1. Lecture 16 Atmospheric Aerosols
ATS 621 Fall 2012
Lecture 16 Atmospheric Aerosols

2. National Ambient Air Quality Standards

3. PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 2008
Our National Air, EPA Report, 2008
PM10 (particles < 10 mm)
PM2.5 (particles < 2.5 mm)
Yellow and red sites are in violation of national air quality standard:
150 mg m-3 for daily PM mg m-3 for annual PM2.5
Modest decline in PM2.5 over last decade (< 20%)

4. U.S. population density

5. These are PRIMARY emissions only

6. ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol: dispersed condensed phases suspended in a gas
Size range: mm (molecular cluster) to 100 mm (small raindrop)
Soil dust
Sea salt
Environmental importance: health (respiration), visibility, climate,
cloud formation, heterogeneous reactions, long-range transport of nutrients…

7. Particle size distribution
Coarse particles
Produced by mechanical processes
Size range 2 to 100 µm
Fine particles
Produced by high energy processes
Have a very high surface area
Have a tendency to grow in size
Accumulation mode
Typically in the μm size range
Produced from agglomeration of nuclei mode particles, growth by condensation of smaller particles, and direct emissions
Nuclei mode
Particles smaller than 0.08 µm
Produced from nucleation/condensation of molecules
Grow in size by agglomeration / condensation

8. Multi-modal particle size distribution
Concept due to Whitby (c. 1975)

9. Range of particle sizes Particles = stable clusters of 100’s to many, many molecules
Size determines
Atmospheric lifetime
Effectiveness of light scattering
Deposition in human lung

10. Particle size Expressed as aerodynamic equivalent diameter
Reference to a spherical particle of uniform density that falls at its terminal velocity
Small particles (fine; < 1 µm aerodynamic diameter)
Behave approximately as gases: Subject to Brownian motion; Follow fluid streamlines; Capable of coagulation with each other or larger particles; Settle out slowly (usually negligible loss process)
Larger particles (coarse; > 1 µm aerodynamic diameter)
Strongly affected by gravity; Settle out more rapidly (usually an important loss process)
Relative to numbers present,
Most particles very small (< 0.1 µm)
Relative to particle mass/volume,
Most volume/mass is associated with particles > 0.1 µm

11. Particle residence times in the atmosphere
Williams et al., ACPD, 2002.

12. Inhalability and Particle Size
Total suspended particles (TSP)
PM10 – thoracic particles
PM2.5 – respirable particles

13. Particle Size Affects Deposition Efficiency and Location

14. AEROSOL IMPACTS ON VISIBILITY
Visibility degradation by aerosols at Glacier National Park, Montana
7.6 µgm-3
12.0 µgm-3
21.7 µgm-3
65.3 µgm-3
(previous) U.S. air quality standard
Glacier
National
Park
Natural aerosol concentrations are typically less than 2 mg m-3

15. HOW COMPOSITION AND SIZE FIT TOGETHER…
Image from: C. Leck

16. Airborne Particulate Matter (PM)
Smoke
Primary Soot
Bacteria
Pollen
Fugitive Dust
Mass Frequency
(slide courtesy J. Volckens)

17. FINE PARTICLE GROWTH AT BLODGETT FOREST
“Banana Plot”
Nucleated particles are…organics? + sulfuric acid? +
ammonia or amines?
[Lunden et al., 2006]

18. Common Particulate-Phase Species
Nitrate (NO3-)
Ammonium (NH4+)
Sulfate (SO4 2-)
Sea salt (Na+Cl-)
“Dusts”
Metals
Black carbon (“soot”)
many organic species
If low vapor pressures, partition to condensed phase (semivolatile)
Volatilization / recondensation probably happening for organic species, perhaps for metals such as Hg and Pb
WATER (also semivolatile)
PM is BOTH primary AND secondary

19. FINE AEROSOL COMPOSITION IN NORTH AMERICA
Annual mean PM2.5 concentrations (NARSTO, 2004)

21. Front Range “average” PM2.5 aerosol

22. ANNUAL MEAN PM2.5 CONCENTRATIONS (2002) derived from MODIS satellite instrument data
SURFACE
AEROSOL
0.47 mm
0.65 mm
2.13 mm

23. DUST: MOST IMPORTANT(?) NATURALLY EMITTED AEROSOL
Sources: arid / semi-arid regions
Emission in both fine and coarse mode, depends on surface properties and wind speed.
Resulting lifetime ~weeks
Dust Emissions (2001)
g m-2 y-1
[Fairlie et al. 2007]
[Husar et al., 2002]

24. DESERT DUST CAN BE TRANSPORTED ON INTERCONTINENTAL SCALES
April 16, 2001: Asian dust!
clear day
Glen
Canyon,
Arizona
Annual mean PM2.5 dust (mg m-3), 2001
Asia
Sahara
Most fine dust in the U.S. (except in southwest) is of intercontinental origin

25. MEAN SEA SALT AEROSOL CONCENTRATIONS
Lower marine boundary layer (0-100 m)
[Alexander et al. 2005]

26. CARBONACEOUS AEROSOL SOURCES
ORGANIC CARBON (OC)
ELEMENTAL CARBON (EC)
GLOBAL
22 Tg yr-1
130 Tg yr-1
= BSOA
UNITED
STATES
0.66 Tg yr-1
2.7 Tg yr-1

27. BLACK CARBON EMISSIONS
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING

28. WILDFIRES: A GROWING AEROSOL SOURCE
S. California fire plumes,
Oct
Total carbonaceous (TC) aerosol
averaged over U.S. IMPROVE sites
Interannual variability is driven
by wildfires

29. SECONDARY ORGANIC AEROSOL PRODUCTION FROM BIOGENIC VOC EMISSIONS
Nucleation
(oxidation products)
Oxidation Reactions
(OH, O3,NO3)
Growth
Condensation on pre-existing aerosol
Over 500 reactions to describe the formation of SOA precursors, ozone, and other photochemical pollutants [Griffin et al., 2002; Griffin et al., 2005; Chen and Griffin, 2005]

30. BIOGENIC HYDROCARBONS
Isoprene (C5H8)
Monoterpenes(C10H16)
Sesquiterpenes (C15H24)
Anthropogenic SOA-precursors = aromatics (emissions are 10x smaller)
"Trees cause more pollution than automobiles do.“
(when talking about ozone in 1981)

31. PRIMARY BIOLOGICAL AEROSOL PARTICLES (PBAP)
BACTERIA
VIRUSES
POLLEN
FUNGUS
PLANT
DEBRIS
ALGAE
Very large and likely short-lived
These particles have not traditionally been considered part of the OA budget, but this has been revised in recent years.
Not much is known about emissions, processing, climate effects.

32. GLOBAL SULFUR BUDGET [Chin et al., 1996] (flux terms in Tg S yr-1)
SO42-
t = 3.9d
SO2
t = 1.3d
cloud 42 OH
H2SO4(g) 4 18 8
NO3 OH
(CH3)2S 64
DMS
t = 1.0d 10
dep
27 dry
20 wet
dep
6 dry
44 wet 22
Phytoplankton
Volcanoes
Combustion
Smelters

33. GLOBAL SULFUR EMISSION TO THE ATMOSPHERE
2001 estimates (Tg S yr-1):
Industrial 57 Volcanoes 5
Ocean Biomass burning 1
[Chin et al. 2000]

34. FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS
Sulfate always forms an aqueous aerosol
Ammonia dissolves in the sulfate aerosol totally or until titration of acidity, whichever happens first
Nitrate is taken up by aerosol if (and only if)
excess NH3 is available after sulfate titration
HNO3 and excess NH3
can also form a solid aerosol
if RH is low
Thermodynamic rules:
Highest concentrations in industrial Midwest
(coal-fired power plants)
Observed
aerosol
acidity in US

35. GLOBAL EMISSIONS OF AMMONIA
[Bouwman et al., 1997]
GLOBAL
UNITED STATES 55
2.8
Ammonia,
Tg N yr-1

36. SULFATE-NITRATE-AMMONIUM AEROSOLS IN U.S. (2001)
Highest concentrations in industrial Midwest
(coal-fired power plants)

37. STRATOSPHERIC AEROSOL
PSCs (nitric acid / water vapor)
Injection of volcanic ash
(SiO2, Al2O3, Fe2O3) as well as gases
(H2S, SO2, HCl)
TROPOPAUSE
Transport of long-lived S gases (eg. COS)
Aerosols in the stratosphere are long-lived due to absence of precipitation and “layered” transport (due to stability)

38. SURFACE AEROSOL NUMBER CONCENTRATION
GLOMAP: 2 moment sectional model simulating sulfuric acid / sea salt
Dec
July
Continental: > 250 cm-3
Urban/polluted: > 2000 cm-3
Marine BL: ~ 200 cm-3
[Spracklen et al., 2006]

39. Particle size distributions
We use frequency functions to describe the particle size distribution
Number of particles per unit volume, per interval of the x variable (usually x=diameter):
The AREA under the curve has meaning (total # concentration of particles between the two diameters)
The value of the function itself at a particular point has little physical meaning
we often choose x=log(diameter)
We can convert the number distribution readily into surface area or volume distributions by multiplying by area or volume per particle

40. Normalize N in each bin by width of bin
Example “data”
Normalize N in each bin by width of bin

41. TYPICAL AEROSOL SIZE DISTRIBUTION
ultrafine
fine
coarse
accumulation
PM PM10
N=number concentration (particles/cm3)

42. Mean diameters

43. The lognormal distribution
N is the total number concentration of particles.

44. The cumulative distribution

46. Aerosol water contents (Kreidenweis et al., ERL, 2008)
Relative Humidity

47. RAOULT’S LAW water saturation vapor pressure
over pure liquid water surface
water saturation vapor pressure
over aqueous solution of water
mixing ratio xH2O
An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio

48. UPTAKE OF WATER BY AEROSOLS

49. RELATIVE HUMIDITIES FOR DELIQUESCENCE/CRYSTALLIZATION OF AEROSOLS