Aerosols and Climate Postgraduate lecture course 06/02/09 © Imperial College LondonPage 1.

Aerosols and Climate

Postgraduate lecture course 06/02/09

© Imperial College LondonPage 1

Outline


Part I

What is an aerosol?

How do aerosols affect climate?

Future climate?

Part II

Measuring aerosol from the ground, air and space

What is an aerosol?


Definition:

An aerosol is a suspension of tiny particles in air

Characteristics:

Origin: Natural or anthropogenic

Size: nanometers – 10 m+

Concentrations: Typical~1000-10000 cm-3 but up to 109 cm-3

Climate impact: diverse!

What is an aerosol? I: Origin


Natural

Anthropogenic

And they can mix Same aerosol type can be produced both naturally and anthropogenically

Primary and secondary

© Imperial College LondonPage 5Courtesy G. Mann

What is an aerosol? I: Origin


IPCC, 2001

Sources in kg km-2 hr-1

Page 7

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…

10000© Imperial College London

AccumAitken CoarseNucln

10-3 10-2 10-1 100 101 102

Cloud and precipitation physics

Atmospheric electricity

Atmospheric radiation and optics

Air chemistry and pollution

Aerosol diameter (m)

What is an aerosol? III: Lifetime


Diameter (m)

Designation Nucleation/Aitken nuclei Accumulation Coarse

Sources

Sinks

Liftetime

Combustion

Gas-to-particle conversions

Windblown dustsFly-ash,

sea-salt, pollensCoagulation of

Aitken nuclei

Coarse emissions from industriesCloud droplet

evaporation

CoagulationCapture by cloud particles

Precipitation scavenging

Dry fallout

Less than an hour in polluted air or clouds

Days Hours to days

Minutes to hours

10-3 10-2 10-1 100 101 102 103


What is an aerosol? IV: Concentrations

Typically vary due to location

But also vary for same generic aerosol ‘type’ due to meteorological conditions


Role in the climate system: one example


Direct Radiative forcing

CO2 increase

Lower atmosphere warms

Stratosphere cools

Positive 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, Ts ~ RF where is the climate sensitivity parameter



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



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!



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…


Calculating key aerosol optical properties

Assumption of particle shape + appropriate scattering code

PROCESSING

Mass extinction coefficient, ke

Single-scattering albedo, o

Scattering phase function

OUTPUTS

Size distribution

Chemical composition (complex refractive index)

INPUTS

Particle diameter (m)

Peak extinction

e.g. Spheres: Mie theory

Spheroids: T-Matrix

NB size parameter: 2r/


Single scattering albedo

o = ks / ke and ke = ks + ka

where ks is the mass scattering coefficient and ka is the mass absorption coefficient

NB: Instead of ke (ks, ka) can also use:

Extinction coefficient, e = 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

Particle diameter (m)

n = 1.5-0.005i

n = 1.37-0.001i


The phase functionDefinition: ‘The angular distribution of scattered light intensity at a given

wavelength’

Isotropic scattering, P(,,m) = 1

Forward scatter

Back scatter

where is the scattering angle, m is the complex refractive index and dis an element of area

4

1m)dA,,(P4

1

cossinsincoscoscos oo

Taking o = 0 and assuming spherical scatterers:

2

0 0

1d d sin)(P4

1


The phase function

Definition: ‘The angular distribution of scattered light intensity at a given wavelength’

Related terms:

Asymmetry Parameter, g

Backscatter ratio, b

Gives idea of scatter direction

+ve: forward scatter

0: isotropic

-ve: back scatter

Proportion of radiation scattered into backwards hemisphere

0

d cossin)m,,(P2

1m),g(

0

2

d sin)m,,(P

d sin)m,,(P

)m,(b


Critical single-scattering albedo

How can we estimate whether forcing is positive or negative for a given set of conditions?

Haywood and Boucher, 1999

b = 0.1 b = 0.2 b = 0.3



What about LW?

Only really an issue for aerosol types with large coarse mode population: most interest on mineral dust from anthropogenic activity…

NB1: Forcing magnitude strongly dependent on surface/atmospheric temperature contrast: sign can change

Reduction in OLR

Positive forcing

Local surface and atmospheric heating +LW

-LW

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



…but some work suggests urban pollution, pollen outbreaks etc. also directly affect LW

Lubin et al., 1994 Spankuch et al., 2000


And Feedbacks…

A ‘simple’ example: absorbing dust over desert in daytime

+LW

-LWFeedback effect on OLR

Reduces reflected SW flux at TOA and surface

Reduces OLR, enhances downwelling flux at surface

Instantaneous

Surface cools atmosphere warms

Surface and atmosphere warmIndividual response

Atmos warms, surface?Combined response


Enough of clear-skies: cloud-aerosol effects or Indirect aerosol forcing

COOLING OVERALL COOLING WARMING

IPCC, 2007Glaciation Indirect Effect


More tenuous perhaps:

Observations: no of droplets does increase but concurrent widening of cloud droplet spectrum: ‘First Dispersion Effect’

Rotstayn and Liu, 2003

Could partially offset 1st indirect effect but countered by Lu and Seinfeld, 2006

Simulations suggest an associated increased precipitation amount:

‘Second dispersion Effect’

(e.g. Roelofs and Jongen, 2004)


Highly uncertain in terms of climate impact

Efficacy = i/CO2

IPCC, 2007


Future Climate?

Courtesy K. Carslaw

Mitchell et al., 1995

Consistent with:

(a) Global Dimming

Stanhill and Cohen, 2001


Future Climate?

Courtesy K. Carslaw

Consistent with:

(b) Global Brightening

Mitchell et al., 1995

Wild et al., 2005


Future Climate?

Andreae et al., 2005

Projected changes will exacerbate GHG effects

IPCC TAR prediction

Key: understanding present day forcing and sensitivity of climate to aerosols…


Observing aerosol

dz

e055055 dz k

Average of nine model predictions

055


Observing aerosol – Ground based

AErosol ROBotic NETwork (AERONET)

http://aeronet.gsfc.nasa.gov/

20071994



AERONET measurements directly provide:

∞ V() and, Beer-Lambert:

)m)(exp(R

R)(V)(V TOT

2

mo

s

dzds

Langley plots:

Leads to

)(V'ln))((m)V(ln oTOT

m

dy/dx

And

()a = ()TOT - ()t - ()r

Plane parallelm = sec s

In reality,

s < 60°, m ~ sec s

Measurements at several s, so also provides Ångström coefficient,

j

i

aj

ai

a

ln

)()(

ln

~lnd

)(lnd



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

))(m),r(N(I),(Ior )),(P);();((I),(I aa aoaa

Then: variability in clear-sky downward solar radiance assumed dominated by aerosol

© Imperial College London

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 PCASPSID-1/SID-2FFSSP

0.05-1.5 m1-30 m1.5-20 m

Aerosol optical properties TSI NephelometerRadiance research PSAP

= 0.45,0.55,0.7 m = 0.568 m

Aerosol chemical comp Filters 2 ranges for inorganics and carbon

Broadband irradiance BBRs 0.3-3 or 0.7-3 m

Spectral radiances SWSARIES

303.4-1706.5 nm3.33-18.18 m

Spectral irradiances SHIMS 303.4-1706.5 nm

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

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

FFSSP: Fast Forward Scattering Spectrometer Probe

As above, but only considers ‘forward’ scattered light


In situ size

Page 35

SID2

FFSSP

PCASP

GERBILS campaign, June 2007

Courtesy S. Osborne


In situ optical properties

Nephelometer

Measures total scatter and hemispheric backscatter

Known light source, known path length: obtain scattering coefficient

Page 36

Particle soot absorption photometer (PSAP)

Measures absorptance through a filter

Provides absorption coefficient

Combination of two allows derivation of extinction coefficient


In situ optical properties

Page 37

Courtesy S. Osborne and B. Johnson

Excellent detail and opportunity to study aerosol case studies but ‘snapshot’ in nature


Observations of aerosol – from space

Page 38

Geostationary: rotates with Earth. Limited view but excellent time resolution

Basics: 3 types of orbit

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


Surface

representation

Atmospheric

profile

Aerosol representation

Radiative Transfer code

Channel filter functions

Simulated quantity, (s,v,r,)

Set of geometries and s

SUN

r

LUT GENERATION

Observed quantity, (s,v,r

Retrieved

geometrical restrictions



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)

But which points to perform retrieval on?



Page 41

Longest established method: Narrow band radiometers

Different spectral regions exploited for different purposes

(i) Visible reflectancesoch

chch F

L

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

Jacobowitz et al., 2003


12:00 UTC 04/03/04

3gen OPAC Nonspher DesMODIS (Terra)

SEVIRI

3. Dust detection and loading


Brindley and Ignatov, 2006



Page 44



(ii) Visible reflectances may also be used over other dark targets but extra information is required

Kaufman et al., 1997

Relies on relationship between s in different spectral channels



MODerate Imaging Spectroradiometer (MODIS)



Page 46



(iii) UV radiances: exploits low surface reflectance in this regime (bar snow)

Development of UV aerosol index, UVAI

)R(I

Ilog100UVAI

*354

calc354

obs354

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



Page 47



(iv) IR radiances: avoids reflectance issue, exploits contrast between Tsfc and

Tdust. Sensitive to dust height, atmospheric profile and surface emissivity

AOD from SEVIRI, 1200 UTC, 18th June, 2007

e.g. Legrand et al., 2001, Brindley, 2007



Page 48

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

70.5° backward viewR (nadir), G (70.5 ° forward), B(70.5 ° back) Diner et al., 2001



Page 49

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 m), G (0.670 m), B ( 0.865 m)

‘Natural’ light Polarised light

Note: larger particles have smaller polarisation signal

Polarisation also affected by shape of aerosol

e.g. Deuzé et al., 2001, 2002



Page 50

Alternatives:

(vii) Use active techniques

Example: Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)

532 nm total

532 nm perp

1064 nm total

Cirrus

Dust Biomass Measures backscatter.

LIDAR ratio, = extinction/backscatter

Requires modelling of expected for different atmospheric components


Breon et al., 2002

Possibility to investigate indirect effects…

POLDER



…and semi-direct effects

MODIS



UK Air Quality Network

Attempts to relate PM measurements to AOT

Pere et al., 2009

Observations of aerosol – from space Relationship with air quality…



Page 54

The Future?

GLORY: aims to measure total solar irradiance and aerosol/cloud properties

Cirrus

Dust Biomass

Data product Range Uncertainty

AOD 0-5 0.02 (ocean) 0.04 (land)

Effective radius 0.05-5 m 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

Mishchenko et al., 2007


Cirrus

Dust Biomass

Problems

1. Continental aerosol with diameters > ~ 0.2 m have a size distribution which can be approximated by:

where N is the number concentration, D is the aerosol diameter, C is a constant. Sketch this distribution as a function of D on a log-log scale and interpret . Derive expressions for dN/dD and dV/d(log D) assuming spherical particles. For continental aerosol ~3. 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 m experience a ‘Stokes drag force’ given by 3Dv, where 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 m 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 = 1.7 x 10-5 kg m-1s-1]

D log CD) d(log

dN log


Cirrus

Dust Biomass

Problems

3. Define single-scattering albedo, o and explain what it means. What is the value of o 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 to cool or warm the surface?

Some of the vegetation is now burnt and 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 o? 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?


Cirrus

Dust Biomass

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 m is measured at 54 mV, while by 4 pm this reading had dropped to 40 mV. If Vo(0.44 m) is known to be constant at 75 mV what is the aerosol optical depth at this wavelength? Corresponding measurements at 0.87 m are 52 and 45 mV, with Vo(0.87 m) 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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Aerosols and Climate Postgraduate lecture course 06/02/09 © Imperial College LondonPage 1.

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