Total Ozone Mapping Spectrometer Algorithm Research Articles

Validation of OMI total ozone retrievals from the SAO ozone profile algorithm and three operational algorithms with Brewer measurements

Abstract. The accuracy of total ozone computed from the Smithsonian Astrophysical Observatory (SAO) optimal estimation (OE) ozone profile algorithm (SOE) applied to the Ozone Monitoring Instrument (OMI) is assessed through comparisons with ground-based Brewer spectrometer measurements from 2005 to 2008. We also compare the three OMI operational ozone products, derived from the NASA Total Ozone Mapping Spectrometer (TOMS) algorithm, the KNMI (Royal Netherlands Meteorological Institute) differential optical absorption spectroscopy (DOAS) algorithm, and KNMI's Optimal Estimation (KOE) algorithm. The best agreement is observed between SAO and Brewer, with a mean difference of within 1% at most individual stations. The KNMI OE algorithm systematically overestimates Brewer total ozone by 2% at low and mid-latitudes and 5% at high latitudes while the TOMS and DOAS algorithms underestimate it by ~1.65% on average. Standard deviations of ~1.8% are calculated for both SOE and TOMS, but DOAS and KOE have higher values of 2.2% and 2.6%, respectively. The stability of the SOE algorithm is found to have insignificant dependence on viewing geometry, cloud parameters, or total ozone column. In comparison, the KOE–Brewer differences are significantly correlated with solar and viewing zenith angles and show significant deviations depending on cloud parameters and total ozone amount. The TOMS algorithm exhibits similar stability to SOE with respect to viewing geometry and total column ozone, but has stronger cloud parameter dependence. The dependence of DOAS on observational geometry and geophysical conditions is marginal compared to KOE, but is distinct compared to the SOE and TOMS algorithms. Comparisons of all four OMI products with Brewer show no apparent long-term drift, but seasonal features are evident, especially for KOE and TOMS. The substantial differences in the KOE vs. SOE algorithm performance cannot be sufficiently explained by the use of soft calibration (in SOE) and the use of different a priori error covariance matrices; however, other algorithm details cause fitting residuals larger by a factor of 2–3 for KOE.

Comparison of TOMS retrievals and UVMRP measurements of surface spectral UV radiation in the United States

Abstract. Surface noontime spectral ultraviolet (UV) irradiances during May-September of 2000–2004 from the total ozone mapping spectrometer (TOMS) satellite retrievals are systematically compared with the ground measurements at 27 climatological sites maintained by the USDA UV-B Monitoring and Research Program. The TOMS retrievals are evaluated by two cloud screening methods and local air quality conditions to determine their bias dependencies on spectral bands, cloudiness, aerosol loadings, and air pollution. Under clear-sky conditions, TOMS retrieval biases vary from −3.4% (underestimation) to 23.6% (overestimation). Averaged over all sites, the relative mean biases for 305, 311, 325, and 368 nm are respectively 15.4, 7.9, 7.6, and 7.0% (overestimation). The bias enhancement for 305 nm by approximately twice that of other bands likely results from absorption by gaseous pollutants (SO2, O3), and aerosols that are not included in the TOMS algorithm. For all bands, strong positive correlations of the TOMS biases are identified with aerosol optical depth, which explains nearly 50% of the variances of TOMS biases. The more restrictive in-situ cloud screening method reduces the biases by 3.4–3.9% averaged over all sites. This suggests that the TOMS biases from the in-situ cloud contamination may account for approximately 25% for 305 nm and 50% for other bands of the total bias. The correlation coefficients between total-sky and clear-sky biases across 27 sites are 0.92, 0.89, 0.83, and 0.78 for 305, 311, 325, and 368 nm, respectively. The results show that the spatial characteristics of the TOMS retrieval biases are systematic, representative of both clear and total-sky conditions.

A quarter century of TOMS total column ozone measurements over Brazil

The Earth's atmosphere has been probed since 1978 by the total ozone mapping spectrometer (TOMS) experiment. In polar orbits around the Earth, Nimbus 7, Meteor 3, and Earth Probe satellites have carried TOMS to measure the backscattered solar ultraviolet radiation, which is processed by Version 8 TOMS algorithm for the retrieval of column ozone values. These TOMS data and their behavior are reported and investigated in this work for some sites in the equatorial and tropical zones of Brazil. Comparison with measurements from Brewer and Dobson spectrophotometers in some cases is provided. An average difference as low as 1.5% between TOMS and Brewer measurements was obtained. In addition, more than two decades of TOMS monthly average column ozone measurements were applied to the calculation of ozone trends. Although small and in general negative, these trends were statistically significant for all sites in the winter season.

Total Ozone Mapping Spectrometer measurements of aerosol absorption from space: Comparison to SAFARI 2000 ground‐based observations

The capability to detect the presence of absorbing aerosols in the atmosphere using space‐based near‐UV observations has been demonstrated in the last few years, as indicated by the widespread use by the atmospheric sciences community of the Total Ozone Mapping Spectrometer (TOMS) aerosol index as a qualitative representation of aerosol absorption. An inversion procedure has been developed to convert the unique spectral signature generated by the interaction of molecular scattering and particle absorption into a quantitative measure of aerosol absorption. In this work we evaluate the accuracy of the near‐UV method of aerosol absorption sensing by means of a comparison of TOMS retrieved aerosol single scattering albedo and extinction optical depth to ground‐based measurements of the same parameters by the Aerosol Robotic Network (AERONET) for a 2‐month period during the SAFARI 2000 campaign. The availability of collocated AERONET observations of aerosol properties, as well as Micropulse Lidar Network measurements of the aerosol vertical distribution, offered a rare opportunity for the evaluation of the uncertainty associated with the height of the absorbing aerosol layer in the TOMS aerosol retrieval algorithm. Results of the comparative analysis indicate that in the absence of explicit information on the vertical distribution of the aerosols, the standard TOMS algorithm assumption yields, in most cases, reasonable agreement of aerosol optical depth (±30%) and single scattering albedo (±0.03) with the AERONET observations. When information on the aerosol vertical distribution is available, the accuracy of the retrieved parameters improves significantly in those cases when the actual aerosol profile is markedly different from the idealized algorithmic assumption.

Errors resulting from assuming opaque Lambertian clouds in TOMS ozone retrieval

Accurate remote sensing retrieval of atmospheric constituents over cloudy areas is very challenging because of insufficient knowledge of cloud parameters. Cloud treatments are highly idealized in most retrieval algorithms. Using a radiative transfer model treating clouds as scattering media, we investigate the effects of assuming opaque Lambertian clouds and employing a Partial Cloud Model (PCM) on Total Ozone Mapping Spectrometer (TOMS) ozone retrievals, especially for tropical high-reflectivity clouds. Assuming angularly independent cloud reflection is good because the Ozone Retrieval Errors (OREs) are within 1.5% of the total ozone (i.e., within TOMS retrieval precision) when Cloud Optical Depth (COD)⩾20. Because of Intra-Cloud Ozone Absorption ENhancement (ICOAEN), assuming opaque clouds can introduce large OREs even for optically thick clouds. For a water cloud of COD 40 spanning 2– 12 km with 20.8 Dobson Unit (DU) ozone homogeneously distributed in the cloud, the ORE is 17.8 DU in the nadir view. The ICOAEN effect depends greatly on solar zenith angle, view zenith angle, and intra-cloud ozone amount and distribution. The TOMS PCM is good because negative errors from the cloud fraction being underestimated partly cancel other positive errors. At COD⩽5, the TOMS algorithm retrieves approximately the correct total ozone because of compensating errors. With increasing COD up to 20–40, the overall positive ORE increases and is finally dominated by the ICOAEN effect. The ICOAEN effect is typically 5–13 DU on average over the Atlantic and Africa and 1–7 DU over the Pacific for tropical high-altitude (cloud top pressure ⩽300 hPa ) and high-reflectivity (reflectivity ⩾ 80%) clouds. Knowledge of TOMS ozone retrieval errors has important implications for remote sensing of ozone/trace gases from other satellite instruments.

On the accuracy of Total Ozone Mapping Spectrometer retrievals over tropical cloudy regions

Motivated by the desire to accurately derive tropospheric ozone from Total Ozone Mapping Spectrometer (TOMS) measurements, we investigate several aspects of these observations in the presence of highly reflecting clouds. Using the collocated Temperature Humidity InfraRed (THIR) measurements of cloud‐top pressures, we identify three TOMS algorithm errors resulting from the inaccurate assignment of cloud‐top pressure. The most significant error results from the inappropriate tropospheric ozone amount added below cloudy scenes to complete the total ozone column. After accounting for the cloud‐height errors, we find significant total ozone column excesses of 10–15 Dobson units (DU) (1 DU = 2.6867×1016 molecules cm−2) over high‐altitude, highly reflecting clouds compared with clear area observations. After accounting for additional algorithm errors involving the tropospheric ozone climatology and considering potential dynamical, photochemical, and NIMBUS‐7/Earth Probe calibration errors, approximately 4–9 DU excesses over cloudy scenes remain. We speculate that the TOMS algorithm approximation of clouds as opaque Lambertian reflecting surfaces may account for a significant portion of these unexplained excesses. The excess ozone due to calibration error and unknown sources will affect the tropospheric ozone derived from TOMS measurements using clear/cloudy difference techniques.

Satellite estimation of spectral surface UV irradiance: 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surface UV irradiance from satellite ozone and reflectivity data from a clear‐sky case to a cloudy atmosphere and snow‐covered surface. Two methods are compared for the estimation of cloud‐transmission factor CT, the ratio of cloudy to clear‐sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite‐derived CT from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground‐based CT estimations from the Canadian network of Brewer spectrometers for the period 1989–1998. For snow‐free conditions the TOMS derived CT at 324 nm approximately agrees with Brewer data with a correlation coefficient of ∼0.9 and a standard deviation of ∼0.1. The key source of uncertainty is the different size of the TOMS FOV (∼100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly CT averages were smaller than for daily values. The plane‐parallel cloud method produces a systematic CT bias relative to the Brewer data (+7% at low solar zenith angles to −10% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds and snow/ice if the regional snow albedo RS is known from outside data. Since RS varies on a daily basis, using a climatology will result in additional error in the satellite‐estimated CT. The CT error has the same sign as the RS error and increases over highly reflecting surfaces. Finally, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of CT. Both effects reduce CT compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.

Effect of marine stratocumulus on TOMS ozone

The algorithm used to correct total O3 from the total ozone mapping spectrometer (TOMS) for cloud effects is based on the measured reflectivity, a climatological cloud top height, and an assumed tropospheric O3 column amount below clouds. In regions of persistent subtropical marine stratocumulus it is assumed that this introduces a positive error into total O3 because these clouds are lower than the assumed mean cloud height used in the algorithm. This appears to be confirmed by high correlation between Nimbus 7 TOMS total O3 and reflectivity data for typical regimes of persistent stratus, as identified by the international satellite cloud climatology project (ISCCP) observations. The TOMS total O3 overestimate has been computed using Nimbus 7/solar backscattered ultraviolet total O3 derived using temperature humidity infrared radiometer (THIR) data for years 1979–1984. A functional relationship between the THIR/non‐THIR total O3 difference and reflectivity is used with TOMS reflectivity to modify Nimbus 7 TOMS O3 data for selected regions and periods. The correction diminishes or eliminates a number of apparent O3 maxima, with reductions of up to 20 Dobson units (DU) in total O3 on daily maps and ∼5 DU on monthly mean O3 maps. Significant correlation between corrected TOMS O3 and reflectivity data remains because low‐altitude O3 is retrieved more efficiently over a high‐albedo surface. It is also possible that dynamical influences leading to stratocumulus formation bring O3‐enriched air into the area. These results imply that although good arguments can be made for the use of TOMS total O3 as a proxy for tropospheric O3 in the tropics, caution must be exercised in the use of daily and even monthly O3 maps in the vicinity of clouds. Further research into the TOMS algorithm in cloudy regions is required to derive reliable estimates of tropospheric O3.

Total ozone from the TIROS operational vertical sounder during the formation of the 1987 “ozone hole”

Total ozone maps obtained from the infrared radiances measured by the TOVS/HIRS2 instrument on board the NOAA 10 satellite are used to study the formation of the 1987 Antarctic “ozone hole.”We use in this study an improved version of the retrieval algorithm described by Muller and Cayla (1983), now calibrated for middle and high latitudes and taking into account the unusually depleted ozone profiles of the Antarctic spring. Error analysis indicates that our method has an accuracy of the order of 5–7% in clear sky conditions. Values determined from the TIROS operational vertical sounder (TOVS) are in good agreement with Dobson measurements in the mid‐latitudes and with the ozonesondes launched from the Antarctic stations during the Airborne Antarctic Ozone Experiment (AAOE). The agreement with the total ozone mapping spectrometer (TOMS) data at mid‐latitudes is also good, but significant differences are found in early September in the high latitudes. In particular, the large‐scale zonally symmetric minimum representative of the ozone hole appears later in the TOVS maps than in the TOMS data. The ozone hole was already apparent in the TOMS map on the first days of September, while TOVS detected only localized ozone deep minima associated with optically thick polar stratospheric clouds (PSCs) and did not observe any circular depletion structure until September 17. This discrepancy seems to be the consequence of high solar zenith angles and climatological errors in the TOMS algorithm, which tends to underestimate the ozone content in late winter. It is only in mid‐September that TOVS data show a rapid ozone decrease affecting the whole vortex. The low ozone amounts are first recorded in the vicinity of the PSCs detected in the ozone field and then spread into the vortex. TOVS observations suggest that a rapid ozone decrease might take place during or just after the formation of major water ice PSCs. Since the vortex is exposed to UV sunlight in mid‐September, this could be the direct consequence of both a sudden increase of free chlorine and an efficient denitrification occurring during type 2 PSC events. It is concluded that since the algorithm presented in this paper allows reliable ozone determinations in middle and high latitudes and accurate type 2 PSC detection, measurements from TOVS could play an important role in the ozone layer monitoring, especially in the wintertime polar regions where UV techniques are ineffective or affected by the lack of intense sunlight.

A new self‐calibration method applied to TOMS and SBUV backscattered ultraviolet data to determine long‐term global ozone change

The currently archived (1989) total ozone mapping spectrometer (TOMS) and solar backscattered ultraviolet (SBUV) total ozone data (version 5) show a global average decrease of about 9.0% from November 1978 to November 1988. This large decrease disagrees with an approximate 3.5% decrease estimated from the ground‐based Dobson network. The primary source of disagreement was found to arise from an overestimate of reflectivity change and its incorrect wavelengths dependence for the diffuser plate used when measuring solar irradiance. Both of these factors have led to an overestimate of the rate of atmospheric ozone depletion by SBUV and TOMS. For total ozone measured by TOMS, a means has been found to use the measured radiance‐irradiance ratio from several wavelengths pairs to construct an internally self consistent calibration. The method uses the wavelength dependence of the sensitivity to calibration errors and the requirement that albedo ratios for each wavelength pair yield the same total ozone amounts. Smaller errors in determining spacecraft attitude, synchronization problems with the photon counting electronics, and sea glint contamination of boundary reflectivity data have been corrected or minimized. New climatological low‐ozone profiles have been incorporated into the TOMS algorithm that are appropriate for Antarctic ozone hole conditions and other low ozone cases. The combined corrections have led to a new determination of the global average total ozone trend (version 6) as a 2.9±1.3% decrease over 11 years (October 1978 to November 1989). Version 6 data are shown to be in agreement within error limits with the average of 39 ground‐based Dobson stations and with the world standard Dobson spectrometer 83 at Mauna Loa, Hawaii. The global average ozone trend from version 6 data shows the presence of varying short‐period trends (1979 to 1983, −0.33%/yr; 1983 to 1986, −0.91% yr, and 1986 to 1990, +0.16%/yr) that are partially masked in the original version 5 trends.