Ultraviolet Spectroscopy of Stratospheric Molecular O3

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1 Ultraviolet Spectroscopy of Stratospheric Molecular O3 Christine Predaina * Colorado Space Grant Consortium, University of Colorado Boulder, Colorado, The study of O3 (Ozone) remains a current topic of research today, as people from scientists to political policy makers concern themselves with the issues of ozone depletion. A considerable amount of attention has been paid particularly to atmospheric ozone destruction catalyzed by the production of man-made chlorofluorocarbons (CFCs) and nitrogen oxides. The most effective means of observing stratospheric ozone changes is through the use of space based spectroscopy. Spectral analysis of backscattered ultraviolet light yields information about O3 abundances in the atmosphere. O3 is the molecule responsible for a majority of ultraviolet light reflection in the Earth's atmosphere. Although extensive O3 spectral analysis has been done on current and past orbital missions, a full characterization of ozone spectroscopy is not complete. Baseline laboratory tests of ozone are necessary to the complete understanding of ozone processes in the atmosphere but are difficult to execute and are therefore ongoing. In this discussion the spectroscopy of stratospheric ozone in the ultraviolet spectral range will be discussed within the context of the purpose of the Citizen Explorer 1 mission. It will be supported by a summary of past and present orbital observation missions, present laboratory experimentation, and fundamental understanding of spectroscopy and the O3 molecule. Nomenclature UV = Ultraviolet CFCs = Chlorofluorocarbons λ = wavelength θ = satellite zenith angle, as seen from the ground θ0 = solar zenith angle φ = azimuth angle Ω = column ozone amount P0 = pressure at the reflecting surface R = effective reflectivity at the reflecting surface Sb = fraction of radiation reflected from surface that atmosphere reflects back to surface Id = total amount of direct and diffuse radiation reaching surface at P0 f = fraction of radiation reflected toward satellite in direction θ that reaches satellite, I. Introduction tratospheric ozone depletion is a controversial political and economic issue as well as a complex scientific issue. S There exists a well known link between man made chlorofluorocarbons (CFC s) and the depletion of stratospheric ozone. CFCs came into wide use in the 1950s as refrigerants, blowing agents for creating foam insulation, and as industrial cleaning agents. These man made chemicals contain chlorine which damages ozone when airborne. It is believed that more than 80% of airborne chlorine comes from the decay of human-made compounds. 2 The decrease of stratospheric ozone results in an increased amount of solar ultraviolet (UV) radiation that reaches the surface of the Earth. This is of interest to political policy makers as UV radiation poses multiple human health risks including: skin cancer, eye damage, and the effects of breathing photochemical smog created by UV reacting with pollution. It has been estimated that a one percent decline in ozone levels can lead to a two percent * Science Team Lead, Citizen I Explorer Satellite, 520 UCB, University of Colorado at Boulder. Colorado Space Grant Consortium 1

2 rise in human skin cancer cases. 2 Statistics like these spur political motivation and public interest in scientific research to help explain and try to combat the human effects that damage the ozone layer. The study of the ozone layer has become a worldwide endeavor. Numerous research groups use instruments based in space, on the ground, on airborne balloons, and on aircraft to study and measure the ozone in the stratosphere and all the chemicals that affect its production and destruction. Such research has become one of the highest priorities for environmental science in recent times and is commonplace among popular culture from media to schools. 2 Total ozone column density is difficult to calculate because there are many variables that need to be taken into consideration to obtain a measurement that is representative of the actual ozone abundance. Some of these variables are aerosols, surface albedo, clouds, the season, O3 absorption, and uncertainty in the instrumentation. 5 This list of variables does not include the data that is required just to carry out an ozone density calculation such as: Ozone absorption coefficients as a function of temperature for the wavelengths. Atmospheric Rayleigh scattering coefficients. Climatological temperature profiles. Climatological ozone profiles. Solar zenith angle. Satellite zenith angle at the IFOV. Angle between the solar vector and the scan plane at the IFOV. Pressure at the reflecting surface. 4 These values are all either determined for each individual calculation or are generalized for all calculations. In order to even make initial guesses at ozone abundances a great deal of modeling and precalculating must be done to set up an algorithm that can make reasonable estimates of ozone density. This precalculation requires knowledge of the instrumentation being used for spectroscopy and a basic understanding of the O 3 molecule that will be studied. II. Citizen Explorer I The primary focus of this discussion is in regards to the Citizen Explorer 1 mission. The Citizen Explorer I satellite (CX1) is built around a set of scientific instruments designed to measure ozone concentration in the Earth s stratosphere using ultraviolet spectroscopy. The satellite has two scientific instruments: a spectrophotometer and a photometer. The spectrophotometer is designed to measure backscattered UV light from the Earth's atmosphere over the UV spectra range of 280 nm to 350 nm. The photometer is designed to measure a calibrating wavelength of 365 nm which acts as the spacecraft s albedo and surface reflectivity instrument. The data from the onboard instruments will be combined with data from ground based instrumentation primarily measuring aerosols. These Ground based observations will be conducted by K-12 students all over the world using aerosol detectors made at the University of Colorado and UV detectors purchased from Sunsor, Inc. None of the science that will be done with CX1 is original or contributes significantly to the body of knowledge concerning atmospheric phenomena. The mission is being patterned after a NASA program called TOMS, the Total Ozone Mapping Spectrometer. 4 This mission does however provide a way to teach K-12 students about the importance of ozone and atmospheric studies while at the same time serving an engineering challenge to undergraduates at the University of Colorado. Since this scientific research is not original it relies heavily on what has been accomplished by other scientists and engineers and the body of knowledge they have already assembled concerning spectral analysis of O 3 and its chemical properties. III. Spectroscopy Spectroscopy is the study of the interaction of electromagnetic radiation with matter, in the case of CX1 this matter is molecular O 3. A diffraction grating in the spectrophotometer is used to split radiation into various wavelengths where those individual wavelengths can be individually analyzed. Atmospheric spectroscopy uses these and other instruments to gain an understanding of the composition of the stratosphere. Colorado Space Grant Consortium 2

3 The most effective means of observing stratospheric ozone changes is through the use of space based spectroscopy. Spectral analysis of backscattered ultraviolet light yields information about changing molecular O 3 abundances in the atmosphere. The complicated aspect of this type of spectroscopy is determining how much of the light detected by the spectrophotometer is reflected by the Earth s surface and how much is scattered from the atmosphere. This means that the reflecting properties of the surface must be known, which is done by the photometer in the CX1 system. Reflectivity of light off of a surface is independent of wavelength, absorption by a molecule is dependant on wavelength. Therefore, radiances at two wavelengths, one sensitive to atmospheric ozone and one not, can be used to derive atmospheric ozone abundances and reflectivity. This technique is the pair determination method used in the TOMS spacecraft which will be discussed in detail later. 4 However, using the pair determination method imposes constraints on the total wavelength range such that each individual measurement only contains less than 5 nm of wavelengths and that the ability to make measurements of adjacent wavelength pairs is possible. Also required is the calibration of the instrument response function to have an uncertainty of less than 10 % which is difficult at best to accomplish on a limited budget such as the one CX1 has to work with. These requirements were imposed on TOMS and have dictated the design of the CX1 scientific instrumentation. 4 There are many aspects beyond spectroscopy that affect the ozone calculation. The aerosols, the clouds, the surface albedo, the ozone profile, the ozone spectroscopic data, and the instrument slit function are all expected to influence the ozone calculation. This means that careful consideration must go into choosing what wavelengths to do spectral analysis on that will minimize the effects of some or all of these variables. This leads to the selection of several pair ratios to be used in the same calculation to help estimate the uncertainty of the ozone measurement and calculation. 4 This selection is best done with a working understanding of the chemical processes O 3 undergoes in the stratosphere. IV. Chemistry of Stratospheric Ozone To understand how to detect and measure O 3 in the stratosphere it is important to understand the chemical processes that are at work to create and destroy the molecule. O 3 is a molecule of oxygen composed of three loosely bound atoms. O 3 is the molecule that plays an important role in stratospheric chemistry due to the fact that it photodissociates into O 2 + O when it is exposed to ultraviolet radiation. It is this photodissociation that occurs in the stratosphere that absorbs the harmful radiation and protects the Earth s surface. Ultraviolet light then also creates ozone by ionizing free oxygen atoms which then combine with oxygen molecules to create ozone. O 3 in the stratosphere is destroyed when it combines with either chlorine or nitrogen oxides. A single chlorine molecule can destroy 100,000 ozone molecules in its lifetime. 2 Highly energized ultraviolet light and cosmic rays break down CFCs, releasing active chlorine, which in turn destroys ozone molecules. The following is a commentary including chemical reaction equations explaining how these photochemical reactions take place in the stratosphere to create and destroy O 3. A. The creation of ozone When diatomic oxygen in the stratosphere absorbs ultraviolet radiation with wavelengths less than 240 nm, it breaks apart into two oxygen atoms. O 2 (gas) 2 O(gas) (light wavelength < 240 nm) The resulting free oxygen atoms are ionized by UV light and combine with O 2 molecules to form ozone. (1) O(gas) + O 2 (gas) O 3 (gas) (2) Colorado Space Grant Consortium 3

4 This reaction is exothermic which means it gives off heat. Therefore, the net effect of the previous two reactions is the conversion of three molecules of O 2 into two molecules of O 3 with the simultaneous byproduct of converting light energy to heat. B. How ozone protects the Earth Ozone absorbs ultraviolet radiation with wavelengths as long as 290 nm. This radiation causes the ozone to break apart into O 2 molecules and oxygen atoms. O 3 (gas) O 2 (gas) + O(gas) (light wavelength < 290 nm) This reaction is also exothermic and its overall effect is the conversion of light energy into heat. This means that ozone in the stratosphere prevents highly energetic radiation from reaching the Earth's surface and converts the energy of this radiation to heat. (3) C. The catalytic destruction of ozone There are two main reactions that are known to contribute to the decomposition of ozone in the stratosphere: chlorine from CFC s and nitrogen oxides. Manmade CFCs are chemically unreactive: they are nontoxic, noncorrosive, nonflammable, and very stable. These were the very reasons why they were originally used in fire extinguishers, as propellants in aerosols, solvents in electronics manufacture, and as foaming agents in plastics. 3 In addition naturally occurring nitrogen oxides also contribute to the decomposition of O 3. This chemical reaction breaks down ozone and produces oxygen and nitrogen oxide. This chemical reaction breaks down ozone and produces oxygen and chlorine monoxide. O 3 O 2 + O O 3 O 2 + O NO + O 3 NO 2 + O 2 Cl + O 3 ClO + O 2 NO 2 + O NO + O 2 ClO + O Cl + O 2 Net: 2 O 3 3 O 2 (4) Net: 2 O 3 3 O 2 (5) Because NO is regenerated in the third step, a single molecule of NO can assist in the destruction of many ozone molecules. N 2 O released from soil rises unchanged in the lower atmosphere until it is decomposed by UV radiation in the stratosphere. A fraction of the N 2 O is converted to the NO that catalytically destroys ozone. V. The TOMS Algorithm Once the spectroscopy data is collected and the chemical reactions are understood all of the data must be analyzed. This analysis is done based on the sixth version of the TOMS algorithm that combines all of the variables and measured values into one calculation to determine the total ozone abundance at a given measurement. To interpret the radiance measurements made by the TOMS instrument for example requires an understanding of how the Earth s atmosphere scatters ultraviolet radiation at different solar zenith angles. Incoming solar radiation undergoes absorption and scattering in the atmosphere by atmospheric elements such as ozone and aerosols and by what is known as Rayleigh scattering. Radiation that penetrates to the troposphere is scattered by clouds and Colorado Space Grant Consortium 4

5 aerosols, and radiation that reaches the ground is scattered by surfaces of widely varying reflectivity. 4 information that is included in the TOMS algorithm. This is all A. TOMS wavelength pairs The Citizen Explorer I Satellite is designed to use the TOMS wavelength pair method for measuring ozone column density. This method compares two wavelengths or wavelength ranges; one where the ozone absorption is very strong and the other where the ozone absorption is weak. This relative spectroscopy is what allows the Citizen Explorer s low budget scientific instruments to produce usable data even for a complicated calculation. The following is a high level overview of the TOMS algorithm that CX1 will use as a model for data processing. B. Selecting wavelength pairs A basic understanding of which ultraviolet wavelengths absorb a lot of ozone and which absorb very little ozone is necessary. Wavelength pairs are chosen that are either very sensitive to the differential ozone absorption or are not very sensitive at all. The comparison of these pairs collected over time yields the change in the ozone density. Here is an example of two pairs selected for their relative sensitivity to differential ozone absorption: 340 nm Pair 1= 305.5nm very sensitive to differential ozone absorption Pair 2= 323.5nm 305.5nm less sensitive to differential ozone absorption and to differential aerosol extinction The wavelength used for the denominator in the pairs is 305 nm is chosen where the ozone absorption is very strong and the signal at the ground level high enough. The other wavelengths that are used for the numerator are 340 nm and 323 nm where the ozone absorption is weak. The comparison of these two pairs over multiple measurements will yield ozone abundances. C. Calculating Ozone column density The backscattered radiance emerging from the top of the atmosphere as seen by a TOMS instrument can be modeled by solving the function Im, which is the sum of light from atmospheric backscatter Ia, and reflection of the incident radiation from the reflecting surface (cloud, earth etc) Is: Im (λ, θ, θ0, Ω, P0, R) = Ia (λ, θ, θ0, φ, Ω, P0) + Is (λ, θ, θ0, φ, Ω, P0, R) (6) The surface reflection can be expressed as follows: I s,, 0,,P 0, R = RT,, 0,,P 0 1 RS b,,p 0 T,, 0,,P 0, R =I d,, 0,,P 0 f,,,p 0 (7) (8) The ozone algorithm therefore uses ratio of radiance to irradiance in the form of the N-value, defined as follows: N = 100log 10 I F (9) Colorado Space Grant Consortium 5

6 The N-value provides a unit for backscattered radiance that has a scaling comparable to the column ozone; the factor of 100 is to produce a convenient numerical range. In practical application, rather than calculate N-values separately for each measurement, detailed calculations are performed for a grid of total column ozone amounts, vertical distributions of ozone, solar and satellite zenith angles, and two choices of pressure at the reflecting surface. The calculated N-value for a given scene is then obtained by interpolation in this grid of theoretical N values. 4 This was done for the CX1 mission through the use of a program known as SBDART which creates lookup tables populated with data for these variables generated by a model. This model is then used with the measurements from the spacecraft to determine the ozone density. The I/F for the entire band, A(λ0), is then given by the following expression: (10) A(λ) = at wavelength λ, F(λ) = solar flux at wavelength λ, I(λ) = earth radiance at wavelength λ, and S(λ) = Instrument response function at wavelength λ. To calculate the radiances for determining ozone the height and reflectivity of the reflecting surface must be known. The TOMS algorithm assumes that reflected radiation can come from two levels, ground level or cloud level, both of which have statically modeled values initially. The modeled ground and cloud radiances combined with the photometer radiance are then compared with the measured radiance from the spectrophotometer. If Iground Imeasured Icloud, and snow/ice is assumed not to be present, an effective cloud fraction f is defined by: To summarize, the ozone derivation is a two-step process. In the first step, an initial estimate is determined using the difference between N-values using a set of wavelength pairs, one that is significantly absorbed by ozone, and the other is insensitive to ozone. This is used with the modeled look up tables from SBDART. In the second step, N-values are calculated using this ozone estimate. In general, these calculated values will not equal the measured N-values. The differences, in the sense Nmeas Ncalc, are known as residues. 4 The two pair wavelengths as described above are compared to a wavelength which is insensitive to ozone, in the case of Citizen Explorer this wavelength is 365 nm measured by the photometer. The separation of the 365-nm wavelength from the pair wavelengths is far larger than the separation between the pairs providing a long baseline for determining wavelength dependences. This will over time produce a data set that will represent the column ozone density over the locations measured. Acknowledgments I would like to thank Steve Wichman for his constant support and encouragement for years of writing technical papers for conferences and symposia. (11) Colorado Space Grant Consortium 6

7 References 1 Mason N. J.; Pathak S. K., Spectroscopic studies of ozone- the Earth's UV filter, Contemporary Physics, vol. 38, no. 4, 1 July 1997, pp Remsberg, Ellis E., HALOE: Tracking Ozone Loss From Space, NASA FS LaRC, August Shakhashiri, Bassam Z., Chemical of the week - Ozone, Wisconsin-Madison [cited 25 March 2005]. 4 Richard D. McPeters, P. K. Bhartia, Arlin J. Krueger, and Jay R. Herman, Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) Data Products User's Guide 5 Stamnes et al., "Derivation of total ozone abundance and cloud effects from spectral irradiance measurements", Applied Optics, Vol. 30, No. 30, October Colorado Space Grant Consortium 7

Ozone: Earth s shield from UV radiation

Outline Ozone: Earth s shield from UV radiation Review electromagnetic radiation absorptivity by selective gases temperature vs. height in atmosphere Ozone production and destruction natural balance anthropogenic