Preliminary Results for HCHO and BrO from the EOS-Aura Ozone Monitoring Instrument

Transcription

1 Preliminary Results for HCHO and BrO from the EOS-Aura Ozone Monitoring Instrument Thomas P. Kurosu a, Kelly Chance a and Christropher E. Sioris a a Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA ABSTRACT The Ozone Monitoring Instrument (OMI), a nadir-viewing near-uv/visible CCD spectrometer, was launched on NASA s EOS-Aura satellite platform on 15 July 2004 into a sun-synchronous, polar orbit with an equator crossing time of 13:45h (ascending node). OMI measurements cover the spectral region of nm with a spectral resolution between 0.42 nm and 0.63 nm and a nominal ground footprint of km 2 at nadir. Global coverage is achieved in one day. The very high spatial resolution of OMI measurements set a new standard for trace gas and air quality monitoring from space. Combined with daily global coverage, this significantly advances our ability to answer outstanding questions on air chemistry, including the determination of BrO sources in mid and low latitudes, BrO O 3 anti-correlations as a function of latitude, and the production of formaldehyde in cities of the developing world. We give an overview of the OMI instrument and introduce the operational trace gas retrieval scheme for BrO, HCHO, and OClO that is based on a direct, non-linear fitting approach of observed radiances, including corrections for spectral undersampling. We present first results for tropospheric BrO and HCHO, an important element in air quality monitoring. Only limited results are currently available for OClO, an element in the destruction cycle of polar stratospheric ozone, due to the lack of OMI observations at a time of the year where OClO loading is significantly above the detection limit from space. Keywords: OMI, EOS-Aura, bromine, formaldehyde, VOC, atmospheric composition, air quality, satellite remote sensing, hyperspectral 1. THE OMI INSTRUMENT On 15 July 2004 the Ozone Monitoring Instrument (OMI) was launched on the EOS Aura platform into a sun-synchronous, polar orbit with an equator crossing time of 1345h in the ascending node. OMI is a nadir viewing UV/Vis instrument observing continuously from 270 to 500 nm, and thus similar in scope to the European Space Agency s Global Ozone Monitoring Experiment (GOME), 1 from which it derives much of its heritage. Compared to GOME, OMI has up to three times coarser spectral resolution but at improved spectral sampling, and a more than 40 times smaller ground footprint of km 2 at nadir. 2 OMI will routinely observe, on a daily global scale, ozone, 3 aerosol 4 and cloud 4 6 products, NO 2, 7, 8 HCHO, 7 BrO, 7 OClO, 7 and SO 2. 7 The spectral range of nm is dived into three channels: UV-1 from 270 to 310 nm (with an average spectral resolution of 0.42 nm at FWHM), UV-2 from 310 to 365 nm (0.42 nm), and the visible channel from 365 to 500 nm (0.63 nm). Details on spectral sampling and detector design can be found in Ref. 2 Figure 1 (left image) shows a visualization of the OMI swath over the islands of Hawaii. The viewing angle of ±57 at the satellite, which corresponds to about ±67 at the surface, translates into a total swath of 2,600km across track. This swath is divided into 60 across-track pixels that range in width from 24km at nadir to about 120km at the edges of the swath. 2 Nominal operations of OMI were started on 29 September 2004, six days after the optical bench reached its target temperature of 265K. Repeated cool-down and warm-up cycles provided cold-detector data as early as 17 August, but those observations were made with non-optimized instrument settings and are therefore difficult to utilize for optically Further author information: (Send correspondence to T.P. Kurosu) T.P. Kurosu: tkurosu@cfa.harvard.edu, Telephone: K.Chance: kchance@cfa.harvard.edu, Telephone: C.E. Sioris: csioris@cfa.harvard.edu, Telephone: Passive Optical Remote Sensing of the Atmosphere and Clouds IV, edited by Si Chee Tsay, Tatsuya Yokota, Myoung-Hwan Ahn, Proc. of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 Figure 1. Example of an OMI ground pixel, showing a segment of a complete OMI granule (orbit 01146) covering Hawaii. Across-track pixel sizes range from km 2 (along across track) in the center to about km 2 at the edges of the track. weak absorbers like BrO, OClO, and formaldehyde (absorption features are at the 0.5% level or less in many cases). Similarly, warm detector data (303K) can not be used for the retrievals of these gases. Results presented in this paper are derived from level 1b data generated with the operational level 0-to-1 processor 2 Version 5, which are available starting 23 September. As a consequence, very few results for OClO are available at this time, since OClO concentrations rapidly drop below the detection limit from space with the onset of the polar spring. 9 In the next section we present an overview of the OMI operational retrieval algorithm for BrO, HCHO, and OClO, followed by a section with first results of tropospheric BrO (in the Antarctic) and HCHO (global). 2. THE RETRIEVAL ALGORITHM In the operational retrieval procedure for BrO, HCHO, and OClO, slant column abundances Ω s are determined by fitting measured radiance, beginning with the measured irradiance, molecular absorption cross sections, correction for Ring effect, 10 effective albedo (which includes effects from Rayleigh scattering), and a low-order closure polynomial. All spectral fitting proceeds via a Gauss Newton based non-linear least squares minimization procedure. 11 Key elements in the retrievals are solar and radiance wavelength calibrations, on-line computation of an undersampling correction, and the core fitting itself. The retrieval proceeds in three steps: (1) A solar wavelength calibration determines the relative shift between the OMImeasured solar radiance and an irradiance reference spectrum, 12 and also retrieves the shape and width of the instrument slit function, unless the pre-flight laboratory measurements of the OMI slit function [R. Dirksen, private communication] are used. In the current versions of the algorithm, lab-measured slit function values are used in the BrO and HCHO retrievals, while an asymmetric Gaussian is fitted to the data in the retrieval of OClO. (2) Radiance wavelength calibration determines the wavelength shift between a representative OMI radiance measurement and the solar reference spectrum, and interpolates all reference spectra and cross sections to a common wavelength grid. Both steps (1) and (2) are performed for all 60 across-track pixels. (3) The radiance fitting retrieves slant column quantities of the target gas individually for each pixel. Ozone absorbs in all of the BrO, HCHO, and OClO fitting windows and must be taken into account. As has become apparent during the work of formaldehyde detection from GOME the stability of the HCHO retrieval is greatly improved by pre-fitting O 3 and using it linearly constrained by the determined fitting uncertainties. In similar fashion, HCHO retrieval is further improved by pre-fitting BrO Air Mass Factor Scheme In the case of BrO and HCHO, post-processing after the radiance fit converts the slant columns Ω s to vertical column abundances Ω v by ways of a molecule-specific Air Mass Factor (AMF): AMF = Ω s /Ω v. The reprocessing of earlier observations is currently in progress. Proc. of SPIE Vol

3 Figure 2. Left: OMI UV-2 slit function in the BrO fitting region (black dots), its Nyquist-sampled portion (light-gray line), and the undersampled portion (dark-gray line). Right: As before, but for twice the OMI sampling frequency. AMF values are calculated using the LIDORT radiative transfer model, 16 accounting for atmospheric scattering, surface reflection, satellite viewing geometry, and the vertical distribution of the target gas. For BrO (mainly stratospheric), a Gaussian vertical concentration profile 17 is assumed. For HCHO (tropospheric) the AMF formulation is more intricate, as it is strongly influenced by tropospheric clouds Undersampling correction Remote sensors commonly suffer from spectral undersampling: The instrument transfer function (ITF) is not sampled highly enough to preserve the information content incoming to the detectors. When the measurement is resampled to another wavelength grid, as is routinely done during wavelength calibration, the undersampled portion of the signal introduces spectral structures that can severely limit the accuracy of the retrieval. The sampling theorem (Nyquist theorem) states that a signal may be uniquely reconstructed, without error, if the sampling rate is equal to, or greater than, twice the highest frequency component in the signal. In other words, the highest frequency that is correctly sampled corresponds to half the number of sampling points all higher frequencies are undersampled. The importance of an undersampling correction was demonstrated for GOME by Refs. 18 and 19, and it has been shown to be an important consideration in the retrieval of weak absorbers like BrO, HCHO, and OClO. In these cases the spectral aliasing introduced by undersampling is of similar magnitude as the spectral signature of the absorbers. In a recent study, 20 the effect of undersampling on satellite sensors has been quantified in more detail. The discussion below is a short summary of Ref. 20, which was recently accept for publication. The extensive pre-flight characterization of the OMI ITF makes it possible to perform a theoretical undersampling study before actual measurements are available. OMI sampling rates are 0.15 nm/pixel ( 2.8 pixels/fwhm) in the UV and 0.21 nm/pixel ( 3 pixels/fwhm) in the visible. The left panel in Figure 2 shows the UV-2 channel instrument transfer function (ITF, black dots) in the fitting window of BrO, which has been derived from a polynomial fit to the pre-flight characterization measurements. Also shown are the portions of the ITF that are fully sampled according to the Nyquist theorem (light-gray line), and the undersampled part (dark-gray line). The right panel in Figure 2 shows the same quantities, but for a sampling frequency twice that of OMI, illustrating that higher sampling rates eliminate the undersampled portion of the slit function almost entirely. For the retrieval of BrO, HCHO and OClO (as well as the pre-fitted O 3 ), an undersampling correction is computed online during the radiance wavelength calibration, and is included in each radiance fit as a reference spectrum. This approach has been shown to greatly reduce the fitting RMS by accounting for large parts of instrument-specific residuals. It is important to note that a well-characterized ITF from pre-flight measurements does not affect the undersampling of the actual measurements, since the latter are taken in space with the available sampling rate of the detector. 118 Proc. of SPIE Vol. 5652

4 As an alternative to undersampling, for BrO, HCHO and OClO a radiance spectrum can be used instead of the OMImeasured solar spectrum. In this case the trace gas loading is retrieved relative to that of the radiance reference. The advantage of this approach lies in the elimination of most (but not all) undersampling effects, since now the retrieval does not involve resampling between radiance and irradiance grid, and wavelength shifts between different radiance spectra are usually small. Obviously, great care must be taken in the selection of such a radiance reference in order to minimize the error introduced by any residual loading of the target gas in the reference spectrum. It must be noted that this approach is only suitable for gases that are weakly absorbing. It will fail for strongly absorbing targets like O 3. The use of a radiance reference also improves the retrievals of BrO, HCHO and OClO by reducing cross-track calibration errors from which the current versions of the OMI operational algorithms still suffer [G. Jaross, private communication]. It is expected that future versions of the OMI operational data processor will provide improved cross-track calibration. In the meantime, results from OMI will exhibit, to a higher or lesser extent, the pronounced cross-track dependence that is apparent in the results presented in the next section. All results shown below have been derived with this radiance reference method BrO 3. FIRST RESULTS BrO was first measured from space by GOME, 18 in the nm region. Although thought of as a primarily stratospheric gas, lower tropospheric ozone destruction in the Arctic polar sunrise has been coupled with bromine chemistry associated with the ice pack. 23 Observations from ER-2 24 and GOME 25 show the presence of enhanced tropospheric BrO in the Arctic and Antarctic spring. Compared to GOME, OMI measurements of BrO are of higher spatial resolution, which, coupled with cloud determination, permit the location and persistence of enhanced polar tropospheric BrO to be studied in synergy with ozone in order to quantify the effects on tropospheric O 3. The expected uncertainty of OMI BrO column measurements, excluding systematic contribution from cross section uncertainties, is 12%, corresponding to molec/cm 2 vertical column density. 7 Figure 3 shows first retrievals of BrO from OMI during the Antarctic Spring. The top left panel shows one day (14 orbits) of OMI BrO measurements, for viewing angles <60, and binned to a grid with preference given to smaller-angle viewing geometries. Enhanced columns of BrO over much of the Antarctic shelf-ice are apparent, with isolated hot spots clearly discernible. Across-track variation, apparent as streaks in the data, is due to the less than optimum across track radiance calibration presently implemented in the OMI operational data processor for the level 1b product. Relative retrieval error in % is shown in the top right panel: Over Antarctica, fitting uncertainties are generally around 25 30%, and as low as 15% for the BrO hot spots; towards lower latitudes the relative fitting error increases, at least in part due to the diminishing BrO column amounts. The extent of the ice shelf in September 2004, including the median ice extent, 21 is provided in the bottom right panel of Figure 3. Although parts of the BrO enhancements may well be artifacts arising from correlations with enhanced surface albedo over ice, the clearly visible hot spots close to the edges of the ice shelf are uncorrelated with albedo and show the potential of BrO retrieval from OMI. A zoom of one such hot spot (south of New Zealand, close to Antarctica s Cape Adare) is shown in the lower left panel of Figure 3 to illustrate the resolving power of the OMI instrument HCHO HCHO is a principal intermediate in the oxidation of hydrocarbons in the troposphere, providing an important indicator of biogenic activity. In the remote marine troposphere it may serve as a useful proxy for tropospheric OH. HCHO was first measured from space by GOME, in the nm range. 26 HCHO is prominent in the Southeastern U.S. in summertime (from isoprene oxidation) 13, 15, 27 and is also a prominent product of biomass burning. 26 OMI measures HCHO with higher spatial resolution and with better temporal coverage, allowing for improved characterization of sources and transport. The expected uncertainty of OMI HCHO column measurements under biogenicallyenhanced conditions, excluding systematic contribution from cross section uncertainties, is 22%, corresponding to molec/cm 2 vertical column density. 7 Figure 4 (top panel) presents a global HCHO retrieval from OMI. Shown is a 26 day average between 24 September and 19 October 2004 that computed from HCHO geometric vertical column with a cloud contamination of 20% cloud fractional coverage or less. Biomass burning events in the Amazon and in Central Africa Proc. of SPIE Vol

5 Figure 3. Top left: OMI total geometric columns of tropospheric BrO released from breaking-up shelf ice in Antarctica, 29/30 September 2004 (orbits ). Top right: Relative fitting errors in %. Bottom left: Zoom of BrO columns from orbit OMI, across Antarctica s Cape Adare. Bottom right: Sea ice index21 for September 2004, including mean ice extent (pink line); available at index. 120 Proc. of SPIE Vol. 5652

6 Figure 4. OMI total (geometric AMF) column of tropospheric formaldehyde (HCHO), averaged over 24 Sep 19 Oct 2004 (orbits ), screened for cloud fractional cover 20%.22 Top: Global distribution of HCHO. Bottom: Zoom of South-East Asia, with slightly enhanced color scale. Proc. of SPIE Vol

7 stand out as large areas of elevated HCHO columns. No strongly elevated HCHO is found in the northern hemisphere, since neither biomass burning nor isoprene emissions are significant after late August to early September. The bottom panel of Figure 4 shows a zoom from the global HCHO retrieval over South-East Asia. It illustrates the outstanding capability of OMI to identify regions of biomass burning and anthropogenic VOC emissions. Areas of biomass burning (northern Australia, Borneo, New Guinea, etc.) stand out, as do industrial centers like Jakarta. As in the case of BrO, across-track variation apparent as streaks in the data is due remaining calibration problems in the OMI operational data processor. The high spatial resolution of these observations are unprecedented from space. They will provide an important tool for isolating VOC emissions and biomass burning estimates and will be useful for improving process studies of the terrestrial biosphere to a level better than possible so far from GOME [P. Palmer, private communication] OClO OClO is a useful indicator for chlorine activation in the stratosphere. It was first measured from space by GOME 9 in the nm range. To within current GOME measurement and retrieval uncertainties, it is found entirely within the polar vortices. In addition to continuing the measurement record, providing an indicator for the trend in stratospheric chlorine loading, OMI measures OClO at higher spatial resolution than GOME, which will help to elucidate the details of its formation, persistence, and correlation with PSCs and temperature. For OClO the data products are slant column densities. This is due to the fact that OClO is generally found at very high solar zenith angles, so the determination of appropriate air mass factors for a given case requires substantial off-line analysis. As has been noted above, OClO is rapidly photolyzed during the onset of the polar spring, and concentrations quickly drop below the detection limit currently achieved from space. First retrievals from OMI performed for measurements in late September confirm the values of OClO slant column densities of molec/cm 2 at a solar zenith angle of 90 stated by Ref. 9. Further study currently awaits the availability of reprocessed OMI measurements from early September, and observations of high-latitude air masses in the Arctic during the northern hemispheric winter. ACKNOWLEDGMENTS This work was funded by a National Aeronautics and Space Administration Ozone Monitoring Instrument Science Team contract. The authors would like to thank the OMI Science Team for helpful discussions. Thomas Kurosu would like to thank Marghi Hopkins, Peter Leonard and Mike Linda at SSAI for support with OMI data processing, provision of OMI L1b data, and all matters related to righting things when PGEs go wrong. REFERENCES 1. J. P. Burrows and K. V. Chance, SCIAMACHY and GOME: The Scientific Objectives, in Optical Methods in Atmospheric Chemistry, SPIE, P. F. Levelt and R. Noordhoek, OMI Algorithm Theoretical Basis Document Volume I: OMI Instrument, Level 0-1b Processor, Calibration & Operations, Tech. Rep. ATBD-OMI-01, Version 1.1, August P. K. Bhartia, OMI Algorithm Theoretical Basis Document Volume II: OMI Ozone Products, Tech. Rep. ATBD- OMI-02, Version 2.0, August P. Stammes and R. Noordhoek, OMI Algorithm Theoretical Basis Document Volume III: Clouds, Aerosols, and Surface UV Irradiance, Tech. Rep. ATBD-OMI-03, Version 2.0, August J.R. Acarreta, J. de Haan, and P. Stammes, Cloud pressure retrieval using the O 2 O 2 absorption band at 477 nm, Journal of Geophysical Research 109(D05204), J. Joiner, A.P. Vasilkov, D.E. Flittner, J.F. Gleason, and P.K. Bhartia, Retrieval of cloud pressure and oceanic chlorophyll content using Raman scattering in GOME ultraviolet spectra, Journal of Geophysical Research 109(D01109), K. Chance, OMI Algorithm Theoretical Basis Document Volume IV: OMI Trace Gas Algorithms, Tech. Rep. ATBD-OMI-04, Version 2.0, August K.F. Boersma, H.J. Eskes, and E.J. Brinksma, Error analyses for tropospheric NO 2 retrieval from space, Journal of Geophysical Research 109(D04311), Proc. of SPIE Vol. 5652

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