Modelling the generation
of marine aerosols at the sea
surface*
E dw ard
C.
O C E A N O L O G IA , 26, 1988
PL ISSN 0078-3234
W hitecaps
M arine aerosol generation
Air-sea interaction
M onahan
University o f Connecticut,
M arine Sciences Institute,
Connecticut
M anuscript received February 24, 1987, in final form D ecem ber 28, 1987.
Abstract
The developm ent o f a m odel for the estim ation o f the local, instantaneous rate o f sea spray
production is presented.
1. Introduction
It has to date proved impossible to devise a m ethod to m easure directly
the actual flux of aerosols up out of the sea surface, in circumstances where
the surface was not sheltered, or conditions not otherwise modified from the
natural state. Since, under all but the highest wind conditions, the m arine
aerosol particles are prim arily produced by the bubble m ediated mechanisms
described by B lanchard (1963), it seems appropriate to construct a model
that explicitly describes the droplets produced by oceanic whitecaps ie by the
transient clusters of bubbles that appear on the sea surface as a consequence
of wave breaking.
2. The model
The basic form of the sea surface aerosol generation model, where
dF0
or
represents the num ber of aerosol droplets, per m icrom eter increment of
* The paper presented on 8 O ctober 1986 at the Seminar on Clim ate and Air-Sea
Interaction at the Institute o f O ceanology o f P olish Academ y o f Sciences, Sop ot (Poland).
droplet radius, produced per square meter of sea surface, per second, is as
follows (M onahan et al, 1982):
dF0
IF
. 8E
7F’
<"
where W is the fraction of the sea surface from which whitecap bubble
dE
clusters disappear per second, and — is the num ber of particles, per
or
m icrom eter radius increment, produced during the decay of a unit area
(1 m 2) of whitecap. Now W is given by (M onahan, 1971):
W = W t ~ 1,
(2)
where W is the instantaneous fraction of the sea surface covered by whitecaps, and x is the tim e constant that defines the exponential rate of decay of
the area of an individual whitecap. It should be noted that W is a quantity
that can only be evaluated from observations at sea (eg M onahan, 1971;
T oba and Chaen, 1973; Bortkovskii, 1983) while i, whose value should
ideally be assessed from cine-film or video observations of oceanic whitecaps,
can also be determ ined from observations of small-scale whitecaps produced
in the laboratory. A typical value of t is 3.5 seconds.
8E
The quantity — is evaluated from m easurem ents m ade in the laboratory
whitecap sim ulation tank. The most recent results were obtained using a 3 m
long tank with a deep central well. Two solitary waves, or under certain
conditions, two bores, are caused to collide in the center of the tank
(M onahan el al, 1982). The tank is covered with a plexiglas hood and the air
beneath this hood is continuously circulated through filters so that there are
negligible num bers of aerosol droplets present before a breaking/colliding
wave event. The aerosol droplets injected into this enclosed volume of air
during the decay of the whitecap produced in the sim ulation tank by a
breaking wave are counted using a variety of aerosol counters/detectors.
Specifically, dE/dr is given by:
!-*(!> ·"·
where A
the total num ber of aerosol particles, per m icrom eter radius
increment, generated as a consequence of a single breaking wave event, and
A 0 is the initial surface area, in square meters, of the resulting whitecap.
A typical value for A 0 is 0.35 m 2.
The most recent version of
' s g*ven by (M onahan et al, 1983):
AC5) = 4·40' 105''_3(1+0-°57r105)·iO1·19e B l,
(4)
where r is the radius, in m icrometers, of the m arine aerosol droplets in
equilibrium at 8 0 % relative hum idity, and B is given by:
(0 .3 8 0 - log r)
0.650
The last term that must be quantified before this model can be used to
assess the actual m agnitude of the local, instantaneous, rate of m arine
aerosol production is W, the instantaneous whitecap coverage. W hitecap
coverage is strongly dependent on 10 m elevation wind speed, U, for the
reasons set forth in W u (1979). M onahan and O ’M uircheartaigh (1980)
obtained the following optim al expression describing the dependence of W
upon U:
W = 3.84· 10~6 · I / 3,41.
(5)
3. Conclusions
The model described above has been used, with some success, as the sea
surface source function in com puter models of the sea salt aerosol population
of the m arine atm ospheric boundary layer by Burk (1984) and by Stram ska
(1982, 1987).
Essentially, the same com posite expression has recently been derived
using a som ew hat different modelling approach involving the initial bubble
plume volume instead of the initial whitecap area (M onahan, 1986). A quite
different, com plem entary approach to modelling the production of marine
aerosols by whitecaps, incorporating m easurem ents obtained in the lab o rato ­
ry from a covered weir/waterfall, has been set forth by C ipriano (1979) and
Cipriano and Blanchard (1981).
All of the models developed to date deal prim arily with the production of
the supra-m icrom eter, essentially jet-droplet, aerosol. Recently, the gross
production of sub-m icrom eter, prim arily film-droplet, aerosol from lab o rato ­
ry whitecaps has been m easured (Cipriano et al, 1987). The actual size
spectrum of the film-droplets arising from the decay of a whitecap is yet to
be described. At very high wind speeds, spume drops are produced directly
by the m echanical disruption of wave crests (M onahan et al, 1986). The
m agnitude of this supplem entary source of large spray drops has not yet
been established.
Any future model of the aerosol production m ediated by bubbles should
take into account the observation that oceanic whitecap coverage depends
on environm ental factors in addition to wind speed (Bortkovskii, 1983;
M onahan and O ’M uircheartaigh, 1986).
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