Transcript No Slide Title

Aerosols and Climate
Postgraduate lecture course
06/02/09
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Outline
Part I
What is an aerosol?
How do aerosols affect climate?
Future climate?
Part II
Measuring aerosol from the ground, air and space
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What is an aerosol?
Definition:
An aerosol is a suspension of tiny particles in air
Characteristics:
Origin: Natural or anthropogenic
Size: nanometers – 10 mm+
Concentrations: Typical~1000-10000 cm-3 but up to 109 cm-3
Climate impact: diverse!
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What is an aerosol?
I: Origin
Primary and secondary
Natural
Same aerosol
type can be
produced both
naturally and
anthropogenically
And they can mix
Anthropogenic
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Courtesy G. Mann
What is an aerosol?
I: Origin
Sources in kg
km-2 hr-1
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IPCC, 2001
What is an aerosol?
II: Size and shape
Different representations of size
distribution mean different
emphasis: most mass is in
coarse mode but these may not
be the most climatologically
important aerosols…
Nucln
Aitken
Accum
Coarse
Cloud and precipitation physics
Atmospheric electricity
Atmospheric radiation and optics
Air chemistry and pollution
10-3
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10-2
10-1
100
101
Aerosol diameter (mm)
102
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10000
What is an aerosol?
III: Lifetime
Diameter (mm)
10-3
Designation
10-2
10-1
Nucleation/Aitken nuclei
100
101
Accumulation
102
Coarse
Combustion
Sources
Sinks
Liftetime
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Gas-to-particle
conversions
Coagulation of
Aitken nuclei
Cloud droplet
evaporation
Coagulation
Capture by
cloud particles
Less than an
hour in polluted
air or clouds
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Windblown
dusts
Fly-ash,
sea-salt,
pollens
Coarse
emissions
from industries
Precipitation
scavenging
Dry fallout
Days
Hours
to
days
Minutes to
hours
103
What is an aerosol?
IV: Concentrations
Typically vary due to
location
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But also vary for same
generic aerosol ‘type’ due to
meteorological conditions
Role in the climate system: one example
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Direct Radiative forcing
Radiative forcing (RF): ‘the net change in total irradiance at the tropopause to an
applied perturbation after allowing for stratospheric temperatures to readjust to
radiative equilibrium but holding all other atmospheric variables fixed’
Here net total irradiance (SW + LW) has the convention down – up
For WMGG, DTs ~ l RF where l is the climate sensitivity parameter
CO2 increase
Lower atmosphere warms
Stratosphere cools
Positive forcing
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Direct Radiative forcing
For aerosol it is more complicated: depends on aerosol properties plus
characteristics of underlying surface
Case I: Scattering aerosol over dark surface
Reduced SW radiation at surface,
more SW radiation reflected to space
Negative forcing
Local surface and atmospheric
cooling
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Direct Radiative forcing
For aerosol it is more complicated: depends on aerosol properties plus
characteristics of underlying surface
Case II: Absorbing aerosol over bright surface
Less SW radiation reaches surface,
more absorbed in atmosphere, less
reflected to space
Positive forcing
Local surface cooling and
atmospheric warming – Oops!
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Direct Radiative forcing
What about a scattering aerosol over a bright surface?! How can we estimate
whether forcing is positive or negative for a given set of conditions?
Concept of critical single-scattering albedo
First we need to back-track slightly and introduce some key
aerosol parameters…
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Calculating key aerosol optical properties
ke (m2 g-1)
Size distribution
Chemical composition
(complex refractive index)
e.g. Spheres: Mie theory
Spheroids: T-Matrix
PROCESSING
Particle diameter (mm)
Peak extinction
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Assumption of particle
shape + appropriate
scattering code
Mass extinction coefficient, ke
Single-scattering albedo, wo
Scattering phase function
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INPUTS
NB size
parameter:
2pr/l
OUTPUTS
Single scattering albedo
wo = ks / ke
and ke = ks + ka
where ks is the mass scattering coefficient and ka is the mass absorption
coefficient
wo
n = 1.5-0.005i
n = 1.37-0.001i
Particle diameter (mm)
NB: Instead of ke (ks, ka) can also use:
Extinction coefficient, be = attenuation of radiation per unit path length (m-1)
Extinction cross section, Ae = Qe x geometric area of particle (m2)
where Qe is extinction efficiency
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The phase function
Definition: ‘The angular distribution of scattered light intensity at a given
wavelength’
1
4p
p P(, l, m)dA  1
4
where  is the scattering angle, m is the
complex refractive index and dA is an element
of area
cos  cos o cos  sin  o sin  cos

Taking o = 0 and assuming spherical scatterers:
1
4p
2p p

  P( ) sin  d d  1
0 0
Forward
scatter
Back
scatter
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Isotropic scattering,
P(,l,m) = 1
The phase function
Definition: ‘The angular distribution of scattered light intensity at a given wavelength’
Related terms:
Asymmetry Parameter, g
p
1
g( l , m)   P( , l , m) sin  cos d
20
Gives idea of scatter direction
+ve: forward scatter
0: isotropic
-ve: back scatter
Backscatter ratio, b
p
p P( , l , m) sin  d
b (l , m ) 
2
p
 P( , l , m) sin  d
0
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Proportion of radiation
scattered into backwards
hemisphere
Critical single-scattering albedo
How can we estimate whether forcing is positive or negative for a given
set of conditions?
b = 0.1
b = 0.2
b = 0.3
Haywood and Boucher, 1999
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Direct Radiative forcing
What about LW?
Only really an issue for aerosol types with large coarse mode population:
most interest on mineral dust from anthropogenic activity…
-DLW
+DLW
Reduction in OLR
Positive forcing
Local surface and
atmospheric heating
NB1: Forcing magnitude
strongly dependent on
surface/atmospheric
temperature contrast:
sign can change
NB2: Natural emissions of mineral dust and volcanic
material also affect the LW: impact on radiation field
generally termed ‘Direct Effect’ as not strictly a forcing;
ditto for impact of natural aerosols in SW
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Direct Radiative forcing
…but some work suggests urban pollution, pollen outbreaks etc. also directly
affect LW
Spankuch et al., 2000
Lubin et al., 1994
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And Feedbacks…
A ‘simple’ example: absorbing dust over desert in daytime
Reduces reflected
SW flux at TOA and
surface
Instantaneous
Individual response
Surface cools
atmosphere warms
Combined response
-DLW
+DLW
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Reduces OLR,
enhances downwelling
flux at surface
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Surface and
atmosphere warm
Atmos warms,
surface?
Feedback
effect on OLR
Enough of clear-skies: cloud-aerosol effects
or Indirect aerosol forcing
Glaciation Indirect Effect
COOLING
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OVERALL COOLING
IPCC,
2007
WARMING
More tenuous perhaps:
Observations: no of droplets does
increase but concurrent widening of cloud
droplet spectrum:
‘First Dispersion Effect’
Simulations suggest an
associated increased
precipitation amount:
‘Second dispersion Effect’
(e.g. Roelofs and Jongen, 2004)
Rotstayn and
Liu, 2003
Could partially offset 1st indirect effect
but countered by Lu and Seinfeld, 2006
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Highly uncertain in terms of climate impact
IPCC,
2007
Efficacy = li/lCO2
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Future Climate?
Mitchell et al., 1995
Consistent with:
(a) Global Dimming
Courtesy K. Carslaw
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Stanhill and Cohen,
2001
Future Climate?
Mitchell et al., 1995
Consistent with:
(b) Global Brightening
Courtesy K. Carslaw
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Wild et al., 2005
Future Climate?
Projected changes
will exacerbate
GHG effects
IPCC TAR
prediction
Andreae et al.,
2005
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Key: understanding
present day forcing
and sensitivity of
climate to aerosols…
Observing aerosol
 055   k e055  dz
dz
055
Average of nine model predictions
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Observing aerosol – Ground based
AErosol ROBotic
NETwork
(AERONET)
http://aeronet.gsfc.nasa.gov/
1994
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2007
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Observing aerosol – Ground based
AERONET measurements directly provide:
(l): I(l) ∞ V(l) and, Beer-Lambert:
2
R 
V(l )  Vo (l ) m  exp( (l )TOT  m)
 R 
Leads to
ln V( l )  m( (l )TOT )  ln V' o (l )
s
ds
dz
Plane parallel, m = sec s
In reality,
s < 60°, m ~ sec s
Langley plots:
And
(l)a = (l)TOT - (l)t - (l)r
dy/dx
Measurements at several ls, so also
provides Ångström coefficient, a
a 
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m
d ln  (l ) a
~
d ln l
ln
 (li ) a
 (l j ) a
l
ln i
lj
Observing aerosol – Ground based
AERONET measurements also used to retrieve:
size distribution, single scattering
albedo, phase function and complex
index of refraction
Idea: simultaneously invert radiances
measured at a number of
wavelengths and scattering angles
so uses diffuse and direct beam
measurements
Then: variability in clear-sky downward solar
radiance assumed dominated by aerosol
I(, l)  I( a (l);woa (l); Pa (, l))
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or
I(, l)  I(Na (r), ma (l))
Observations of aerosol – in situ
Balloon or aircraft based instrumentation
One example: FAAM BAE-146
Type of measurement
Instrument
Size range, wavelengths etc
Aerosol microphysics
PMS PCASP
SID-1/SID-2
FFSSP
0.05-1.5 mm
1-30 mm
1.5-20 mm
Aerosol optical properties
TSI Nephelometer
Radiance research PSAP
l = 0.45,0.55,0.7 mm
l = 0.568 mm
Aerosol chemical comp
Filters
2 ranges for inorganics and
carbon
Broadband irradiance
BBRs
0.3-3 or 0.7-3 mm
Spectral radiances
SWS
ARIES
303.4-1706.5 nm
3.33-18.18 mm
Spectral irradiances
SHIMS
303.4-1706.5 nm
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Plus other ‘standard’ meteorological measurements…
In situ size (and shape)
PCASP: Passive Cavity Aerosol Spectrometer Probe
Measures angular distribution of light scattered out of
HeNe beam focussed on particle laden air stream
Uses Mie theory to relate scattering pattern to particle
size
FFSSP: Fast Forward Scattering Spectrometer Probe
As above, but only considers ‘forward’ scattered light
SID1/2: Small Ice Detector
Similar idea to above
Isolates single particles and measures angular
scattering using array of detectors
Variation in detector response cf mean value provides
particle shape information
In situ size
GERBILS campaign, June 2007
PCASP
FFSSP
SID2
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Courtesy S. Osborne
In situ optical properties
Nephelometer
Measures total scatter and
hemispheric backscatter
Known light source, known
path length: obtain scattering
coefficient
Particle soot absorption photometer
(PSAP)
Measures absorptance through a filter
Provides absorption coefficient
Combination of two allows derivation
of extinction coefficient
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In situ optical properties
Courtesy S. Osborne and
B. Johnson
Excellent detail and opportunity to study aerosol case studies but ‘snapshot’
in nature
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Observations of aerosol – from space
Basics: 3 types of orbit
Geostationary: rotates with Earth. Limited view but excellent time resolution
Sun-synchronous or Polar: Provides coverage of whole globe within ~ 6 days
dependent on inclination. Always crosses given latitude band at same local times
Inclined or Precessing: Generally a low angle of inclination. Limits latitude
regions sampled but increases sampling rate
Most (but not all) ‘aerosol’ instruments in polar or near polar orbit
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Observations of aerosol – from space
Surface
representation
Atmospheric
profile
Aerosol
representation
Set of geometries and s
Radiative Transfer code
SUN
Channel filter
functions
r
LUT
GENERATION
Simulated quantity, c(s,v,r,l)
geometrical
restrictions
Retrieved l
Observed quantity, c(s,v,r)
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But which points to perform
retrieval on?
5th March, 2007, 14:00 UTC
GERB cloud mask:
Oops!
Dust flag: restores points
incorrectly identified as
cloud. ‘Biased’ towards
thicker plumes (055 > ~ 0.5)
(Brindley and Russell, 2006)
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Observations of aerosol – from space
Longest established method: Narrow band radiometers
Different spectral regions exploited for different purposes
(i) Visible reflectances
 ch 
p  L ch
Foch
Requires a large contrast between
surface and aerosol reflectance,
and surface bi-directional
reflectance function (BDRF) to be
well known: good for ocean
Longest record (1979+) from AVHRR
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Jacobowitz et al., 2003
12:00 UTC 04/03/04
SEVIRI
ODIS
erra)
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3gen
OPAC
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Nonspher
Des
3. Dust detection and loading
Brindley and Ignatov, 2006
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Observations of aerosol – from space
Longest established method: Narrow band radiometers
Different spectral regions exploited for different purposes
(ii) Visible reflectances may also be used over other dark targets but extra
information is required
Kaufman et al., 1997
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Relies on relationship between s in different
spectral channels
Observations of aerosol – from space
MODerate Imaging
Spectroradiometer (MODIS)
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Observations of aerosol – from space
Longest established method: Narrow band radiometers
Different spectral regions exploited for different purposes
(iii) UV radiances: exploits low surface reflectance in this regime (bar snow)
Development of UV aerosol index, UVAI
obs
 I 354

UVAI  100log calc * 
 I 354 (R 354 ) 
An ‘error’ caused by presence of aerosol
Positive for absorbing aerosols
Qualitative, but correlated to aerosol
optical depth, and a long-term record
e.g. Torres et al., 2007
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Observations of aerosol – from space
Longest established method: Narrow band radiometers
Different spectral regions exploited for different purposes
(iv) IR radiances: avoids reflectance issue, exploits contrast between Tsfc and
Tdust. Sensitive to dust height, atmospheric profile and surface emissivity
e.g. Legrand et al.,
2001, Brindley, 2007
AOD from SEVIRI, 1200 UTC, 18th June, 2007
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Observations of aerosol – from space
Alternatives:
(v) Exploit directional behaviour of aerosol scattering: viewing different angles
allows differentiation between surface structure and different aerosol types
Example: Multi-angle Imaging SpectroRadiometer (MISR)
nadir
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R (nadir), G (70.5 ° forward),
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70.5° backward
view B(70.5 ° back)
Diner et al., 2001
Observations of aerosol – from space
Alternatives:
(vi) Exploit directional behaviour plus change in polarisation caused by aerosols
Example: POLarisation and Directionality of the Earth’s Reflectances (POLDER)
R (0.443 mm), G (0.670 mm), B ( 0.865 mm)
Note: larger particles
have smaller
polarisation signal
Polarisation also
affected by shape of
aerosol
‘Natural’ light
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Polarised light
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e.g. Deuzé et al., 2001, 2002
Observations of aerosol – from space
Alternatives:
(vii) Use active techniques
Example: Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)
Cirrus
Dust
532 nm total
532 nm perp
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1064 nm ©total
Biomass
Measures backscatter.
LIDAR ratio, h =
extinction/backscatter
Requires modelling of
expected h for different
atmospheric components
Observations of aerosol – from space
Possibility to investigate
indirect effects…
POLDER
Breon et al., 2002
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Observations of aerosol – from space
…and semi-direct effects
MODIS
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Observations of aerosol – from space
PM2.5
UK Air Quality
Network
Attempts to relate PM
measurements to AOT
Pere et al., 2009
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PM10
Relationship with air quality…
Observations of aerosol – from space
The Future?
GLORY: aims to measure total solar irradiance and aerosol/cloud properties
Data product
Dust
Range
Uncertainty
0-5
0.02 (ocean) 0.04
(land)
Effective radius
0.05-5 mm
10 %
Effective variance
0-3
40 %
Real Refractive
index
1.3-1.7
0.02
Single-scattering
albedo
0-1
0.03
Morphology
Spherical, irregular
dust, soot clusters
N/A
Cirrus
AOD
Biomass
Mishchenko et al., 2007
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Problems
1. Continental aerosol with diameters > ~ 0.2 mm have a size distribution which can be
approximated by:
 dN 
log 
  C  b log D
d(log
D)


where N is the number concentration, D is the aerosol diameter, C is a constant. Sketch
Cirrus
this distribution as a function of D on
a log-log scale and interpret b. Derive expressions
for dN/dD and
dV/d(log D) assuming spherical
particles. For continental aerosol b~3.
Dust
Biomass
What does this imply about the mass distribution of the aerosol? Which types of aerosol
would you therefore expect to dominate the total aerosol mass loading in the Earth’s
atmosphere?
2. Aerosol with diameter D between ~ 2 to 40 mm experience a ‘Stokes drag force’ given
by 3phDv, where h is the viscosity of the air, and v is the velocity of the aerosol through
air. Neglecting the density of the air compared to the density  of the aerosol, derive an
expression for the terminal velocity vs of the aerosol. Hence calculate the terminal
velocity of aerosol with diameters 2 and 10 mm respectively. What does this imply about
the evolution of a size distribution associated with a particular aerosol event as it moves
away from its source? What assumptions have you made?
[Terminal velocity or ‘settling velocity’ is achieved when the forces acting on a falling
particle balance. Take  = 103 kg m-3 and h = 1.7 x 10-5 kg m-1s-1]
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Problems
3. Define single-scattering albedo, wo and explain what it means. What is the value of wo
for a purely scattering aerosol?
Consider the case of a hypothetical purely scattering aerosol layer which scatters equally
in all directions. Show that the corresponding asymmetry parameter is equal to zero.
What is the backscatter ratio? If the aerosol was placed over a vegetated surface with
reflectance 0.2 would you expect it Cirrus
to cool or warm the surface?
Some of the Dust
vegetation is now burnt and Biomass
the aerosol layer becomes mixed with soot. It’s
properties change such that the magnitude of the absorption coefficient is equivalent to 10
% of the scattering coefficient and the backscatter ratio is 0.1.
What is the new value of wo? Does the aerosol warm or cool the surface?
A wind picks up and transports the mixed aerosol over a nearby desert. What would the
surface reflectance need to be in order for a positive radiative forcing to occur? Is this
likely?
4. Briefly describe the concepts of global dimming and global brightening. What factors
need to be accounted for when correlating surface based measurements of solar radiation
with aerosol amount?
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Problems
5. A hand-held microtops sun photometer works in an analogous manner to AERONET
measurements. What would you expect the main difficulties to be in obtaining an
accurate aerosol optical depth measurement on (a) land, (b) ship. How, practically, could
these be reduced?
An operator outside the Albert Hall took a set of microtops cloud-free measurements
during the course of 21st June, 2008.
At 12 pm, V at 0.44 mm is measured at 54 mV, while
Cirrus
by 4 pm this reading had dropped to 40 mV. If Vo(0.44 mm) is known to be constant at 75
Dust
Biomass
mV what is the aerosol optical depth at this wavelength? Corresponding measurements
at 0.87 mm are 52 and 45 mV, with Vo(0.87 mm) constant at 68 mV. What are the aerosol
Ångström coefficients at the two times? What does this suggest about changes in the
aerosol size distribution over time?
[You may assume a negligible contribution to the total optical depth from molecular scatter
and trace gas absorption]
6. Briefly outline the different techniques currently used to observe aerosol from space.
What are the main strengths and weaknesses associated with each method?
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