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 (σ(sp) and σ(bsp), respectively) upon increasing and decreasing controlled RH, maximum particle diameter (D(p)) 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 (σ(ap)) and single scattering albedo (ω). Parameters derived from the measurements include the dependence of σ(sp) and σ(bsp) on RH (f(RH)), the hemispheric backscatter fraction (b), and the Angstrom exponent (a), all as functions of λ and particle D(p) size cut. During polluted periods, for D(p) ≤ 10 μm, and at λ = 550 nm, means and standard deviations of aerosol parameters included σ(sp) = 75.1 ± 30.5 Mm^-1 at controlled RH = 27%, f(RH = 82%) = 1.46 ± 0.10 and 1.22 ± 0.06 for σ(sp) and σ(bsp), 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 σ(sp) and σ(bsp) 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 σ(sp) from sub-micrometer D(p) 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 hygrosonic 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.