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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