1 UV Remote Sensing of O3 and SO2 The Ozone Layer • The stratospheric ozone layer is a consequence of molecular photodissociation • UV-C radiation dissociates molecular oxygen: O2 + hv (λ < 0.2423 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 O2 molecules to produce ozone: O + O2  O3 • Ozone is then dissociated by UV radiation: O3 + hv (λ < 0.32 µm)  O + O2 • Ozone is also destroyed by this reaction: O3 + O  O2 + O2 The Chapman Reactions2 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 O2 • UV-A radiation (λ > 0.32 µm) is transmitted to the lower atmosphere • Plus a small fraction of UV-B (0.31-0.32 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 temperatures3 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 Breathe4 Satellite trace gas retrievals in the UV 550 440 330 220 110 October 1, 1994 October 1, 1999 October 1, 2003 October 1, 2006 Antarctic Ozone from TOMS and OMI Dobson Units http://macuv.gsfc.nasa.gov/5 Pinatubo SO2 cloud Pintatubo (Philippines) erupted in June 1991 and produced the largest SO2 cloud measured to date (i.e. since 1978) Maximum SO2 column: ~800 DU Total SO2 Mass: ~20 Mt Satellite viewing geometry Solar zenith angle Sensor zenith angle Solar azimuth angle Sensor azimuth angle N d Absorbing gas6 Beer’s Law in this case € Iλ= I0,λexp(−σλN d m)m = sec θs + sec θsat = airmass factor (AMF) UV SO2 and O3 absorption spectra Flyspec, UV camera OMI has ~720 UV channels, compared to 6 on TOMS7 Ideal Gas Law • P = pressure (Pa), V = volume taken up by gas (m3), n = number of moles, R = gas constant (8.314 J mol-1 K-1), T = temperature (K) • k = Boltzmann constant (1.38×10-23 J K-1), N = number of molecules, NA = Avogadro constant (6.022×1023 molecules mol-1) € PV = nRTPV = NkT€ k =RNA• The equation of state of an ideal gas – most gases are assumed to be ideal • 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 = 101.325 kPa (1 atm) and T = 273.15 K (0ºC) • Sometimes P = 101.325 kPa and T = 293.15 K (20ºC) • At STP (101.325 kPa, 273.15 K) each cm3 of an ideal gas (e.g., air) contains 2.69×1019 molecules (or 2.69×1025 m-3) • This number is the Loschmidt constant and can be derived by rearranging the ideal gas law equation: • At higher altitudes, pressure is lower and the number density of molecules is lower • Mean molar mass of air = 0.02897 kg mol-1 (air is mostly N2) € N =PVkT8 Column density • Another way of expressing the abundance of a gas is as column density (Sn), which is the integral of the number density along a path in the atmosphere • The unit of column density is molecules cm-2 • The integral of the mass concentration is the mass column density Sm (typical units are µg cm-2) • Usually the path is the entire atmosphere from the surface to infinity, called the total column, giving the total (vertical) atmospheric column density, V: € Sn= cn(s) dspath∫€ Sm= cm(s) dspath∫€ V = cn(z) dz0∞∫Dobson Units • A Dobson Unit [DU] is a unit of column density used in ozone research, and in measurements of SO2 • Named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone (~1920 – 1960) • 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)9 Dobson Units • So 1 DU is defined as a 0.01 mm thickness of gas at STP • We know that at STP (101.325 kPa, 273.15 K) each cm3 of an ideal gas (e.g., air, ozone, SO2) contains 2.69×1019 molecules (or 2.69×1025 m-3) • So a 0.01 mm thickness of an ideal gas contains: 2.69×1019 molecules cm-3 × 0.001 cm = 2.69×1016 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