Chemical evolution and dispersion of ship plumes in the remote marine boundary layer: investigation of sulfur chemistry

In this study a Lagrangain photochemical box model has been used to explore gaseous/heterogeneous sulfur plume chemistry initiated by ship traffic in the MBL. The results indicate that although SO2 ship emission rates have an impact on the concentration levels of SO2, there is no evidence of non-linear loss processes in this chemical system. This finding as well as others have demonstrated that the major loss pathway for SO2 is via heterogeneous loss to sea-salt aerosol. This loss rate, however, can be greatly influenced by the MET condition selected for the MBL. For example, the “stable” MET condition leads to the maximum loss rate, whereas the “unstable” condition gives the minimum value. In the case of H2SO4(g), the “stable” MET condition provides both the highest SO2 and OH concentrations, and hence, the formation rate is also maximized. It was found that increases in aerosol surface area (i.e., heterogeneous loss) as well as shifts to higher latitudes (e.g., lower T and higher O3 levels) tended to increase the rate of loss of SO2 due to enhancements in the rates of heterogeneous chemistry. However, the single largest factor that influenced the rate of SO2 loss was the presence of BL clouds where oxidation via scavenged H2O2 was the dominant process. Quite significant also was the finding that photochemical processes, because of their production of OH, O3, and H2O2, are strongly coupled to SO2 sea-salt heterogeneous reactions, cloud heterogeneous reactions, and gas-phase reactions to form H2SO4. Not surprisingly, therefore, model predictions of the rate of formation of new particles within ship plumes were found to be quite dependent on the intensity of photochemistry.