Upper Atmospheric NO from SCHIAMACHY: Simulations and Instrument Capabilities
C. Muller , J.C. Lambert, M. Van Roozendael Belgian Institute for Space Aeronomy
IASB-BIRA avenue Circulaire,3 B-1180 Brussels, Belgium Email:
[email protected]
ABSTRACT The SCanning Imaging Absorption spectroMeter for Atmospheric ChartograpHY (SCIAMACHY) operates in eight channels covering the UV, the visible and two infrared regions. Recent developments in the testing of the instrument now enable not only the full use of channel 1 (240 nm-314 nm) at a required high level of performance but also its extension to 220 nm. This instrumental improvement allows new objectives to be addressed in the upper stratosphere, in addition to the already proposed mesospheric and thermospheric investigations of nitric oxide. Simulations will show the instrument capabilities for these studies and will be compared with the latest instrument test data obtained before instrument delivery. The operation modes corresponding to these NO observations will also be described. The capabilities of SCIAMACHY for mapping the total column of upper atmospheric NO will be investigated as well as possibilities to infer NO vertical distribution and transfer properties between the different atmospheric regions. . ENVISAT objectives. The ESA ENVISAT satellite is now (September 2000) scheduled for a launch in June 2001, the satellite and payload are undergoing integrated tests in the ESA ESTEC facilities in the Netherlands. Three instruments have the study of atmospheric chemistry as their main objective: the stellar occultation U.V.-visible instrument: GOMOS, the thermal infrared sounder MIPAS, and SCIAMACHY. SCIAMACHY (Burrows et al, 1988) was proposed as part of the first ENVISAT payload. It was accepted by ESA as an “Announcement of opportunity” instrument. SCIAMACHY is now a cooperative programme of Germany, the Netherlands and Belgium. The primary objective of SCIAMACHY is to determine vertical and horizontal distributions of important atmospheric constituents and parameters (ozone and other trace species, aerosols, radiance, irradiance, clouds, temperature and pressure) from measurements of radiance combining scattered, absorbed and reflected light from the Earth’s atmosphere and Earth’s surface. Radiance measurements will be performed in different viewing geometries: nadir, limb and solar and lunar occultation. These measurements will contribute to the better understanding of major climate and environmental issues: • • • • • •
Tropospheric pollution including industrial emission and biomass burning Troposphere/stratosphere exchange processes Stratospheric ozone chemistry Climate change - chemistry interactions Volcanic eruptions Solar variability.
The two other instruments aim at similar objectives, centred on the zones where an active biosphere interacts with the atmosphere. A detailed SCIAMACHY description can be found in Burrows et al (1998). Nitric oxide vertical distribution Nitric oxide has relevance in all atmospheric zones: in the troposphere, where it is a direct pollutant and a precursor of tropospheric ozone; in the stratosphere where it contributes to the ozone layer equilibrium and finally in the upper mesosphere and thermosphere where its photo ionisation by the solar Lyman α radiation leads to the formation of the ionosphere and thus influences radio propagation (Nicolet, 1945, 1965, Nicolet and Aiken,1960). Thermospheric NO was measured by its resonant fluorescent emissions in the γ and δ bands by rocket and space means since the middle of the sixties (Barth, 1964) by rocket and space borne instruments. Its relation to solar activity was rapidly put into evidence, however, below 80 km, Rayleigh scattering dominates the NO emission and thus this technique did not bring any information on the NO situation in the stratosphere and mesosphere. The urgent necessity of proving the presence of nitrogen oxides in the natural stratosphere led to its first determinations in infrared solar occultation from balloons and high altitude aircrafts (Ackerman et al, 1973). This observation programme was continued from space by the SPACELAB payloads: ESA Grille and NASA ATMOS leading to the distribution shown on figure 1(Laurent et al, 1985).
Fig. 1: Comparison of the November 1983 Spacelab 1 Grille spectrometer result with the ATLAS 3 ATMOS results obtained in similar conditions above the Southern polar regions, the ATMOS results are from version 3 (Gunson and Irion, 2000) and the grille results are from Muller et al (1988) with supplemental points above 95 km originating from a solar CO spectrum obtained during the same solar occultation (Muller et al, 1999). Figure 1 shows the main features of the NO distribution: a stratospheric maximum, a minimum at the stratopause and extremely variable conditions above 100 km, the maximum at the tropopause in the ATMOS data (Gunson and Irion, 2000) is still under investigation by the ATMOS team. Similar profiles in infrared occultation were obtained by the HALOE instrument on board UARS and confirm these findings despite a larger error for the thermospheric mesospheric part (Russell. et al, 1993) The infrared emission limb sounder CRISTA (covered essentially the upper part of the mesospheric distribution as well as the limb UV sounders: MAHRSI (Stevens et al, 1997), ISO (Torr et al,1995), SME and SNOE (Barth et al,1999). High quality observations of NO distribution have thus been
obtained for more than thirty years. However, three areas of insufficient coverage and accuracy can be identified: the troposphere, the upper stratosphere where the value of the NO minimum is actually not known as all remote sensing techniques are influenced by the lower and upper maximums and finally, despite all the existing work, the thermospheric part where small spatial scale variations seem to appear. Diurnal variations vary also considerably with altitude, from almost instantaneous equilibrium with NO2 in the stratosphere to lifetimes of the order of one day in the thermosphere. Long term variations of the stratospheric maximum and its spatial distribution are, of course, among the nominal ENVISAT atmospheric objectives. ENVISAT and SCIAMACHY capabilities. At ENVISAT proposal time, MIPAS was the only payload instrument covering a specific spectral NO interval: the 5.3 µ m infrared fundamental band which has several uncontaminated lines including the 1914.993 cm-1 doublet in which it was first observed in the stratosphere in 1973 and which was used by the Grille spectrometer for its determination from SPACELAB 1 in 1983 (Laurent et al, 1985). GOMOS and SCIAMACHY have to rely on the nitric oxide γ bands to access NO, unfortunately, the GOMOS spectral range begins at 250 nm and the nominal SCIAMACHY range begins only at 240 nm, leaving out the more intense bands all situated between 200 and 230 nm. The MIPAS observation will require careful retrieval in order to separate the stratospheric part from the much warmer thermospheric emissions. Observations in the cold upper troposphere and lower troposphere will probably prove to be extremely difficult. Stratospheric NO disappears entirely at night while thermospheric NO has a much longer lifetime, the measurement of diurnal variations in this range is one of the objectives of the AALIM proposal (Muller et al, 1999). However, MIPAS covers the ν3 NO2 band and as well as HNO3 bands and will thus be able to perform perfectly its stratospheric NOy monitoring objective. As GOMOS and SCIAMACHY were by design excellent NO2 monitors in their visible range, the original spectral range did not include a possibility for NO measurement. During SCIAMACHY testing, the verification and suppression of straylight in the 240-300 nm range let to check the quality of the 218 to 240 nm corresponding to unused pixels of the detector and it was found that this interval could be used leading to a possible detection of the γ 1-0 band at 226 nm. This interval is especially important because it includes the NO γ fundamental band centred at 226 nm, which has the highest absorption by the non-excited nitric oxide and thus should allow its observation in the stratosphere and troposphere. Unfortunately, atmospheric scattering prevents the use of this interval in the troposphere by any means except long path DOAS (Reisinger, 1999). In the stratosphere, the surest way to assess NO signal will be by solar occultation where signal reappears already weakly at 40 km. of altitude to become comfortable above 50 km. The stratospheric NO maximum around 40 km of altitude would give a 20% absorption, the UV would be thus far more efficient for the study of the presently little known upper part of the NO stratospheric distribution between 40 and 60 km of altitude. At higher altitude, in the mesosphere, a second maximum reappears between 80 and 120 km of altitude and is much easier to detect as NO is distributed among the upper level states and appears thus also in other bands which can then be detected in the nominal channel 1 range of SCIAMACHY, this mesospheric and thermospheric study is the subject of specific A.O. proposals relating to airglow and auroral studies (Muller et al, 1999). The interpretation of solar occultation will be based on the determination of slant columns using either the cross-section used in figure 2 (Reisinger, 1999) or more elaborate line by line determinations as shown on figure 3. As the entire gamma fundamental bands are included in the observed feature, pressure and temperature dependence can be neglected in a first approximation and the obtained slant columns can then be fitted to determine a vertical distribution of NO in the upper stratosphere and lower mesosphere. The use of the limb emission data outside occultations will depend on the actual performance of the instrument in flight. As the 226 nm range was not originally planned, the influence of its observation on nominal operations has still to be investigated; if possible, the very specific weighting functions of limb observations would allow global mapping of NO from the upper stratosphere to the thermosphere using a technique similar to the one described for occultation. SCIAMACHY has also a lunar occultation capability. This nighttime measurement will provide important information on the thermospheric NO absorptions and emissions and the way they perturb the observation of daytime stratospheric NO.
Fig. 2: Modtran 3-7 computation, the 50 km curve still receive significant signal due to the design of the instrument which allows the detection of backscattered radiation. The transmission spectrum of NO is generated using the cross-sections of A.Reisinger used in long path NO monitoring (Reisinger, private communication, 1999).
1.0
Transmission
0.8
0.6
0.4
0.2
0.0
224
225
226
227
wavenumber (nm)
Fig. 3: spectrum of the γ 1-0 bands of NO computed for an optical path of 1.E 16 molecules of NO and for a temperature of 220 K, the resolution of 1cm-1 is 15 times better than the SCIAMACHY resolution.
The nadir data presents a more complicated problem; abundant thermospheric NO can be produced on small spatial and temporal scales by different energy deposition mechanisms ranging from solar flares to intrusions of magnetospheric filaments. This NO content will completely mask the lower altitudes both in absorption and in emission. This NO emission has been successfully observed during the SSBUV flights (Mc Peters, 1995) and has shown to be related to various aspects of solar activity. Its modelling is important for the main SCIAMACHY objectives as this operation can improve the accuracy of the ozone retrieval algorithms. Due to the intensity of the thermospheric emissions, it is not hoped that accurate stratospheric columns will ever be determined by this technique; however, the thermospheric column will be retrieved using modelling techniques similar to the ones used by Mc Peters (1995). γ bands are present in the spectra of the the ESA GOME instrument and the regions where they are present are currently filtered out of the GOME retrievals. Preliminary results of GOME NO data retrieval, currently in progress at BIRA-IASB, are shown in figure 4.
Fig. 4: Average over 54 nadir GOME spectra showing the position of the γ bands of NO present in the channel 1 spectral interval, these emissions originate in the lower thermosphere.
Conclusions. While being able to continue and improve observations already well begun by SSBUV and a series of limb sounders exemplified by SME, SCIAMACHY will be original in its solar occultation mode by providing the upper part of the stratospheric maximum and the value of the minimum preceding the mesospheric-thermospheric maximum, the analysis of this profile will contribute to confirm or infirm the thirty years old theory (Crutzen,1970) assigning the photodissociation of nitrous oxide originating from the troposphere as the main stratospheric NOx source. In this respect, the NO study with SCIAMACHY fully fits the ENVISAT basic objectives.
Acknowledgments This work is part of the Belgian SCIAMACHY preparation program financed through PRODEX by the OSTC. (Federal office of science , technology and cultural policy) . REFERENCES Ackerman, M., Frimout, D., Muller, C., Nevejans, D., Fontanella, J.C., Girard, A. and Louisnard, N., Stratospheric nitric oxide from infrared spectra, Nature, 245, 205-206, 1973. Barth, C.A., Rocket measurements of the nitric oxide dayglow, J. Geophys.Res., 69, 3301-3303, 1964. Burrows J. P., K. V. Chance, P. J. Crutzen, H. van Dop, J. C. Geary, T. J. Johnson, G. W. Harris, I. S. A. Isaksen, G. K. Moortgat, C. Muller, D. Perner, U. Platt, J.-P. Pommereau, H. Rodhe, E. Roeckner, W. Schneider, P. Simon, H. Sundqvist, and J. Vercheval 1988 "SCIAMACHY - A European proposal for atmospheric remote sensing from the ESA Polar Platform" Published by Max-Planck-Institut für Chemie, 55122 Mainz, Germany, July 1988. Barth, C.A., Bailey, S.M.,and Solomon, S.C., Solar-terrestrial coupling: solar soft x-rays and thermospheric nitric oxide, Geophys.Res.Let., 26, 1251-1254, 1999. Grossmann,K.U., Kaufmann, M. and Vollmann,K., Thermospheric Nitric Oxide Infrared Emissions Measured by CRISTA , Adv. Space Res. Vol.19, pp. 591-594, 1997. Gunson,M.R., Irion,F.W. ATMOS version 3, Jet.Propulsion Laboratory, Pasadena, California, http://remus.jpl.nasa.gov/atmosversion3/atmosversion3.html ,2000. Laurent,J., Lemaître,M.P., Besson,J., Girard,A., Lippens,C., Muller,C., Vercheval,J. & Achkerman,M., Middle atmospheric NO and NO2 observed by means of the Spacelab One grille spectrometer, NATURE, 315, 126-127, 1985. Muller, C., I. Aben, W.J. van der Zande and W.Ubachs, Uses of the ENVISAT payload for mesospheric and thermospheric investigations: the AALIM proposal, in Proc. European Symposium on Atmospheric Measurements from Space (ESAMS), ESA/ESTEC, The Netherlands, 18-21 January 1999, ESA WPP-161, Vol. 2, 623-626, 1999.
Lambert, J.C.,. Lippens,C.. and Van Roozendael, M., Atmospheric NO from space, EGS, Nice, 2000. Muller,C.,
Nicolet, M., Contribution à l’étude de l’ionosphère, Mém. Mus. Hist. Nat. Belg., 19, 124pp, 1945. Nicolet, M., Ionospheric processes and nitric oxide, J. Geophys. Res., 70, 691-701, 1965. Nicolet , M. and Aiken, A.C., The formation of the region D of the ionosphere, J. Geophys. Res., 65, 1469-1483, 1960. Russell, J. M. III, L. L. Gordley, J. H. Park, S. R. Drayson, D. H. Hesketh, R. J. Cicerone, A. F. Tuck, J. E. Frederick, J. E. Harries, and P. J. Crutzen: The Halogen Occultation Experiment, J. Geophys. Res., 98, 10,777-10,797, 1993. data available at : http://haloedata.larc.nasa.gov . Stevens, M.H., Nitric oxide γ band fluorescent scattering and self absorption in the mesosphere and lower thermosphere, J. Geophys.Res., 100, 14735-14742, 1995. Stevens, M.H., R.R. Conway, J.G. Cardon, J.M. Russell III, MAHRSI Observations of Nitric Oxide in the Mesosphere and Lower Thermosphere, Geophys. Res. Let., 24, 3213-3216, 1997. Torr, M.R., Torr, D.G., Chang, T., Richars, P., Swift, W. and Li, N., Thermospheric nitric oxide from the ATLAS 1 and Spacelab 1 missions, J. Geophys. Res., 100, 17389-17413, 1995.
