Transcript Chem. 31 – 9/15 Lecture

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
Z (km)
T (°C)
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
only one
opening
L
Low P systems – have
high winds near center
H
High P systems – have low
winds over large centers
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: 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
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 NO2 (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” 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
Tropospheric Chemistry
Oxidation 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
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
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)
Tropospheric Chemistry
Oxidation 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)
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 → 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
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 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
Tropospheric Chemistry
Formation 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
Tropospheric Chemistry
Formation 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
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 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)
Tropospheric Chemistry
Strategies 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)
Tropospheric Chemistry
Strategies 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
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 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
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 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)