In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 2. Effects of mixing height on aerosol properties

The goal of this study was to determine under what meteorological conditions, if any, measurements of aerosol properties made at the Earth's surface are representative of aerosol properties measured within the column of air above the surface. Specifically, this study uses instrumentation from the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site to determine how the observed mixing height (MH) and the degree of mixing affects the vertical variation of aerosol extensive and intensive properties. The MH was estimated by applying the Heffter [1980] technique with a critical lapse rate of 0.001 K m^-1 to the available radiosonde data. This estimate was then used to determine whether each vertical aerosol profile flight leg from 59 flights was within or above the atmospheric boundary layer (ABL). Although the aerosol extensive properties, such as light absorption by particles σ_ap, light scattering by particles σ_sp, and total extinction by particles σ_ext measured within the ABL were highly correlated with the surface measurements (R² = 0.74, 0.88, and 0.88, respectively), the airborne measurements of σ_ap were systematically lower (slope = 0.77). By contrast, the σ_sp and σ_ext regression line slopes were 0.99 and 1.00. The aerosol extensive properties measured above the ABL were poorly correlated with surface values and were almost an order of magnitude lower than the simultaneous measurements made within the ABL. Aerosol intensive properties such as the single-scattering albedo ̄w_o, the hemispheric backscatter fraction b, and the Ångström exponent å exhibited a similar behavior (R² = 0.74, 0.61, and 0.55, respectively) but had regression line slopes within the ABL of 0.88, 0.75, and 0.62, respectively. There were no statistically significant correlations between the intensive aerosol properties measured above the ABL and those measured at the surface. Limiting the analysis to well-mixed days with near-neutral static stability did not significantly improve the results. Thus the hypothesis that the best agreement between surface measurements and those within the ABL will occur when the atmosphere is well mixed was not supported by this data set.