5 Until 1964 the Chapman reactions were thought to be the principal processes governing the ozone balance in the stratosphere. However, measurements indicated that the actual concentration of ozone is smaller than that predicted by about a factor of two to four. Predictions of ozone concentrations by the Chapman mechanism compared with observations at Panama, 1970.
6 Catalytic Ozone Destruction. For a chemical to significantly affect the overall concentration of ozone it either must be present in great abundance or must be involved in a catalytic cycle. Breakthroughs in the 1970s identified a number of NATURAL catalytic processes that all have the form: X + O 3 XO + O 2 XO + O X + O 2 net :O 3 +O 2O 2 Where X = H, OH, NO, Cl or Br.
7 CFCs: Molina and Rowland Nature 249, 810-812 (1974)
8 Importance of reservoirs If the catalysts carried on unabated they would themselves destroy the ozone layer, but happily there are termination reactions that lead to the formation of reservoir species e.g. Cl + CH 4 HCl + CH 3 ClO + NO 2 ClONO 2 Active (radicals)Inactive (reservoirs)
The ozone hole 27 th Anniversary Farman et al. Nature 1985
10 What might have happened if we did nothing about CFCs? Newman et al. Atm. Chem. Phys., 9, 2113 (2009)
11 What might have happened if we did nothing about CFCs? Newman et al. Atm. Chem. Phys., 9, 2113 (2009)
12 Take home message 17% of globally averaged ozone gone by 2020 67% by 2065 Collapse of lower strat. tropical ozone by 2060 UV levels double by 2060
17 Ozone Chemistry- low NO x environment O 3 + hν O ( 1 D) + O 2 λ ~ 330 nm O ( 1 D) + M O ( 3 P) + M O ( 1 D) + H 2 O 2 OH CO + OH CO 2 + H H + O 2 + M HO 2 + M HO 2 + O 3 OH + 2O 2 Net: CO + O 3 CO 2 + O 2 Δ[O 3 ] [NO x ]
18 Ozone Chemistry- higher NO x environment Δ[O 3 ] [NO x ] Examples: Outflow from pollution centres and biomass burning regions
19 Ozone Chemistry- higher NO x environment CO + OH CO 2 + H H + O 2 + M HO 2 + M HO 2 + O 3 OH + 2O 2 NO + HO 2 NO 2 + OH NO 2 + hν NO + O( 3 P) O( 3 P) + O 2 + M O 3 + M Net: CO + 2O 2 CO 2 + O 3 Δ[O 3 ] [NO x ]
20 Ozone Chemistry- higher NO x environment CO + OH CO 2 + H H + O 2 + M HO 2 + M HO 2 + O 3 OH + 2O 2 NO + HO 2 NO 2 + OH NO 2 + hν NO + O( 3 P) O( 3 P) + O 2 + M O 3 + M Net: CO + 2O 2 CO 2 + O 3 Δ[O 3 ] [NO x ]
21 Ozone Chemistry- higher NO x environment CO + OH CO 2 + H H + O 2 + M HO 2 + M HO 2 + O 3 OH + 2O 2 NO + HO 2 NO 2 + OH NO 2 + hν NO + O( 3 P) O( 3 P) + O 2 + M O 3 + M Net: CO + 2O 2 CO 2 + O 3 Δ[O 3 ] [NO x ]
NOx Limiting Environment Δ[O 3 ] [NO x ] NO 2 + OH HNO 3
What about the indoor environment? Three differences 1.No meteorology – slower movement of air (in general) ventilation rates important 2.More surfaces (surface:volume ratio) deposition and emission of chemicals much more important 3.Light levels very different (less UV indoors but may have higher total light levels than outdoors?)
1.Different ventilation rates Some ideas from experiments in London
Marylebone Rd. Gloucester Place DAPPLE #02– Summer 2004
Roof conc. approx 10 times greater than Indoor Longer transport time to Indoor position Longer decay time Results Indoor (2004)
Ventilation rates Mechanically ventilated – sealed building, air sucked in and pumped out, regulated by levels of CO 2. Outdoor air pumped in but filtered to remove particles. Naturally ventilated – leaky buildings (most dwellings and many offices), too hot, open a window, too cold, close off room. Inevitable ingress of outdoor air.
2. Emissions and deposition 1.Many chemicals released indoors are also released outdoors but depending on confinement and ventilation, exposure levels indoors may be much much higher and last for much longer. 2.Typical sources indoors include; fabrics, varnishes, paint, cooking, heating systems, air fresheners! Cleaning products (with added scents)
Some examples of chemicals indoors Paradichlorobenzene Moth crystals, room deodorants Methylene chloride Paint removers, solvent usage Formaldehyde Pressed wood products, foam Styrene Insulation, textiles, disinfectants, plastics, paints Acetaldehyde Glues, deodorants, fuels, preventives, mold growth on leathers Acrolein Component of oak-wood, by-product of the combustions of wood, kerosene and cotton
Some examples of chemicals indoors Toluene diisocyanate Polyurethane foam, aerosols Benzene Smoking Tetrachloroethylene Wearing or storing dry-cleaned clothes Chloroform Chlorinated water (showering, washing clothes, dishes) 1,1,1-trichloroethane Wearing or storing dry-cleaned clothes, aerosols sprays, fabric protectors Carbon tetrachloride Industrial strength cleaners
3. Light levels 1.Glass will cut off high energy UV light from the Sun and will reduce visible light levels. General shading by buildings will also reduce light levels in general. 2.All this means that Chemistry is much slower indoors than outdoors but the build up can be greater because of the slow air movement.