ix aerosol

8/14/2019 IX Aerosol

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VIII. Aerosols

Size distribution

Formation and ProcessingCompositionAerosol phase chemistry



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)



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)



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



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



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



• 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



A process oriented view ofaerosol size distribution



• hygroscopic aerosolsgrow/shrink with RH(with hysteresis!)

• aerosol size strongly affects

light scattering cross-section

deliquescence

efflorescence

Humidity and aerosol size...



Removal mechanisms... gravitational settling

• 10 m particle 1000 cm hr-1

• 1 m particle 10 cm hr-1

coarse particles



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



Why is there an “accumulation” mode?

impaction, settling

diffusion,

coagulation



So lifetimes are ….

• Aitken nuclei – hours to days(diffusion/coagulation)

• Accumulation mode – weeks

• Coarse mode – hours to days(deposition)

• Ultrafine – minutes to hours



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



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)



• 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…



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



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



Because Keq2/H+>>1 nearly all nitric acid will exist as nitrate.

Th ch mic l p rsp ctiv ch mic l siz



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)



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



“brown carbon”: sugars

alcohols

aromaticsdi/tri acids

ketoacids

hydroxyacids

soot – “elemental carbon” formed in flames

little spectral dependence

carbon-only

Organic aerosols - burning



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)



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



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



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)

ix aerosol

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