Abstract. Hourly-averaged aerosol radiative properties measured over the years 2010–2013 at four continental North American NOAA/ESRL Federated Aerosol Network sites – Southern Great Plains in Lamont, OK (SGP), Bondville, IL (BND), Appalachian State University in Boone, NC (APP), and Egbert, Ontario, Canada (EGB) were analyzed to determine regional variability and temporal variability on several timescales, how this variability has changed over time at the long-term sites (SGP and BND), and whether systematic relationships exist for key aerosol properties relevant to radiative forcing calculations. The aerosol source types influencing the four sites differ enough so as to collectively represent rural, anthropogenically-perturbed air conditions over much of continental North America. Seasonal variability in scattering and absorption coefficients at 550 nm (σsp and σap, respectively) and most aerosol intensive properties was much larger than day of week and diurnal variability at all sites for both the sub-10 μm and sub-1 μm aerosols. Pronounced summer peaks in scattering were observed at all sites, accompanied by broader peaks in absorption, higher single-scattering albedo (ω0), and lower hemispheric backscatter fraction (b). Amplitudes of diurnal and weekly cycles in absorption at the sites were larger for all seasons than those of scattering. The cycle amplitudes of intensive optical properties on these shorter timescales were minimal in most cases. In spite of the high seasonality in ω0 and b, the co-variation of these two intensive properties cause the corresponding seasonal cycle in monthly median direct radiative forcing efficiency to be small, with changes of only a few percent at all sites. Median sub-10 μm aerosol σsp values for SGP and BND for the 2010–2013 time period were ~25% lower for all months than during the late 1990s period studied by Delene and Ogren (2002), consistent with the trends reported in other North American studies. There were even larger reductions in sub-1 μm aerosol σsp, leading to a larger coarse-mode influence at both sites. Similar reductions in median σap were observed at BND but median σsp changed little at SGP relative to the earlier observations of D&O2002, leading to lower ω0 at SGP. Most intensive properties and their variability were similar for both periods but median b was larger for all months of the 2010–2013 period at BND and nearly all months at SGP, indicating a shift toward smaller accumulation-mode particles. Systematic relationships between aerosol radiative properties were developed and applied to provide information on aerosol source types and processes at the four sites but some key relationships varied noticeably with season, indicating that the use of such relationships for model evaluation and inversion of remote sensing data must consider their seasonality.
