A study of processes that govern the maintenance of aerosols in the marine boundary layer

We systematically evaluate the relative influence of sources and sinks of particles in the remote marine boundary layer (MBL) to elucidate the principal factors that govern MBL aerosol behavior. Processes considered include: (1) surface flux of dimethyl sulfide (DMS); (2) gas-phase oxidation of DMS to SO2; (3) gas-phase oxidation of SO2 to H2SO4; (4) mass transfer of SO2 and H2SO4 to pre-existing particles; (5) humogenous nucleation of H2SO4/H2O; (6) entrainment of air from the free troposphere; (7) deposition to the sea surface; (8) cycling of air through clouds and rain scavenging; (9) oxidation of SO2 in sea salt aerosols and cloud droplets; and (10) sea-salt particle production at sea surface. The average aerosol number concentration is found to be quite sensitive to the rate of entrainment of aerosol-containing air from the free troposphere. The path that leads to the greatest accumulation of non-sea-salt (nss) sulfate involves SO2 (rather than H2SO4) absorption into existing particles. Because of scavenging of SO2 and H2SO4 by sea-salt aerosol, a considerable fraction of nss-sulfate is internally mixed with sea-salt aerosol. Under the conditions assumed in this study, MBL aerosol number concentration is dominated by free tropospheric aerosol under virtually all conditions, 89% in the base case, and even 69% at a 17 ms-1 wind speed. Aerosol mass, on the other hand, is dominated by sea-salt particles, 62% in the base case and 98% at a wind speed of 17 m s-1. Evaporation of cloud droplets provides 4.6% of the particle number in the base case, but 28% of the particle mass. At the high nucleation rate case considered here, there is notably little change in the overall contributions to aerosol number and mass from the base case; only about 5% of the total particle number is provided by nucleation events. Variation in precipitation frequency also has only a minor effect on the overall contributions. One concludes that the MBL aerosol is remarkably robust in the face of ever-changing conditions. Free tropospheric aerosol entrainment tends to sustain particle number concentrations, and sea salt emissions maintain most of the aerosol mass. Cloud processing, while not a major contributor to aerosol number, does provide, except under high wind conditions, the order of 20% of the aerosol mass. Although nucleation occurs only infrequently and does not contribute appreciably to long-term average aerosol number or mass, nucleation is, nonetheless, the mechanism that replenishes aerosol number in brief, intense episodes when aerosol surface area levels are substantially reduced by precipitation. The relative influence of particle sources and sinks in the remote marine boundary layer (MBL) is evaluated to elucidate the principal factors governing MBL aerosol behavior. Processes considered include: dimethyl sulfide (DMS) surface flux; gas phase DMS oxidation to SO2; gas phase SO2 oxidation to H2SO4; SO2 and H2SO4 mass transfer to preexisting particles; H2SO4/H2O nucleation; air entrainment from the free troposphere; deposition to the sea surface; air through clouds cycling and rain scavenging; SO2 oxidation in sea salt aerosols and cloud droplets; and sea-salt particle production at sea surface. These processes combined result in a remarkably robust MBL aerosol in the face of ever-changing conditions.