School of somethingFACULTY OF OTHER

Lecture 1: Introduction

Ken Carslaw

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A short history of aerosol and climate1783 Laki Eruption. Benjamin Franklin attributes global cooling to volcanic haze

1800’s aerosols are the smallest known sub-division of matter

1859 Tyndall explains blue colour of sky in terms of light scattering by particles

1871 JW Strutt (3rd Lord Rayleigh) explains light scattering

1880 Aitken discovered role of aerosol in cloud drop formation

1898 Aitken: First evidence of new particle formation in the atmosphere.

1950s: Aerosol as pollution

1950s Aerosol is a regional pollution issue (LA, London…). Smog.

1958 CE Junge "unpolluted areas... no longer exist" in Western Europe

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A short history of aerosol and climate1960s-70s: Anthropogenic aerosol and climate?

1964 Nicolai Fuchs published The Mechanics of Aerosols

1967 Aerosol turbidity spread over 1000’s km. McCormick and Ludwig "Climate Modification by Atmospheric Aerosols."

1968-70. Aerosol cooling could counteract CO2 warming (Bryson).

1970: First calculation of cooling by aerosol (JM Mitchell). 2/3 of cooling since 1940 due to volcanoes

1970. Hubert Lamb “volcanic dust probably not main influence”

1970 Journal of Aerosol Science founded

1971 Potential aerosol cooling of 3.5K, bigger than CO2 (Rasool and Schneider)

BUT, discussion about whether aerosol cools or warms

1976 Wide regional effect of sulfate aerosol on climate (Bolin and Charlson)

1977 Aerosol effect on cloud albedo. Net cooling. (Twomey)

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A short history of aerosol and climate

1980s: First global models with aerosol effects

Late 1970s. First global models including aerosol. James Hansen and others.

1981. Climate model with volcanic aerosol, solar and CO2 forcings explains 20th C T changes (Hansen)

1982 The American Association of Aerosol Research formed

1990: First IPCC report: “neither the sign nor magnitude of the [aerosol] climatic feedback can be quantitatively estimated”

1990s: Aerosol a central part of climate models. Increasing confidence

1991: SO4 aerosols roughly balancing CO2 warming (Charlson et al., 1990, 1991, Hansen and Lacis, 1990)

1991: Pinatubo eruption. Hansen et al (1992) predict 0.5K cooling. Roughly correct.

1992. Biomass burning aerosol impacts (Penner)

1994/5: Model of 20th century climate with CO2 and SO4 aerosol (Taylor and Penner, Mitchell et al.)

1995. 2nd IPCC. Several aerosols in climate models. “Human influence on climate discernable”

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A short history of aerosol and climate

2000s: Sophisticated models and observations

2001. 3rd IPCC. “Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Aerosol most uncertain forcing.

2000/01: Potential large warming from soot aerosol

2001: First “microphysical” global aerosol models

2002: Term “Global dimming”

2005: Global brightening!

2007: 4th IPCC. Aerosol still most uncertain forcing…

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Aerosol distribution and properties

Spatial distribution

Mass, number

Size distribution

What’s important for climate?

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Global aerosol distribution

Dust

Biomass burning

Pollution

Sea spray

Remer et al., Global aerosol climatology from the MODIS satellite sensors, JGR, 2008

Aerosol Optical Depth

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Seasonal variability

MODIS satellite aerosol optical depth climatology

Remer et al., JGR, 2008

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Particle concentrations at the surface

LocationNumber (>3 nm)

cm-3

Mass<2.5 m

g m-3

Mass<10 m

g m-3

Urban 105-4x106 8-100 (12-20) 30-300 (40)*

Polluted continental

2000-10,000 2-8 10-40

Remote continental

50-10,000 0.5-2.5 2-10

Marine 100-400 1-4 10

European annual mean exposure limit

European recommended limit

1 g m-3 ~ 0.75 parts per billion (ppbm) at STP

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European surface level particulate mass <10 m

2004, European Environment Agency

From the European Air Quality monitoring network

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Aerosol vertical profile

Profile of aerosol extinction:

Decays from boundary layer to ~3-4km, then follows molecular

Typical for surface sources

Well mixed

exponential

constant

BOUNDARYLAYER

FREE TROPOSPHERE

Sasano, Applied Optics, 1996

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Aerosol number vertical profile

Number increases with altitude

Typical Z-shaped profile over continents

Numerous small particles are invisible

Z

Remote Pacific

Europe

POLLUTED BOUNDARYLAYER

CLEAN BOUNDARY LAYER

Clarke et al.

Schroder et al.

Cirrus

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Particle size distributionsModesNucleation, Aitken, accumulation and coarse

Not always present

Air qualityParticulate matter (PM), particulates

PM1 – mass less than 1 mm aerodynamic diameter

PM2.5 and PM10

Air quality and healthCoarse > 2.5m

Fine <2.5mm

Ultrafine < 0.1m

Cloud Physics:Giant > 2m

Ultra giant > 10 m

0

4e3

8e3

0

50

100

0

4

8

0.01 0.1 1.0 10.0

Diameter / m

NucleationAitken

Accumulation

Coarse

PM1 PM10PM2.5

Ultrafine fine coarse

cm-3

m2 c

m-3

m3 c

m-3

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Size distribution representation

1-3- mcm particles/)( rfdr

dN 1-3- mcm particles/)( rf

dr

dN

1-3- mcm particles/)( rfdr

dN

0

3000

6000

0

100

200

0

60

120

0.01 0.1 1.0 10.0

Diameter / m

0

5e4

1e5

0

300

600

0

8

16

0 2 4 10

Diameter / m6 8

cm-3 m-1

m2 cm-3 m-1

3-

10

cm particles/10ln)1(

10lnlog dr

dNr

drr

dN

rd

dN1-3- mcm particles/)( rf

dr

dN

m3 cm-3 m-1

cm-3

m2 cm-3

m3 cm-3

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Urban

0

105

2.105

0

500

1000

020

40

60

0.01 0.1 1.0 10.0

Diameter / m

Dominated by emissions

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Polluted continental

0

4000

8000

0

50

100

0

4

8

0.01 0.1 1.0 10.0

Diameter / m

Accum. modeEmitted particles plus complex regional processes

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Remote continental

0

4000

8000

0

100

200

0

4

8

0.01 0.1 1.0 10.0

Diameter / m

Remote doesn’t mean clean!

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Free troposphere and stratosphere

0

50

100

0

10

20

0

2

4

UT

0.01 0.1 1.0 10.0

Diameter / m

0

10

20

0

1

2

0

0.05

0.1

0.01 0.1 1.0 10.0

Diameter / m

Volcanically perturbed

20

3

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Arctic

0.01 0.1 1.0 10.0

Diameter / m

0

100

200

0

2

3

0

0.2

0.4

Arctic haze

Very aged aerosol

Two periods:

Spring: polluted (Arctic haze)

Summer: clean

G. Shaw, Bull. Am. Meteorol. Soc., 76, 2403-2413, 1995.

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Marine

0

100

200

0

50

100

0

10

20

0.01 0.1 1.0 10.0

Diameter / mHeintzenberg et al., Size distribution and chemical composition of marine aerosols: a compilation and review, Tellus, Ser. B, 52B, 1104–1122, 2000.

Varies strongly between clean and polluted

Multiple modes from sea spray production processes

…but there are other sources even in clean regions

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Desert

0

100

200

0

500

1000

0

1000

2000

0.01 0.1 1.0 10.0

Diameter / m

Aged dust plume

Dominated by emissions

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Composition

0.01 0.1 1.0 10.0

Diameter / m

FINE

H, NH4, SO4, NO3

Organic carbon

Elemental carbon

Metals

(Fe, Pb, Cd, V, Zn etc)

COARSE

Dust (CaCO3, Mg, Si, Al, Fe)

Coal dust

NaCl

Pollen, spores

Biological debris

See lectures by Sandro Fuzzi.

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What’s important for climate?

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Light extinction

0.01 0.1 1.0 10.0

Diameter / m

Continental

Arctic

Marine

Ext

inct

ion

Effi

cie

ncy

volu

me

volu

me

volu

me

n = 1.5-0.005i

n = 1.37-0.01i

Number

Light scattering efficiency depends in a complex way on • size distribution of

particles• chemical composition

(refractive index)• particle shape (sphere

or not)

Light absorption efficiency depends on• mass of absorbing

material• mixing of absorber with

other material

Area

See later lectures

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Cloud drop formation

Cloud drop number depends on (in order of

importance):

• Particle numbers and sizes

• Cloud dynamics

• Particle chemical composition

0.01 0.1 1.0 10.0

Diameter / m

See lectures by Graham Feingold

cloud condensation nuclei

0.01 0.1 1.0 10.0

Diameter / m

Only a subset of particles affect climate

Only a subset of particles affect climate

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Long term change

IPCC, 2001

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CCN change?

1850

2000

Leeds GLOMAP model

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Next 2 lecturesSources and sinksFactors that control the size distribution

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Definition

An aerosol is a dispersion of solid and liquid particles suspended in gas

It is a singular noun describing the mixture of particles. E.g. “the aerosol over Asia…”

Refer specifically to aerosol particles or particles.

E.g., the “the particle growth rates…” rather than “the aerosol growth rate…”.