CHAPTER 2 . The Chemical and Physical Properties of Marine Aerosols: An Introduction
55
2.3.4
MSA and nss-SO~In remote ocean regions, the concentrations of nss-SO~- and MSA are often correlated
and yield a characteristic ratio that suggests a link between DMS emissions and nssSO~- production. Figure 2.7 shows scatter plots of nss-SO~- against MSA from three
ocean stations: America Samoa (Savoie et al.1994), Mawson Station,Antarctica (Savoie
et al. 1993) and Bermuda (Savoie and Prospero, unpublished data). At American Samoa and Mawson, remote sites where anthropogenic impacts are very small, the scatter plots yield relatively well-defined regression lines which suggest that the production of nss-SO~- is linked to DMS emissions. In the low and mid-latitudes, the mass
ratio of nss-SO~-/MSA tends to be about 12-15; this is the case for the data from American Samoa (Fig. 2.7). In contrast, in the high latitudes it is about 3 as observed at Palmer
Station (Fig. 2.7). The fact that the slopes of the regressions are markedly different
reflects the different gas-to-particle conversion processes that apply in these two very
different environments (i.e. tropical vs. the high latitudes). The fact that the ratio of
nss-SO~-/MSA is relatively low in the high latitudes compared to that in the low and
mid-latitudes is often cited as verification of the temperature dependence of the reaction of DMS with OR, although there is a continuing debate about the role of temperature in causing the observed differences (Berresheim et al. 1995).
At ocean sites impacted by transport from pollution sources, there is no clear relationship between MSA and nss-SO~-, because pollution-source SO~- overwhelms the
contribution form oceanic sources. The lack of correlation between MSA and nss-SO~at North Atlantic sites is clearly evident in Fig. 2.5; the nss-SO~- time series at Reimaey
appears very different from that of MSA and shows the dominance of pollution transport, which is evident as frequent "spikes" in the time series. The same situation is
obtained at Bermuda (Fig. 2.5). Only at Barbados one can see at times a linkage between MSA and nss-SO~- in the time series (Fig. 2.5). Nonetheless, it is possible to discern the impact of oceanic DMS-SO~- at polluted sites such as Bermuda. Scatter plots
usually show a well-defined lower boundary as seen in Fig. 2.7 for Bermuda. A
line drawn along this boundary yields a nss-SO~-/MSA slope of about 15, similar
to that shown for American Samoa in Fig. 2.7. Thus, when pollution levels are low,
the ocean DMS source is still significant and detectable at Bermuda. Data from other
sites in the North Atlantic (and from other oceanic regions impacted by pollution
sources) yield scatter plots of nss-SO~-/MSA are similar to that shown for Bermuda in
Fig. 2.7.
2.3.5
New-Particle Production from OMS over the Oceans
While it is now generally accepted that DMS plays a central role in the atmospheric
sulphur cycle of the oceans, there are many uncertainties in the process and the ultimate impact on climate. The first problem has to do with the process of the biological
processes in the water column that lead to the production of DMS and the physical
processes that control the subsequent transfer from the ocean to the atmosphere. As
55
2.3.4
MSA and nss-SO~In remote ocean regions, the concentrations of nss-SO~- and MSA are often correlated
and yield a characteristic ratio that suggests a link between DMS emissions and nssSO~- production. Figure 2.7 shows scatter plots of nss-SO~- against MSA from three
ocean stations: America Samoa (Savoie et al.1994), Mawson Station,Antarctica (Savoie
et al. 1993) and Bermuda (Savoie and Prospero, unpublished data). At American Samoa and Mawson, remote sites where anthropogenic impacts are very small, the scatter plots yield relatively well-defined regression lines which suggest that the production of nss-SO~- is linked to DMS emissions. In the low and mid-latitudes, the mass
ratio of nss-SO~-/MSA tends to be about 12-15; this is the case for the data from American Samoa (Fig. 2.7). In contrast, in the high latitudes it is about 3 as observed at Palmer
Station (Fig. 2.7). The fact that the slopes of the regressions are markedly different
reflects the different gas-to-particle conversion processes that apply in these two very
different environments (i.e. tropical vs. the high latitudes). The fact that the ratio of
nss-SO~-/MSA is relatively low in the high latitudes compared to that in the low and
mid-latitudes is often cited as verification of the temperature dependence of the reaction of DMS with OR, although there is a continuing debate about the role of temperature in causing the observed differences (Berresheim et al. 1995).
At ocean sites impacted by transport from pollution sources, there is no clear relationship between MSA and nss-SO~-, because pollution-source SO~- overwhelms the
contribution form oceanic sources. The lack of correlation between MSA and nss-SO~at North Atlantic sites is clearly evident in Fig. 2.5; the nss-SO~- time series at Reimaey
appears very different from that of MSA and shows the dominance of pollution transport, which is evident as frequent "spikes" in the time series. The same situation is
obtained at Bermuda (Fig. 2.5). Only at Barbados one can see at times a linkage between MSA and nss-SO~- in the time series (Fig. 2.5). Nonetheless, it is possible to discern the impact of oceanic DMS-SO~- at polluted sites such as Bermuda. Scatter plots
usually show a well-defined lower boundary as seen in Fig. 2.7 for Bermuda. A
line drawn along this boundary yields a nss-SO~-/MSA slope of about 15, similar
to that shown for American Samoa in Fig. 2.7. Thus, when pollution levels are low,
the ocean DMS source is still significant and detectable at Bermuda. Data from other
sites in the North Atlantic (and from other oceanic regions impacted by pollution
sources) yield scatter plots of nss-SO~-/MSA are similar to that shown for Bermuda in
Fig. 2.7.
2.3.5
New-Particle Production from OMS over the Oceans
While it is now generally accepted that DMS plays a central role in the atmospheric
sulphur cycle of the oceans, there are many uncertainties in the process and the ultimate impact on climate. The first problem has to do with the process of the biological
processes in the water column that lead to the production of DMS and the physical
processes that control the subsequent transfer from the ocean to the atmosphere. As