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

Abstract

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) homogenous 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 m s⁻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.

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