50
J. M. Prospero
Bubble rupture produces aerosol droplets in a size range that extends from under
about 0.01 11m diameter to more than 10 11m diameter (Fitzgerald 1991; O'Dowd et al.
1997). The concentration and size distribution of sea salt aerosols is dependent on a
number of factors, especially the wind speed and the altitude above the sea surface
(Fitzgerald 1991). Table 2.1 shows that on a mass basis, sea salt aerosol is dominant at
all sites except Izana, which is located on a mountain (altitude 2360 m). As shown in
Fig. 2.1, the size distribution of sea salt is such that the total surface area and volume
distribution peaks in the supramicrometre size range (O'Dowd et al. 1997). Thus, sea
salt aerosol offers a large surface area for chemical and physical reactions with other
aerosol species and with gases (O'Dowd et al.1997). Such large particles have relatively
high settling velocities. For example, a 10 11m diameter sea salt aerosol particle has a
settling velocity of 0.307 cm S-1 (0.265 km d- 1 ). Consequently, most of the sea salt mass
has a relatively short residence time in the atmosphere, and much of the very large
annual production rate (I 000-3 000 Tg yr- 1 ) is rapidly returned to the ocean. As we
shall see in a later section, sea salt aerosol can serve as a very effective mechanism for
removing many atmospheric species, both gaseous and aerosol, from the marine
boundary layer (MBL).
2.2.2
The Contribution of Sea Salt to Submicrometre Aerosol
Although most of the sea salt aerosol mass is in the size fraction above 1 11m diameter,
a small but significant fraction of the sea salt aerosol is in the submicrometre fraction
(O'Dowd et al. 1999). Quinn et al. (2000) report on measurements of submicrometre
aerosol made in the Pacific Ocean. They collected the aerosol fraction below 111m diameter and analyzed it for a wide range of soluble components including nss-SO~- and
sea salt.
Quinn et al. (2000) also weighed each of the filters so as to obtain a total aerosol
mass below 111m. The total concentration of measured ions is always less than the total weighed mass; the difference is referred to as the "residual mass" which includes
unanalyzed components such as mineral dust, organic material, soot, etc. Figure 2.6
shows the concentration of these three components (nss-SO~-, sea salt, and residual
mass) for various latitude bands in the Pacific Ocean. A number of features stand out
in the figure. First of all, in most latitude bands the concentration of submicrometre
sea salt is comparable to that of nss-SO~-. The exceptions are in the mid-latitude North
Pacific (20-40
0
N), where submicrometre sea salt is much lower and the high latitude
(40-60 0 S) South Pacific, where it is much higher. The relative sea salt concentrations
reflect the large differences in wind speeds in these latitude bands. In the case of nssSO~-, continental sources impact on the North Pacific, whereas over most of the South
Pacific the only significant source of nss-SO~- is the oxidation of DMS (see below).
The other notable feature of this figure is that the residual mass is very large in most
latitude bands, usually comparable to (and in some cases greater than) those of
nss-SO~- and sea salt. One might expect such a result for the North Pacific because of
the impact of the transport of soil dust, soot, and organics from the continents, especially Asia as evident in Table 2.1. But the residual mass is also quite large in the South
Pacific where there is relatively little land mass and the transport distances are very
great.
J. M. Prospero
Bubble rupture produces aerosol droplets in a size range that extends from under
about 0.01 11m diameter to more than 10 11m diameter (Fitzgerald 1991; O'Dowd et al.
1997). The concentration and size distribution of sea salt aerosols is dependent on a
number of factors, especially the wind speed and the altitude above the sea surface
(Fitzgerald 1991). Table 2.1 shows that on a mass basis, sea salt aerosol is dominant at
all sites except Izana, which is located on a mountain (altitude 2360 m). As shown in
Fig. 2.1, the size distribution of sea salt is such that the total surface area and volume
distribution peaks in the supramicrometre size range (O'Dowd et al. 1997). Thus, sea
salt aerosol offers a large surface area for chemical and physical reactions with other
aerosol species and with gases (O'Dowd et al.1997). Such large particles have relatively
high settling velocities. For example, a 10 11m diameter sea salt aerosol particle has a
settling velocity of 0.307 cm S-1 (0.265 km d- 1 ). Consequently, most of the sea salt mass
has a relatively short residence time in the atmosphere, and much of the very large
annual production rate (I 000-3 000 Tg yr- 1 ) is rapidly returned to the ocean. As we
shall see in a later section, sea salt aerosol can serve as a very effective mechanism for
removing many atmospheric species, both gaseous and aerosol, from the marine
boundary layer (MBL).
2.2.2
The Contribution of Sea Salt to Submicrometre Aerosol
Although most of the sea salt aerosol mass is in the size fraction above 1 11m diameter,
a small but significant fraction of the sea salt aerosol is in the submicrometre fraction
(O'Dowd et al. 1999). Quinn et al. (2000) report on measurements of submicrometre
aerosol made in the Pacific Ocean. They collected the aerosol fraction below 111m diameter and analyzed it for a wide range of soluble components including nss-SO~- and
sea salt.
Quinn et al. (2000) also weighed each of the filters so as to obtain a total aerosol
mass below 111m. The total concentration of measured ions is always less than the total weighed mass; the difference is referred to as the "residual mass" which includes
unanalyzed components such as mineral dust, organic material, soot, etc. Figure 2.6
shows the concentration of these three components (nss-SO~-, sea salt, and residual
mass) for various latitude bands in the Pacific Ocean. A number of features stand out
in the figure. First of all, in most latitude bands the concentration of submicrometre
sea salt is comparable to that of nss-SO~-. The exceptions are in the mid-latitude North
Pacific (20-40
0
N), where submicrometre sea salt is much lower and the high latitude
(40-60 0 S) South Pacific, where it is much higher. The relative sea salt concentrations
reflect the large differences in wind speeds in these latitude bands. In the case of nssSO~-, continental sources impact on the North Pacific, whereas over most of the South
Pacific the only significant source of nss-SO~- is the oxidation of DMS (see below).
The other notable feature of this figure is that the residual mass is very large in most
latitude bands, usually comparable to (and in some cases greater than) those of
nss-SO~- and sea salt. One might expect such a result for the North Pacific because of
the impact of the transport of soil dust, soot, and organics from the continents, especially Asia as evident in Table 2.1. But the residual mass is also quite large in the South
Pacific where there is relatively little land mass and the transport distances are very
great.