Measurements of the Climateforcing Properties of Atmospheric Aerosols

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Measurements of theClimate-forcing Properties of
Atmospheric Aerosols

• John Ogren
• NOAA Climate Monitoring and Diagnostics Laboratory

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Aerosol Effects on Climate

• Direct Effects
• Aerosols scatter and absorb visible and infrared
  radiation
• Could cause warming or cooling
• Depends on size distribution and index of
  refraction of the particles, as well as albedo of
  the underlying surface

• Indirect Effects
• Cloud condensation nuclei (CCN) and ice nuclei
  (IN) are precursors for all cloud particles in
  the atmosphere
• CCN and IN influence cloud optical and
  microphysical properties
• Possible effects include changes in Earth's
  albedo and changes in hydrological cycle

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Estimates of Global Mean Radiative Forcing
Confidence level High Low Low Low Very Very Very
Very low low low low
"The balance of evidence suggests a discernible
human influence on global climate (IPCC, 1996)"
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Placeholder (4 slides)

• Kiehl and Briegleb, GHG forcing
• Kiehl and Briegleb, sulfate forcing
• Kiehl and Briegleb, GHGsulfate forcing
• Santer et al., spatial pattern matching

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CAVEAT

• Aerosol cooling and greenhouse warming cannot
  offset each other, because
• The effects occur at different times and in
  different parts of the atmosphere,
• Leading to a change in the differential
  heating/cooling that drives the general
  circulation of the atmosphere.

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Uncertainties Of Parameters Used To Estimate
Direct Aerosol Forcing Of Climate
Source Penner et al. (1994)
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Scientific Questions

• What are the means, variabilities, and trends of
  the climate-forcing properties of different types
  of aerosols?
• What chemical species are responsible for these
  properties?
• What are the factors that control these
  properties?

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Sampling Strategy forCMDL Aerosol Monitoring
Program

• We can never achieve adequate global coverage
  with a ground-based sampling program.
• Recognizing this, our strategy is to characterize
  key optical, chemical, and microphysical
  properties of a number of aerosol types that are
  needed for development and validation of global
  models and satellite data retrieval algorithms.

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Key Aerosol Types For Evaluating Climate Forcing
By Anthropogenic Aerosols
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CMDL Aerosol Network
Barrow
Sable Is.
Hungary
Bondville
Mauna Loa
So. Great Plains (ARM)
Samoa
South Pole
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Parameters controlling aerosol forcing
D daylight fraction S0 solar constant Tat atmosphe
ric transmission Ac cloud fraction Rs surface
albedo
DF average aerosol forcing at top of
atmosphere (TOA) d aerosol optical depth
Source Haywood and Shine (1995)
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Linking up-scatter optical depthto aerosol
chemical composition
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Aerosol climate-forcingproperties observed by
CMDL

• Routinely
• sspd, d, w0d
• wavelength dependence of sspd, d
• and at regional sites
• bd (surrogate for b)
• M (coarse/fine, total and ions)
• a (mass and ions)

• Intensive campaigns
• fs (RH), fb (RH)
• vertical profiles of sspd, w0d, bd, l-dependence
  
• Missing
• fw (RH), f (RH,z)
• a (z)
• M (carbon, dust)

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KEY FEATURES OF CMDLAEROSOL SAMPLING SYSTEM

• Heated inlet, so that all size cuts and optical
  measurements are performed at a low relative
  humidity. The sample is heated just enough to
  maintain the RH below 40 and the temperature
  below 40 C.
• Measurements performed on two size-fractions, Dp
  lt 1.0 mm (fine particles) and 1 lt Dp lt 10 mm
  (coarse particles) diameter.
• Optical and chemical measurements are
  synchronized so that they can be related
  quantitatively.
• Real-time contamination control using Ntot, wind
  speed, and wind direction.
• Highly-automated so that minimal operator
  attention is required.

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NOAA Aerosol Sampling Inlet

• 10 m agl inlet stack, 20 cm diameter, 50 cm/s
  flow velocity
• 2.4 m stainless steel tube, 5 cm diameter, 130
  cm/s velocity, heated to RH ? 40
• Split into 5 lines, 1.9 cm dia, 30 lpm each
• 10 mm impactor, then switched 1 mm impactor
• Filtered pump exhaust

Barrow, Alaska
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NOAA Aerosol Sampling System

• TSI integrating nephelometer (ssp, sbsp, 3
  wavelengths)
• Radiance PSAP (sap, 550 nm)
• Real-time contamination control using TSI CN
  counter and winds
• Sample handling in glove box

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NOAA Aerosol Sampling Rack

• Switched impactor for D lt 1 mm and D lt 10 mm size
  ranges.
• Large particle (1 lt D lt 10 mm) impactor and
  8-filter holder for submicrometer particles.
• Gravimetric and IC analyses.
• Automated CO2 span check system for nephelometer.

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Arctic Haze has Decreasedat Barrow, Alaska since
the mid-1980s
Parameter plotted is atmospheric aerosol light
scattering coefficient at 550 nm (1/Mm)
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Aerosol single-scattering albedoat Bondville,
Illinois
Values are daily averages for l550 nm, RHlt40,
Dlt1 mm
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Aerosol Chemical Compositionat Bondville,
Illinois (1995)
Speciation of other based on 4-weeks of data
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Sensitivity of aerosol forcing to observed
variation in aerosol properties at Bondville
frequency
forcing per unit optical depth (W m-2)
IPCC (1995) temperature change predictions used a
single value and considered only scattering.
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3-D Isentropic Airmass Trajectories

• Sector boundaries guided by SO2 emission field
• Cases were included only if the trajectory stayed
  in a single sector for five days prior to
  arriving within 200 km of Sable Island
• Data averaged for 2 hours, centered on trajectory
  arrival time

Clean Continental
Polluted Continental
Marine
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Wavelength-dependence of aerosol scattering is
controlled by coarse/fine mix
Ångström exponent (å)

• Daily averages from Sable Is., 8/92-6/97,
  550/700 nm wavelengths

f
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Aerosol Extensive Properties atSable Island by
Trajectory Sector
Light scattering (Mm-1)
Light absorption (Mm-1)
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Aerosol Intensive Properties atSable Island by
Trajectory Sector
hemispheric backscatter
single-scattering albedo
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Hygroscopic growth at Sable Island

• polluted

• marine
• polluted

fb(RH)
fssp(RH)
relative humidity
relative humidity
1. Chemical composition controls hygroscopic
growth.
2. Backscatter fraction decreases with increasing
RH.
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Aerosol Forcing Efficiency (W m-2)at Sable
Island by Trajectory Sector
absorption effect (D lt 1 mm)
size dependence
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Calculation of aerosol opticaldepth from surface
observations
fs(RH) 1.5 ? 0.2
calculated fromssp, f(RH), lidar
measured with shadowband radiometer
Measurements from Southern Great Plains site.
Lidar and radiometer data provided by DOE/ARM
program.
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Surface aerosol properties in southeast USA may
represent the column
altitude (m agl)
Results are for RH lt 40, D lt 1 mm.
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NOAA/CMDL Measurements of Single-scattering
Albedo
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Aerosol RadiativeProperties in Mexico City
single-scattering albedo
light scattering coefficient (Mm-1)
Black bars denote night-time. Observations were
at a wavelength of 550 nm and relative humidity lt
40.
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CMDL Aerosol Monitoring Highlights

• Observations of aerosol climate forcing
  properties reveal pronounced differences for
  different aerosol types, with variabilities that
  are large compared with the values used in
  current models.
• More species than sulfate make significant
  contributions to aerosol radiative forcing.
• Aerosol hygroscopic growth depends strongly on
  chemical composition.
• Surface measurements can be representative of the
  vertical column.

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Conclusions

• DF/Dd is a useful concept for combining different
  measurements and for comparing results from
  different cases or sites
• Consideration of scattering only suggests that
  some models may be underestimating the sulfate
  forcing by about 30
• Aerosol radiative properties exhibit considerable
  variability, in both upscatter fraction and
  single-scattering albedo, which is not included
  in current models