Chem. 253

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 Weeks Group Assignment
• On ozone hole
• Should be shorter than last week
• Todays 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 (doesnt occur - OH conc. is too low)
• 2HO2 ? H2O2 O2 (cleaner regions)
• OH NO2 ? HNO3 (polluted regions)
• Note dependence on XX 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

• Cant 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)