A multi-year study of lower tropospheric aerosol variability and systematic relationships from four North American regions

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 1996–2009 at BND and 1997–2009 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 PM₁ (sub-1 μm particulate matter) scattering and absorption coefficients at 550 nm (σ_sp and σ_ap, respectively) and most of the other PM₁ 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 (ω₀) and maximum hemispheric backscatter fraction (b). The co-variation of ω₀ 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), PM₁ 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 PM₁₀ 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, although relationships involving absorption Ångström exponent vary with site and season.