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

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

Tropospheric Chemistry What 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)

Tropospheric Chemistry What 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 T (°C) Z (km)

Tropospheric Chemistry What 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 L Low P systems – have high winds near center H High P systems – have low winds over large centers only one opening

Tropospheric Chemistry What 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

Tropospheric Chemistry What 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)

Tropospheric Chemistry What is Smog and What Causes It? Secondary Air Pollutants –An air pollutant forms through atmospheric reactions –Examples: NO 2, 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: NO x, VOCs, and sunlight

Tropospheric Chemistry What 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 NO 2 (absorbs lower visible wavelengths) –low visibility is not necessarily the cause of health effects, but high aerosol concentrations are associated with health problems

Tropospheric Chemistry Oxidation 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” O 2 reaction (2NO 2 + O 2 → 2NO 2 ), under polluted conditions, reaction is insignificant Faster oxidation requires other oxidants (OH - daytime, O 3, and NO 3 – nighttime are most prevalent) OH is the most widespread initiator of oxidation

Tropospheric Chemistry Oxidation in the Atmosphere OH formation Reaction: O 3 + h → O 2 + O* O* + H 2 O → 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 + O 2, which ends odd O in O only Chapman mechanism) –normally initiation and termination are slow steps

Tropospheric Chemistry Oxidation in the Atmosphere OH Reactions – radical Propagation – Example 1: OH + CO –CO is a pollutant from incomplete combustion –Toxic at relatively high concentrations (replaces O 2 in hemoglobin) reactions: 1) CO + OH → HOCO (unstable free radical) 2) HOCO + O 2 → CO 2 + HOO (HO 2 ) net: CO + OH + O 2 → CO 2 + HO 2 (transforms 1 OH to 1 HO 2 )

Tropospheric Chemistry Oxidation in the Atmosphere OH Reactions – radical Propagation – Example 2: OH + CH 4 –CH 4 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) CH 4 + OH → CH 3 + H 2 O (abstraction reaction) 2) CH 3 + O 2 → CH 3 O 2 3) CH 3 O 2 + NO → CH 3 O + NO 2 4) CH 3 O + O 2 → HCHO + HO 2 Note: HCHO will react further (fast relative to CH 4 ; slow vs. other intermediates)

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

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

Tropospheric Chemistry Oxidation 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 → H 2 O 2 (doesn’t occur - OH conc. is too low) –2HO 2 → H 2 O 2 + O 2 (cleaner regions) –OH + NO 2 → HNO 3 (polluted regions) Note: dependence on [X][X’] means control on “overreactive” cycle

Break for Group Activity

Tropospheric Chemistry Formation of Ozone Back to Formation of Tropospheric Ozone (a major secondary pollutant) Have discussed two ingredients, what about NO x ? Roles of NO x Recycles HO 2 to OH NO + HO 2 → OH + NO 2 Produces ozone through photolysis NO 2 + h → NO + O O + O 2 + M → O 3 + M

Tropospheric Chemistry Formation of Ozone Overview of simple cycle: NO x + CO + h Steps: 1. CO + OH + O 2 → CO 2 + HO 2 (2 rxns) 2. NO + HO 2 → OH + NO 2 3. NO 2 + h → NO + O 4. O + O 2 + M → O 3 + M Net: CO + 2O 2 + h → CO 2 + O 3

Tropospheric Chemistry Formation of Ozone Source of NO X Mostly combustion (cars and power plants are most significant) Thermal NO X formation N 2 + O 2 → 2NO postive  H, but also positive  S – favored at high T anytime air is heated to high T, some NO forms (high NO X observed over lava fields in Hawaii, lightning is a significant natural NO X source) Fuel/Oxidant NO sources –N in fuel (some in coal) gives higher NO emissions –why using N 2 O, CH 3 NO 2 for cars is a bad idea Natural Sources – not significant in urban areas

Tropospheric Chemistry Formation 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 HO 2 and RO 2 radicals needed to convert NO to NO 2 Generally, alkanes react slower than alkenes Alkenes also react with O 3 Aldehydes can cause additional radical formation through photolysis HCHO + h → H + HCO (also → H 2 + CO)

Tropospheric Chemistry Strategies to Limit Ozone Production Can’t reduce sunlight easily, so NO X or HC reductions are possible Initial regulation focused on car emissions in which going to more efficient and lean (excess O 2 ) combustion, which reduces HCs but can increase NO X (until improvements in catalysts) Two other reactions affect the strategy: NO + O 3 → NO 2 + O 2 (keeps O 3 low in downtown regions) and OH + NO 2 → HNO 3 (limits OH reactivity)

Tropospheric Chemistry Strategies to Limit Ozone Production HC and NO X Limitation Regimes In low hydrocarbon high NO X conditions, reducing NO X can increase ozone (at least locally) Reduction of hydrocarbons will have a limited effect in downwind regions and where natural hydrocarbons are significant

Tropospheric Chemistry Strategies 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 NO X 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

Tropospheric Chemistry Strategies 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 NO x : e.g. HNO 3 + h → OH + NO 2 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)