Direct aerosol radiative forcing based on optical measurements at contrasting, climatically important sites

To characterize direct aerosol radiative forcing of climate ( DARF), aerosol optical properties were measured at two contrasting locations, a clean marine Southern Hemisphere site (Cape Grim Tasmania) and an anthropogenically perturbed Northern Hemisphere site (Sagres, Portugal). Total light scattering and backscattering coefficients by particles (sigma sp and sigmabsp, respectively) and their dependence upon controlled relative humidity (RH), particle diameter ( Dp) size cut, and wavelength of light were measured using a controlled RH nephelometer system. Results from Cape Grim demonstrated the predominate influence of sea-salt aerosol in regard to all aerosol optical properties and for both sub- and super-micrometer Dp size cuts. At Sagres, polluted aerosol predominated and even during times of "clean" air masses of a marine origin, evidence of an anthropogenic: influence was observed. As such, much greater magnitude and wavelength dependence of aerosol light scattering resulted at Sagres. Whereas the marine aerosol at Cape Grim was virtually a purely light scattering aerosol, the perturbed aerosol at Sagres had a significant light absorbing component though too was dominated by light scattering. Furthermore, aerosol hygroscopic growth was smaller in magnitude and monotonic at Sagres in comparison to the deliquescent marine aerosol at Cape Grim. At Sagres, evidence of hysteresis in growth in sigmasp as a function of RH (different pathways for increasing and decreasing controlled RH) was greatest at RH = 65% with enhanced growth of ∼20% during "clean" and ∼10% during "polluted periods." Based on laboratory and field measurements from Sagres, hysteresis effects are presumed to be greater at Cape Grim as individual humidograms from marine aerosol at Sagres demonstrated increases up to 67%. Using these measurements and literature data, at Cape Grim DARF is estimated as ∼0 while DARF at Sagres = --3.3 Wm--2 with an estimated uncertainty +/-1.2 Wm--2. DARF at Sagres is ∼50% larger in magnitude but opposite in sign (i.e. causing a net cooling) to global and annual mean greenhouse gas radiative forcing. However, instantaneous DARF varied substantially in time, and at Sagres was as significant as 40 Wm--2 for broadband calculations. These results are available as inputs to global climate models to enhance prediction of aerosol-climate effects.