Is aerosol production within the remote marine boundary layer sufficient to maintain observed concentrations?
To evaluate the impact of aerosols on climate we must consider the aerosol dynamics of the remote marine atmosphere. Marine aerosols are subject to losses due to precipitation, dry deposition, and coagulation; yet, observed remote marine aerosol concentrations and size distributions are relatively constant. This maintenance of the aerosol distribution requires a particle source. This work focuses on the potential of H2SO4 nucleation within the marine boundary layer (MBL) to supply these particles. Spatial and temporal variability in meteorology and species concentrations are considered in a mathematical model to evaluate the effect of natural deviations from average MBL conditions on the highly nonlinear aerosol system. A dynamic, vertically dimensioned, size-resolved aerosol model is used with parameterized heterogeneous chemical processes. The results suggest that MBL nucleation may be an important source of new particle number in the remote MBL. However, though our model shows that typical remote MBL aerosol distributions can on average be maintained by MBL nucleation and sea-salt emissions, large oscillations in particle number concentration occur. Because such oscillations are only occasionally reported in measurements, MBL nucleation may not be the dominant source of new particles in the remote MBL. The nucleation events, which cause these oscillations, are predicted to occur at the top of the MBL after rain and/or entrainment of clean free tropospheric air. Predictions are particularly sensitive to the H2SO4 accommodation coefficient, nucleation tuner, and washout efficiency. Reduction of the accommodation coefficient is shown to increase the predicted accumulation mode concentration because the nucleation rate is enhanced. Entrainment of clean free tropospheric air is shown to increase the frequency of nucleation events within the MBL and may help to explain the observed correlation between subsidence and MBL small particles. A small constant addition of particles from the free troposphere, ocean, etc. suppresses H2SO4 nucleation and can lead to a reduction in total predicted aerosol number. This is because nucleation events require low total aerosol surface area and the constant addition of particles reduces the severity of aerosol surface area minimums. Larger external aerosol sources can maintain the observed remote MBL aerosol distribution. However, the temporal and spatial variability of such sources could have a large impact on aerosol concentrations and requires further investigation. Copyright 1999 by the American Geophysical Union.
Capaldo, KP; Kasibhatla, P; Pandis, SN
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