Air Pollution and Atmospheric Chemistry

1. Air Pollution and Atmospheric Chemistry Sasha Madronich Atmospheric Chemistry Division National Center for Atmospheric Research Boulder, Colorado USA 27 July 2004, Boulder

2. Components of Air Quality Models • Spatial and Temporal Grids • Horizontal domain (local; regional; global) • Vertical extent (PBL; troposphere; trop+strat+mesosphere) • Time span (day or week episode; interannual; climatologic) • Chemical Inputs • Natural emissions • Anthropogenic emissions • Inflow from model boundaries • Initial conditions • Chemical Transformations • Gas phase • Condensed phase (aerosols, clouds) • Transport • Horizontal advection • Vertical diffusion and convection • Update environment (T, P, H2O, hn) • Deposition • Wet (rain, snow) • Dry (gas & aerosol on surfaces) • Solution forward in time • Coupled non-linear stiff differential equations

3. Earth’s Atmosphere • Composition • 78% nitrogen • 21% oxygen • 1-2% water (gas, liquid, ice) • trace amounts (<< 1%) of many other species, some natural and some “pollutants” • Reactivity dominated by • oxygen chemistry • solar photons • To understand fate of pollutants, must first understand oxygen photochemistry

4. Energetics of Oxygen in the Atmosphere DHf (298K) kcal mol-1 Excited atoms O*(1D) 104.9 Ground state atoms O (3P) 59.6 Ozone O3 34.1 “Normal” molecules O2 0 Increasing stability

5. Atmospheric OxygenThermodynamic vs. Actual

6. Photochemistry • Thermodynamics alone cannot explain atmospheric amounts of O3, O, O* • Need • energy input, e.g. O2 + hn O + O (l < 250 nm) • chemical reactions, e.g. O + O2 (+ M)  O3 (+ M) = Photochemistry

7. WMO, 2002

8. Chapman, 1930’s: Pure oxygen photochemistry O3 production: O2 + hn (l < 240 nm)  2 O O + O2 + M  O3 + M O3 destruction: O3 + hn (l < 800 nm)  O + O2 O + O3 2 O2 Correctly predicts vertical profile shape, but too much O3. Stratospheric Odd Oxygen (Ox = O + O3)

9. Bates and Nicolet, 1950’s: Hydrogen-containing “contaminants” Formation of excited oxygen atoms: O3 + hn (l<330 nm)  O2 + O* Formation of HOx radicals from H2O and CH4: H2O + O*  OH + OH CH4 + O*  OH + CH3 Catalytic destruction of O3 by HOx: O3 + OH  O2 + HO2 O + HO2 O2 + OH O3 + HO2 2 O2 + OH Better, but still too much O3 Stratospheric Odd Hydrogen (HOx = OH + HO2)

10. Crutzen, 1970: Nitrogen containing “contaminants” Formation of excited oxygen atoms: O3 + hn (l<330 nm)  O2 + O* Formation of NOx radicals from N2O: N2O + O*  NO + NO Catalytic destruction of O3 by NOx: O3 + NO  O2 + NO2 O + NO2 O2 + NO works for natural stratosphere Stratospheric Odd Nitrogen (NOx = NO + NO2)

11. Rowland and Molina, 1974: Chlorofluorocarbons (CFCs) can make it to stratosphere because they are not destroyed in troposphere: Formation of chlorine atoms from photolysis of chlorofluorocarbons: CH3Cl + hn  CH3 + Cl CF2Cl2 + hn  CF2Cl + Cl Catalytic destruction of O3 by Clx: O3 + Cl  O2 + ClO O + ClO  O2 + Cl Stratospheric Halogens (Cl, Br, I, …)

12. Stratospheric Reservoirs • Formation of less-reactive reservoirs: Cl + CH4 HCl + CH3 ClO + NO2 + M  ClONO2 + M OH + NO2 + M  HNO3 + M • Reservoirs can either be removed by diffusion to troposphere, or can be transformed back to reactive species. • Strong reactivation of halogens occurs on surfaces of polar stratospheric clouds.

14. SOLAR SPECTRUM UNEP, 2002

15. Detrimental Effects of UV Radiation • Human and animal health • Skin cancer, skin ageing, sunburns • Ocular damage • Immune system suppression • Reduced Growth in Plants • Terrestrial (agriculture, forests) • Marine (less phytoplankton) • Air Quality • More UV means more urban ozone, secondary aerosols • Materials • Degradation of plastics (PVC, PC)

16. Global UV Changes (1990’s/1980’s) All conditions (ozone and cloud changes) Clear sky (ozone change only)

17. Atmospheric Halogens are Decreasing or Stabilizing WMO, 2002

18. The Future Avoided WMO, 2002

19. WMO, 2002

20. Tropospheric Ozone Formation – how? • Urban ozone (O3) is generated when air containing hydrocarbons and nitrogen oxides (NOx = NO + NO2) is exposed to UV radiation (Haagen-Smit, 1950’s). • Laboratory studies show that O3 is made almost exclusively by the reaction: O2 + O + M  O3 + M • But troposphere lacks short-wavelength photons (l<250 nm) needed to break O2 directly. So: what is the source of tropospheric O atoms??

21. Tropospheric O3 - From NO2? • NO2 photolysis is a source of O atoms: NO2 + h ( < 420 nm)  NO + O O + O2 + M  O3 + M • Two problems: Reversal by NO + O3 NO2 + O2 Usually O3 >> NO2 • Makes some O3, but not enough!

22. Tropospheric O3 Formation – Need hn, HCs, NOx • Initiation by UV radiation (Levy, 1970): O3 + h ( < 330 nm)  O*(1D) + O2 O*(1D) + H2O  OH + OH • Hydrocarbon consumption (oxygen entry point): OH + RH  R + H2O R + O2 + M  ROO + M • Single-bonded oxygen transferred to NOx: ROO + NO  RO + NO2 • NOx gives up oxygen atoms (as before): NO2 + h ( < 420 nm)  NO + O O + O2 + M  O3 + M

23. Tropospheric O3 Formation – Secondary Reactions • Propagation RO + O2 R’CO + HO2 HO2 + NO  OH + NO2 more O3, OH • Termination OH + NO2 + M  HNO3 + M HO2 + HO2 + M  H2O2 + M HO2 + O3  OH + 2 O2 slows the chemistry

24. Tropospheric Chemical Mechanisms • This talk: 15 reactions • Typical 3D model used for air quality: 100 - 200 reactions • Typical 0D (box) models used for sensitivity studies: 5,000 - 10,000 reactions • Fully explicit (computer-generated) mechanisms: 106 - 107 reactions

25. Hydrocarbon Chemistry is Complex! Aumont and Madronich, 2003

27. Consequences of tropospheric O3 chemistry - 1 • Surface O3 pollution Urban: 100-500 ppb Regional: 50-100 ppb Global background increase 10-20 ppb  35-45 ppb in NH 10-20 ppb  25-35 ppb in SH • Damage to health and vegetation • Greenhouse role of O3 • Changes in global oxidation capacity

28. California EPA, 2004

29. Consequences of tropospheric O3 chemistry - 2 • Formation of peroxides and acids: HO2 + HO2 H2O2 + O2 OH + NO2 + M  HNO3 + M OH + SO2  …  H2SO4 H2O2(aq) + SO2(aq)  …  H2SO4(aq) • Damage to vegetation and structures (acid precipitation) • Sulfate aerosol formation (visibility, climate)

30. Consequences of tropospheric O3 chemistry - 3 • Products of hydrocarbon oxidation CO2 (minor compared to direct emissions) CO (~ 1/2 of total global emissions) Oxygenated organics: aldehydes, ketones, alcohols, organic acids, nitrates, peroxides • Damage to health, vegetation • Secondary organic aerosol formation (health, visibility, climate) • Changes in global oxidation capacity

31. Global Oxidation (self-cleaning) Capacity Solar UV radiation Oxidation, e.g.: CH4 + OH  … CO2 + H2O Insoluble  Soluble Carboxylic acids H2SO4, SO4= Halocarbons Emissions Deposition (dry, wet) HNO3, NO3- CH4 CmHn NO NO2 HCl, Cl- CO SO2

32. Consequences of tropospheric O3 chemistry - 4 • OH increase because of increasing emissions of NOx? • OH increase because of increasing UV radiation? OR • OH decrease because of increasing emissions of CO, CmHn, SO2, and other reduced compounds? • Decreased OH (oxidizing capacity) implies generally higher amounts of most pollutants including: • Higher amounts of greenhouse gases • Higher amounts of substances that deplete the ozone layer • More global spread

33. How Climate Change Can Affect Pollution - 1 • Changes in Anthropogenic and Biogenic Emissions: • Anthropogenic emissions of ozone precursor compounds (CO, NOx, SOx, NMHC) and aerosols are expected to increase over the next 50 years. • Biogenic emissions of NMHCs and CO are expected to be affected significantly by future changes in temperature, relative humidity and photosynthetically available radiation (PAR).

34. How Climate Change Can Affect Pollution - 2 • Changes in Transport: • Modification of inter-continental transport of pollutants. • Modification of moist convective activity, with associated changes in wet removal processes and vertical redistribution of pollutants. • Modification of the boundary-layer height and ventilation rates. • Modification of stratosphere-troposphere exchange, with consequently different inputs of ozone to the troposphere.

35. How Climate Change Can Affect Pollution - 3 • Changes in Chemically Relevant Environmental Variables: • Increased temperatures lead to faster kinetics of O3 production. • Changes in H2O, affecting both the gas phase chemistry, e.g. OH production via O(1D) + H2O, and the growth of aerosols near the deliquescence point. • Changes in cloud distributions, with associated changes in aqueous chemical processes (e.g. sulfate formation), NOx production by lightning, wet removal, and photochemistry. • Increased aerosol loading, with associated enhancements of heterogeneous chemistry, and – depending on aerosol type – either increased or decreased photochemistry. • Changes in stratospheric ozone, with associated changes in photochemistry.

36. INTERACTIONS: Climate change & Stratospheric ozone WMO, 2002

37. INTERACTIONS: Climate, Clouds, and UVR: 2130 – Present, SH Summer Madronich, Tie, Rasch, unpubl.

38. INTERACTIONS: Climate & Air Pollutants IPCC, 2001

39. IPCC, 2001

40. INTERACTIONS: Heat, Air Pollution & Health

41. INTERACTIONS: Carbon cycle & Tropospheric O3 Loya et al., Nature, 425, 705, 2003

42. (a very incomplete picture) Good? Bad? Unclear? Stratospheric Ozone Depletion + halocarbons + H2O - T ± H2O + OH + IR cooling + CFC replacement + UV +CFC replacement + CH4, + O3, + soot,+ sulfate, ± clouds Air Quality Climate Change + T, + H2O, ±emissions, ± rain, ± winds, ± clouds