss of processed natural gas within California's South Coast Air Basin using long-term measurements of ethane and methane

Abstract. Methane emissions inventories for Southern California's South Coast Air Basin (SoCAB) have underestimated emissions from atmospheric measurements. To provide insight into the sources of the discrepancy, we analyze records of atmospheric trace gas total column abundances in the SoCAB starting in the late 1980s to produce annual estimates of the ethane emissions from 1989 to 2015 and methane emissions from 2007 to 2015. The first decade of measurements shows a rapid decline in ethane emissions coincident with decreasing natural gas and crude oil production in the basin. Between 2010 and 2015, however, ethane emissions have grown gradually from about 13 ± 5 to about 23 ± 3 Gg yr−1, despite the steady production of natural gas and oil over that time period. The methane emissions record begins with 1 year of measurements in 2007 and continuous measurements from 2011 to 2016 and shows little trend over time, with an average emission rate of 413 ± 86 Gg yr−1. Since 2012, ethane to methane ratios in the natural gas withdrawn from a storage facility within the SoCAB have been increasing by 0.62 ± 0.05 % yr−1, consistent with the ratios measured in the delivered gas. Our atmospheric measurements also show an increase in these ratios but with a slope of 0.36 ± 0.08 % yr−1, or 58 ± 13 % of the slope calculated from the withdrawn gas. From this, we infer that more than half of the excess methane in the SoCAB between 2012 and 2015 is attributable to losses from the natural gas infrastructure