VIII. Aerosols

Size distributionFormation and ProcessingCompositionAerosol phase chemistry

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)

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)

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

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 of aerosol size distribution

• hygroscopic aerosols grow/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 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

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

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

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• 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…

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

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

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

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)

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

“brown carbon”:sugarsalcoholsaromaticsdi/tri acidsketoacidshydroxyacids

soot – “elemental carbon”formed in flameslittle spectral dependencecarbon-only

Organic aerosols - burning

Seasalt aerosols...

seasalt production via bubble bursting...

• film drops (many, small, organics)

• jet drops (fewer, larger)

wind bubbles spray

whitecap coverage W α U3+

The sulfur story (in brief) ...• emissions: fossil fuel SO2, volcanic SO2, oceanic DMS• DMS oxidation ... gas phase ... complex!

(mod. from Yin et al., 1990)

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

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)