PNNL

Abstract

The role of aerosol particles in maintaining a cloudy boundary layer in the remote marine boundary layer is explored. It has previously been shown that precipitation can result in the transition from a closed- to open-cellular state but that the boundary layer cannot maintain this open-cell state without a resupply of particles. Potential sources include wind-driven production of sea salt particles from the ocean, nucleation from the gas phase, and entrainment from the free troposphere. In order to investigate aerosol sources in the marine boundary layer and their role in maintaining an open-cellular cloud state, we have coupled in detail chemical, aerosol, and cloud processes in the WRF/Chem model, and added state-of-the art representations of sea salt emissions and of aerosol nucleation. We introduce the new features of the model and evaluate its ability to reproduce chemical and aerosol observations made in open cells of the southeast Pacific boundary layer during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) over a diurnal cycle. During the daytime, the open-cell circulation facilitates aerosol nucleation by lofting dimethyl sulfide from the ocean surface through open cell wall updrafts into a cloud-scavenged ultra-clean layer below the inversion height, where it is oxidized to SO2 and subsequently to H2SO4. The resulting elevated H2SO4 concentrations, assisted by low H2SO4 sink values in the ultra clean layer initiate aerosol nucleation. The existence of the ultra-clean layer is confirmed by observations. The nanometer-sized newly formed particles take hours to days to grow to sizes large enough to act as cloud condensation nuclei (CCN). In contrast, mechanical production of particles from the ocean surface by surface winds provides a steady source of larger particles that are effective CCN. For the case considered, entrainment of free tropospheric aerosol is a negligible source of CCN. Together these sources act to resupply sufficient aerosol to the marine boundary layer system to sustain an open-cellular cloud state.