Aerosol size distributions in air masses advecting off the east coast of the United States

Abstract

Accurate measurements of the size distribution of atmospheric aerosols in the size (radius) range from 0.006 to 2.5 #m made from a ship within 300 km of the east coast of the United States are presented. As air advects from land to sea, there is a rather rapid decay of particles smaller than 0.05 #m during the first day or so. After this initial decay of small particles the size distribution was often found to remain remarkably stable for hours. Significant changes in the size distribution were often associated with changes in air masses and meteorological conditions. Some of the changes can be explained qualitatively. Others, such as a pronounced double l•ak which occurred occasionally in the fine-particle range, are more difficult to understand. Possible physical mechanisms which might produce a double-l•aked distribution are discussed, including the increase in the mass of cloud condensation nuclei as a result of the chemical formation of new material from gases absorbed in cloud droplets during a cloud formationevaporation cycle. Using the measured size distributions, volume extinction coefficients were calculated for the wavelength range 0.3-10 #m. The calculations compare favorably to the measured scattering coefficients at 0.55-#m wavelength. Particles in the radius range 0.1-0.3 #m made the largest contribution to the total extinction at optical wavelengths on nearly all occasions. 1980; Zhulanov et al., 1982].The very low concentration of small particles often encountered in the marine environment requires more sensitive instrumentation than that employed in urban studies. Because of the increased sensitivity and accuracy achievable with the differential mobility size analyzer, its recent development is especially important for marine aerosol measurements. The differential mobility size analyzer was used in the studies by Hoppel [1979] and Haaf and Jaenicke [1980] in the marine environment and is also used in this study to obtain size distributions at radii below 0.5 #m.The total concentration of particles in continental air is of the order of 10,000 particles/cm 3. Over the remote oceans the This paper is not subject to U.S. copyright. Published in 1985 by the American Geophysical Union. Paper number 4D 1281. total concentrations are often of the order of 100 particles/cm 3. This implies that there are processes which remove particles from an air mass as it moves from continental to marine regions. In 1980, the USNS Hayes made a research cruise off the east coast of the United States which was sponsored jointly by the Environmental Biology Branch and Atmospheric Physics Branch of the Naval Research Laboratory (NRL). The purpose of our participation in the cruise was to investigate the evolution of the size distribution in air masses advecting off the east coast of the United States with special emphasis on submicron particles. The cruise track is shown in Figure 1, where the boxes indicate stations on which the ship stopped from 4 to 10 hours for biological sampling. INSTRUMENTATIONAerosol size distri...

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