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 (sigma(sp) and sigma(bsp), respectively) upon increasing and decreasing controlled RH, maximum particle diameter (D-p) size cut, and wavelength of scattered light (lambda). An aethalometer was used to measure black carbon concentrations ([BC]) to yield estimates of light absorption by particles (sigma(ap)) and single scattering albedo (omega). Parameters derived from the measurements include the dependence of sigma(sp) and sigma(bsp) on RH (f(RH)), the hemispheric backscatter fraction (b), and the Angstrom exponent ((a) over circle), all as functions of lambda and particle D-p size cut. During polluted periods, for D-p less than or equal to 10 mu m, and at lambda = 550 nm, means and standard deviations of aerosol parameters included sigma(sp) = 75.1 +/- 30.5 Mm(-1) at controlled RH = 27%, f(RH = 82%) = 1.46 +/- 0.10 and 1.22 +/- 0.06 for sigma(sp) and sigma(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 sigma(sp) and sigma(bsp) by a factor of 4 to 7, increased wavelength dependence evident from an increase in (a)over dot from 0.57 to 1.48, shift from 0.32 to 0.56 of the fraction of sigma(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 similar to 1% increases in omega, and an estimated range for omega at Sagres during ACE-2 was 0.91 < omega < 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.