chem. 253 – 2/25 lecture. announcements i return hw 1.3 + group assignment last week’s group...
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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 ChemistryNucleation 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 m)
Cloud Chemistry- Nucleation of Cloud Droplets
aerosol size distribution (mass based)
log dp (m)0.01 1010.1
0.01 log dp (m)1010.1
Nucleation efficiency
0
100
hygroscopic aerosol
Soot, soil
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 ChemistryNucleation 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: 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 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 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
• 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(in-
cloud)– 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?
Source pH
400 ppmv CO2 5.61
1 ppbv SO2 5.38
1 ppbv HNO3; LWC = 0.5 g/m3 4.09
5 g/m3 NH4HSO4 aerosol; LWC = 0.5 g/m3
4.06
4 Independent Senerios
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 SO3
2- + O3 → SO42- + O2 fast
reaction is faster at high pH because more S(IV) is present in reactive forms
Cloud Chemistry- Reactions in Clouds
1E-14
1E-12
1E-10
1E-08
1E-06
1E-04
1E-02
S(I
V)
loss
rate
(M
/s)
2 3 4 5 6 7 pH
O3 HCHO H2O2
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 1pH = 4.0
drop 2pH = 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 Dissertationdiffusion 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: Consequencesof Polluted Air
• Effects Covered in Chapter 4 Include: Haze, Acid Precipitation, and Health Effects
• We will cover health effects when covering toxicology
Chapter 4: Consequencesof 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: Consequencesof Polluted Air - Haze
• Light scattering is most mass efficient (most scattered light per g of aerosol) for dp ~
• 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: Consequencesof 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: Consequencesof 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: Consequencesof 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