School of somethingFACULTY OF OTHER

Lecture 2: Aerosol sources and sinks

Ken Carslaw

2

Key issues

What are the relative source strengths and distributions?

How quickly is aerosol produced and removed?

How do these factors change with particle size?

Following lecture: how are aerosol properties altered between emission and removal?

3

Primary and secondary particles

Primary particles are emitted directly into the atmosphere

Secondary particles are formed in the atmosphere (by condensation or nucleation of gaseous precursors).

One person’s primary is another’s secondary

E.g., global model: urban particles may be treated as primary because they are formed below the grid scale. I.e., they can be primary if formed within a source (e.g., an engine, city, etc.)

4

Primary, Secondary and Aged Primary

Primary particles

Secondary particles

Emitted gases

gases

coagulation

condensation

Aged primary particles

Can contain primary and secondary matter

chemistry

Source SourceACPD Discussion by U. Poeschl: http://www.cosis.net/copernicus/EGU/acpd/5/S5095/acpd-5-S5095.pdf

Amusing article on definitions: Schwartz, Henry’s law and sheep’s tails, Atmospheric environment, 22, 2331-2332, 1988. Reply: Clegg and Brimblecombe, p2332-2333.

5

Primary and secondary emissions

Primary

Dust (including re-suspended), combustion products of elemental and organic carbon (biomass burning, wildfires, vehicles), sea spray, primary biological particles (spores, etc)

Secondary

Ammonia ammonium (dissolution)

SO2, Dimethyl sulfide oxidation sulfate (H2SO4)

Nitrogen oxides oxidation nitrate (HNO3)

Volatile organic compounds (VOCs) -> oxidation -> low vapor pressure organic products (secondary organic aerosol, SOA)

From natural and anthropogenic sources

6

Quantifying emissions

Active emissions

(Depend on the environment)

Sea spray, dust – wind speed

DMS – wind speed and biological activity etc.

Biogenic VOCs – temperature, biological activity

Passive emissions

(Depend on emission factors, energy use, etc)

Anthropogenic NOx, SO2, black carbon

7

Sulfur dioxide

Domestic 9 Tg/a Power plants 48 Tg/a

Industry 39 Tg/a Volcanic SO2 = 25-35 Tg/a

Biogenic equiv SO2 = 36Tg/a

SO2 (m=64) H2SO4 (m=98)

Dimethyl sulfide

8

Organic matter

Biomass burning = 34Tg/a Fossil fuel 3Tg/a

Biofuel = 9Tg/a

Biogenic SOA = 10 – 100’s Tg/a

(see Donahue)

9

Sea spray

Global sea spray mass production rate Total = 8000 Tg/a

10

Sea spray size distribution

Marine aerosol production: a review of the current knowledge, O'Dowd and De Leeuw, Phil Trans Roy Soc A, 365, 2007

Wind speed = 8 m/s

1-2-

10

s m particleslog rd

dFNumber, area, volume

~1.3% of sea spray is in the accumulation mode ~ 100 Tg/a

11

Biomass burning size distribution

normalized number distributions of biomass in different burning areas

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

10 100 1000 10000 100000Diameter (nm)

Brazil - aged

Africa - aged

Africa - young

N.America-flamingN.America-smolderingN. America-youngBrazil-young

Brazil - aged

South Atlantic -agedCanada - aged

Brazil - aged

dN

/dlo

gD

fro

m lognorm

al

fitt

ing

12

Traffic emissions size distribution

20-30 nm30-50 nm

~80 nmPutaud et al, Aerosol Phenomenology, 2003

Rural Urban Kerbside

13

Aerosol production / emission rates

14

Sea spray flux versus wind speed

Flux is proportional to 10 m wind speed cubed310uF

15

Sea spray production rates

Integrated flux in a size range

At 10-100 nm:

Mean concentration after one day

N10-100nm ~ 350 cm-3

N1m ~ 3.5 cm-3

rrd

dFF log

log

F1 km

1 m2

htFN -3m/

-1-26 sm 104F

Assume steady flux into 1 km deep well mixed boundary layer with u = 8 ms-1

16

Secondary aerosol production rates

))/][log(1(100

206.0)][1/(][

MOH MMk

123.3310 103;)/300(103

T

SO2 + OH + M H2SO4

kOH ~ 10-12 cm3 molec-1 s-1

OH ~ 106 molec cm-3

SO2 gas phase chemical lifetime ~ 106 s ~ 10 days

Pham et al., JGR, 1995 * see Donahue!

NO2 + OH HNO3

NO2 lifetime ~ 1 day OH+-pinene organic aerosol*kOH = 1.2×10−11 exp(444/T)

Monoterpene lifetime ~ 0.4 days

17

Sulfate aerosol production in clouds

SO2

-24322 SOO/OH...

322 HSOHOHSOevaporationcloud

Involatile H2SO4 remains in particles

SO2

H2SO4 (gas)

H2SO4 (particles)

45

12

deposition

42

~4 times as much SO4 from clouds as from gas phase oxidation:

SO2 lifetime ~ 2.5 days

+ OH

18

Aerosol removal (scavenging) processes

Dry deposition – diffusion to and deposition on surface

Wet deposition

In-cloud or “nucleation” scavenging

Impaction

19

Dry deposition

Deposition velocity over forest

10 cm s-1 lifetime of 1 km deep well mixed boundary layer aerosol ~ 3h

0.1 cm s-1 lifetime of ~12 days

Brownian diffusion

Gravitational settling

1 m/s

20 m/s

Accumulation mode!

20

Wet scavenging

In-cloud scavenging

Below-cloud scavenging

21

Wet scavenging

Characteristic time scale for the conversion of cloud droplets into raindrops in precipitating clouds ~ 3 hours

PROBLEMS:

Cloud-scale processes

Particle size dependence

In-cloud scavenging

22

Wet scavenging

Below-cloud scavenging

Particle diameter, m

10-3 10-2 10-1 100 101

Scav

engi

ng c

oeffi

cien

t, h

r-1

10-5

10-4

10-3

10-2

10-1

100

101

102

R=0.1 mm h-1

R=1.0 mm h-1

R=10 mm h-1

R=100 mm h-1

3nm: 0.5-20h

10 m: 0.5-10h

100nm: 10days-weeks

23

Global aerosol production and loss timescales

24

Production of global aerosol mass

~1 month to reach steady state

Based on Leeds GLOMAP global aerosol model

25

Decay of global aerosol mass and number

Mass lifetime ~ 3 days

10% remains after 1 month

Arctic

Number lifetime ~ 10 days

Switch off all emission processes in a global model

Global models predict a ~factor 2 difference in aerosol lifetime between US, Asia & Europe

Based on Leeds GLOMAP global aerosol model

26

Importance of aerosol lifetime

It is short compared to most greenhouse gases

CO2 – 100 y

CH4 – 11y

Aerosols do not accumulate in the atmosphere

In the long term can expect GHGs to dominate forcing

27

Pb210 tracer to quantify deposition lifetimes

Huge model diversity in remote regions due to differences in deposition

Rasch et al., Tellus, 2000