Continuous measurements of aerosol size distributions were obtained during January--March and November--December periods of 1988 at Mauna Loa Observatory, Hawaii. These periods were chosen in order to characterize aerosol physicochemistry during periods representative of ''low-dust'' atmospheric conditions and periods associated with appreciable Asian dust transport to that site. Size distributions for particles with diameters between 0.15 and 7.6 μm were accumulated in 256 size bins of a laser optical particle counter for 3-hour intervals during most of the period. The aerosol sample stream was heated to selected temperatures in order to provide size-discriminated measurements of aerosol volatility. Resulting data were used to assess the variability in aerosol concentrations and properties related to aerosol backscatter values at a wavelength of 10.6 μm, βCO2, in the mid-troposphere. Low aerosol concentrations, considered representative of mid-tropospheric air, occurred in downslope flow between midnight and sunrise. Our measurements for these time periods suggest that βCO2 varied from a low of about 5¿10-12m-1sr-1 to a high of 5¿10-8m-1sr-1. Coarse particles with diameters between 1.0 and 5.0 μm account for most of the derived values of βCO2 at all but the highest and lowest aerosol mass concentrations. Volatile aerosol appears to dominate aerosol mass during the cleanest periods but was a small fraction of the total during dust events. During low-dust periods the aerosol volatile at 320 ¿C often appears as coatings on refractory (e.g., dust) aerosol and is expected to influence measured βCO2 values. This low dust condition is also expected to be most representative of aerosol at higher altitudes in the free troposphere. We estimate that minimum values of βCO2 at about 8 km should usually fall in the range of 1--3¿10-12m-1sr-1 and be dominated by a sulfate aerosol. ¿1991 American Geophysical Union