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 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)

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 movementT (°C)

Z (km)

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

L

Low P systems – have high winds near center

H

High P systems – have low winds over large centers

only one opening

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

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)

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

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

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

Tropospheric ChemistryOxidation in the Atmosphere

• OH formation Reaction:O3 + h → 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 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)

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)

Tropospheric ChemistryOxidation in the Atmosphere

• OH Reactions – radical Propagation – Example 3: OH + butane

From Seinfeld and Pandis (Atmospheric Chemistry and Physics)

Tropospheric ChemistryOxidation in the Atmosphere

• OH Reactions – radical Propagation – Example 4: OH + propene

From Seinfeld and Pandis (Atmospheric Chemistry and Physics)

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

Break for Group Activity

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 OHNO + HO2 → OH + NO2

• Produces ozone through photolysisNO2 + h → NO + O

O + O2 + M → O3 + M

Tropospheric ChemistryFormation of Ozone

• Overview of simple cycle: NOx + CO + h

• Steps:1. CO + OH + O2 → CO2 + HO2 (2 rxns)

2. NO + HO2 → OH + NO2

3. NO2 + h → NO + O

4. O + O2 + M → O3 + M

Net: CO + 2O2 + h → CO2 + O3

Tropospheric ChemistryFormation of Ozone

• Source of NOX

• Mostly combustion (cars and power plants are most significant)

• Thermal NOX formationN2 + O2 → 2NO

postive H, but also positive S – 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 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 photolysisHCHO + h → H• + HCO• (also → H2 + CO)

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)

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

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

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 + h → 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)