Prof.
Production mechanism, number concentration, size
distribution, chemical composition, and optical
properties of sea spray aerosols
[Source: daily mail]
Bubbles rise to the top and burst
producing residual particles
Markus Petters
Workshop, Raleigh June 4-6 2012
Wind entrains air into
surface waters, leading
to bubble formation
[Source: wikipedia]
The role of marine aerosols in determining
cloud albedo, precipitation, and structure
[Petters et al., JGR, 2006]
Quotes from an Andreae et al. (2009).
“The number size distribution of the sea salt aerosol extends well down into the
submicron range and indeed commonly peaks there”
It had long been thought that most marine sub-micron particles are primarily
comprised of sulphate derived from the oxidation of dimethyl sulfide emitted
from ocean surface and SO2 from long range transport of continental sources
and ship emissions.”
“… numerous recent studies have revealed a substantial organic presence even
in remote marine air.”
[O’Dowd et al. , Nature, 2004]
Hygroscopicity (diameter growth factor) as a
proxy for aggregate chemical composition
Organics: 𝜿 = 𝟎 − 𝟎. 𝟐
gf @ 80% = 1-1.2
Sulfates: 𝜿 = 𝟎. 𝟔 − 𝟎. 𝟕
gf @ 80% = 1.5-1.6
Internally mixed particle: 𝜿 = ∑𝝐𝒊 𝜿𝒊
 Low (< 0.6) measured hygroscopicities indicate
substantial contribution of organic compounds
Sea Salt: 𝜿 = 𝟏. 𝟑
gf @ 80% = 1.8
Model prediction over remote oceans:
0.5 < 𝜿 < 1.2
[Pringle et al., ACP, 2010, ECHAM/MESSy Atmospheric Chemistry (EMAC) model]
Measurements show generally 𝜿 < 0.5.
• Where is all the sea salt?
• Does sea-spray contribute to aerosol number?
• Does sea spray include a dominant organic
source?
Growth factor data from DYCOMS-II
(remote Pacific, 7 flights in July
Histogram of growth factor measurement
taken over the ocean
[Wex et al., GRL, 2010]
[Snider and Petters , ACP 2008]
Bottom up approach: lab studies combined
with bubble spectra
[Blanchard and Syzdek, JGR, 1988]
[Wu et al., 1992]
Bottom up approach: aerosol number
production/bubble scatter widely – do we
accept such large uncertainties?
[Blanchard and Syzdek, JGR, 1988]
[Lewis and Schwartz, 2004]
Organic coatings on bubbles
• The thickness of the organic coating for
oceanic bubbles ranges from 0.01 m
for lipids to 1 m for proteins such as
glycoproteins
• Coating occurs within a few seconds for
bubbles < 300 m
• Coatings stabilize the bubble potentially
through their surfactant properties
• Large bubbles (mm size) are not
completely coated
• Important for ocean optical properties
[Thorpe 1982, Yount 1979 and others]
[Chalmers and Bavarian, 1991]
Trying to understand production of organics
in the laboratory
filtered air
Aerosol
measurement
[Bates et al., 2012, JGR in press]
Organic fractions from sea-spray are all over the
map, but many studies show that it is difficult to emit
organics
Keene et al., JGR 2007
Facchini et al., GRL, 2008
Wex, Fuentes et al., AMET/AMT, 2010
Bates al. JGR, 2012
Modini et al. ACP, 2010
To what level do we need to
decompose the system? Single
bubble, single plume, single
whitecap, single model gridbox?
[Deane and Stokes, Nature, 2002]
Field vs. Laboratory Measurements
•
•
•
Very challenging measurement
environment
Difficult to isolate sea-spray from secondary
aerosol and free- troposphere entrainment
Difficult to control for key variables (wind
speed, sea-water composition, wind fetch,
wet and dry deposition, evolution of size
distribution, atmospheric turbulence …)
Courtesy of Jeff Reid
•
•
•
Studies in great detail are possible,
working towards mechanistic
understanding
Precise control over variables (bubble
size, water composition, surface layer)
Main critique: experiments are not
representative (bubble plume,
atmospheric turbulence …)
http://termserv.casaccia.enea.it/term/brivido/brivido.html
Different questions may need different foci
• Mass emissions (optical properties)
• Number emissions (CCN properties: size + chemical
composition)
• “Special” emissions (surfactants affecting CCN
properties, ice nuclei emissions)