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Lecture 16 Atmospheric Aerosols

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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)

20

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

45

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

50


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