1. Lecture 17 Atmospheric Aerosols
ATS 621 Fall 2012
Lecture 17 Atmospheric Aerosols

3. All PM2.5, 2011 average ammonium sulfate organic carbon
ammonium nitrate
soil (“dust”)

4. 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
 Ammonium nitrate formation is favored at LOW TEMPERATURE, HIGH RH
Thermodynamic rules:
Highest concentrations in industrial Midwest
(coal-fired power plants)
Observed
aerosol
acidity in US

5. For current monitor readings:
Front Range “average” PM2.5 aerosol
ammonium nitrate = ammonium sulfate
For current monitor readings:

6. 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]

7. 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

8. March May July September PM2.5 soil concentrations, 2011
Big year for wildfires in New Mexico….

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

10. 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

11. BLACK CARBON EMISSIONS
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING

12. 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]

13. 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)

14. 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

15. Radiocarbon (14C) is present at a small but approximately constant level in living (or contemporary) materials but is absent in fossil fuels, which are much older than the radiocarbon half-life of 5730 years.
Schichtel, B. A., W. C. Malm, G. Bench, S. Fallon, C. E. McDade, J. C. Chow, and J. G. Watson (2008), Fossil and
contemporary fine particulate carbon fractions at 12 rural and urban sites in the United States, J. Geophys. Res., 113, D02311,
doi: /2007JD

16. 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.

17. 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

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

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

20. 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)

21. 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]

22. 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

23. Normalize N in each bin by width of bin
Example “data”
Normalize N in each bin by width of bin
Can use dDp or dlnDp as the bin width variable

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

25. Mean diameters Mean diameter of the NUMBER distribution
Mean diameter of the SURFACE AREA distribution
Mean diameter of the VOLUME (~MASS) distribution

26. The lognormal distribution
N is the total number concentration of particles.
Typical values of sg for atmospheric aerosols:

28. Volume distribution is most closely related to light scattering

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

30. UPTAKE OF WATER BY AEROSOLS

32. PM Welfare Effects: Visibility and Haze
Visual Range: 250 km
Visual Range: 25 km

33. PM10 Levels and Visibility in Beijing
WHO Guideline: 50 ug/m3
averaged over 24 hrs
8 ug/m3
12 July
26 ug/m3 15 July
32 ug/m3
20 July
104 ug/m3
5 August
191 ug/m3
7 August
278 ug/m3
10 August
(slide courtesy J. Volckens)

34. Visibility Human ability to see is a function of
Characteristics of object observed
Amount and distribution of light
Concentration of suspended particles and gases
Particles & gases scatter & absorb light causing
Appearance of haze
Decrease in contrasts
Change in perceived color (absorption / scatter of certain λ’s)
Scattering usually has a greater effect on visibility than absorption does
2 & 3 affect contrast between 1 & 4

35. Rayleigh scattering (isentropic) applies to gases, very small particles

36. Haze particle

37. Contrast that can be discerned by typical viewer
Koschmieder equation I I0
Assumes:
Homogeneous atmosphere
Uniform sky brightness
Viewing horizontally
Neglected curvature of Earth
Contrast that can be discerned by typical viewer
This equation relates visual range to the total extinction coefficient

38. Components of the extinction coefficient, σext
σext = σRayleigh + σabs,gas + σabs,part + σscat,part
Visible light: 0.4 μm < λ < 0.7 μm
Consider components:
σRayleigh
Due to gases (small particles also scatter this way); scattering α λ-4
-> shorter wavelengths scatter more (blue sky in clean conditions; red sky at sunset)
At sea level and 0.45 μm , σRayleigh ~ 0.03 km-1
-> Lv = 130 km (81 miles)
σabs,gas
NO2 is the only important absorber – absorbs most strongly at blue λs
-> colors plumes yellow, red or brown
σabs,part
Absorption = f(composition, size distribution)
-> soot is most important (5-40% of total visibility reduction)
-> overall soot is ~3x as efficient as sulfate, nitrate or organic aerosol species in terms of visibility reduction per unit mass of airborne particles
-> new data suggest organic species contribute to light-absorbing carbon (“brown carbon”) – prevalent in wood smoke

39. Scattering by particles
σscat,part
Particle scattering is ~60 – 95% of visibility reduction
Scattering importance (generally): sulfate > organic carbon > nitrate
- Particle WATER CONTENT is very important!
Most complicated to compute
Very small particles act like gases
Very large particles (Dp >> λ) extinguish light according to 2x their cross-sectional area (the “extinction paradox” – light scattered at very small angles to the beam is still seen when close to the object, but not when at a distance)
Intermediate-sized particles (“Mie scattering”) have the most effective light scattering mechanism
Extinction cross section can be up to ~4 – 5 times the cross sectional-area
0.1 – 1 μm are of similar λ to visible light: the “accumulation mode”

40. Scattering efficiency of a spherical particle
m=1.5
m= i
Diameter in microns

41. Scattering by particles
The scattering coefficient due to N particles, each of radius r, is
Can compute this for each ‘section’ of a size distribution to get the overall sscat
Similar for absorption by particles and total extinction due to particles
In practice, we often use “mass scattering efficiencies” and “mass absorption efficiencies” to compute overall extinction from measured mass concentrations

42. Effect of refractive index
Mass scattering efficiency

43. Effects of relative humidity on visibility
More aerosol mass is present (in the form of condensed water in the aerosol phase)  visibility is reduced
The SIZES of the particles shift  generally to a more efficient scattering diameter, but can also grow OUT of the most effective scattering range if already very large
The refractive index DECREASES  this decreases the extinction coefficient, but effect is much smaller than the other effects

45. Estimated Annual Visual Range http://vista. cira. colostate
ATS 555 F10 Lecture 3