Marine Aerosols, Clouds and Climate

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Marine Aerosols, Clouds and Climate

• An Overview

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Definition of Marine Aerosol?

• Natural marine aerosol sea-salt, sulfates,
  organics

• Advected marine aerosol soil-dust, sulfates,
  organics, black carbon, nitrates. Large amounts,
  some deposited to ocean and some transported long
  distances

From Husar et al. 1997
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Distribution of Marine Aerosols

• Northern Hemisphere marine environment is
  polluted, even away from major sources

From Heintzenberg et al. 2000
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Marine Aerosols and Clouds

• Clouds transport and transform marine aerosols,
  through chemical and physical processes
• Marine clouds cool the surface by reflecting
  solar radiation

Modified from Hobbs 2000
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Marine aerosols Sulfate

• Dimethylsulfide gas from phytoplankton released
  to atmosphere produces SO2, H2SO4 eventually
  sulfate aerosol
• New sulfate aerosol can act as cloud condensation
  nuclei, or CCN
• Clouds w/ more drops reflect more light, cool
  surface--feedback??

CLAW hypothesis, courtesy M. Walkington
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Marine aerosols Sulfate

• New data suggests sea-salt or clouds scavenge
  much of the SO2 produced from DMS. Then converted
  to SO42- (internally mixed). Cloud drop N less
  affected. 

• In regions w/ low sea-salt and clouds, new H2SO4
  particles can form (then partially neutralized)
• A significant source of new particles is actually
  free troposphere near convection, where low
  particle surface area and cold temperatures favor
  new particle formation
• New particles grow and are entrained into
  boundary layer, where they can act as new CCN

H2SO4
From Clement et al. 2001
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New Particle Production in RICO
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Marine Aerosols Sulfate

• Limited S isotopic measurements of aerosol
  indicate less than 35 of S in No Hem is biogenic

From Heintzenberg et al. 2000
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Marine aerosols Sea-salt

• Sea-salt in various sizes produced by bubbles
  breaking waves

• Hygroscopic, liquid at high humidity and can
  dominate light scattering in remote areas (direct
  effect)

From O'Dowd et al. 1997
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Marine aerosols Sea-salt

• Submicron sea-salt important in increasing
  number of droplets
• Large and "giant" sea salt low in conc. but
  important in forming large cloud drops, which
  deplete water through drizzle

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Sea-salt Effects of Giant Nuclei

• Giant sea-salt particles produced by high wind
  speeds are predicted to dramatically increase
  rain rate

From Jensen, 2004
Anthropogenic giant nuclei?
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Marine aerosols Sea-salt/Sulfate

• SO2 is taken up by sea-salt aerosol and by cloud
  drops formed on sea-salt and is converted to
  sulfate ("non-sea-salt sulfate"). "Internally
  mixed" aerosol is mix of sea-salt and NSS-sulfate.

• Smaller sea-salt particles have higher
  surface/volume ratio and so react more quickly w/
  avail. SO2
• This reactivity is important for clouds and
  climate

.04 ?m
5 ?m
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Marine aerosols Sea-salt/Sulfate

• Internal mixing of sea-salt sulfate is
  important because it reduces the number of
  potential new aerosol particles, which reduces
  cloud drop number

From O'Dowd et al. 1999
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Marine aerosols Organics

• Large amount of aerosol mass may be organic, esp.
  in So Hem.
• Wide open research area, speciation and
  cloud-forming abilities virtually unknown

• Truth or fiction? Organic coatings on sea-salt
  or sulfate CCN may slow droplet growth and reduce
  cloud-forming ability

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Marine Aerosols Organics

• Some natural organics, by also produced by
  industry, biomass, transported long distances
• Organics mix w/ sulfate in polluted regions

Courtesy Jim Anderson, ASU
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Organics and Clouds

• EELS reveals that some nuclei primarily C, with
  smaller amounts of N and S

• Cloud droplets downwind of India evaporated and
  their nuclei analyzed by electron microscopy

• At least some organic aerosols are good cloud
  nuclei, challenge is determining which ones and
  incorporating into models

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Marine Aerosols Dust "The dust falls in such
quantities as to dirty everything on board, and
to hurt people's eyes vessels even have run on
shore owing to the obscurity of the atmosphere".
--Darwin, 1823
From Heintzenberg et al. 2000
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Marine Aerosols Dust

• Nutrients in soil dust (Fe, Mn) important for
  growth of ocean microorganisms
• Pathogens may affect coral reefs
• aspergillosis on sea fan

• Dust transported long distances over oceans.
  African dust reaches Caribbean even US East
  Coast

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Marine Aerosols Dust

• Soil dust may act as a CCN (if coated w/ soluble
  substances) or an ice nucleus
• Possible influence on tropical cloud systems?

From Dunion et al. 2004
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Marine Aerosols, Clouds Climatefirst indirect
aerosol (Twomey) effect

• Marine clouds naturally have low droplet number,
  large droplet size, drizzle a lot
• Advected marine aerosols perturb this state
• Aerosols act as CCN
• more CCN --gt more, smaller droplets
• Smaller drops reflect more sunlight (increase
  cloud reflectance or albedo), cool earth. Assumes
  constant liquid water path
• One of largest uncertainties in prediction of
  global climate change

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Marine Aerosols, Clouds ClimateIs indirect
effect valid for marine stratus?DYCOMS-II expt,
NE Pacific
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DYCOMS-II Expt., NE Pacific
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DYCOMS-II Expt., NE Pacific

• Polluted clouds have smaller, more numerous
  drops, but are they more reflective?
• Variations in liquid water path obscure cloud
  brightening effect of increased aerosol (Cotton
  seminar)

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Summary

• Ocean important source of sea-salt, sulfate
  aerosol, but advected aerosols dominate over much
  of Northern Hemisphere oceans
• Atm. chemistry, ocean productivity and cloud
  properties are perturbed by these advected
  aerosols
• Interactions between these aerosol types
  important
• Increased aerosol DOES affect marine cloud
  properties (increase number, decrease size of
  drops). However, liquid water path differences
  may act to partially offset cooling effect on
  climate.

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Areas Needing Research

• Organic aerosols source (natural vs.
  anthropogenic), speciation, water solubility,
  mixing state, new particle formation
• Sea-salt size distribution, mixing state w/
  sulfate and/or organics, giant nuclei N conc.
• Sulfates source (isotopic measurements), mixing
  state w/ sea-salt, organics
• Dust impact on cloud formation, amount deposited
  to oceans, role of different nutrients and impact
  on ocean productivity