Chem. 253 – 2/25 Lecture
Announcements I
• Return HW 1.3 + Group Assignment
• Last Week’s Group Assignment
– most did reasonably well
• New HW assignment (1.5 – posted on
website)
• Next Wednesday
–
–
–
–
Will have HW due
No Group Assignment
Exam 1
Justin will take over (covering water chemistry)
Announcements II
• Exam 1
– On all topics covered through today
– Will review topics to know at end of lecture
– Exam will be mix of short answer questions
(multiple choice or fill in the blank) + work out
problems
– I may post an example exam (if I can find a
relevant copy)
• Today’s Lecture Topics – Tropospheric
Chemistry
– Finish up cloud chemistry (Chapter 3 and 4)
– Atmospheric Effects (Chapter 4)
Sulfur, Aerosol, and Cloud Chemistry
Review of Main Concepts I
• Aerosols
– suspension of particles in a gas
– particle size range is related to formation and
growth
– three main sizes (ultrafine mode – new particles
from gas phase, accumulation mode – processed
particles, and coarse mode – from mechanical
production)
– distributions are log normal and can be defined
based on number, surface area, or mass
– four main chemical classes (sea-salt, soil dust,
sulfate, and organic)
– both primary and secondary sources
Sulfur, Aerosol, and Cloud Chemistry
Review of Main Concepts II
• Sulfur Chemistry
– both natural and anthropogenic sources
– mostly emitted as SO2, but reduced S is also
important
– predominant pathway is oxidation to H2SO4
– gas phase oxidation occurs through 2-step OH
reaction
– gas phase H2SO4 production can lead to new
particle formation (although mostly leads to
growth of existing particles)
– aqueous phase SO2 oxidation adds mass to
accumulation mode sized particles
Sulfur, Aerosol, and Cloud Chemistry
Review of Main Concepts III
• Cloud/Precipitation Chemistry
– Will review + add new material
Cloud/Precipitation 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
• 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)
• While larger particles are more efficient at
nucleation, there are a lot more small particles,
so number of droplets formed is dominated by
the accumulation mode (100 nm < d < 2.5 mm)
Cloud Chemistry
- Nucleation of Cloud Droplets
aerosol size distribution (mass based)
0.01
100
0.1
log dp (mm)
1
10
Nucleation efficiency
Soot, soil
hygroscopic aerosol
0
0.01
0.1
log dp (mm)
1
10
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
Nucleation Example Problems
• Why is an RH over 100% required for cloud
droplet nucleation?
• Why is nucleation efficiency higher in less
polluted regions (for a given particle size)?
• 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?
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 (if reacting very fast in
droplets)
• Henry’s Law:
KH

X

PX
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 constant (good assumption for SO2 and CO2)
Then [X] = KH∙PX where PX comes from mixing ratio
• 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
• 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.
• Example problem (high KH case): Determine the pH and
aqueous NO3- concentration (in M) if air containing 1
ppbv HNO3 enters a cloud with a pressure of 0.90 atm, a
T = 293K, and a LWC of 0.50 g/m3. Assume 100%
scavenging.
Break for Group Activity
Cloud Chemistry
-
Overview of Scavenging
• Gases scavenged are almost always in
Henry’s law equilibrium
• We will assume one of two cases occurs:
– so little scavenging that Px(pre-cloud) = Px(incloud)
– or 100% scavenging (complete transfer from
gas phase to aqueous phase)
• Aerosol scavenging depends on size and
type of particles (with typical lower end of
around 100 nm)
Cloud Chemistry
-
What determines pH?
• It is complicated
• Strong acids (HNO3(g) and H2SO4(l))
provide [H+], tempered by NH3 and other
bases
• Both SO2 and CO2 can add acidity through
reaction of H2XO3 with water
• In many senarios, including “background”
locations, neither SO2 nor CO2 significantly
contribute to pH
Cloud Chemistry
-
What determines pH?
4 Independent Senerios
Source
pH
400 ppmv CO2
5.61
1 ppbv SO2
5.38
1 ppbv HNO3; LWC = 0.5 g/m3
4.09
5 mg/m3 NH4HSO4 aerosol; LWC
= 0.5 g/m3
4.06
Cloud Chemistry
-
A Modeled Example
example including ammonium bisulfate, sulfur dioxide and carbon dioxide
Equilibrium pH where
sum of anion charge =
sum of cation charge
Calculation method is fairly
complex (uses systematic
method)
Cloud Chemistry
-
Reactions in Clouds
• Cloud reactions are important for water
soluble species because of higher
concentrations in clouds
• Only sulfur chemistry covered here
Cloud Chemistry
- Reactions in Clouds
• Reaction of S(IV) and H2O2
-
HSO3- + H2O2 → HSO4- + H2O (acid catalyzed)
Rate = k[HSO3-][H+][H2O2]
Rate = k’[H2O2]PSO2
Effectively pH independent
Cloud Chemistry
- Reactions in Clouds
• Reaction of S(IV) and Ozone
- Two main reactions:
HSO3- + O3 → HSO4- + O2 moderately fast
SO32- + O3 → SO42- + O2 fast
reaction is faster at high pH because more
S(IV) is present in reactive forms
Cloud Chemistry
-
Reactions in Clouds
1 E -0 2
S(IV) loss rate (M/s)
1 E -0 4
1 E -0 6
1 E -0 8
1 E -1 0
1 E -1 2
1 E -1 4
2
3
4
5
6
pH
O3
H CH OH 2 O 2
7
Cloud Chemistry
- Reactions in Clouds
• Oxidation of S(IV)
– H2O2 is more important oxidant
in acidic clouds
– O3 can be important in cleaner
air
– Bulk models underestimate O3
reaction
– Under certain conditions,
reactions can be diffusion
limited
drop 1
pH = 4.0
drop 2
pH = 6.0
pH of combined
drop = 4.30
rate ratio (H2O2/O3) at
combined pH ~ 1000
rate ratio (H2O2/O3)
from independent
reactions in two drops
~ 0.5
Cloud/Precipitation Chemistry
- Incorporation of Pollutants
Precipitation Chemistry
• Precipitation Formation
– Cloud droplets are
collected by collisions
with rain droplets or
snow crystals and
transfer their contents
– Snow crystals also can
form mainly through
diffusion from water
vapor and are very
clean
From Mosimann ETH Dissertation
diffusion growth (top) to high
degree of riming (bottom)
Precipitation Chemistry
• Precipitation Formation
– In addition to in-cloud transfer, pollutants
can be incorporated from below cloud
scavenging
– This tends to be best for aerosols by
snow and gases by rain
– Precipitation pollutants are typically
somewhat lower than low-level cloud
concentrations
Chapter 4: Consequences
of Polluted Air
• Effects Covered in Chapter 4 Include:
Haze, Acid Precipitation, and Health
Effects
• We will cover health effects when covering
toxicology
Chapter 4: Consequences
of Polluted Air - Haze
• How do aerosols affect visibility and what
factors contribute to reduce visibility?
• Loss of light transmission (as in
spectroscopy) can occur due to scattering
or absorption
– usually aerosol scattering is most important
– NO2 absorption and soot absorption
contribute to a lesser extent
Chapter 4: Consequences
of Polluted Air - Haze
• Light scattering is most mass efficient (most
scattered light per g of aerosol) for dp ~ l
• Thus accumulation or fine aerosol mass is good
indicator for poor visibility
• High humidity also makes problem worse due to
hygroscopic growth of aerosol particles
• Meteorological conditions trapping pollutants or
contributing to photo-oxidation also make
visibility worse
Chapter 4: Consequences
of Polluted Air – Acid Rain
• Main contributors are strong acids HNO3 and
H2SO4
• These species form slowly (e.g. relative to
ozone), so worst places are downwind of major
NOx and SO2 sources
• Besides pollution sources, two other factors are
important:
– atmospheric neutralization
– soil chemistry
Chapter 4: Consequences
of Polluted Air – Acid Rain
• Neutralization by Atmospheric Bases
– NH3 (from fertilizers and animal excretions)
– CaCO3 (in soil dust)
• Soil Also Allows Run-off Neutralization
– Occurs in soils containing carbonates (limestone,
marble, etc.)
• Acid Rain More Strongly Affects Soils with Weak
Buffer Capacity
– Granite or quartz bedrock regions can’t buffer acidic
precipitation
– This results in acidic lakes
Chapter 4: Consequences
of Polluted Air – Acid Rain
• Problems with Acidified Water and Soils
• Plant growth in lakes is reduced, which can
affect whole ecosystem
• Additionally, Al and other metals are mobilized
at lower pH due to shift in:
Al(OH)3(s) ↔ Al3+ + 3OH• Many such metals are toxic to fish at higher
concentrations