VIII. Aerosols Size distribution Formation and Processing Composition Aerosol phase chemistry.
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Transcript of VIII. Aerosols Size distribution Formation and Processing Composition Aerosol phase chemistry.
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VIII. Aerosols
Size distributionFormation and ProcessingCompositionAerosol phase chemistry
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Importance of aerosols
• human health
air quality, airborne pathogen transport
• climate change
direct/indirect effects
aerosol optical properties, aerosol/cloud
interactions
• geochemical cycles
metals, nutrients, organics
• acidification (sulfur, nitrogen)
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Terminology
• Aerosol – a dispersion of solid and liquid particles suspended in gas (air).
note: “aerosol” is defined as the dispersion of both particles and gas, but in common practice it is used to refer to the particles only!
• Primary aerosol – atmospheric particles that are emitted or injected directly into the atmosphere.
• Secondary aerosol – atmospheric particles that are created by in situ aggregation or nucleation from gas phase molecules (gas to particle conversion).
Either type may be natural or anthropogenic or both
How much aerosol is there? typically ~10’s of ug/m3 (air density ~1kg/m3)
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Global Particle Production (Table 2.19 from Seinfeld and Pandis)
Source Estimate Flux (Tg/yr) Particle Size CategoryPrimary
Soil dust (mineral aerosol) 1000-3000 Mainly coarseSea salt 1000-10000 Coarse
Volcanic dust 2-10000 CoarseBiological debris 26-80 Coarse
SecondarySulfates from biogenic gases 80-150 Fine
Sulfates from volcanic SO2 5-60 FineOrganic matter from biogenic VOC 40-200 Fine
Nitrates from NOx 15-50 Fine and coarseTotal Natural 2200-23500 Best estimate 3100AnthropogenicPrimary
Industrial dust etc. (except soot) 40-130 Fine and coarseSoot 5-20 Mainly fine
SecondarySulfates from SO2 170-250 Fine
Biomass burning 60-150 FineNitrates from NOx 25-65 Mainly coarse
Organics from anthropogenic VOC 5-25 FineTotal anthropogenic 300-650 Best estimate 450Total 2500-24000 Best estimate 3600
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Aerosol Size Distributions
Number distribution nn(Dp)=dN/dDp
Surface area distribution ns(Dp)= dS/dDp
S=Dp2
Volume distribution nv(Dp)=dV/dDp
V=(/6)*Dp3
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Log-normal distributions
Aitken mode
Accumulation mode
Coarse mode
Number distribution nn(log Dp)=dN/d log Dp
Surface area distribution ns(log Dp)= dS/d log Dp
Volume distribution nv(log Dp)=dV/d log Dp
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• Aitken mode – 0.01-0.1 m• Accumulation mode – 0.1-1
m• Coarse mode - >1 m
and sometimes, the elusive• nucleation mode <0.01
um
The Aerosol Modes
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A process oriented view of aerosol size distribution
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• hygroscopic aerosols grow/shrink with RH (with hysteresis!)
• aerosol size strongly affects light scattering cross-section
deliquescence
efflorescence
Humidity and aerosol size...
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Removal mechanisms... gravitational settling
• 10 m particle 1000 cm hr-1
• 1 m particle 10 cm hr-1
coarse particles
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fine particles
Diameter (μm) Distance diff used in 1 s (cm) .001 0.2 .01 0.02 .1 .002 1 .0004 10 .0001
You can estimate the distance a particle will diffuse in a given time from the equation:
where D is the diffusion coefficient
Dt)cm(cetandis
Diffusion/Coagulation
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Why is there an “accumulation” mode?
impaction, settling
diffusion, coagulation
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So lifetimes are ….
• Aitken nuclei – hours to days (diffusion/coagulation)
• Accumulation mode – weeks• Coarse mode – hours to days
(deposition)• Ultrafine – minutes to hours
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Secondary organic aerosol formation
• VOC oxidized to less-volatile OC• Partitioning to aerosol phase depends
on vapor pressure– High equilibrium vapor pressure high
tendency to stay in gas phase– Low equilibrium vapor pressure partitions
to aerosol phase – non-volatiles
• Large organics (C> 6) tend form aerosols while organics C<6 do not.
• Oligomerization on/in acid aerosol
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Aqueous Aerosol
• Thermodynamic partitioning (AgAaq)
– liquid water content (L=g of H2O/m3 of air)• L=0.1-0.3 in clouds• L=0.02-0.5 in fogs
– Henry’s law constant (H)• HA=[A] (M)/A (atm)
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• HO2=1.3x10-3 M/atm• HO3=1.1x10-2 M/atm• HNH3=62 M/atm• HH2O2=7x104 M/atm• HH2CO=2.5 M/atm
Exercise: Calculate the concentration of ozone in pure water in equilibrium with 10 ppbv ozone, assume ideal gas.
A few Henry’s law constants…
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Formaldehyde…
constantlaw sHenry' effective the is *H
63005.22530*H
H2530]COH[2530]C(OH)[HCO][H
H*
2530K 2.5H )OH(CHCOHCOH
COH
)aq(2
COH
22(aq)2
eqA
22)aq(2)g(2
22
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Acids…
87
eq*
HNO*
3
eqHNO3HNO3)(33
HNO3HNOeq3
23)(3
53HNO)(3)(3
53HNO
105.110
4.151
]H[1
][
]H[1H][][][
]H[
H]NO[
4.15
M/atm 2.1x10H
M/atm) 2.1x10(H solubler very wateis acid Nitric
3HNO3HNO
2
3HNO3HNO
33
2
3
32
HM
MH
KHH
HHNO
KNOHNOHNO
K
MKHNOHNO
HNOHNO
HNOtotal
aqtotal
eqaq
aqg
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Because Keq2/H+>>1 nearly all nitric acid will exist as nitrate.
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The chemical perspective ... a chemical size distribution
1. chemical size distributions resemble mass, not number
2. sulfate and organics dominate the accumulation mode, but there’s a surprising amount of seasalt
3. there are a lot of unidentified organics
4. the coarse mode has the expected mechanically generated aerosols, but also nitrate and sometimes sulfate
Mas
s
(C. Leck)
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Dust (mineral aerosols)diameter size: 2-300 µmmain material: sand, silt, clayincludes essential trace metals such as Feconsists of insoluble and soluble fractions
Mineral Dust
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“brown carbon”:sugarsalcoholsaromaticsdi/tri acidsketoacidshydroxyacids
soot – “elemental carbon”formed in flameslittle spectral dependencecarbon-only
Organic aerosols - burning
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Seasalt aerosols...
seasalt production via bubble bursting...
• film drops (many, small, organics)
• jet drops (fewer, larger)
wind bubbles spray
whitecap coverage W α U3+
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The sulfur story (in brief) ...• emissions: fossil fuel SO2, volcanic SO2, oceanic DMS• DMS oxidation ... gas phase ... complex!
(mod. from Yin et al., 1990)
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SO2 oxidation in the gas phase is simple...
but most SO2 oxidation occurs in the aqueous phase...
OHSOHOH2SOSOHOOHOSO
HOSOOHSO
2422332
M22
2M
2
HSOOHHSO
HHSOOHSO
SOSO
23
8~pK23
34~pK
22
)aq(2)g(2
2
1
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heterogeneous oxidation of SO2
• in-cloud oxidation– weakly buffered, pH ~4
– oxidation by H2O2
• seasalt aerosols– strongly buffered by carbonate system
– rapid oxidation by O3
– slower oxidation by H2O2 (also OH, halogen radicals...)
– growth of existing particles, inhibits nucleation of new particles
(Chameides and Stelson, 1992)