Aerosols_power point presentation for envilonmental science

Is about Aerosols

1. Atmospheric Aerosol
Basics
AOSC 637
RUSSELL R. DICKERSON

2. Atmospheric Aerosols
Bibliography
Seinfeld & Pandis, Atmospheric Chemistry and Physics, Chapt. 7-13
Finlayson-Pitts & Pitts, Chemistry of the Upper and Lower Atmosphere,
Chapt. 9.
Classic papers:
Prospero et al. Rev. Geophys. Space Phys., 1607, 1983; Charlson et al. Nature
1987; Charlson et al., Science, 1992.
Recent Papers:
Ramanathan et al., Science, 2001; Andreae and Crutzen, Science, 1997;
Dickerson et al., Science 1997; Jickells et al., Global Iron Connections
Between Desert Dust, Ocean Biogeochemistry and Climate, Science, 308 67-
71, 2005.

3. EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS
TO RETURN TO “NATURAL” VISIBILITY LEVELS BY 2064
•
Acadia National Park
clean day moderately polluted day
http://www.hazecam.net/
…will require essentially total elimination of anthropogenic aerosols!

4. INTRODUCTION:
Particles are one of the most important and certainly the most visible aspects of air
pollution. The effects span the areas of health (1% increase in mortality per 10 μg m
-3
); acid
rain, visibility degradation, radiation and photochemistry and cloud microphysics changes
(and thus climate changes), and the Antarctic ozone hole. For a view into the "bad old
days" see Killer Smog by William Wise.
NOMENCLATURE:
Particle refers to a solid or liquid, larger than a molecule, diameter > 0.01 μm, but small
enough to remain in the atmosphere for a reasonable time, diameter < 100 μm.
Particulate is an adjective, in spite of what EPA tries to say.
Aerosol is a suspension of particles in a gas.

5. Particles, like gases, are characterized by chemical content, usually expressed in g m
-3
,
but unlike gases, particles also have a characteristic size. We may want to start discussion
the characteristics of atmospheric aerosols by addressing the question "What is the mean
diameter of the particles?" The answer to this question changes with your point of view.
A. Size Number Distribution
If your concern is the mass of some pollutant that is being transported through the air for
biogeochemical cycles, then you want to know the mean diameter of the particles with the
mass or volume. In other words, "What size particles carry the most mass?”
If your concern loss of visibility then you want to know the diameter of the particles that
have the largest cross section or surface area. In other words, "What size particles cover
the largest surface area?"
If your concern is cloud formation or microphysics then you want to know the range of
diameters with the largest number of particles. In other words, "What is the size of the
most abundant particles?"
If your concern is human health then you need to know about both the mass and number
of the particles, because only a certain size particle can enter the lungs.

6. Here we define the number distribution function, f
n
(D
p
), and the number of
particles with diameter between D
p and D
p + dD
p in a cm
3
of air as follows:

f
n
(D
p
) dD
p
(particles cm
-3
/m)

The total number of particles, N, is given by the following integral (everywhere
we integrate from 0 to infinite diameters):

N =  f
n
(D
p
) dD
p
(particles cm
-3
)

7. We can define a surface area distribution function, f
s
(D
p
), for spherical particles as
follows:

f
s
(D
p
)dD
p
= D
p
2
f
n
(D
p
) (m
2
m
-1

cm
-3
)

This gives the surface area of particles in a size range per cm
3
of air. The total surface
area of the particles, S, is given by the integral over all diameters:
S

=  f
s
(D
p
) dD
p
=  D
p
2
f
n
(D
p
) dD
p


(m
2
cm
-3
)

8. Likewise the volume distribution function and the total volume:
f
v (D
p) dD
p = {/6} D
p
3
f
n (D
p) (m
3
m
-1
cm
-3
)
V =  f
v
(D
p
) dD
p
=  /6 D
p
3
f
n
(D
p
) dD
p
(m
3
cm
-3
)
The distributions based on log D
p
can be defined in a similar manner, where
n(log D
p
)dlogD
p
is the number of particles in one cm
3
with diameter from D
p
to D
p
+

log D
p.

The total number is:
N

=
 n(log D
p
)

d(logD
p
) (particles cm
-3
)

The normalized distribution functions based on log D
p
for surface area and volume are
similar. For the differential number of particles between D
p
and D
p
+ dD
p
we use the
notation dN, and likewise dS and dV, we can represent the size distribution functions as -
n (log D
p
)

=

{dN} / {N

dlogD
p
}
n
s
(log D
p
)

=

{dS} / {S

dlogD
p
}
n
v
(log D
p
)

=

{dV} / {V

dlogD
p
}
This is the common notation for expressing the variation in particle number, surface area or
volume with the log of the diameter.

9. B. Chemical Composition
The bimodal nature of the size-number distribution of atmospheric particles suggests at
least two distinct mechanisms of formation, and the chemical composition of the particles
reflects their origins.
Fine particles have a diameter smaller than about 2.5 m, and are produced
by the condensation of vapors, accumulation, and coagulation. They have a chemical
composition that reflects the condensable trace gases in the atmosphere: SO
2
, NH
3
, HNO
3
,
VOC’s, and H
2
O. The chemical composition is water with SO
4
-2
, NO
3
-
, NH
4
+
, Pb, Cl
-
, Br
-
,
C(soot), and organic matter; where biomass burning is prevalent, K
+
.
Coarse Particles have a diameter greater than about 2.5 m, are produced by
mechanical weathering of surface materials. Their lifetimes, controlled by fallout and
washout, are generally short. The composition of particles in this size range reflects that of
the earth's surface - silicate (SiO
2
), iron and aluminum oxides, CaCO
3
and MgCO
3;
over the
oceans , NaCl.

10. ORIGIN OF THE ATMOSPHERIC AEROSOL
Soil dust
Sea salt
Aerosol: dispersed condensed matter suspended in a gas
Size range: 0.001 m (molecular cluster) to 100 m (small raindrop)
Environmental importance: health (respiration), visibility, radiative balance,
cloud formation, heterogeneous reactions, delivery of nutrients…

11. AEROSOL NUCLEATION
# molecules1 2 3 4
G
cluster sizeCritical
cluster size
Surface
tension
effect
Thermo
driving
force

12. Atmospheric AerosolsAtmospheric Aerosols

13. Aerosol DistributionsAerosol Distributions
Number
•cloud formation
Surface
•visibility
Volume
•mass
Mass & Number
•human health

14. TYPICAL U.S. AEROSOL SIZE DISTRIBUTIONS
Fresh
urban
Aged
urban
rural
remote
Warneck [1999]

15. SAMPLE AEROSOL SIZE DISTRIBUTION ( MARINE AIR)
Sea
salt
Sulfate
(natural)

16. COMPOSITION OF PM2.5 (NARSTO PM ASSESSMENT)
Dust

17. C. Optical Properties and Visibility
The optics of aerosol science follow the most rigorous physics, but traditionally defined
visibility is the distance at which a large dark object, such as a hill or a barn can just be
seen. A more quantitative definition can be obtained by considering the change in intensity
of light reflecting off an object as a function of the scattering of light by the atmosphere.

I/I

=

e
(-bX)

Where I is the intensity of light, b (or b
ext
) is the extinction coefficient with units of m
-1
, and
X is the distance in m. The limit to visibility for the human eye is a 2% change in intensity
relative to the background or:
I/I

=

0.02

18. Radiation and fine particles

19. Atmospheric VisibilityAtmospheric Visibility
(absorption & scattering)
1.Residual
2.Scattered away
3.Scattered into
4.Airlight

20. The extinction coefficient represents the sum of the extinctions from gases and
particles, each of which can in turn be divided into extinction due to absorption or
scattering.
b
ext
= b
gas
+

b
particles
b
ext
= b
abs
+

b
scatt
b
abs
(gases) = Beer's Law absorption
b
scatt
(gases) =

Rayleigh Scattering
b
abs
(particles) = Usually

< 10% of extinction
b
scatt
(particles) =

Mie Scattering

=

(b
sp
)

The ultimate limit to visibility in a very clean atmosphere is Rayleigh scattering, but
Mie scattering usually dominates. The range of b
sp
is 10
-5

m
-1
to 10
-3

m
-1
.
Aerosol optical depth (AOD),  is a measure of the extinction caused by scattering
and absorption:
I/I
0 = exp (-b
ext x)
 = b
ext
x

=  b
ext
(z) dz (unitless)

21. Single scattering albedo, , is a measure of the fraction of aerosol extinction caused
by scattering:
= b
sp
/(b
sp
+ b
ap
)
Angstrom exponent, , is a measure of the size number distribution. the name of the
exponent in the formula that describes the dependency of the aerosol optical thickness
on wavelength. Fresh urban aerosols for dust storms

22. Optical Properties of Small Particles
m = n + ik
m = complex index of refraction
n = scattering (real part)
k = absorption (imaginary part)
The real part of the index of refraction is only a weak function of
wavelength, while the imaginary part, ik, depends strongly on
wavelength.

23. Refractive indices of aerosol
particles at  = 589 nm
m = n + ik
Substance n k
Water 1.333 10
-8
Ice 1.309 10
-8
NaCl 1.544 0
H
2
SO
4
1.426 0
NH
4HSO
4 1.473 0
(NH
4)
2SO4 1.521 0
SiO
2 1.55 0
Black Carbon (soot)1.96 0.66
Mineral dust ~1.53 ~0.006

24. The scattering cross section is the product of the mass loading, and the surface area per unit
mass; note the ln of 0.02 is about -3.9, thus:
Visibility ≈

3.9(b
sp
)
-1
b
sp
= Sm

Where
b
sp
is the scattering coefficient in units of m
-1
m is the mass loading in units of g m
-3
S is the surface area per unit mass in units of m
2
g
-1
For sulfate particles, S is about 3.2 m
2
g
-1
where the humidity is less than about 70%; for other
materials it can be greater.
Visibility =

3.9/(3.2 m)
=

1.2 /(m)

25. Example: Visibility improvement during the 2003 North
American Blackout
Normal conditions over Eastern US during an air pollution episode:
b
sp ≈ 120 Mm
-1
= 1.2 x 10
-4
m
-1
at 550 nm
b
ap = 0.8 x 10
-5
m
-1

b
ext = 1.28 x 10
-4
m
-1
Visual Range ≈ 3.9/b
ext
= 30 km
During blackout
b
sp = 40 Mm
-1
= 0.4 x 10
-4
m
-1
b
ap = 1.2 x 10
-5
m
-1
b
ext
= 0.52 x 10
-4
m
-1
Visual Range = 3.9/b
ext = 75 km

26. Example: Visibility improvement during the 2003 North
American Blackout
Single scattering albedo, , normal = 1.20/1.28 = 0.94
Blackout = 0.4/0.52 = 0.77
With the sulfate from power plants missing, and the soot
from diesel engines remaining the visual range is up, but
the single scattering albedo is down. Ozone production
inhibited.
See: Marufu et al., Geophys Res. Lett., 2004.

27. Extinction Coefficient as a PM2.5 Surrogate
PM
2.5
= 7.6 g/m
3
PM
2.5 = 21.7 g/m
3
PM
2.5 = 65.3 g/m
3
Glacier National Park images are adapted from Malm, An Introduction to
Visibility (1999) http://webcam.srs.fs.fed.us/intropdf.htm

28. ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000
NARSTO PM Assessment, 2003
PM10 (particles < 10 m) PM2.5 (particles < 2.5 m)
Red circles indicate violations of national air quality standard:
50 g m
-3
for PM10 15 g m
-3
for PM2.5

29. Aircraft obs. →
AOD ~ 0.1
←Model calcs.
NO
2 + hυ (+O
2) → NO + O
3

30. AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean

31. AEROSOL OBSERVATIONS FROM SPACE

  
                                                                                                  
 
                    
Biomass fire haze in central America (4/30/03)
Fire locations
in red
Modis.gsfc.nasa.gov

32. BLACK CARBON EMISSIONS
Chin et al. [2000]
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING

33. RADIATIVE FORCING OF CLIMATE, 1750-PRESENT
“Kyoto also failed to address two major pollutants that have an impact on
warming:  black soot and tropospheric ozone.  Both are proven health
hazards.  Reducing both would not only address climate change, but also
dramatically improve people's health.” (George W. Bush, June 11 2001 Rose
Garden speech)
IPCC [2001]

34. Climate radiative forcing – revised 2002.

35. What does the UN Inter-government Panel on Climate
Change say?

36. ASIAN DUST INFLUENCE IN UNITED STATES
Dust observations from U.S. IMPROVE network
April 16, 2001
Asian dust in western U.S.
April 22, 2001
Asian dust in southeastern U.S.
Glen
Canyon,
AZ
Clear day
April 16, 2001: Asian dust!
0 2 4 6 8
g m
-3

37. LONGITUDE
A
L
T
I
T
U
D
E

(
k
m
)
100E 150E 150W 100W
TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES
Subsidence
over western U.S.
Source region
(inner Asia)
Asian plumes
over Pacific
GEOS-CHEM Longitude cross-section at 40N, 16 April, 2001
0
5
10
ASIA UNITED STATES
T.D. Fairlie, Harvard

39. Trends NE China versus NE US
50
100
150
200
250
300
1996199719981999200020012002200320042005
E
l
e
c
t
r
i
c
i
t
y

G
e
n
e
r
a
t
i
o
n

o
r

S
O
2

E
m
i
s
s
i
o
n
s

0.2
0.4
0.6
0.8
1
1.2
1.4
S
O
2

C
o
l
u
m
n

C
o
n
t
e
n
t

(
N
o
r
m
a
l
i
z
e
d
)
China Elect. Generation USEPA Emissions
Satellite China Satellite USA

40. [email protected]
40
GOME & SCIA NO
2
: US Power Plant NOx reductions
•After 2000, clear
decrease (> 30%) in
NO
2 in Ohio River
Valley area. (SIP-Call)
•Little change over
East Coast
1996 2000
2005

41. NO
x Emissions Trends and Satellite NO
2 Data, 1996 - 2005
R
2
= 0.84
2.00
2.50
3.00
3.50
4.00
4.50
5.00
0 100000 200000 300000 400000 500000 600000
Power Plant Emissions in Ohio River Valley (tons/season)
G
O
M
E
/
S
C
I
A

c
o
l
u
m
n

N
O
2

42. Point
Source
Emissions
1999
Frost et
al.,
JGR,
2006

43. Emissions rates (CEMS) for the
third quarter (July to September)
in 1999 (open squares) and 2003
(solid circles). Average emission
rates at a particular plant are the
ratio of total quarterly mass
emissions (in pounds) divided by
total quarterly heat input (in
million British Thermal Units).
Frost et al. JGR, 2006.

44. Aerosols in the Atmosphere
Abundance and size
•Aerosol concentration is highly variable in space and time. Concentrations
are usually highest near the ground and near sources.
•A concentration of 10
5
cm
-3
is typical of polluted air near the ground, but
values may range from 2 orders of magnitude higher in very polluted
regions to several lower in very clean air.
•Radii range from ~ 10
-7
cm for the for small ions to more than 10 µm (10
-3
cm) for the largest salt and dust particles.
•Small ions play almost no role in atmospheric condensation because of
the very high supersaturations required for condensation.
•The largest particles, however, are only able to remain airborne for a
limited time

45. Aerosol Size Naming Convention
Usually divided into three size groups ( D - diameter)
1.Aitken Nuclei  2 x 10
-3
µm < D 2 µm
2.Large Nuclei 2

µm < D 2.5 µm
(also called the accumulation mode)
3. Giant Nuclei D > 2.5 µm

46. Other Naming Convention
•Nucleation mode  D ≤ 0.1 µm
•Accumulation or coagulation mode  0.1 µm < D ~ 1 µm
Thought to be most important in natural cloud formation
•Coarse Particle Mode  D ~ 1 µm

47. Origins of Atmospheric Aerosols
1.Condensation and sublimation of of vapors and the formation of
smokes in natural and man-made combustion.
2.Reactions between trace gases in the atmosphere through the
action of heat, radiation, or humidity.
3.The mechanical disruption and dispersal of matter at the earth’s
surface, either as sea spray over the oceans, or as mineral dusts
over the continents.
4.Coagulation of nuclei which tends to produce larger particles of
mixed constitution

48. Aerosol Makeup
•Typical substances formed in large quantities by condensation
following combustion include ashes, soot, tar products, oils as
well as sulfuric acid and sulfates. These particles are primarily
within the range of Aitken nuclei.
•Mechanical disintegration, by wind and water, of rocks and soil
produces particles with diameters > 0.2 µm. These fall primarily
in the large nuclei range.
•According to Jaenicke (Science, 308 p. 73, 2005) about 25% of
the number of particles with diameter greater than 0.2 µm are
biogenic. (remains to be verified).

49. Aerosol Makeup - continued
•Chemical reactions between nitrogen, oxygen, water vapor and
various trace gases (e.g., sulfur dioxide, chlorine, ammonia, ozone,
and oxides of nitrogen) primarily produce particles in the Aitken
and Large range.
Examples
Formation of ammonium chloride from NH
3 and HCl
Oxidation of SO
2 to H
2SO
4
Reaction of sulfur dioxide, ammonia, and water to produce
ammonium sulfate particles.
Production of higher oxides of nitrogen through the action of
heat, ozone or ultraviolet radiation

50. Cloud Condensation Nuclei - CCN
•Comprises a small fraction of the total aerosol population
•Sea salt is the predominant constituent of CCN with D > 1µm
•For 0.1 µm < D < 1 µm, the main component is thought to be sulfate,
which may be present as sulfuric acid, ammonium sulfate, or from
phytoplankton produced dimethylsulfide (see Charlson et al.,
Nature, 326, 655-661).

51. Activity Spectrum
Let N
c
be the number of particles per unit volume that are
activated to become cloud droplets.
Data from cloud chamber measurements are often parameterized
as
N
c
= C (S-1)
k
where C and k are parameters that depend on air mass type.
Rogers gives:
Maritime air: 30 < C < 300 cm
-3
; 0.3 < k < 1
Continental air:300 < C < 3000 cm
-3
; 0.2 < k < 2
Thus, for the same saturation ratio, one would expect to find small
numbers of CCN per unit volume in maritime air and large
numbers per unit volume in continental air.

52. Long term trend in PM concentrations in Mid
Atlantic Region from the IMPROVE and
CASTNET samplers

53. OC trend – 0.05 g m
-3
yr
-1
OM trend ~ – 0.08 g m
-3
yr
-1

54. 1988-2004 EC trend -0.05 g m
-3
yr
-1

55. 1988-2006 EC trend -0.04 g m
-3
yr
-1

56. DRI doesn’t trust the data from 2000-2004.

57. Washington, DC Nitrate
IMPROVE every 3
rd
day observations (1988-2006)
linear trend and 30-point running mean.
0
1
2
3
4
5
6
7
8
9
10
1
9
8
8
1
9
8
9
1
9
9
0
1
9
9
1
1
9
9
2
1
9
9
3
1
9
9
4
1
9
9
5
1
9
9
6
1
9
9
7
1
9
9
8
1
9
9
9
2
0
0
0
2
0
0
1
2
0
0
2
2
0
0
3
2
0
0
4
2
0
0
5
2
0
0
6
N
it
r
a
t
e
(
u
g
/m
3
)
1988-2006 NO
3
-
trend -0.02 g m
-3
yr
-1

58. Washington, DC IMPROVE 1989-2006 Sulfate
0
5
10
15
20
25
30
35
J a
n
- 8
9
J u
l - 8
9
J a
n
- 9
0
J u
l - 9
0
J a
n
- 9
1
J u
l - 9
1
J a
n
- 9
2
J u
l - 9
2
J a
n
- 9
3
J u
l - 9
3
J a
n
- 9
4
J u
l - 9
4
J a
n
- 9
5
J u
l - 9
5
J a
n
- 9
6
J u
l - 9
6
J a
n
- 9
7
J u
l - 9
7
J a
n
- 9
8
J u
l - 9
8
J a
n
- 9
9
J u
l - 9
9
J a
n
- 0
0
J u
l - 0
0
J a
n
- 0
1
J u
l - 0
1
J a
n
- 0
2
J u
l - 0
2
J a
n
- 0
3
J u
l - 0
3
J a
n
- 0
4
J u
l - 0
4
J a
n
- 0
5
J u
l - 0
5
J a
n
- 0
6
J u
l - 0
6
S
u
l
f
a
t
e
(
u
g
/
m
3
)
Trend – 0.094 g m
-3
yr
-1

59. 1989-2006 Dust – 0.022 g m
-3
yr
-1

60. 1989-2006 PM2.5 – 0.22 g m
-3
yr
-1

61. Seasonal cycle in PM concentrations in Mid
Atlantic Region from the IMPROVE and
CASTNET samplers