The extinction-to-backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of S for tropospheric aerosols is extremely limited. Here we review that knowledge and extend it using a recently developed in situ technique that employs a 180¿ backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, S was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant air mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64¿4 sr for the stagnant category and 40¿9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to S. ¿ 2000 American Geophysical Union