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 + hn → 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 Chemistry
Particulate 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 mg m-3)
• PM2.5 = particulate matter under 2.5 mm in diameter
– PM is traditionally collected using inlet (e.g.
removes particles larger than 2.5 mm 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
Aerosol particles reduce visibility by scattering light
View from my window on typical day
Picture on unusually clear day from
CSUS internet site
View of mountains blocked by particle scattering
Aerosols – Effects on Climate
Direct Effect of aerosols - aerosols scatter more light back to space,
leading to cooling at the earth’s surface.
Example: Star Fire, Aug., 2001
smoke region looks
lighter due to light
scattered back to
space
www.osei.noaa.gov/Events/Fires/
Aerosols
Effects on Clouds/Climate
Example of clouds modified by ship exhaust
http://www-das.uwyo.edu/~geerts/cwx/notes/chap08/contrail.html
Aerosols – Effects on Health
High aerosol concentrations correlate with
hospital visits
Brauer and
HishamHashim, 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)
n (D )  Ae
(D D )2 / 2 D2
Mean diameter = 34 nm; Standard deviation (σ) = 5 nm
NormalSize
SizeDistribution
Distribution
Normal
14
10
10
8
8
6
6
4
4
2
2
Particle
Diameter
Size
(nm) (nm)
80
76
72
70
68
64
60
60
56
52
48
50
44
40
40
36
32
28
30
24
20
20
16
12
8
10
4
0
00
-2
0
NumberdN/dD
in size range
12
Aerosols –
Log Normal Distributions
• Log normal distribution – appears as a normal
distribution when x-axis is plotted on log scale
 ln D  ln D g 2 
n (D ) 
exp 

(2 )1 / 2 ln D
2 ln2  D


N
Geometric Mean
Diameter = 23 nm;
Geometric Standard
Deviation (σ) = 1.8
Log-Normal Distribution
250
dn/dlogD
200
150
100
50
0
1
10
100
Diameter (nm)
1000
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
Same aerosol, number distribution is dominated by smaller
particles, mass distribution is dominated by larger particles
For Number:
Distributions
dN/dlogD and dM/dlogD
250
200
150
Number
Mass
100
50
Geometric
Mean Diameter
= 23 nm;
Geometric
Standard
Deviation (σ) =
1.8
For Mass:
0
1.0
10.0
100.0
D (nm)
1000.0
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)
source
sink
ultrafine mode
(dominates #)
(Whitby, 1978)
accumulation mode
sources: 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 Chemistry
Sources
• 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 Chemistry
Sources
• 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 Chemistry
Reactions
• 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)
• 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 Chemistry
Reactions
• 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.
a-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)
2. 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):
PH O  PH O  X H O
2
2
2
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
XH O 
2
n (H 2O )
n (H 2O )  i  n (solute )
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 Chemistry
Nucleation 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:
where KH = constant (at

X
given T) and X = molecule
KH 
of interest
PX
-
Cloud Chemistry
Scavenging of Gases: “unreactive” gases
• When LWC and KH are relatively low, we can assume
that PX is constant
Then [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 gas
Reaction:
Equation:
SO2(g) + H2O(l) ↔ H2SO3(aq)
H2SO3(aq) ↔ H+ + HSO3HSO3- ↔ H+ + SO32-
KH = [H2SO3]/PSO2
Ka1 = [H+][HSO3-]/[H2SO3(aq)]
Ka2 = [H+][SO32-]/[HSO3-]
Note: concentration of dissolved SO2 = [S(IV)]
= [H2SO3] + [HSO3-] + [SO32-] = [H2SO3](1 + Ka1/[H+] + Ka1Ka2/[H+]2)
“Effective” Henry’s law constant
= KH* = KH(1 + Ka1/[H+] + Ka1Ka2/[H+]2) = function of pH
Cloud Chemistry
Some 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.