CHAPTER 2 . The Chemical and Physical Properties of Marine Aerosols: An Introduction
57
stated above, Kettle et al. (1999) could not find any systematic relationships that could
explain the observed distributions.
A second major uncertainty has to do with the chemical reactions of DMS in the
atmosphere and the subsequent conversion of these products to the aerosol phase. DMS
has a relatively short lifetime, about one day in the MBL under typical OH concentrations. Because of the high concentration (and large surface area) of sea salt aerosol in
the MBL and the high mobility of the newly-formed ultrafine particles, DMS reaction
products can rapidly diffuse to the existing aerosol phase as suggested in Fig. 2.1.
Sievering et al. (1992) suggest that a large fraction of the DMS-SOz reacts directly with
sea salt aerosols; because of the large settling velocity of these particles, this fraction
of the DMS-SO~- is rapidly recycled back to the ocean surface. From the standpoint of
climate processes, the critical question is: How much of the DMS-SOrSO~- goes into
the formation of new aerosol particles in the size range of (roughly) 0.1-1.0 Ilm diameter - that is, particles that are efficient both as scatterers of light and as cloud-droplet nucleating particles? It is only through the production of new particles in this size
range that DMS-SOrSO~- can have a significant impact on radiation and on cloud
nucleating processes. If the DMS reaction products (i.e. SOz and SO~-) end up on the
surfaces of existing particles, primarily sea salt particles, then the DMS-SO~- source
will have little impact on climate. A number of major field campaigns have attempted
to address this issue (Bates et al. 1998; Raes et al. 2000). The general feeling at this time
is that under most conditions, there is relatively little production of new particles in
the MBL from DMS oxidation, because a large fraction of the DMS-SO~- ends up on
large sea-salt particles (Andreae et al. 1999, O'Dowd et al. 1997). However, recent work
suggests that the direct reaction of SOz with sea salt is not as important as suggested
by Sievering et al. (1992); van den Berg et al. (2000), using a sophisticated MBL chemical-physical model, found that the reaction of SOz with sea salt aerosol is quite complicated and that most SOz is oxidized in cloud droplets, not on sea salt aerosol.
The conclusion that new particle production is hindered in the MBL is generally
consistent with recent studies that show that the concentration of nss-SO~- over the
oceans cannot be readily related to ocean water concentrations of DMS. Furthermore,
it has not been possible to relate year-to-year variations in the concentrations of MSA
and nss-SO~- to observable climate changes. For example, measurements made over
the equatorial Pacific throughout the 1980s and 1990S do not show any systematic
change, despite the occurrence of major EI Nino events during that time period (Bates
and Quinn 1997).
Studies suggest that new particle production from DMS takes place primarily in
the outflow from clouds that tap into the MBL (Perry and Hobbs 1994, 1995; Clarke
et al.1998). MBL air rising through the clouds is stripped of particles and reactive gases
by cloud droplets. DMS, which is relatively insoluble and unreactive in the cloud environment, emerges from the top of the cloud into the middle and upper troposphere;
here, because of the low particle concentrations and intense sunlight, the photochemical reaction products of DMS have a high probability of combining (condensing and
coagulating) to form new particles. These newly-formed ultra-fine particles (smaller
than about O.Olllm diameter) are subsequently brought down into the MBL by subsiding air. In the MBL, the particles can grow and ultimately serve as cloud-nucleating particles that can contribute to the formation of new clouds, thereby continuing
57
stated above, Kettle et al. (1999) could not find any systematic relationships that could
explain the observed distributions.
A second major uncertainty has to do with the chemical reactions of DMS in the
atmosphere and the subsequent conversion of these products to the aerosol phase. DMS
has a relatively short lifetime, about one day in the MBL under typical OH concentrations. Because of the high concentration (and large surface area) of sea salt aerosol in
the MBL and the high mobility of the newly-formed ultrafine particles, DMS reaction
products can rapidly diffuse to the existing aerosol phase as suggested in Fig. 2.1.
Sievering et al. (1992) suggest that a large fraction of the DMS-SOz reacts directly with
sea salt aerosols; because of the large settling velocity of these particles, this fraction
of the DMS-SO~- is rapidly recycled back to the ocean surface. From the standpoint of
climate processes, the critical question is: How much of the DMS-SOrSO~- goes into
the formation of new aerosol particles in the size range of (roughly) 0.1-1.0 Ilm diameter - that is, particles that are efficient both as scatterers of light and as cloud-droplet nucleating particles? It is only through the production of new particles in this size
range that DMS-SOrSO~- can have a significant impact on radiation and on cloud
nucleating processes. If the DMS reaction products (i.e. SOz and SO~-) end up on the
surfaces of existing particles, primarily sea salt particles, then the DMS-SO~- source
will have little impact on climate. A number of major field campaigns have attempted
to address this issue (Bates et al. 1998; Raes et al. 2000). The general feeling at this time
is that under most conditions, there is relatively little production of new particles in
the MBL from DMS oxidation, because a large fraction of the DMS-SO~- ends up on
large sea-salt particles (Andreae et al. 1999, O'Dowd et al. 1997). However, recent work
suggests that the direct reaction of SOz with sea salt is not as important as suggested
by Sievering et al. (1992); van den Berg et al. (2000), using a sophisticated MBL chemical-physical model, found that the reaction of SOz with sea salt aerosol is quite complicated and that most SOz is oxidized in cloud droplets, not on sea salt aerosol.
The conclusion that new particle production is hindered in the MBL is generally
consistent with recent studies that show that the concentration of nss-SO~- over the
oceans cannot be readily related to ocean water concentrations of DMS. Furthermore,
it has not been possible to relate year-to-year variations in the concentrations of MSA
and nss-SO~- to observable climate changes. For example, measurements made over
the equatorial Pacific throughout the 1980s and 1990S do not show any systematic
change, despite the occurrence of major EI Nino events during that time period (Bates
and Quinn 1997).
Studies suggest that new particle production from DMS takes place primarily in
the outflow from clouds that tap into the MBL (Perry and Hobbs 1994, 1995; Clarke
et al.1998). MBL air rising through the clouds is stripped of particles and reactive gases
by cloud droplets. DMS, which is relatively insoluble and unreactive in the cloud environment, emerges from the top of the cloud into the middle and upper troposphere;
here, because of the low particle concentrations and intense sunlight, the photochemical reaction products of DMS have a high probability of combining (condensing and
coagulating) to form new particles. These newly-formed ultra-fine particles (smaller
than about O.Olllm diameter) are subsequently brought down into the MBL by subsiding air. In the MBL, the particles can grow and ultimately serve as cloud-nucleating particles that can contribute to the formation of new clouds, thereby continuing