Aerosol Optical properties at Sagres, Portugal during ACE‐2

Aerosol light scattering and absorption properties relevant to climate change were measured as part of the Aerosol Characterization Experiment 2 (ACE-2, 15 June to 25 July 1997) at Sagres, Portugal, a site receiving anthropogenically perturbed air masses from Europe. A controlled relative humidity (RH) nephelometry system measured the dependence of the total light scattering and backscattering coefficients by particles (σsp and σbsp , respectively) upon increasing and decreasing controlled RH, maximum particle diameter (Dp) size cut, and wavelength of scattered light (λ). An aethalometer was used to measure black carbon concentrations ([BC]) to yield estimates of light absorption by particles (σap) and single scattering albedo (ω). Parameters derived from the measurements include the dependence of σsp and σbsp on RH (f(RH)), the hemispheric backscatter fraction (b), and the Angstrom exponent (A), all as functions of λ and particle Dp size cut. During polluted periods, for Dp≤10 μm, and at λ=550 nm, means and standard deviations of aerosol parameters included σsp=75.1±30.5 Mm−1 at controlled RH=27%, f(RH=82%)=1.46±0.10 and 1.22±0.06 for σsp and σbsp , respectively, and b=0.113±0.017 and 0.094±0.015 at controlled RH=27% and 82%, respectively. Transition from “clean” to polluted periods was characterized by a mean increase in σsp and σbsp by a factor of 4 to 7, increased wavelength dependence evident from an increase in A from 0.57 to 1.48, shift from 0.32 to 0.56 of the fraction of σsp from sub-micrometer Dp particles, and suppression of f(RH) by 14 to 20%. Onset of polluted periods and aerosol hygroscopic growth each resulted in ∼1% increases in ω, and an estimated range for ω at Sagres during ACE-2 was 0.91<ω< 0.97 considering uncertainty of ±0.02, aerosol hygroscopic growth, and air mass influences. Evidence for the influence of hysteresis in f(RH) was greatest at RH=65% with mean increases of 20% during “clean” periods and 10% during polluted periods. These measurements contribute to characterizing ground-level aerosol optical properties for a site that receives “clean” and anthropogenically perturbed aerosol.