UV Remote Sensing of O 3 and SO 2. The Ozone Layer

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1 UV Remote Sensing of O 3 and SO 2 The Ozone Layer The stratospheric ozone layer is a consequence of molecular photodissociation UV-C radiation dissociates molecular oxygen: O 2 + hv (λ < µm) O + O The large amount of oxygen in the atmospheric column absorbs most solar radiation at λ < 0.24 µm by this mechanism The free oxygen atoms from the above reaction then combine with other O 2 molecules to produce ozone: O + O 2 O 3 Ozone is then dissociated by UV radiation: O 3 + hv (λ < 0.32 µm) O + O 2 Ozone is also destroyed by this reaction: O 3 + O O 2 + O 2 The Chapman Reactions 1

2 The Ozone Layer Fortunately for life on Earth, ozone absorbs strongly between 0.2 and 0.31 µm via electronic transitions removing most UV-B and UV-C not absorbed by O 2 UV-A radiation (λ > 0.32 µm) is transmitted to the lower atmosphere Plus a small fraction of UV-B ( µm) responsible for sunburn Widening of this UV-B window (due to ozone depletion) would have serious impacts on life Absorption of solar radiation by ozone also locally warms the atmosphere to a much higher temperature than would be possible if ozone was absent hence the increase in T in the stratosphere Hence in an atmosphere without free oxygen, and hence without ozone, the temperature would decrease with height until the thermosphere. There would be no stratosphere, and weather would be vastly different... Ozone hole Antarctic ozone hole on Sept 11, 2005 Observed by Ozone Monitoring Instrument (OMI) Ozone destruction peaks in the Spring, as UV radiation returns to the polar regions Catalyzed by the presence of CFC compounds (which supply chlorine), and by polar stratospheric clouds (PSCs) at very cold temperatures 2

3 Ozone is not just in the stratosphere.. The UV-A radiation that reaches the troposphere is a key player in tropospheric chemistry Photochemical reactions involving unburned fuel vapors (organic molecules) and nitrogen oxides (produced at high temperatures in car engines) produce ozone in surface air (tropospheric ozone) Ozone is good in the stratosphere, but a hazard in the troposphere (it is a strong oxidant that attacks organic substances, such as our lungs) Ozone is a major ingredient of photochemical smog λ < 0.4 µm Los Angeles: sunshine (UV) + cars + trapped air = smog Atmospheric Constituents The Ozone Layer The Air We Breathe 3

4 Satellite trace gas retrievals in the UV Antarctic Ozone from TOMS and OMI October 1, October 1, 2003 October 1, Dobson Units October 1,

5 Pinatubo SO 2 cloud Maximum SO 2 column: ~800 DU Total SO 2 Mass: ~20 Mt Pintatubo (Philippines) erupted in June 1991 and produced the largest SO 2 cloud measured to date (i.e. since 1978) Satellite viewing geometry Solar zenith angle Sensor zenith angle d Absorbing gas N Solar azimuth angle Sensor azimuth angle 5

6 Beer s Law in this case I λ = I 0,λ exp( σ λ N dm) m = sec θ s + sec θ sat = airmass factor (AMF) UV SO 2 and O 3 absorption spectra Flyspec, UV camera OMI has ~720 UV channels, compared to 6 on TOMS 6

7 Ideal Gas Law The equation of state of an ideal gas most gases are assumed to be ideal PV = nrt PV = NkT k = R N A P = pressure (Pa), V = volume taken up by gas (m 3 ), n = number of moles, R = gas constant (8.314 J mol -1 K -1 ), T = temperature (K) k = Boltzmann constant ( J K -1 ), N = number of molecules, N A = Avogadro constant ( molecules mol -1 ) Neglects molecular size and intermolecular attractions States that volume changes are inversely related to pressure changes, and linearly related to temperature changes Decrease pressure at constant volume = temperature must decrease (adiabatic cooling) Ideal gases Standard temperature and pressure (STP): varies with organization Usually P = kpa (1 atm) and T = K (0ºC) Sometimes P = kpa and T = K (20ºC) At STP ( kpa, K) each cm 3 of an ideal gas (e.g., air) contains molecules (or m -3 ) This number is the Loschmidt constant and can be derived by rearranging the ideal gas law equation: N = PV kt At higher altitudes, pressure is lower and the number density of molecules is lower Mean molar mass of air = kg mol -1 (air is mostly N 2 ) 7

8 Column density Another way of expressing the abundance of a gas is as column density (S n ), which is the integral of the number density along a path in the atmosphere S n = The unit of column density is molecules cm -2 The integral of the mass concentration is the mass column density S m (typical units are µg cm -2 ) S m = path c n (s) ds Usually the path is the entire atmosphere from the surface to infinity, called the total column, giving the total (vertical) atmospheric column density, V: V = path 0 c m (s) ds c n (z) dz Dobson Units A Dobson Unit [DU] is a unit of column density used in ozone research, and in measurements of SO 2 Named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~ ) The illustration shows a column of air over Labrador, Canada. The total amount of ozone in this column can be conveniently expressed in Dobson Units (as opposed to typical column density units). If all the ozone in this column were to be compressed to STP (0ºC, 1 atm) and spread out evenly over the area, it would form a slab ~3 mm thick 1 Dobson Unit (DU) is defined to be 0.01 mm thickness of gas at STP; the ozone layer represented above is then ~300 DU (NB. 1 DU also = 1 milli atm cm) 8

9 Dobson Units So 1 DU is defined as a 0.01 mm thickness of gas at STP We know that at STP ( kpa, K) each cm 3 of an ideal gas (e.g., air, ozone, SO 2 ) contains molecules (or m -3 ) So a 0.01 mm thickness of an ideal gas contains: molecules cm cm = molecules cm -2 =1 DU Using this fact, we can convert column density in Dobson Units to mass of gas, using the cross-sectional area of the measured column at the surface For satellite measurements, the latter is represented by the footprint of the satellite sensor on the Earth s surface 9