1. LECTURE 13 Atmospheric Aerosols
AOSC 434
AIR POLLUTION
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, , 2005.

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

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 mg 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. fn(Dp) dDp (particles cm-3/mm)
Here we define the number distribution function, fn (Dp), and the number of particles with diameter between Dp and Dp + dDp in a cm3 of air as follows:
fn(Dp) dDp (particles cm-3/mm)
The total number of particles, N, is given by the following integral (everywhere we integrate from 0 to infinite diameters):
N =  fn(Dp) dDp (particles cm-3 )

7. S =  fs(Dp) dDp = p Dp2 fn (Dp) dDp (mm2 cm-3)
We can define a surface area distribution function, fs(Dp), for spherical particles as follows:
fs(Dp)dDp = pDp2 fn(Dp ) (mm2 mm-1 cm-3 )
This gives the surface area of particles in a size range per cm3 of air. The total surface area of the particles, S, is given by the integral over all diameters:
S =  fs(Dp) dDp = p Dp2 fn (Dp) dDp (mm2 cm-3)

8. fv (Dp) dDp = {p/6} Dp3 fn (Dp) (mm3 mm-1 cm-3 )
Likewise the volume distribution function and the total volume:
fv (Dp) dDp = {p/6} Dp3 fn (Dp) (mm3 mm-1 cm-3 )
V =  fv(Dp) dDp =  p/6 Dp3 fn(Dp) dDp (mm3 cm-3)
The distributions based on log Dp can be defined in a similar manner, where
n(log Dp)dlogDp is the number of particles in one cm3 with diameter from Dp to Dp + log Dp. The total number is:
N =  n(log Dp) d(logDp) (particles cm-3 )
The normalized distribution functions based on log Dp for surface area and volume are similar. For the differential number of particles between Dp and Dp + dDp we use the notation dN, and likewise dS and dV, we can represent the size distribution functions as -
n (log Dp) = {dN} / {N dlogDp }
ns (log Dp) = {dS} / {S dlogDp }
nv (log Dp) = {dV} / {V dlogDp }
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 mm, 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: SO2, NH3, HNO3, VOC’s, and H2O. The chemical composition is water with SO4-2, NO3-, 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 mm, 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 (SiO2), iron and aluminum oxides, CaCO3 and MgCO3; over the oceans , NaCl.

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

11. AEROSOL NUCLEATION # molecules 1 2 3 4 DG Surface tension effect DG
Surface tension effect
Critical cluster size
cluster size
Thermo driving force

12. Atmospheric Aerosols

13. Aerosol 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. From USEPA (Neil Frank)

19. DI/I = e(-bDX) C. Optical Properties and Visibility
The optics of aerosol science follow the most rigorous physics. 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.
DI/I = e(-bDX)
Where I is the intensity of light, b (or bext) 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:
DI/I = 0.02

20. Radiation and fine particles

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

22. bext = bgas + bparticles bext = babs + bscatt
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.
bext = bgas + bparticles
bext = babs + bscatt
babs (gases) = Beer's Law absorption
bscatt (gases) = Rayleigh Scattering
babs (particles) = Usually < 10% of extinction
bscatt (particles) = Mie Scattering = (bsp)
The ultimate limit to visibility in a very clean atmosphere is Rayleigh scattering, but Mie scattering usually dominates. The range of bsp is 10-5 m -1 to 10-3 m-1.
Single scattering albedo, w, is a measure of the fraction of aerosol extinction caused by scattering:
w = bsp/(bsp + bap)

23. 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.

24. Refractive indices of aerosol particles at  = 589 nm
m = n + ik
Substance n k
Water
1.333
10-8
Ice
1.309
NaCl
1.544
H2SO4
1.426
NH4HSO4
1.473
(NH4)2SO4
1.521
SiO2
1.55
Black Carbon (soot)
1.96
0.66
Mineral dust
~1.53
~0.006

25. 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(bsp)-1
bsp = Sm
Where
bsp 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 m2g-1
For sulfate particles, S is about 3.2 m2 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)

26. Example: Visibility improvement during the 2003 North American Blackout
Normal conditions over Eastern US during an air pollution episode:
bsp ≈ 120 Mm-1 = 1.2 x 10-4 m-1 at 550 nm
bap = 0.8 x 10-5 m-1
bext = x 10-4 m-1
Visual Range ≈ 3.9/bext = 30 km
During blackout
bsp = 40 Mm-1 = 4.0 x 10-5 m-1
bap = 1.2 x 10-5 m-1
bext = x 10-4 m-1
Visual Range = 3.9/bext = 75 km

27. Example: Visibility improvement during the 2003 North American Blackout
Single scattering albedo, w, 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.

28. Extinction Coefficient as a PM2.5 Surrogate
PM2.5 = 7.6 g/m3
PM2.5 = 21.7 g/m3
PM2.5 = 65.3 g/m3
Glacier National Park images are adapted from Malm, An Introduction to Visibility (1999)

29. ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U. S
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, NARSTO PM Assessment, 2003
PM10 (particles > 10 mm)
<
PM2.5 (particles > 2.5 mm)
<
Red circles indicate violations of national air quality standard:
50 mg m-3 for PM mg m-3 for PM2.5

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
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING
Chin et al. [2000]

33. The understanding of different influences on climate have markedly improved leading to very high confidence that that the globally averaged net of effect of human activities since 1750 has been one of warming.
The combined radiative forcing due to increases in carbon dioxide, methane and nitrous oxide is W m-2 and is rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years, with the carbon dioxide radiative forcing increased by 20% from 95-05, the largest increase in the last 200 yrs.
Aerosols produce a cooling effect. Aerosols also can influence cloud lifetime and precipitation.
Additional anthropogenic contributions to radiative forcing come from other sources as well, including Tropospheric ozone changes due to ozone-forming chemicals

36. Asian dust in western U.S. Asian dust in southeastern U.S.
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.
mg m-3
Glen
Canyon, AZ
April 16, 2001: Asian dust!
Clear day

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

39. The future is hard to predict.

42. What really happened? National Emission Trends
Generally, gradual decrease for all major air pollutants
Method change for monitoring VOCs caused a bump around 2002 (counting more species)
Courtesy of Dr. Hao He

43. Major NOx controls
Implemented after 2000

44. NASA Aura OMI Shows Air Quality is Improving
• OMI nitrogen dioxide data indicate a 30-40% decrease in the pollutant’s levels from 2005 to 2011.
• NO2 levels have dropped through the implementation of emission control devices on coal-burning power plants and more fuel-efficient cars.
• NASA AQAST members are working with state air quality agencies to demonstrate the effectiveness of their efforts to improve air quality and to find novel uses of satellite data for air quality applications.
US NOx emissions have decreased by about 50% since 2000 through a combination of controls on power plants and automobiles. For instance, see for a description of the impact of emission control device on
The Ozone Monitoring Instrument (OMI) is on the Aura satellite ( which just celebrated 10 years since launch.
The OMI NO2 data shown in the images were produced by the NASA GSFC OMI Science Team (614). The retrieval algorithm was developed by Lok Lamsal (GSFC; 614) and the data processed by Yasuko Yoshida (GSFC; 614), both of whom were partially funded by the NASA Air Quality Applied Sciences Team (AQAST) via Bryan Duncan (AQAST member). The images were created by the NASA Goddard's Scientific Visualization Studio/T. Schindler.
OMI data also show that sulfur dioxide (SO2), a precursor to acid rain and sulfate aerosols, has decreased significantly in the US over the Aura record:

45. The bad news: PM2.5 only fell modestly.
SO2
PM2.5
Why? Sulfate was dominant and has a longer lifetime than SO2.
SOA becoming dominant; it also has a lifetime of
~10 d.

46. 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 105 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

47. Aerosol Size Naming Convention
Usually divided into three size groups ( D - diameter)
1. Aitken Nuclei ® 2 x 10-3 µm < D £ 0.2 µm
2. Large Nuclei ® 0.2 µm < D £ 2.5 µm
(also called the accumulation mode)
3. Giant Nuclei ® D > 2.5 µm

48. 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

49. Origins of Atmospheric Aerosols
Condensation and sublimation of of vapors and the formation of smokes in natural and man-made combustion.
Reactions between trace gases in the atmosphere through the action of heat, radiation, or humidity.
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

50. 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).

51. 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 NH3 and HCl
Oxidation of SO2 to H2SO4
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

52. 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 dimethyl sulfide (see Charlson et al., Nature, 326, ).

53. Activity Spectrum
Let Nc be the number of particles per unit volume that are activated to become cloud droplets.
Data from cloud chamber measurements are often parameterized as
Nc = C (S-1)k
where C and k are parameters that depend on air mass type.
Rogers gives:
Maritime air: < C < 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.

54. How aerosols affect the radiative properties of clouds
How aerosols affect the radiative properties of clouds.By nucleating a larger number of smaller cloud drops, aerosols affect cloud radiative forcing in various ways.
How aerosols affect the radiative properties of clouds.By nucleating a larger number of smaller cloud drops, aerosols affect cloud radiative forcing in various ways. (A) Buffering in nonprecipitating clouds. The smaller drops evaporate faster and cause more mixing of ambient air into the cloud top, which further enhances evaporation. (B) Strong cooling. Pristine cloud cover breaks up by losing water to rain that further cleanses the air in a positive feedback loop. Aerosols suppressing precipitation prevent the breakup. (C) Larger and longer-lasting cirrus clouds. By delaying precipitation, aerosols can invigorate deep convective clouds and cause colder cloud tops that emit less thermal radiation. The smaller ice particles induced by the pollution aerosols precipitate more slowly from the anvils. This can cause larger and longer-lasting cirrus clouds, with opposite effects in the thermal and solar radiation. The net effect depends on the relative magnitudes.
D Rosenfeld et al. Science 2014;343:
Published by AAAS

55. Summary Aerosols are the most visible of air pollution classes.
The PM2.5 (annual average) standard to protect health is 12 mg/m3.
Number, surface and volume (mass) all play a role.
Log-normal distributions.
Single scattering albedo w = bsp/(bsp + bap)
Composition reflects sources and sinks.
Trends have been downward over North America.