AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON 2014

AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON 2014
LECTURE 15 AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON 2014

STRATOSPHERIC POLLUTION
Without ozone in the atmosphere there could be no life as we know it on the surface of the Earth. All of the atmospheric ozone, that is the “ozone column” is only about 0.3 atm cm. In other words, if all the air were squeezed out of the atmosphere, and the remaining ozone were brought to STP, it would be only 0.3 cm thick. Murphy’s Law is strictly obeyed by NOx pollution in the atmosphere. Chemistry of the stratosphere different from troposphere. Table 15.1 Solar intensity at the Earth’s surface assuming 0.30 atm cm (300 D.U.) ozone. Note that the maximum flux is about 7x10¹⁵ (photons/(cm²s)/10 nm).

Layers in the atmosphere
Copyright © R. R. Dickerson & Z.Q. Li

Where O₃ stops absorbing, sunlight begins to reach the surface of the Earth. Hartley (1880) measured the ozone spectrum. Fabry and Buisson (1913) measured the solar spectrum at the Earth’s surface and concluded that the UV radiation reaching the surface of the Earth must be controlled by ozone in the upper atmosphere, they even made an accurate estimate of the amount of ozone! Today we will examine the various catalytic cycles that control the level of ozone in the stratosphere. We will calculate the O₃ abundance for a highly simplified atmosphere containing only O₂ and N₂. λ (nm) σ (atm⁻¹cm⁻¹) I/Io 250 305 1.0x10⁻⁴⁰ 275 162 1.0x10⁻²¹ 300 9.5 6.0x10⁻² 325 0.27 9.2x10⁻¹

Folic acid (vitamin B-9)

If you have a weak stomach Go get a cup of coffee for the next 3 min.

Too little UV radiation means rickets;
UV converts cholesterol to Vitamin D. UVC to 290 nm UVB to 320 nm UVA to 400 nm Copyright © R. R. Dickerson & Z.Q. Li 10

VII. A) OZONE CATALYTIC CYCLES
Chapman Reactions (1931) O₂ + h → 2O (1) O + O₂ + M → O₃ + M† (2) O₃ + h → O₂ + O (3) O + O₃ → 2O₂ (4) By way of qualitative analysis, Reactions (1) plus (2) produce ozone. 2 x ( O + O₂ + M → O₃ + M ) (2) 3 O₂ + h → 2 O₃ NET

While Reactions (3) plus (4) destroy ozone
While Reactions (3) plus (4) destroy ozone. O₃ + h → O₂ + O (3) O + O₃ → 2O₂ (4) 2O₃ + h → 3 O₂ NET Reactions (3) plus (2) add up to a null cycle, but they are responsible for converting solar UV radiation into transnational kinetic energy and thus heat. This cycle causes the temperature in the stratosphere to increase with altitude. Thus is the stratosphere stratified. O + O₂ + M → O₃ + M* (2) NULL NET By way of quantitative analysis, we want [O₃]ss and [O]ss and [Ox]ss where “Ox” is defined as odd oxygen or O + O₃. The rate equations are as follows.

(a) (b) (a+b) From the representation for O atom chemistry: In the middle of the stratosphere, however, R₃ >>2 R₁ and R₂ >> R₄ thus: (I) This does not mean that R₄ is unimportant, but it can be ignored in an approximation of [O]ss at the altitude of the ozone layer. The ratio of [O] to [O₃] can also be useful:

(II) Reactions 2 and 3 set the ratio of O to O₃, while Reactions 1 and 4 set the absolute concentrations. Now we will derive the steady state ozone concentration fro the stratosphere. From the assumption that Ox is in ready state we know: R₁ = R₄ Thus j(O₂)[O₂] = k₄[O][O₃] Substituting from (I), the steady state O atom concentration: or

SAMPLE CALCULATION At 30 km

This is almost a factor of ten above the true concentration
This is almost a factor of ten above the true concentration! What is wrong? There must be ozone sinks missing. 2) Bates and Nicolet (1950) “HOx” Odd hydrogen “HOx” is the sum of OH and HO₂ (sometimes H and H₂O₂ are included as well). HO₂ + O₃ → OH + 2O₂ (5) OH + O₃ → HO₂ + O₂ (6) 2O₃ → 3O₂ NET The following catalytic also destroys ozone. HO₂ + O → OH + O₂ (7) O + O₃ → 2O₂ NET

The second catalytic cycle speeds up Reaction 4, that is it effectively increases k₄. Note that any loss of odd oxygen is the same as loss of ozone. These catalytic losses are still insufficient to explain the observed ozone concentration. 3) Crutzen (1970); Johnston (1971) “NOx” Odd nitrogen or “NOx” is the sum of NO and NO₂. Often “NOx” is used as “odd nitrogen” which includes NO₃, HNO₃, 2N₂O₅, HONO, PAN and other species. This total of “odd nitrogen” is better called “NOy” or “total reactive nitrogen.” N₂ and N₂O are unreactive. NO + O₃ → NO₂ + O₂ O + NO₂ → NO + O₂ O + O₃ → 2O₂ NET This is the major means of destruction of stratospheric ozone. The NOx cycle accounts for about 70% of the ozone loss at 30 km. We will calculate the implied steady ozone concentration later.

4) Stolarski & Cicerone (1974) “ClOx” Cl + O₃ → ClO + O₂ ClO + O → Cl + O₂ O + O₃ → 2O₂ NET This reaction scheme is very fast, but there is not much ClOx in the stratosphere … yet. Today ClOx accounts for about 8% of the ozone loss at 30 km. If all these catalytic destruction cycles are added together, they are still insufficient to explain the present stratosphere O₃ level. The general for of a catalytic ozone destruction cycle is: X + O₃ → XO + O₂ XO + O → X + O₂

McElroy, Salawitch, et al. (1986) Cl + O₃ → ClO + O₂
Molina and Molina (1987) 2(Cl + O₃ → O₂ + ClO) ClO + ClO + M → (ClO)₂ + M (ClO)₂ + hv → Cl + ClOO ClOO + M → Cl + O₂ + M 2O₃ → 3O₂ NET McElroy, Salawitch, et al. (1986) Cl + O₃ → ClO + O₂ Br + O₃ → BrO + O₂ ClO + BrO → Cl + Br + O₂ 2O₃ → 3O₂ NET Copyright © R. R. Dickerson & Z.Q. Li 20

Table 15.2 Stratospheric ozone destruction cycles
Sources Sinks Reservoirs HOx H₂O,CH₄,H₂ HNO₃ · nH₂O H₂SO₄ · nH₂O H₂O,H₂O₂ NOx N₂O + O(¹D) HNO₃ HO₂NO₂,ClONO₂ ClOx CH₃Cl,CFC HCl HCl, HOCl The sinks involve downward transport to the troposphere and rainout or other local loss. Note that some sinks are also reservoirs: HCl + OH → H₂O + Cl

Antarctic Ozone Hole In the Antarctic winter there is no sunlight and even in the spring there is too little UV to generate enough O atoms to destroy ozone. The annual loss of ozone over Antarctica is driven by heterogeneous chemistry and visible radiation. A good current review Is provided by Solomon Nature, 1990, and “Scientific Assessment of Ozone Depeation :1991” (WMO). The destruction of ozone is usually moderated by the production of chlorine nitrate, an important reservoir species. NO₂ + ClO + M → ClONO₂ + M In the Antarctic winter, heterogeneous reactions “denitrify” the stratosphere (Molina et al., Science, 1987). Molecular chlorine is only weakly bound, and can be dissociated by visible radiation.

Cl + O₃ → O₂ + ClO ClO + ClO + M → (ClO)₂ + M (ClO)₂ + hv → Cl + ClOO ClOO + M → Cl + O₂ + M 2O₃ → 3O₂ NET Two types of Polar Stratospheric Clouds (PSC’s) exist. Type I = HNO₃ · 3H₂O Nitric acid trihydrate, formed at T ≤ 195K Type II = H₂O Ice formed at T ≤ 190K They move NOy species from the vapor phase to the condensed phase as HNO₃. The are involved in catalytic cycles with chlorine and bromine compounds that speed the reaction of ozone with itself to form oxygen. They move chlorine from the reservoir species HCl and ClONO₂ to ClOx.

Polar Stratospheric Clouds (PSCs)

World Production of CFCs

From Farman et al., Nature 1985.

Antarctic Ozone Loss: Hole cannot get wider or deeper.
TOMS OMI Ground Based After Farman et al., Nature, 315, 207, 1985 • Models now provide good overall simulation of Antarctic ozone loss. • Scientific understanding of polar ozone depletion led to international ban of CFC production

ALTITUDE (km) 5 10 15 20 25 30 35 D. Hofmann, NOAA CMDL OZONE ABUNDANCE (PARTIAL PRESSURE, mPa) OCTOBER AVERAGE 282 DU SEP 29, 1999  90 DU OZONE PROFILES, SOUTH POLE: UPDATE Ozone Hole Update, II Copyright © R. R. Dickerson & Z.Q. Li

Accommodation Coefficients
Condensed phase has lower entropy than gas phase. Accommodation coefficients (reaction probabilities) should be greater at lower temperatures. Copyright © R. R. Dickerson & Z.Q. Li

Heterogeneous Chemistry: Faster at low temperatures
In all cases,  must be measured in the laboratory (thanks, RJS 2010) ATLAS 3 : Space Shuttle Atlantis Mission Specialist: Ellen Ochoa Reaction probabilities given for various surface types, with formulations of various degrees of complexity, in Section 5 of the JPL Data Evaluation. Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatment of aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc. Copyright © R. R. Dickerson & Z.Q. Li

Summary of Ozone Hole Formation
Threat to DNA-based life forms. Not predicted by any models First observed by Farman et al., (Nature 1985). Ozone destruction nearly complete. Halogen (Cl & Br) reactions are responsible. Polar stratospheric Clouds play a central role. Multiphase (heterogeneous) reactions denitrify stratosphere. Reaction rates depend on accommodation coefficients, f(T). Replacement of CFC’s should heal ozone hole. Copyright © R. R. Dickerson & Z.Q. Li

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