Abstract. Hourly averaged aerosol optical properties (AOPs) measured over the years 2010-2013 at four continental North American NOAA Earth System Research Laboratory (NOAA/ESRL) cooperative aerosol network sites -Southern Great Plains near Lamont, OK (SGP), Bondville, IL (BND), Appalachian State University in Boone, NC (APP), and Egbert, Ontario, Canada (EGB) are analyzed. Aerosol optical properties measured over 1996 at BND and 1997 at SGP are also presented. The aerosol sources and types in the four regions differ enough so as to collectively represent rural, anthropogenically perturbed air conditions over much of eastern continental North America. Temporal AOP variability on monthly, weekly, and diurnal timescales is presented for each site. Differences in annually averaged AOPs and those for individual months at the four sites are used to examine regional AOP variability. Temporal and regional variability are placed in the context of reported aerosol chemistry at the sites, meteorological measurements (wind direction, temperature), and reported regional mixing layer heights. Basic trend analysis is conducted for selected AOPs at the long-term sites (BND and SGP). Systematic relationships among AOPs are also presented.Seasonal variability in PM 1 (sub-1 µm particulate matter) scattering and absorption coefficients at 550 nm (σ sp and σ ap , respectively) and most of the other PM 1 AOPs is much larger than day of week and diurnal variability at all sites. All sites demonstrate summer σ sp and σ ap peaks. Scattering coefficient decreases by a factor of 2-4 in September-October and coincides with minimum single-scattering albedo (ω 0 ) and maximum hemispheric backscatter fraction (b). The covariation of ω 0 and b lead to insignificant annual cycles in top-of-atmosphere direct radiative forcing efficiency (DRFE) at APP and SGP. Much larger annual DRFE cycle amplitudes are observed at EGB (∼ 40 %) and BND (∼ 25 %), with least negative DRFE in September-October at both sites. Secondary winter peaks in σ sp are observed at all sites except APP. Amplitudes of diurnal and weekly cycles in σ ap at the sites are larger for all seasons than those of σ sp , with the largest differences occurring in summer. The weekly and diurnal cycle amplitudes of most intensive AOPs (e.g., those derived from ratios of measured σ sp and σ ap ) are minimal in most cases, especially those related to parameterizations of aerosol size distribution.Statistically significant trends in σ sp (decreasing), PM 1 scattering fraction (decreasing), and b (increasing) are found at BND from 1996 to 2013 and at SGP from 1997 to 2013. A statistically significant decreasing trend in PM 10 scattering Ångström exponent is also observed for SGP but not BND. Most systematic relationships among AOPs are similar for the four sites and are adequately described for individual seasons by annually averaged relationships, althoughPublished by Copernicus Publications on behalf of the European Geosciences Union.

Abstract. Several types of filter-based instruments are used to estimate aerosol light absorption coefficients. Two significant results are presented based on Aethalometer measurements at six Arctic stations from 2012 to 2014. First, an alternative method of post-processing the Aethalometer data is presented, which reduces measurement noise and lowers the detection limit of the instrument more effectively than boxcar averaging. The biggest benefit of this approach can be achieved if instrument drift is minimised. Moreover, by using an attenuation threshold criterion for data post-processing, the relative uncertainty from the electronic noise of the instrument is kept constant. This approach results in a time series with a variable collection time ( t) but with a