Tropospheric Chemistry: Smog Formation and Causes

1. Chem. 253 – 2/11 Lecture Made change to slide #25 (last slide)

2. Announcements I • Return HW 1.1 (discuss what was graded) + Group assignment • New HW assignment (1.3 – posted on website) • This Week’s Group Assignment • On ozone hole • Should be shorter than last week • Today’s Lecture Topics – Tropospheric Chemistry • What is smog? • The OH radical and oxidation pathways • Tropospheric ozone formation

3. Tropospheric ChemistryWhat is Smog and What Causes It? • Air Pollution Events – a reason to study tropospheric chemistry • Poor Air Quality • Poor visibility • Increased respiratory health problems + increased deaths in particularly bad events • Historical Poor Air Quality Episodes • London (1200s, 1600s, 1800s, 1950s) • Industrial towns (Meuse, Belgium; Donora, PA, 1930s-40s) • Los Angeles (1960s) • More recently (San Joaquin Valley, Mexico City)

4. Tropospheric ChemistryWhat is Smog and What Causes It? • Commonalities of Past Air Pollution Episodes • Stagnant or trapped air • Combustion sources (coal or hydrocarbons) • Relatively high combustion source density • Limited Ventilation - sources • Reduced vertical mixing – causes: • Inversions from a) radiational cooling (particularly in fall and winter), b) cool surfaces (e.g. marine air in LA, fog/snow in mountain valleys) • Large scale high pressure systems typically have downward air movement Z (km) T (°C)

5. Tropospheric ChemistryWhat is Smog and What Causes It? • Limited Ventilation - sources • Reduced horizontal mixing – causes: • High pressure systems – typically have weak surface winds • Geographical constraints from mountains/valleys example: San Joaquin Valley only one opening L H Low P systems – have high winds near center High P systems – have low winds over large centers

6. Tropospheric ChemistryWhat is Smog and What Causes It? • Combustion Sources of Past Air Pollution Episodes • Earlier events typically occurred in winter in coal burning regions or with industrial emissions (smelter towns) • Primary pollution sources were big problems (e.g. coal soot and sulfur gases) • With strong inversions, increasing smoke stack height helped trapped cold air

7. Tropospheric ChemistryWhat is Smog and What Causes It? • Primary Air Pollutants • An air pollutant as emitted from source (or conversion within seconds to minutes of emission) • Examples: fly ash, soot, sulfur dioxide, polyaromatic hydrocarbons • Problems are generally close to sources (within km of emission sources) • Solutions to problems • reduce source (e.g. fireplace bans for woodsmoke) • dilution (allow wood smoke under good ventilation conditions)

8. Tropospheric ChemistryWhat is Smog and What Causes It? • Secondary Air Pollutants • An air pollutant forms through atmospheric reactions • Examples: NO2, tropospheric ozone, peracetyl nitrate, sulfate aerosol • Problems are much less restricted (Sacramento area ozone is highest in foothill communities) • Solutions require detailed understanding of issues • Ozone example: 3 ingredients needed: NOx, VOCs, and sunlight

9. Tropospheric ChemistryWhat is Smog and What Causes It? • Visibility Impairment • poor visibility is often a result of secondary pollution episodes • measured by extinction (absorbance plus scattering) • perception of visibility problems is not uniform (much easier to see pollution when above polluted air than when in it) • most visibility reduction is due to aerosol particles with some from NO2 (absorbs lower visible wavelengths) • low visibility is not necessarily the cause of health effects, but high aerosol concentrations are associated with health problems

10. Tropospheric ChemistryOxidation in the Atmosphere • Oxygen is a Thermodynamically Unstable Gas • hydrocarbons and metals typically would be more stable in their oxidized forms • However, it is stable kinetically • for even a “fast” O2 reaction (2NO2 + O2→ 2NO2), under polluted conditions, reaction is insignificant • Faster oxidation requires other oxidants (OH - daytime, O3, and NO3 – nighttime are most prevalent) • OH is the most widespread initiator of oxidation

11. Tropospheric ChemistryOxidation in the Atmosphere • OH formation Reaction: O3 + hn→ O2 + O* O* + H2O → 2OH • Free Radical Cycles (also applies to most stratospheric reactions) • initiation steps (formation of one or two free radicals – those shown above) • radical propagation steps (reactions passes radical on) • radical termination steps (similar to O + O2, which ends odd O in O only Chapman mechanism) • normally initiation and termination are slow steps

12. Tropospheric ChemistryOxidation in the Atmosphere • OH Reactions – radical Propagation – Example 1: OH + CO • CO is a pollutant from incomplete combustion • Toxic at relatively high concentrations (replaces O2 in hemoglobin) reactions: 1) CO + OH → HOCO (unstable free radical) 2) HOCO + O2→ CO2 + HOO (HO2) net: CO + OH + O2→ CO2 + HO2 (transforms 1 OH to 1 HO2)

13. Tropospheric ChemistryOxidation in the Atmosphere • OH Reactions – radical Propagation – Example 2: OH + CH4 • CH4 is one of the most prevalent (and least reactive) hydrocarbons • Its oxidation is not a major factor for localized air pollution reactions (main path): 1) CH4 + OH → CH3• + H2O (abstraction reaction) 2) CH3• + O2→ CH3O2• 3) CH3O2• + NO → CH3O• + NO2 4) CH3O• + O2→ HCHO + HO2 Note: HCHO will react further (fast relative to CH4; slow vs. other intermediates)

14. Tropospheric ChemistryOxidation in the Atmosphere • OH Reactions – radical Propagation – Example 3: OH + butane From Seinfeld and Pandis (Atmospheric Chemistry and Physics)

15. Tropospheric ChemistryOxidation in the Atmosphere • OH Reactions – radical Propagation – Example 4: OH + propene From Seinfeld and Pandis (Atmospheric Chemistry and Physics)

16. Tropospheric ChemistryOxidation in the Atmosphere • Free Radical Termination Reactions • To stop the free radical cycles, we also need termination steps that involve two free radicals reacting with themselves • Examples • 2OH → H2O2 (doesn’t occur - OH conc. is too low) • 2HO2→ H2O2 + O2 (cleaner regions) • OH + NO2→ HNO3(polluted regions) • Note: dependence on [X][X’] means control on “overreactive” cycle

17. Break for Group Activity

18. Tropospheric ChemistryFormation of Ozone • Back to Formation of Tropospheric Ozone (a major secondary pollutant) • Have discussed two ingredients, what about NOx? • Roles of NOx • Recycles HO2 to OH NO + HO2→ OH + NO2 • Produces ozone through photolysis NO2 + hn→ NO + O O + O2 + M → O3 + M

19. Tropospheric ChemistryFormation of Ozone • Overview of simple cycle: NOx + CO + hn • Steps: 1. CO + OH + O2→ CO2 + HO2 (2 rxns) 2. NO + HO2→ OH + NO2 3. NO2 + hn→ NO + O 4. O + O2 + M → O3 + M Net: CO + 2O2 + hn→ CO2 + O3

20. Tropospheric ChemistryFormation of Ozone • Source of NOX • Mostly combustion (cars and power plants are most significant) • Thermal NOX formation N2 + O2→ 2NO postive DH, but also positive DS – favored at high T anytime air is heated to high T, some NO forms (high NOX observed over lava fields in Hawaii, lightning is a significant natural NOX source) • Fuel/Oxidant NO sources • N in fuel (some in coal) gives higher NO emissions • why using N2O, CH3NO2 for cars is a bad idea • Natural Sources – not significant in urban areas

21. Tropospheric ChemistryFormation of Ozone • Source of volatile organic hydrocarbons • Incomplete combustion source (car engines) • Solvents (e.g. old type of paints) • Natural sources (e.g. isoprene – significant in some locations) • Besides amount, type makes a big difference • Initial reaction rate affects production of HO2 and RO2 radicals needed to convert NO to NO2 • Generally, alkanes react slower than alkenes • Alkenes also react with O3 • Aldehydes can cause additional radical formation through photolysis HCHO + hn→ H• + HCO• (also → H2 + CO)

22. Tropospheric ChemistryStrategies to Limit Ozone Production • Can’t reduce sunlight easily, so NOX or HC reductions are possible • Initial regulation focused on car emissions in which going to more efficient and lean (excess O2 ) combustion, which reduces HCs but can increase NOX (until improvements in catalysts) • Two other reactions affect the strategy: NO + O3→ NO2 + O2 (keeps O3 low in downtown regions) and OH + NO2→ HNO3 (limits OH reactivity)

23. Tropospheric ChemistryStrategies to Limit Ozone Production • HC and NOX Limitation Regimes • In low hydrocarbon high NOX conditions, reducing NOX can increase ozone (at least locally) • Reduction of hydrocarbons will have a limited effect in downwind regions and where natural hydrocarbons are significant

24. Tropospheric ChemistryStrategies to Limit Ozone Production • Emission Reductions – Catalytic Convertors on Cars • Initial focus was to complete hydrocarbon oxidation (less CO and HCs) • Newer catalysts also reduce NOX • Diesel is more problematic • Engines are higher combustion ratio – tend to burn hotter and particulate emissions are higher • A urea based catalyst is now more common

25. Tropospheric ChemistryStrategies to Limit Ozone Production • Regional Ozone Problems • Focus on limiting VOCs is better for reduction in urban areas but can cause greater problems in downwind regions • A reason for this is release of reservoir species back to NOx: e.g. HNO3 + hn→ OH + NO2 • Additionally, natural hydrocarbon sources keep hydrocarbons from dropping too low • Conditions for High Regional Ozone • Typically under warm summertime conditions with high pressures (need sunlight, limited mixing, plus high temperatures reduce wet removal)