Chem. 250 – 10/7 Lecture
Updated 10/30
Instructor: Roy Dixon
My Website for Course:
http://www.csus.edu/indiv/d/dixonr/C250/C250.htm
Announcements - I
A. Lecture Format
1. Powerpoint – overview + graphics + some concepts
2. Blackboard – mainly problem solving
B. Topics I’m Covering
1. Aerosol and Cloud Chemistry (10/7)
2. Precipitation/Water Chemistry (11/18)
3. Metals in the Environment (12/2)
4. Toxicology (12/9)
Note:
- See syllabus for pages from text to read
Announcements - II
C. Exam based on understanding of main
concepts and ability to solve relevant
problems
Note: Today’s lecture is on material I
understand well – so exam questions will
have greater emphasis on lecture than
on text (also text emphasis is out-dated)
Atmospheric Aerosols
• Assigned homework (due next Wed.):
Chapter 2 (12, 14, 15, 17, 18, 19)
Chapter 6 (44a,b, 45, 48)
Atmospheric Aerosols
•
Rationale for Studying
1. Important in biogeochemical cycles (e.g. S
cycle)
2. Direct Effects on Visibility and Climate
3. Effects on Clouds and Precipitation
Formation
4. Effects on Human Health
Atmospheric Aerosol
Visibility Effects
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
Introduction – Climate Effects
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/
Atmospheric Aerosols
Climate Effects
Example of clouds modified by ship exhaust
http://www-das.uwyo.edu/~geerts/cwx/notes/chap08/contrail.html
Atmospheric Aerosols
Health Effects
High aerosol concentrations correlate with
hospital visits
Brauer and
HishamHashim, ES&T,
32, 1998
Atmospheric 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
Atmospheric 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
Atmospheric 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
Atmospheric Aerosols –
Calculation Example
• How many 10 nm particles 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
Atmospheric 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
Atmospheric 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
Atmospheric Aerosols –
Sizes of Various Aerosols
Surface Area Distribution
(3 modes)
(Whitby, 1978)
Atmospheric Aerosols –
Sulfate
•
Originates from Oxidation of SO2
– 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)
Atmospheric Aerosols –
Sulfate
- Aqueous Phase Reactions
- Reactions occur in cloud droplets
- Specific reactions covered later
- Reactions add sulfate only to particles big
enough to nucleate cloud droplets (>50 nm)
- Properties
- Acidic, unless neutralized by NH3(g)
- Water Soluble
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
– Calculation in book (terminal velocity type) is
misleading because mixing in boundary layer
is fast (~ hours)
– 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
- Incorporation of Pollutants
for larger
particles
Removal in Precipitation
Atmospheric Aerosols –
Some Example Problems/Questions
•
If SO2 gas is present at 10 ppbv at P = 0.9 atm and T = 20°C, what would be the
mass concentration (in μg m-3) of a resultant sulfuric acid aerosol? What if it is
converted to ammonium sulfate?
– n/V = C = P/RT= (10 x 10-9)(0.9 atm)/(0.0821 L atm/mol K)(293K)
– C = (3.74 x 10-10 mol/L)(1 mol ammonium sulfate/1 mole SO2)*
(132 g/mol)(106 mmol/mol)(1000 L/m3) = 49 mg/m3
•
The element sodium would be expected to exist primarily in which sized particles?
•
As an organic aerosol ages, what should happened to the ratio of the mass of carbon
to mass of organics?
–
–
•
As an organic aerosol ages, it becomes oxidized so that O makes up a more significant
fraction of the mass. This will decrease the ratio of the mass of C to mass organics.
Under what conditions will aerosol particles become more acidic or less acidic as they
age?
–
•
Coarse particles. This is because Na would be expected to mostly originate from sea-salt,
which is predominantly in the course mode (since sea-salt has a high % Na and few other
sources of Na are significant).
Aerosol particles will become more acidic if there are significant sources of SO2 (which
oxidizes to sulfuric acid). They will become neutralized (more basic) if there is a significant
source of ammonia gas.
Which sulfur dioxide oxidation process leads to ultra-fine (D<50 nm) particles?
–
Gas phase oxidation (Aqueous phase oxidation requires large enough particles for nucleation
of cloud droplets).
Atmospheric Aerosols
Some Example Problems/Questions – cont.
• What is the mole fraction of ammonium sulfate in an
aerosol particle present at a RH of 95%?
At 95% RH, PH2O/P̽H2O = 0.95 = XH2O (mole fraction of water)
0.95 = nH2O/(nH2O + 3nAS) note: nAS = moles ammonium sulfate)
0.95nH2O + 2.85nAS = nH2O
0.05nH2O = 2.85nAS
nH2O/nAS = 57 or nAS/nH2O = 0.0175
XAS = nAS/(nAS + nH2O) = (nAS/nH2O)/[(nAS/nH2O) + 1]
XAS = 0.0175/(0.0175 + 1) = 0.0172 (or 1.7 % by mole)
• What is the mass fraction of water in the above aerosol
particle?
mass ratio = mAS/mH2O = (nAS/nH2O)(132/18) = 0.128
% H2O = mH2O*100/(mH2O + mAS)
= (mH2O/mAS)*100/[(mH2O/mAS) + 1] = 89%
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
• Main mechanisms
- Nucleation of cloud droplets on aerosol
particles
- Scavenging of gases
- Reactions within the droplet
• Incorporation into precipitation
Cloud Chemistry
- Incorporation of Pollutants
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 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 concentration of liquid water in the cloud (also called
liquid water content or LWC).
• The higher the LWC, the lower the concentration
(dilution effect)
• Cloud nucleation leads to heterogeneous cloud droplet
composition – Ignored here for calculations