1. Air Pollution-Tropospheric Ozone

2. Good Ozone and Bad Ozone Stratospheric ozone protect lives on Earth from harmful effects of UV radiation. Tropospheric ozone: –Causing respiratory distress and eye irritation –Destroying plants –Producing cracks in rubber Ozone is a strong oxidant, reacts with molecules containing C=C double bonds, forming epoxides.

3. Two types of air pollutants: primary vs. secondary Primary pollutants: released directly from sources –Examples: CO, SO2, NOx Secondary pollutants: formed through chemical reactions of the primary pollutants and the constituents of the unpolluted atmosphere in the air. –Example: O3

4. Formation of ozone NO 2 + hv  NO + O(1) O + O 2 + M  O 3 + M(2) NO + O 3  NO 2 + O 2 (3) NO 2 + hv  NO + O(1) O + O 2 + M  O 3 + M(2) HO 2. + NO  NO 2 + OH(4) RO 2. + NO  NO 2 + RO. (5) No net O3 formation O3 is formed Net of (1)+(2)+(4): RO2. + O2  O3 + RO. NO2 is capable of absorbing visible light (<400 nm) to produce O.

5. Sources of RO2. : Oxidation of hydrocarbons RH + OH  R. + H2O R. + O2  RO2. A single organic radical can produce many peroxy radicals by successive rounds of O2 combination and fragmentation.

6. Example: Oxidation of carbon monoxide CO +. OH + O 2  CO 2 + HO 2. HO 2. + NO  NO 2 +. OH NO 2 + hv  NO + O O + O 2 + M  O 3 + M Net: CO + 2 O 2 + hv  CO 2 + O 3 The net reaction can be viewed as a catalytic oxidation of CO to CO 2. Net formation of O 3 occurs.

7. Example: Oxidation of methane CH4 +. OH + O2  CH3OO. + H2O CH3OO. + NO  CH3O. + NO2 CH3O. + O2  HCHO + HO2. HO2. + NO . OH + NO2 NO2 + h  NO + O (2x) O + O2 + M  O3 + M (2x) Net: CH4 + 4 O2  HCHO + H2O + 2 O3 The net reaction is that for each mole of methane oxidized, 2 moles of O3 is produced.

8. Necessary ingredients for ozone formation Sunlight NOx (NO, NO2) Hydrocarbons (VOCs: volatile organic carbon) VOCs + NOx + h  O3 + other pollutants Production of O atom Production of RO2, which reacts with NO so that O3 could accumulate.

9. Necessary ingredients for ozone formation CH4 +. OH + O2  CH3OO. + H2O CH3OO. + NO  CH3O. + NO2 CH3O. + O2  HCHO + HO2. HO2. + NO . OH + NO2 NO2 + h  NO + O (2x) O + O2 + M  O3 + M (2x) Net: CH4 + 4 O2  HCHO + H2O + 2 O3 VOC Sunlight

10. Formation of oxidants other than O3 Formation of aldehydes (e.g. formaldehyde) Formation of PAN (peroxyacetyl nitrate) and its analogs ROO. + NO2  ROONO2 (peroxyalkyl nitrate) PAN

11. Photochemical smog Smog derives from a combination of the words smoke and fog.

12. London smog and Los Angeles smog London smog is characterized by high SO2 and particle concentration in the presence of fog. –Also referred as sulfurous smog Los Angeles smog is characterized by high oxidants (mainly O3). It was first recognized in the Los Angeles area. –The term smog is misleading in this case, as smoke and fog are not key components. –The appropriate term is photochemical air pollution.

13. Photochemical air pollution

14. HO2. Radical: Interconversion of. OH and HO2. OH and HO2 are interconverted through a series of reactions involving hydrocarbons and oxides of nitrogen. HO2. + NO . OH + NO2. OH + RCH3  H2O + RCH2. RCH2. + O2  RCH2OO. RCH2OO. + NO  NO2 + RCH2O. RCH2O. + O2  RCHO + HO2. Sources of OH are in effect sources of HO2. under most tropospheric conditions.

15. Sources for. OH radicals: Photolysis of O3 Photolysis of O 3 forms O 1 D, followed by its reaction with water. O 3 + h  O 1 D + O 2  < 320 nm O 1 D + H 2 O  2. OH

16. Sources for. OH radicals: Photolysis of HONO HONO + h . OH + NO < 400 nm Possible sources for HONO include NO2 +H2O OH + NO NO + NO2 + H2O HO2 + NO2 reaction (possibly a contribution from a minor channel of this reaction) direct emissions, for example, from automobiles.

17. Sources for. OH radicals: Photolysis of H2O2 H 2 O 2 + h  2. OH  360 nm H2O2 is formed from the reaction: HO2. + HO2.  H2O2 + O2

18. Sources for HO2. Radicals: formaldehyde Formaldehyde photolysis is a major source of HO2. during the daylight hours. HCHO + h  H. + HCO.  < 370 nm H. + O2 + M  HO2. + M HCO. + O2  HO2. + CO Note: Any process that produces HCO. or H. is a source of HO2. in the troposphere.

19. Nighttime sources for. OH/HO2. Ozone oxidation of alkene species –Ethene + O3  0.12 OH –Isoprene + O3  0.27 OH Thermal decomposition of Peroxyactyl nitrate (PAN) and its analogs of higher carbon. CH3C(O)OONO2  CH3C(O)OO. + NO2 CH3C(O)OO. + NO  CH3C(O)O. +NO2 CH3C(O)O.  CH3. + CO2 CH3. + O2  CH3OO. CH3OO. + NO  CH3O. + NO2 CH3O. + O2  HCHO + HO2.

20. Nighttime sources for. OH/HO2. (Continued) NO3 reaction with hydrocarbons NO3 + RH  HNO3 + R. R. + O2  ROO. ROO. + NO  RO. + NO2 RO. + O2  HO2. + R’CHO

21. Various sources of. OH/HO2. as a function of the time of day

22. Control strategies for ozone O3 is a secondary pollutant  control of O3 requires control of its precursors. Control of VOCs –General too abundant to be brought low enough to be the limiting factor. –In certain areas, VOCs from biological sources could be significant. Control of NOx –Difficult to control as efficient energy conversion requires high combustion temperature.