Retrieval of daytime mesospheric ozone using OSIRIS observations of O₂ (<i>a</i>¹Δ_g) emission

Improving knowledge of the ozone global distributions in the mesosphere–lower thermosphere (MLT) is a crucial step in understanding the behaviour of the middle atmosphere. However, the concentration of ozone under sunlit conditions in the MLT is often so low that its measurement requires instruments with very high sensitivity. Fortunately, the bright oxygen airglow can serve as a proxy to retrieve the daytime ozone density indirectly, due to the strong connection to ozone photolysis in the Hartley band. The OSIRIS IR imager (hereafter, IRI), one of the instruments on the Odin satellite, routinely measures the oxygen infrared atmospheric band (IRA band) at 1.27 µm. In this paper, we will primarily focus on the detailed description of the steps done for retrieving the calibrated IRA band limb radiance (with <10 % random error), the volume emission rate of O2 (a1Δg) (with <25 % random error) and finally the ozone number density (with <20 % random error). This retrieval technique is applied to a 1-year sample from the IRI dataset. The resulting product is a new ozone dataset with very tight along-track sampling distance (<20 km). The feasibility of the retrieval technique is demonstrated by a comparison of coincident ozone measurements from other instruments aboard the same spacecraft, as well as zonal mean and monthly average comparisons between Odin-OSIRIS (both spectrograph and IRI), Odin-SMR and Envisat-MIPAS. We find that IRI appears to have a positive bias of up to 25 % below 75 km, and up to 50 % in some regions above. We attribute these differences to uncertainty in the IRI calibration as well as uncertainties in the photochemical constants. However, the IRI ozone dataset is consistent with the compared dataset in terms of the overall atmospheric distribution of ozone between 50 and 100 km. If the origin of the bias can be identified before processing the entire dataset, this will be corrected and noted in the dataset description. The retrieval technique described in this paper can be further applied to all the measurements made throughout the 19 year mission, leading to a new, long-term high-resolution ozone dataset in the middle atmosphere.

On the detection of a high-altitude peak of atmospheric ozone by the NOMAD/UVIS onboard the ExoMars TGO

&lt;p&gt;The Nadir and Occultation for MArs Discovery (NOMAD) is a spectrometer suite onboard the ExoMars Trace Gas Orbiter (TGO), providing observations in the nadir, limb, and solar occultation modes since April 2018. UVIS, a single spectrometer unit within NOMAD spans the ultraviolet-visible range between 200 nm and 650 nm. It obtained ~ 4000 vertically resolved (&lt; 1 km) solar occultation observations of the martian atmosphere for over a full Mars year (MY, 687 days) starting at MY 34 during late northern summer at L&lt;sub&gt;s&lt;/sub&gt; = 163&amp;#176;. Ozone (O&lt;sub&gt;3&lt;/sub&gt;), a principal component of the martian atmosphere, is highly responsive to the incoming UV flux, and is a sensitive tracer of the odd hydrogen chemistry. Transmittance spectra returned by UVIS sampled the O&lt;sub&gt;3 &lt;/sub&gt;Hartley band around 250 nm and provided unique insights into understanding the vertical, latitudinal and temporal behavior of O&lt;sub&gt;3&lt;/sub&gt;. UVIS detected a high-altitude peak of O&lt;sub&gt;3 &lt;/sub&gt;between 40 and 60 km that is mostly persistent between L&lt;sub&gt;s&lt;/sub&gt; = 340&amp;#176; and ~ 200&amp;#176; at polar latitudes, and is found to be highly dependent on latitude and season. We will present high-resolution results tracking the vertical, latitudinal, diurnal and seasonal evolution of the secondary peak of ozone for a full Mars year. In comparison, we will also provide O&lt;sub&gt;3&lt;/sub&gt; simulations from the GEM-Mars General Circulation Model (GCM) with the purpose of shedding light into understanding the photochemical processes that lead to the presence and disappearance of the high-altitude peak of atmospheric ozone.&amp;#160;&lt;/p&gt;

Biomarker on Callisto

&lt;p&gt;Ozone (O&lt;sub&gt;3&lt;/sub&gt;), regarded as a biomarker, has been observed on the icy satellites Ganymede [1a], Dione and Rhea [1b]. Presence of O&lt;sub&gt;3&lt;/sub&gt; on the icy surfaces of Ganymede, Rhea and Dione are due to energetic processing of oxygen-bearing molecules. Laboratory experiments had shown efficient synthesis of O&lt;sub&gt;3&lt;/sub&gt; in oxygen-bearing molecules such as CO&lt;sub&gt;x&lt;/sub&gt;, SO&lt;sub&gt;x&lt;/sub&gt; and NO&lt;sub&gt;x&lt;/sub&gt;. Most of the experiments used InfraRed (IR) spectroscopy to detect O&lt;sub&gt;3&lt;/sub&gt; [2a]. However, unambiguous O&lt;sub&gt;3 &lt;/sub&gt;detection in planetary objects using IR signatures is difficult due to the presence of silicates. Therefore, the Hartley band of O&lt;sub&gt;3&lt;/sub&gt;, 220 &amp;#8211; 310 nm, was used to find O&lt;sub&gt;3 &lt;/sub&gt;presence on icy surfaces [2b].&amp;#160;&lt;/p&gt; &lt;p&gt;Apart from the three satellites of the outer Solar System, there may be other satellites that might harbour O&lt;sub&gt;3&lt;/sub&gt;. UltraViolet (UV) spectrum of Callisto recorded by the HST was reported to show spectral signatures of SO&lt;sub&gt;2&lt;/sub&gt; [3]. Based on this observation, the irradiation experiments simulating SO&lt;sub&gt;2&lt;/sub&gt; ices on Callisto revealed the coexistence of SO&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;3&lt;/sub&gt;. The spectral signatures in the UV were found to extend from 220 &amp;#8211; 310 nm with a broad peak 255 &amp;#8211; 285 nm, clear indication of O&lt;sub&gt;3&lt;/sub&gt; embedded in the SO&lt;sub&gt;2&lt;/sub&gt; ice matrix. Here we will present the detailed analysis that suggests the presence of O&lt;sub&gt;3&lt;/sub&gt; on Callisto. &amp;#160;&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;&lt;strong&gt;References: &lt;/strong&gt;&lt;/p&gt; &lt;p&gt;[1] Noll et al. [a] (1996) Science, 273, 341. &amp; [b] (1997) Nature, 388, 45.&lt;/p&gt; &lt;p&gt;[2] Sivaraman et al. [a] (2007) &amp;#160;ApJ, 669, 1414. &amp; [b] (2014) Chem Phy Lett, 603, 33.&lt;/p&gt; &lt;p&gt;[3] Noll et al. (1997) GRL, 24, 1139.&lt;/p&gt;

Retrieval of daytime mesospheric ozone using OSIRIS observation of O₂(a¹∆_g) emission

Improving knowledge of the ozone global distributions in the mesosphere-lower thermosphere (MLT) is a crucial step in understanding the behaviour of the middle atmosphere. However, the ozone concentration under sunlit conditions in the MLT is often so low that its measurement requires instruments with very high sensitivity. Fortunately, the bright oxygen airglow can serve as a proxy to retrieve the daytime ozone density indirectly, due to the strong connection to ozone photolysis in the Hartley band. The OSIRIS IR imager (hereafter IRI), one of the instruments on the Odin satellite, routinely measures the oxygen infrared atmospheric band (IRA band) at 1.27 μm. In this paper, we will describe the detailed steps of retrieving the calibrated IRA band limb radiance, the volume emission rate of O2(a1∆g) and, finally, the ozone number density. This retrieval technique is applied to a one-year-sample IRI dataset. The resulting product is a completely new ozone dataset with very high along-track resolution. The performance of the retrieval technique is demonstrated by a comparison of the coincident ozone measurements from the same spacecraft, as well as zonal mean and monthly average comparisons between OS, SMR, MIPAS and ACE-FTS. The consistency of this IRI ozone dataset implies that such a retrieval technique can be further applied to all the measurements made throughout the 19 years-long mission, leading to a long-term, high resolution dataset in the middle atmosphere.

Linearization of the effect of slit function changes for improving Ozone Monitoring Instrument ozone profile retrievals

We introduce a method that accounts for errors caused by the slit function in an optimal-estimation-based spectral fitting process to improve ozone profile retrievals from the Ozone Monitoring Instrument (OMI) ultraviolet measurements (270–330 nm). Previously, a slit function was parameterized as a standard Gaussian by fitting the full width at half maximum (FWHM) of the slit function from climatological OMI solar irradiances. This cannot account for the temporal variation in slit function in irradiance, the intra-orbit changes due to thermally induced change and scene inhomogeneity, and potential differences in the slit functions of irradiance and radiance measurements. As a result, radiance simulation errors may be induced due to convolving reference spectra with incorrect slit functions. To better represent the shape of the slit functions, we implement a more generic super Gaussian slit function with two free parameters (slit width and shape factor); it becomes standard Gaussian when the shape factor is fixed to be 2. The effects of errors in slit function parameters on radiance spectra, referred to as pseudo absorbers (PAs), are linearized by convolving high-resolution cross sections or simulated radiances with the partial derivatives of the slit function with respect to the slit parameters. The PAs are included in the spectral fitting scaled by fitting coefficients that are iteratively adjusted as elements of the state vector along with ozone and other fitting parameters. The fitting coefficients vary with cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The PA spectrum is quite similar in the Hartley band below 310 nm for both standard and super Gaussians, but is more distinctly structured in the Huggins band above 310 nm with the use of super Gaussian slit functions. Finally, we demonstrate that some spikes of fitting residuals are slightly smoothed by accounting for the slit function errors. Comparisons with ozonesondes demonstrate noticeable improvements when using PAs for both standard and super Gaussians, especially for reducing the systematic biases in the tropics and midlatitudes (mean biases of tropospheric column ozone reduced from -1.4∼0.7 to 0.0∼0.4 DU) and reducing the standard deviations of tropospheric ozone column differences at high latitudes (by 1 DU for the super Gaussian). Including PAs also makes the retrievals consistent between standard and super Gaussians. This study corroborates the slit function differences between radiance and irradiance, demonstrating that it is important to account for such differences in the ozone profile retrievals.

Linearization of the effect of slit function changes for improving OMI ozone profile retrievals

We introduce a method that reduces the spectral fit residuals caused by the slit function errors in an optimal estimation based spectral fitting process to improve ozone profile retrievals from the Ozone Monitoring Instrument (OMI) ultraviolet measurements (270–330 nm). Previously, a slit function was parameterized as a standard Gaussian by fitting the Full Width at Half Maximum (FWHM) of the slit function from climatological OMI solar irradiances. This cannot account for the temporal variation of slit function in irradiance, the intra-orbit slit function changes due to thermally-induced change and scene inhomogeneity, and potential differences in the slit functions of irradiance and radiance measurements. As a result, radiance simulation errors may be induced due to using the convolved reference spectra with incorrect slit functions. To better represent the shape of the slit functions, we implement a more generic super Gaussian slit function with two free parameters (slit width and shape factor); it becomes standard Gaussian when the shape factor is fixed to be 2. The effects of errors in slit function parameters on radiance spectra, referred as Pseudo Absorbers (PAs), are linearized by convolving high-resolution cross sections or simulated radiances with the partial derivatives of the slit function with respect to the slit parameters. The PAs are included in the spectral fitting scaled by fitting coefficients that are iteratively adjusted as elements of the state vector along with ozone and other fitting parameters. The fit coefficients vary with cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The total PA spectrum is quite similar in the Hartley band below 310 nm for both standard and super Gaussians, but is more distinctly structured in the Huggins band above 310 nm with the use of super Gaussian slit functions. Finally, we demonstrate that some spikes of fitting residuals are slightly smoothed by accounting for the slit function errors. Comparisons with ozonesondes demonstrate substantial improvements with the use of PAs for both standard and super Gaussians, especially for reducing the systematic biases in the tropics and mid-latitudes and reducing the standard deviations at high-latitudes. Including PAs also makes the retrievals consistent between standard and super Gaussians. This study corroborates the slit function differences between radiance and irradiance demonstrating that it is important to account for such differences in the ozone profile retrievals.

Accurate measurements of ozone absorption cross-sections in the Hartley band

Ozone plays a crucial role in tropospheric chemistry, is the third largest contributor to greenhouse radiative forcing after carbon dioxide and methane and also a toxic air pollutant affecting human health and agriculture. Long-term measurements of tropospheric ozone have been performed globally for more than 30 years with UV photometers, all relying on the absorption of ozone at the 253.65 nm line of mercury. We have re-determined this cross-section and report a value of 11.27 x 10−18 cm2 molecule−1 with an expanded relative uncertainty of 0.86% (coverage factor k= 2). This is lower than the conventional value currently in use and measured by Hearn (1961) with a relative difference of 1.8%, with the consequence that historically reported ozone concentrations should be increased by 1.8%. In order to perform the new measurements of cross-sections with reduced uncertainties, a system was set up to generate pure ozone in the gas phase together with an optical system based on a UV laser with lines in the Hartley band, including accurate path length measurement of the absorption cell and a careful evaluation of possible impurities in the ozone sample by mass spectrometry and Fourier transform infrared spectroscopy. This resulted in new measurements of absolute values of ozone absorption cross-sections of 9.48 x 10−18, 10.44 x 10−18 and 11.07 x 10−18 cm2 molecule−1, with relative expanded uncertainties better than 0.7%, for the wavelengths (in vacuum) of 244.06, 248.32, and 257.34 nm respectively. The cross-section at the 253.65 nm line of mercury was determined by comparisons using a Standard Reference Photometer equipped with a mercury lamp as the light source. The newly reported value should be used in the future to obtain the most accurate measurements of ozone concentration, which are in closer agreement with non-UV-photometry based methods such as the gas phase titration of ozone with nitrogen monoxide.

Tropospheric ozone and ozone profiles retrieved from GOME-2 and their validation

This paper describes and assesses the performance of the RAL (Rutherford Appleton Laboratory) ozone profile retrieval scheme for the Global Ozone Monitoring Experiment 2 (GOME-2) with a focus on tropospheric ozone. Developments to the scheme since its application to GOME-1 measurements are outlined. These include the approaches developed to account sufficiently for UV radiometric degradation in the Hartley band and for inadequacies in knowledge of instrumental parameters in the Huggins bands to achieve the high-precision spectral fit required to extract information on tropospheric ozone. The assessment includes a validation against ozonesondes (sondes) sampled worldwide over 2 years (2007–2008). Standard deviations of the ensemble with respect to the sondes are considerably lower for the retrieved profiles than for the a priori, with the exception of the lowest subcolumn. Once retrieval vertical smoothing (averaging kernels) has been applied to the sonde profiles there is a retrieval bias of 6% (1.5 DU) in the lower troposphere, with smaller biases in the subcolumns above. The bias in the troposphere varies with latitude. The retrieval underestimates lower tropospheric ozone in the Southern Hemisphere (SH) (15–20% or ~ 1–3 DU) and overestimates it in the Northern Hemisphere (NH) (10% or 2 DU). The ability of the retrieval to reflect the geographical distribution of lower tropospheric ozone, globally (rather than just ozonesonde launch sites) is demonstrated by comparison with the chemistry transport model TOMCAT. For a monthly mean of cloud-cleared GOME-2 pixels, a correlation of 0.66 is found between the retrieval and TOMCAT sampled accordingly, with a bias of 0.7 Dobson Units. GOME-2 estimates higher concentrations in NH pollution centres but lower ozone in the Southern Ocean and South Pacific, which is consistent with the comparison to ozonesondes.

Intermediate photofragment distributions as probes of non-adiabatic dynamics at conical intersections: application to the Hartley band of ozone

Quantum dynamics at a reactive two-state conical intersection lying outside the Franck–Condon zone is studied for a prototypical reaction of ultraviolet photodissociation of ozone in the Hartley band.