2005 SOLAS Summer School Introduction to Marine Aerosols

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2005 SOLAS Summer SchoolIntroduction to Marine
Aerosols
Eric S. SaltzmanEarth System ScienceUniv. of
CA, Irvine
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What driving marine aerosol research?

• geochemical cycles
• metals, nutrients, organics
• acidification (sulfur, nitrogen)
• climate change
• direct/indirect effects
• aerosol optical properties, aerosol/cloud
  interactions
• nutrients
• N deposition
• coastal, HNLC
• desert dust and iron deposition
• human health
• air quality, airborne pathogen transport

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Earths energy balance...
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Global mean radiative forcing of climatefor year
2000 relative to 1750 (IPCC)
Aerosol effects on radiation budget radiation

• Direct effects (cloud-free)
• scatter ? cooling
• absorption ? heating
• Indirect effects (clouds)
• more (but smaller) droplets ? scatter (Twomey)
• more droplets ? longer cloud lifetime (Albrecht)
• absorption ? heating ? evaporates clouds

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• The life cycle of marine aerosols...
• What are marine aerosols made of ?
• Where do they come from?
• How long do they stay in the atmosphere? How are
  they removed?
• How do they evolve while in the atmosphere?
• How do they interact with the climate system?
• forcing
• feedback

Note on observational constraints intensive
many param., poor coverage network good
coverage, few param. satellite excellent
coverage, limited interpretability...
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Things we know we know... (or dont)...
...there are known knowns. These are things we
know that we know. We also know there are known
unknowns that is to say we know there are some
things we do not know. But there are also
unknown unknowns. These are things we don't know
we don't know. Donald Rumsfeld

• Unknown knowns...?
• nucleation is rare
• sulfate is the source of CCN (cloud condensation
  nucleii) over the oceans
• sea spray doesnt make submicron aerosols"

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Terminology...

• aerosol - a dispersion of solid and liquid
  particles suspended in gas (air).
• note in common practice, aerosol is used to
  refer to the particles only!
• primary aerosol - emitted directly into the
  atmosphere.
• Saharan dust, sea spray, pollen, plant waxes,
  soot
• secondary aerosol - created by nucleation of new
  particles, aggregation of existing particles, or
  growth of preexisting particles from gas phase
  molecules (gas to particle conversion).
• either type ? natural, anthropogenic, or both
• internal vs. external mixtures in an internally
  mixed aerosol all particles have the same
  composition
• CN condensation nucleii - aerosols that scatter
  light at very high supersaturation levels
• CCN cloud condensation nucleii - aerosols that
  scatter light at very low superaturation levels
• How much aerosol is there?
• typically 10s 100s of ug/m3 (air density
  1kg/m3)

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Aerosol size distributions...
number distribution surface area distribution
volume distribution
(Seinfeld and Pandis)
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the log-normal aerosol size distribution...
Aitken mode
accumulation mode
number distribution surface area distribution
volume distribution
coarse mode
(Seinfeld and Pandis)
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The aerosol modes...

• Aitken mode 0.01-0.1 um
• accumulation mode 0.1-1 um
• coarse mode - gt1 um
• and sometimes, the elusive
• nucleation mode lt0.01 um

(C. Leck)
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Humidity and aerosol size...

• hygroscopic aerosols grow/shrink with RH (with
  hysteresis!)

deliquescence
efflorescence

• aerosol size strongly affects light scattering
  cross-section

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a process-oriented view of the size distribution
... ... reflecting competition between
production/transformation/removal
no ultrafines here... at the time, there was no
instrumentation to detect them
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why accumulation mode?
impaction, settling
diffusion, coagulation
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removal mechanisms... gravitational settling
coarse particles

• 10 um particle ? 1000 cm hr-1
• 1 um particle ? 10 cm hr-1

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Diffusion ...
You can estimate the distance a particle will
diffuse in a given time from the equation
where D is the diffusion coefficient
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The chemical perspective ... a chemical size
distribution

• 1. chemical size distributions resemble mass, not
  number
• 2. sulfate and organics dominate the accumulation
  mode, but theres a surprising amount of seasalt
• 3. there are a lot of unidentified organics
• 4. the coarse mode has the expected mechanically
  generated aerosols, but also nitrate and
  sometimes sulfate

Mass
(C. Leck)
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Marine aerosol system...
cloud processing nucleation
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Mineral Dust

• Dust (mineral aerosols)
• diameter size 2-300 µm
• main material sand, silt, clay
• includes essential trace metals such as Fe
• consists of insoluble and soluble fractions

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Seasalt aerosols...
wind? bubbles? spray whitecap coverage W a
U3!

• seasalt production via bubble bursting...
• film drops (many, small, drags along surface
  organics)
• jet drops (fewer, larger, bulk)
• spume drops (larger still)

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Seasalt aerosols...
Film Drops
Jet Drops
Number
10 ?m
1 ?m
80 nm
20 nm
Particle Diameter (Dry)
(from C. Leck)
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Seasalt number and mass as a function
of surface wind speed
seasalt particle number
seasalt mass
(compiled by Lewis and Schwartz, 2004)
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Seasalt mass fraction as a function of size
(Lewis and Schwartz, 2004)
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Marine aerosol mass fractions...
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The sulfur story (in brief) ...

• emissions fossil fuel SO2, volcanic SO2,
  oceanic DMS
• DMS oxidation ... gas phase ... complex!

(mod. from Yin et al., 1990)
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Happily, SO2 oxidation in the gas phase is
simple...

• but most SO2 oxidation occurs in the aqueous
  phase...
• some basics...

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heterogeneous oxidation of SO2

• in-cloud oxidation
• weakly buffered, pH 4
• oxidation by H2O2
• growth of CN, split Aitken mode
• seasalt aerosols
• strongly buffered by carbonate system
• rapid oxidation by O3
• slower oxidation by H2O2 (also OH, halogen
  radicals...)
• growth of existing particles, inhibits nucleation
  of new particles

(Chameides and Stelson, 1992)
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the single-particle view...
Murphy et al., 1998 Buseck and Posfai, 1999
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Organic aerosols - burning
soot elemental carbon formed in flames little
spectral abs. dependence carbon-only
brown carbon sugars alcohols aromatics di/tri
acids ketoacids hydroxyacids
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Marine-derived organic aerosols...
(ODowd et al., 2004)
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A surprise from the Arctic sea ice...
Bacterium
C. Leck
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More organics in marine aerosols ...
SEM micrograph of an Arctic marine aerosol
sample. Single particles (diameter 1 µm) and
groups of particles are lying between the fibers
of the filter. About half of the particles are
coated by an organic layer.
Negative TOF-SIMS spectra of marine aerosol.
mass 255 ? C15H31COOH (palmitic acid)
C15
C14
C15
C17
C18
Tervahattu et al., 2002
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Iodine and nucleation...

• coastal I emissions
• ? I2O5
• macroalgae
• CH2I2, I2 ?
• broader importance?

ODowd et al., 2002a,b
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Multi-year time series of bulk aerosol chemistry
at island stations...
eg. Prospero et al., 2003
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Modeled aerosol sources...
annual average sources (kg km-2 hr-1)
IPCC, 2001
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Aerosol properties from space...

• mineral dust
• urban pollution
• biomass burning
• aerosol size info

Data from POLDER-1 (IPCC, Deuze et al., 1999)
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Understanding forcing/feedback requires realistic
aerosols in climate models...
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Whats left to do...?

• in situ aerosol/cloud radiation experiments!
• shape non-sphericity
• mixing state
• life cycle of marine aerosol organics, marine
  microlayer...
• gas phase ? aerosol chemistry (sulfur, iodine,
  organics)
• aerosol ? gas phase chemistry (organics,
  halogens)
• depositional fluxes
• coupled aerosol/chemistry/climate models
• marine aerosols are woefully undersampled!

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The end.