Aerosol optical properties at Sagres, Portugal during ACE-2

Aerosol light scattering and absorption properties relevant to climate chan ge were measured as part of the Aerosol Characterization Experiment 2 (ACE- 2, 15 June to 25 July 1997) at Sagres, Portugal, a site receiving anthropog enically perturbed air masses from Europe. A controlled relative humidity ( RH) nephelometry system measured the dependence of the total light scatteri ng and backscattering coefficients by particles (sigma(sp) and sigma(bsp), respectively) upon increasing and decreasing controlled RH, maximum particl e diameter (D-p) size cut, and wavelength of scattered light (lambda). An a ethalometer was used to measure black carbon concentrations ([BC]) to yield estimates of light absorption by particles (sigma(ap)) and single scatteri ng albedo (omega). Parameters derived from the measurements include the dep endence of sigma(sp) and sigma(bsp) on RH (f(RH)), the hemispheric backscat ter fraction (b), and the Angstrom exponent ((a) over circle), all as funct ions 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 d eviations of aerosol parameters included sigma(sp) = 75.1 +/- 30.5 Mm(-1) a t controlled RH = 27%, f(RH = 82%) = 1.46 +/- 0.10 and 1.22 +/- 0.06 for si gma(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 suppres sion of f(RH) by 14 to 20%. Onset of polluted periods and aerosol hygroscop ic growth each resulted in similar to 1% increases in omega, and an estimat ed range for omega at Sagres during ACE-2 was 0.91 < omega < 0.97 consideri ng uncertainty of +/- 0.02 aerosol hygroscopic growth, and air mass influen ces. 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 pollu ted periods. These measurements contribute to characterizing ground-level a erosol optical properties for a site that receives "clean" and anthropogeni cally perturbed aerosol.