ix aerosol
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8/14/2019 IX Aerosol
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VIII. Aerosols
Size distribution
Formation and ProcessingCompositionAerosol phase chemistry
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Importance of aerosols
• human health
air quality, airborne pathogen transport
• climate change
direct/indirect effectsaerosol 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 incommon 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 situaggregation or nucleation from gas phase molecules (gas to particleconversion).
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 Category
Primary
Soil dust (mineral aerosol) 1000-3000 Mainly coarseSea salt 1000-10000 Coarse
Volcanic dust 2-10000 Coarse
Biological debris 26-80 Coarse
Secondary
Sulfates from biogenic gases 80-150 Fine
Sulfates from volcanic SO2 5-60 Fine
Organic matter from biogenic VOC 40-200 FineNitrates from NOx 15-50 Fine and coarse
Total Natural 2200-23500 Best estimate 3100
Anthropogenic
Primary
Industrial dust etc. (except soot) 40-130 Fine and coarse
Soot 5-20 Mainly fine
Secondary
Sulfates from SO2 170-250 Fine
Biomass burning 60-150 Fine
Nitrates from NOx 25-65 Mainly coarse
Organics from anthropogenic VOC 5-25 Fine
Total anthropogenic 300-650 Best estimate 450
Total 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 ofaerosol size distribution
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• hygroscopic aerosolsgrow/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 diffused in 1 s (cm)
.001 0.2
.01 0.02
.1 .002
1 .0004
10 .0001
You can estimate the distance aparticle will diffuse in a giventime 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 onvapor pressure– High equilibrium vapor pressure high
tendency to stay in gas phase
– Low equilibrium vapor pressure partitions toaerosol phase – non-volatiles
• Large organics (C> 6) tend form aerosolswhile 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• H
H2O2
=7x104 M/atm• HH2CO=2.5 M/atm
Exercise: Calculate the concentrationof ozone in pure water in equilibriumwith 10 ppbv ozone, assume ideal gas.
A few Henry’s law constants…
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Formaldehyde…
constantlawsHenry'effectivetheis*H
63005.22530*H
H2530
]COH[2530]C(OH)[HCO][H
H*
2530K 2.5H
)OH(CHCOHCOH
COH
)aq(2
COH
22(aq)2
eq A
22)aq(2)g(2
22
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Acids…
8
7
eq*
HNO
*
3
eq
HNO3HNO3)(33
HNO3HNOeq
3
23)(3
5
3HNO)(3)(3
5
3HNO
105.110
4.151
]H[1
][
]H[1H][][][
]H[
H
] NO[
4.15
M/atm2.1x10H
M/atm)2.1x10(Hsolublervery wateisacid Nitric
3HNO3HNO
2
3HNO3HNO
33
2
3
32
H M
M H
K H H
H HNO
K NO HNO HNO
K
M K H NO HNO
HNO HNO
HNOtotal
aqtotal
eqaq
aq g
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Because Keq2/H+>>1 nearly all nitric acid will exist as nitrate.
Th ch mic l p rsp ctiv ch mic l siz
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The chemical perspective ... a chemical sizedistribution
1. chemical size distributions
resemble mass, not number
2. sulfate and organicsdominate the accumulationmode, but there’s a
surprising amount ofseasalt
3. there are a lot ofunidentified organics
4. the coarse mode has theexpected mechanicallygenerated aerosols, butalso nitrate and sometimes
sulfate
M a s
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
l
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“brown carbon”: sugars
alcohols
aromaticsdi/tri acids
ketoacids
hydroxyacids
soot – “elemental carbon” formed in flames
little spectral dependence
carbon-only
Organic aerosols - burning
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Seasalt aerosols...
seasalt production viabubble bursting...
• film drops (many, small,
organics)• jet drops (fewer, larger)
wind bubbles spray
whitecap coverage W α U3+
Th lf (i b i f)
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The sulfur story (in brief) ...• emissions: fossil fuel SO2, volcanic SO2, oceanic DMS
• DMS oxidation ... gas phase ... complex!
3 3
OH
CH3SCH3
CH3SCH3
2.
CH3SCH
CH3SCH
O
O
CH3SCH
O
CH3SCH
O
3
OH
2.
CH3SCH OO CH3S
CH3SOH
2.
CH3SCH O
CH3SO SO SO2 SO3 SO4H2
CH3SO2 CH3SO3 CH3SO3H
2CH3SCH OOH
..
dimethylsulfone
methanesulfonic acid
sulfuric acid
OH
OH
O2 OH O2
O2
HO2
NO
O3
+ CH 3.
+ CH2O
OH
HO2
H2O
M
O2
H O2
ONO
CH3SCH3
2
NO3
CH 2OHO2,
NO2,
(mod. from Yin et al., 1990)
O id i i h h i i l
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SO2 oxidation in the gas phase is simple...
but most SO2 oxidation occurs in the aqueous phase...OHSOHOH2SO
SOHOOHOSO
HOSOOHSO
24223
32M
22
2M
2
HSOOHHSO
HHSOOHSO
SOSO
23
8~pK23
34~pK
22
)aq(2)g(2
2
1
heterogeneous oxidation of SO
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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 newparticles
(Chameides and Stelson, 1992)
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