Chem. 253 – 2/18 Lecture

Announcements I• Return HW 1.2 + Group assignment• HW 1.2 – some needed to show work

(e.g. conversions from molec cm-3 s-1 to g cm-3 yr-1)

• Last Week’s Group Assignment– scores were lower than expected– some blame from clarity of instructions– some problems were like non-collected

review questions• New HW assignment (1.4 – posted on

website)

Announcements II• Exam 2 Coming Up – Wed after next• This Week’s Group Assignment

– On Photochemical Smog production• Today’s Lecture Topics – Tropospheric

Chemistry– Review of last week’s topics/Additional

Photochemistry– The other unhealthy part to smog –

particulate matter– Gas phase sulfur chemistry– Aerosol chemistry– Clouds and cloud chemistry

Tropospheric Photochemistry

Review of Main Concepts I• Initiation of Oxidation– mostly by OH (O3 and NO3 are less

significant oxidants)• Hydrocarbon + CO Oxidation

– OH initiates oxidation (to CO2 in the case of CO and to carbonyls in the case of alkanes/alkenes)

– Reactions also result in RO2 radicals (R = H, CH3, other alkyl group)

– Reaction rates vary vastly (very slow for CH4, slow for CO and small alkanes, faster for alkenes and aldehydes)

Tropospheric Photochemistry

Review of Main Concepts II• NOx and Ozone Formation– NO is primarily formed from combustion in air

(e.g. cars and power plants)– NO + RO2 → NO2 + [RO] (RO = OH or radical

capable of forming HO2)– Above reaction recycles HO2 to OH and is

needed for O3 production– Tropospheric O3 production:

NO2 + h → NO + O

O + O2 + M → O3 + M • Radical Ending Reactions – Limit Cycles

(OH + NO2 → HNO3, 2HO2 → H2O2 + O2)

Tropospheric Photochemistry

Review of Main Concepts III• Situations can be NOx or VOC Limited– NOx limited means reduction of NOx will be

best for decreasing ozone– VOC limited means reduction in VOCs will

best for decreasing ozone– NOx limited generally occurs under

relatively low NOx conditions and VOC limited under relatively low VOC conditions

– Urban areas tend to be VOC limited (high NOx), while rural areas tend to be NOx limited (low NOx)

Tropospheric Photochemistry

Beyond Ozone• Ozone is Usually of Interest Because of:– health issues– role in generating OH– significant initiator (reacts with alkenes)

• Other Hazardous Compounds– radicals (OH, HO2), NO2, reservoir species (e.g.

peracetyl nitrate, nitric acid, hydrogen peroxide)• Other Health Issue is Aerosols:

– one source is secondary aerosol (produced through OH initiated or related reactions)

– aerosol generated from anthropogenic VOCs and natural VOCs (enhanced by higher OH/ozone)

Tropospheric ChemistryParticulate Matter/Aerosols

• Definitions– Aerosol = suspension of particles in a gas– Particles can be liquid or solid– Particulate Matter = the particle phase of an

aerosol– Particulate Matter is also subdivided based on

particle size• TSP = total suspended particulate matter (typical

units are g m-3)• PM2.5 = particulate matter under 2.5 m in diameter

– PM is traditionally collected using inlet (e.g. removes particles larger than 2.5 m for PM2.5) and filters

Particulate Matter/Aerosols

• Rationale for Studying

1. Important in biogeochemical cycles (e.g.

S cycle)

2. Direct Effects on Visibility and Climate

(covered with Greenhouse gases later)

3. Effects on Clouds and Precipitation

Physics and Chemistry

4. Effects on Human Health

Aerosols – Effects on Visibility

View from my window on typical day

Aerosol particles reduce visibility by scattering light

Picture on unusually clear day from CSUS internet site

View of mountains blocked by particle scattering

Aerosols – Effects on Climate

www.osei.noaa.gov/Events/Fires/

Direct Effect of aerosols - aerosols scatter more light back to Direct Effect of aerosols - aerosols scatter more light back to space, leading to cooling at the earth’s surface.space, leading to cooling at the earth’s surface.Example: Star Fire, Aug., 2001Example: Star Fire, Aug., 2001

smoke region looks lighter due to light scattered back to space

Example of clouds modified by ship exhaustExample of clouds modified by ship exhaust

http://www-das.uwyo.edu/~geerts/cwx/notes/chap08/contrail.html

Aerosols Effects on Clouds/Climate

Aerosols – Effects on HealthHigh aerosol concentrations correlate with hospital visits

Brauer and Hisham-Hashim, ES&T, 32, 1998

Aerosols – Size Matters

• Many Properties of Aerosol Particles Depend on Their Size

• Most Aerosols have Log-Normal Size Distributions

• Common Types of Size Distributions– Number (number of particles of given

size)– Mass (or Volume)– Surface Area

Aerosols –Normal Distribution

• Normal Distribution (not very common)

22 2/)()( DDDAeDn

Normal Size Distribution

0

2

4

6

8

10

12

14

Size (nm)

Nu

mb

er

in s

ize

ra

ng

e

Mean diameter = 34 nm; Standard deviation (σ) = 5 nm

Normal Size Distribution

-2

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80

Particle Diameter (nm)

dN

/dD

Aerosols –Log Normal Distributions

• Log normal distribution – appears as a normal distribution when x-axis is plotted on log scale

D

g

D

DDNDn

2

2

2/1 ln2

lnlnexp

ln)2()(

Log-Normal Distribution

0

50

100

150

200

250

1 10 100 1000

Diameter (nm)

dn

/dlo

gD

Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8

Aerosols –Calculation Example

• How many 10 nm particles (d = 10 nm) would have the same volume as 1 100 nm particles?– N*[(10 nm)3/6] = 1*[(100 nm)3/6]– N = (100/10)3 = 1000

• How many 10 nm particles would have the same surface area as 1 100 nm particle?– N*[(10 nm)2] = 1*[(100 nm)2]– N = 100

Aerosols –N and Mass Distributions

Distributions

0

50

100

150

200

250

1.0 10.0 100.0 1000.0

D (nm)

dN

/dlo

gD

an

d d

M/d

log

D

Number

Mass

Same aerosol, number distribution is dominated by smaller particles, mass distribution is dominated by larger particles

For Number:

Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8

For Mass:

Geometric mean = 65 nm

Break for Group Activity

Aerosols –Sources of Aerosols

• Major Classes (Based on Composition)– Soil Dust (coarse particles)– Sea Salt (coarse particles)– Sulfate (fine particles)– Carbonaceous or Organic (fine particles)

• Classes (Based on Sources)– Primary Sources– Secondary Sources (typically from oxidation of gaseous

precursors)

Note: particle “aging” and physical processes make distinction of particle classes more difficult

Aerosols –Sizes of Various Aerosols

Surface Area Distribution

(3 modes)(Whitby, 1978)

ultrafine mode (dominates #)

source accumulation mode

sinksources: coagulation + vapor deposition growth (both from ultrafine)

sink: washout

coarse mode (dominates mass in boundary layer)

Sulfur Chemistry• Forms

– reduced sulfur (H2S, OCS, CH3SCH3) -2 oxidation state

– partially oxidized (CH3SOCH3, SO2)– fully oxidized (H2SO4, NH4HSO4(s)) (+6

oxidation state)• Chemical Fate in Atmosphere

– Oxidation– Rates depend on stability (slow for OCS, fast

for H2S, CH3SCH3)

Sulfur ChemistrySources

• Natural Sources– Volcanoes (large SO2 source)

• continuous out-gassing • large eruptions (significant source of stratospheric SO2)

– Biota (largest sources is CH3SCH3 in oceans)– Sea-salt (direct in oxidized form)

• Anthropogenic (mostly in form SO2)– coal burning (from coal S – which varies depending

on source)– smelting of metal oxides to metals (e.g. Cu

production)– other fuel combustion/production (H2S with natural

gas, heavier liquid fuels containing S)

Sulfur ChemistrySources

• Note on Text – Anthropogenic sources are ~ 70% of total

• Anthropogenic – control strategies– oxidize S, remove as H2SO4 or through

particle traps– scrubbers (typically basic solutions to trap

SO2 gases)– remove before combustion (done with high S

coal and also for diesel fuel)

Sulfur ChemistryReactions

• Reduced Sulfur Compounds (H2S, CH3SCH3) – mostly oxidized to SO2 through OH initiated

reactions– CH3SCH3 also produces CH3SO3H (a tracer of

natural S)• Sulfur Dioxide

– Gas Phase Reaction:1) SO2 + OH + O2 → SO3 + HO2 (2 steps)

2) and SO3 + H2O(g) → H2SO4 (g)

3) H2SO4 (g) → H2SO4 (s)

4) Step 3 can occur through a) addition to existing particles (growth of particles) or b) formation of new particles (one of very few ways to form new particles via atmospheric reactions)

Sulfur ChemistryReactions

• Sulfur Dioxide– Aqueous Phase Reactions

• First step is dissolution (SO2 (g) + H2O (l) → H2SO3 (aq))

• Then reaction with dissolved oxidants (O3 and H2O2)

– Will Cover In Detail Later– Note that gas phase oxidation and aqueous

phase oxidation results in H2SO4 produced in aerosol particles – but in different sized particles

Atmospheric Aerosols –Carbonaceous

• Primary Sources– Biomass combustion (forest fire smoke)– Inefficient Fossil Fuel Combustion– Mechanically Produced (e.g. from tires)

• Secondary Sources (generally richer in O)– Photooxidation of gaseous precursors (e.g.

-pinene to pinonic acid)– Other (cloud, aerosol reactions)

Atmospheric Aerosols –Carbonaceous - Composition

Rogge et al., ES&T, 1993; Los Angeles Samples

Atmospheric Aerosols –“Aging” of Aerosols

1. Sea-salt and soil dust particles- Acids affect particle composition- Examples:

- CaCO3(s) + 2HNO3(g) → Ca(NO3)2(s) + CO2(g) + H2O(g)

- 2NaCl(s) + H2SO4(aq) → Na2SO2 + 2HCl(g)

• Fine particles- Neutralization of sulfuric acid

- H2SO4(aq) + 2NH3(g) → (NH4)2SO4(s)

- Oxidation/Nitration of Organic Compounds- Aggregation/Growth of particles

Atmospheric Aerosols –Presence of Water

• At relative humidity (RH) less than 100%, many aerosol particles exist at concentrated solutions

• Concentration of solute is related to RH through Raoult’s law (provided particles are large enough):

OHOHOH XPP222

Where: PH2O = the vapor pressure of water, P•H2O = the

saturated vapor pressure of water; PH2O/ P•H2O = RH

XH2O = the mole fraction of water in the solution

)()()(

2

22 soluteniOHn

OHnX OH

i = number of species following dissociation (e.g. for NaCl, i = 2)

Atmospheric Aerosols –Removal of Aerosols

• Dry deposition particles– Most important for coarse particles (D>1

μm)– Settling rate larger for larger particles– Very small particles (<30 nm) can be

removed efficiently to surfaces because they have faster diffusion rates

Atmospheric Aerosols –Removal of Aerosols

• Wet Deposition– Removal in precipitation processes– Major pathway for fine particles but

inefficient for particles with D <50 nm– In-cloud scavenging (1) nucleation of

cloud droplets on aerosol particles and 2) formation of precipitation from cloud droplets)

– Below-cloud scavenging

Cloud Chemistry

• Rationale for Studying- Cloud reactions can be important (e.g. formation of H2SO4)

- Precipitation composition depends on cloud composition- Provide introduction to aqueous chemistry

Cloud Chemistry- Incorporation of Pollutants

Cloud Chemistry- Incorporation of Pollutants

• Main mechanisms- Nucleation of cloud droplets on aerosol particles- Scavenging of gases- Reactions within the droplet

Cloud ChemistryNucleation of Cloud Droplets (some

review?)• Cloud droplets can not form in the absence

of aerosol particles unless RH ~ 300%.• Cloud droplets nucleate on aerosol particles

at RH of ~100.1 to ~101%.• Cloud droplets should nucleate when RH =

100% except that the vapor pressure over a curved surface is less than that over a flat surface (due to water surface tension)

• Smaller particles (d < 50 nm) have more curved surfaces and are harder to nucleate

Cloud Chemistry- Nucleation of Cloud Droplets

• Nucleation more readily occurs with:- Larger particles- Particles with more water soluble compounds (due to growth according to Raoult’s law)- Compounds that reduce surface tension- Smaller aerosol number concentrations (less competition for water so higher RH values)

Cloud Chemistry- Nucleation of Cloud Droplets

• The concentration of constituents incorporated from

nucleation depends on the efficiency of nucleation

and on the liquid water content (or LWC).

• LWC = g liquid H2O/m3 of air

• The higher the LWC, the lower the concentration

(dilution effect)

• Cloud nucleation leads to heterogeneous cloud

droplet composition – Ignored here for calculations

Cloud Chemistry- Scavenging of Gases

• Also Important for covering water chemistry (e.g. uptake of CO2 by oceans)

• For “unreactive” gases, the transfer of gases to cloud droplets depends on: the Henry’s law constant (always)

• In special cases, transfer can depend on LWC (if high), or can be limited by diffusion

• Henry’s Law: X

H PX

K

where KH = constant (at given T) and X = molecule of interest

Cloud Chemistry- Scavenging of Gases: “unreactive” gases

• When LWC and KH are relatively low, we can assume that PX is constantThen [X] = KH∙PX

• When KH is high (>1000 M/atm), conservation of mass must be considered (PX decreases as molecules are transferred from gas to liquid)

• We will only consider 2 cases (low KH case and 100% gas to water case)

Cloud Chemistry- Scavenging of Gases “unreactive” gases

• For compounds with high Henry’s law constants, a significant fraction of compound will dissolve in solution

• fA = 10-6KHRT(LWC) where fA = aqueous fraction (not used in assigned problems)

• When fA ~ 1, can use same method as for cloud nucleation From Seinfeld and Pandis (1998)

Cloud Chemistry- Scavenging of Gases: “reactive” gases

• Many of the gases considered are acidic and react further

• Example: Dissolution of SO2 gasReaction: Equation:SO2(g) + H2O(l) ↔ H2SO3(aq) KH = [H2SO3]/PSO2

H2SO3(aq) ↔ H+ + HSO3- Ka1 = [H+][HSO3

-]/[H2SO3(aq)]

HSO3- ↔ H+ + SO3

2- Ka2 = [H+][SO32-]/[HSO3

-]

Note: concentration of dissolved SO2 = [S(IV)]

= [H2SO3] + [HSO3-] + [SO3

2-] = [H2SO3](1 + Ka1/[H+] + Ka1Ka2/[H+]2)“Effective” Henry’s law constant = KH

* = KH(1 + Ka1/[H+] + Ka1Ka2/[H+]2) = function of pH

Cloud ChemistrySome Example Problems

• Why is a RH over 100% required for cloud droplet nucleation?

• Why is nucleation efficiency higher in less polluted regions?

• An ammonium bisulfate aerosol that has a concentration of 5.0 μg m-3 is nucleated with 50% efficiency (by mass) in a cloud that has a LWC of 0.40 g m-3. What is the molar concentration? What is the cloud pH?

• Example Problem (low KH case): What is the concentration of CH3OH in cloud water if the gas phase mixing ratio is 10 ppbv and a LWC of 0.2 g/m3? The Henry’s law constant is 290 M/atm (at given temp.). Assume an atmospheric pressure of 0.9 atm and 20°C.