Aerosols and climateRob Wood, Atmospheric Sciences

What are aerosols?What are aerosols?

• Solid or liquid particles suspended in air

• Sizes range from a few nm to a few thousand nm⇒ Huge range of masses

Where do aerosols come from?

Estimate of Present-Day Global Emission of Major Aerosol Types (in Tg/year)

Present flux

Source Low High Best

Present flux

Source Low High Best

j yp ( g y )

NaturalPrimary

Soil dust (mineral aerosol)

1,000 3,000 1,500

AnthropogenicPrimary

Industrial dust etc. 40 130 100

Bl k b ( t d 10 30 20aerosol)

Sea salt 1,000 10,000 1,300

Volcanic dust 4 10,000 33

Black carbon (soot and charcoal)

10 30 20

Secondary

Sulphates from SO 120 180 140Biological debris 26 80 50

Secondary

Sulphates from SO2 120 180 140Biomass burning (w/o black carbon)

50 140 80

Nitrates from NOx 20 50 36Sulfates from biogenic gases

60 110 90

Sulfates from volcanic SO2

4 45 12

x 20 50 36Organics from anthropogenic NMHC*

5 25 10

Organic matter from biogenic NMHC*

10 200 55

Nitrates from NOx 10 40 22

1 10 100 103 104 105 106 [nm]

Aerosols and light scattering• Scattering of solar radiation per unit mass is maximum

when particle size is close to solar wavelengths (λ ~ 400-800 nm))

• Particles in the size range 200-2000 nm are most efficient scatterers

• Particle surface area more important than massParticle surface area more important than mass

er m

ass

ffici

ency

pat

terin

g ef

10 100 1000 10000

Particle diameter [nm]

Sca

Surface area and massSurface area and mass

• M = (4π/3) ρ r3M (4π/3) ρ r• A = 4π r2

• A/M ∝ 1/r per m

ass

A/M ∝ 1/r

• when r « λ scattering effic

ienc

y p

when r « λ scattering efficiency increases with r4

catte

ring

e

⇒ r<100 nm particles don’t matter

10 100 1000 10000

Particle diameter [nm]

Sc

[ ]

Aerosols and light scattering

• If aerosol particle is soluble then size willIf aerosol particle is soluble then size will depend upon water uptake

• Water uptake depends upon relative humidity p p p y(RH)

• For given number of particles, scattering will g p , gincrease with RH

• But with no particles there is no scattering despite what the RH is

Size as a function of RH

Chemical composition affects thgrowth

r/rdr

y 3 marine airmass

acto

r rw

th fa 2

grow

1 polluted airmass

RH1 p

What determines scattering?What determines scattering?

• For particles of size > 200 nm, surface area p ,determines scattering from one particle

• Multiply this by the number concentration of i l ll i i lparticles to get overall scattering per unit volume

of air• A ∝ N r2 M ∝ N r3• Atot ∝ N r2, Mtot ∝ N r3

⇒ Atot ∝ N1/3 M2/3

• Need to know both total aerosol mass and the number concentration

Importance of sulfate

• Sulfate aerosols dominate the scattering over continental regionscontinental regions

• Anthropogenic sources comparable to natural sourcessources

• Regionally, this scattering can seriously degrade visibilityvisibility

• Globally, this scattering reduces solar radiation entering the climate system by ≈2 W m-2entering the climate system by ≈2 W m

• Studies suggest that about 0.3-0.8 W m-2 of this scattering are from anthropogenic aerosols, g p g ,mostly sulfate (“direct” effect of aerosols)

Beijing after rain Beijing during dry period

Solar radiation reaching the surfaceSolar radiation reaching the surface

Global Sulfur BudgetRates in Tg S yr-1

SO2

SO42-

τ 3 9dOH

cloud42

2

τ = 1.3d

OHNO3

τ = 3.9dOH

8184H2SO4(g)

(CH3)2S(DMS) 1 0d

OHNO3

10 64 dep27 dry

dep6 dry

τ = 1.0d

22

27 dry20 wet

y44 wet

Phytoplankton Volcanoes Combustion/Smelters

Global Sulfur Emission PatternsGlobal Sulfur Emission Patterns

Chin et al. [2000]

SO2 sources by type

Direct effect of laerosols upon

solar radiation

• Aerosols scatter solar radiationradiation

• More particles ⇒ more scatteringg

• Some scattered radiation returns to spacespace

• Reduction in amount entering climate systemg y

• ⇒ Cooling effect

Aerosol optical depth (measure of scattering)scattering)

How well do we

understand hhuman

influence?

• Direct radiative forcing (effect of g (anthropogenic emissions on aerosolaerosol scattering) from different models

Trends in U.S. NOx AND SO2 EmissionsEmissions

Aerosol-cloud-climate interactions

More aerosol-Climate effects

Cloud droplets form on aerosol particlesCloud droplets form on aerosol particles

• For soluble aerosols, particle size increases with RHp

• When a parcel of air rises, it cools and the RH increases

• When RH>1 the soluble aerosols can “activate” (grow• When RH>1, the soluble aerosols can activate (grow unstably into much larger cloud droplets)

• The RH at which an aerosol activates is stronglyThe RH at which an aerosol activates is strongly dependent upon its size

• Larger aerosols activate first and can consume vapor, g ppreventing the smaller ones from activating

• The number of cloud droplets is strongly controlled by th b f l bl lthe number of soluble aerosols

What determines N in warm clouds?et

[c

m-3

] d

ropl

ratio

n [

Clo

udnc

entr

Cco

n

Aerosol concentration (r>0.1 micron) [cm-3]

Link between number of cloud droplets d l d lb dand cloud albedo

• Similar arguments as for light scattering due to aerosols

• For given amount of liquid water, increased N gives smaller average size of dropletsS f• Smaller droplets have larger surface area

• More reflective cloud, more albedo

• Most well-understood

l l daerosol-cloud-climate effects (indirect(effects)

Global Sulfur Emission PatternsGlobal Sulfur Emission Patterns

Chin et al. [2000]

MODIS, data courtesy of NASA

IPCC, 2007

Putting it all together