Physical and Chemical Properties of Aerosols in the Marine Boundary Layer

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9311213 Covert The formation of sulfate compounds in the marine boundary layer (MBL) from the oxidation of oceanic dimethyl sulfide (DMS) to sulfuric acid (H2SO4) and methane sulfonic acid (MSA) and the subsequent nucleation and condensation of these compounds is a major source of particulate matter in that part of the atmosphere. The partitioning of these compounds between the competing processes of nucleation and condensation determines (along with removal processes) the resultant distribution of aerosol number and mass with particle size. There is strong interaction between these formation processes and the preexisting or resulting aerosol. The characteristic times for particle formation, growth and removal encompass minutes to days depending on atmospheric conditions. Particulate phase sulfur in the MBL and its interaction with marine stratus clouds are hypothesized to have a significant role in global climate either by altering radiative properties, lifetime or geographical extent of the clouds. This research will focus on physics and chemistry of particles in the MBL. The overall project goal will be to determine the extent to which sulfate compounds contribute to functional subsets or modes of the aerosol number and mass size spectrum and the processes by which these particles and particulate mass are formed. The ultrafine mode, (10nm in diameter) is representative of those particles most recently formed in the MBL by condensation from the gas phase. The accumulation mode (80 to 1000nm diameter) is representative of the main mass of sulfate and optically effective particles in the MBL. Cloud condensation nuclei (CCN), those particles effective in cloud nucleation, are a subset of the total particle population defined by size chemistry and water vapor supersaturation. For marine stratocumulus they generally encompass the range greater than ca. 50nm diameter. The seasalt mode consists of seasalt or modified seasalt particles greater than 100nm generated at the surface by breaking films due to wind and wave action. Due to their alkaline chemistry these particles may have a disproportionately large influence on oxidation of SO2 in the MBL, both in and out of cloud. The Atiken mode, which on a number basis loosely covers the range 10 to 80nm, represents those particles that are several days of age and have not been active in cloud processes. Several physical, optical and chemical methods will be used to characterize the distribution of number concentration, chemical composition, hygroscopic and thermal properties of the particles between 0.003 and 10um diameter. Direct measurements of integral optical properties of the aerosol will be made with an integrating nephelometer. Direct measurements of the CCN concentration will be made in a thermal diffusion cloud chamber. There is considerable redundancy in the measurement scheme to constrain the uncertainty of the results. Laboratory measurements will be done to calibrate and intercompare the methods. The field research will be carried out within the context of larger multidisciplinary projects that will address the overall sulfur cycle in which gas phase composition of the MBL will be measured. Field experiments will be done at several locations, along the NW coast of Washington, in the mid-Pacific and in the Southern Hemisphere in the vicinity of Tasmania and the Cape Grim Background Monitoring Station.