METO 621 CHEM Lesson 4

Total Ozone Field March 11, 1990 Nimbus 7 TOMS (Hudson et al., 2003)

Latitudinal Average for Total Ozone March 11, 1990 (Hudson et al., 2003)

Total ozone with seasonal component removed LINEAR FITS Overall (black) 3.2% decade Polar (Blue) 2.5% per decade Mid-latitude (Green) 2.2 per decade Tropical (Red) 1.9% per decade Linear fit from Jan 1979 to May 1991

Total Mass of Ozone The difference between the overall and regime trends can be explained by looking at the equation for the total mass of ozone: M = AΩ 0 = A P Ω P + A M Ω M + A T Ω T +A A Ω A – A =total area between 25 and 60°N, and Ω 0 = overall mean column ozone – A P, A M, A T = regime areas Ω P, Ω M, Ω T = regime mean column ozone One can get a trend if the regime Ω varies with time, or the regime A varies with time, or both

Dynamics versus Chemistry

Effect of scattering and reflection on the photolysis rates As mentioned before most dissociation processes are limited to the ultraviolet. At these wavelengths the Rayleigh cross section is high. Hence scattering can become an important issue in calculating dissociation rates. In addition we must also consider the effect of the radiation that is reflected by the Earth’s surface and clouds. The next figure shows the effect of a change in the earth’s albedo on the dissociation rate at the ground. What is plotted is the enhancement factor i.e. the ratio of the dissociation at a given altitude to that at the top of the atmosphere (ф/ф ∞ ). There is no enhancement at wavelengths below 330 nm, as the solar flux at these wavelengths does not reach the ground.

Enhancement factors vs albedo

Enhancement factors in the stratosphere The following three figures show the enhancement factors for particular wavelengths for three cases (1) no scattering or albedo, (2) with scattering added, and (3) with both scattering and albedo added. The figures are taken from Meier et. al, (1982) The albedo chosen for the calculations is 0.5. There is a broad range observed, from 10% for the ground up to 100% for optically thick non absorbing clouds.

Enhancement factor as a function of altitude for absorption only

Absorption and multiple scattering

With absorption, multiple scattering, and albedo

The Troposphere In the Stratosphere we had high energy photons so that oxygen atoms and ozone dominated the chemistry. In the troposphere we have lower energy photons, and the chemistry is dominated by the OH and NO 3 radicals. OH is generated photochemically (i.e. only during the day), NO 3 is rapidly photolyzed during the day, so it can only survive at night. NO 3 is generally less reactive then OH, its peak concentration is higher. OH provides an efficient scavenging mechanism for both natural and anthropogenic trace constituents

Dry and Wet Deposition Dry deposition – removal of gases and particles by a direct transfer from the atmosphere to the surface. Wet deposition – removal of gases and particles carried to the surface in water – rain, snow, fog etc. Dry deposition is known for SO 2, O 3, CO 2, and SO 3. Wet deposition of gaseous species requires that they be water soluble. Terms used are rainout, or washout. Acid rain is an example of the rainout of sulfurous and nitric acids, produced in polluted atmospheres.

Dry and Wet Deposition

Oxidation and Transformation Let us assume that no methane has been oxidized. Then OH is produced by the following reactions O 3 + h  → O*( 1 D) + O 2 ( 1  g ) O*( 1 D) + H 2 O → OH + OH It should be noted that the O*( 1 D) does not stay around for long, and is quenched to the ground state. The ground state then quickly combines with molecular oxygen to reform ozone. The OH formed reacts mainly with CO and CH4 OH + CO → H + CO 2 OH + CH 4 → CH 3 + H 2 O

Oxidation and Transformation These compounds then react with molecular oxygen H + O 2 + M → HO 2 + M CH 3 + O 2 + M → CH 3 O 2 + M If the concentration of NO is very low then further reactions convert the peroxy radicals to water vapor and carbon dioxide. However if the nitrogen oxides are present then we get HO 2 + NO → OH + NO 2 CH 3 O 2 + NO → CH 3 O + NO 2 This then followed by NO 2 + h  → NO + O O + O 2 + M  → O 3 + M

Oxidation and Transformation

Analogous reactions can be written for the higher hydrocarbons, e.g. C 8 H 18 – octane. If we assign the formula RH to these hydrocarbons then we get RH + OH → R + H 2 O R + O 2 + M → RO 2 + M RO 2 + NO → RO + NO 2 This is the basis of photochemical smog. The photolysis of the resultant NO 2 is the only known way of producing ozone in the troposphere. The RO is further reduced to aldehydes and other organic compounds by OH, all of which can eventually produce ozone.

Oxidation and Transformation

The nitrate radical The nitrate radical NO3 plays a significant role in the troposphere. It is formed by the reaction NO 2 + O 3 → NO 3 + O 2 During the day it is rapidly photolyzed NO 3 + h  → NO 2 + O or NO + O 2 However at night the NO 3 is stable and can react with hydrocarbons NO 3 + RH → HNO 3 + R R can now react with molecular oxygen and begin the oxidation process

The nitrate radical

Chemical lifetimes wrt OH and O 3

Schematic of biogenic emissions