Size-segregated chemical, gravimetric and number distribution-derived massclosure of the aerosol in Sagres, Portugal during ACE-2

During the ACE-2 field campaign in the summer of 1997 an intensive, ground- bared physical and chemical characterisation of the clean marine and contin entally polluted aerosol was performed at Sagres, Portugal. Number size dis tributions of the dry aerosol in the size range 3-10 000 nm were continuous ly measured using DMPS and APS systems. Impactor samples were regularly tak en at 60% relative humidity (RW) to obtain mass size distributions by weigh ing the impactor foils, and to derive a chemical mass balance by ion and ca rbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative h umidity of 60%. A size segregated 3-way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH d erived from number size distribution measurements and impactors samples (we ighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically-determined mass with both number di stribution-derived (slope=1.23/1.09; R-2 > 0.97; depending on the parameter s humidity growth and density) and chemical mass concentration (slope = 1.0 2; R-2 = 0.79) for particles smaller than 3 mu m in diameter. Except for th e smallest impactor size range relatively good correlations (slope = 0.86 1 .42) with small deviations (R-2 = 0.76-0.98) for the different size fractio ns were found. Since uncertainties in each of the 3 methods are about 20% t he observed differences in the size-segregated mass fractions can be explai ned by the measurement uncertainties. However, the number distribution-deri ved mass is mostly higher than the chemically and gravimetrically determine d mass, which can be explained by sampling losses of the impactor, but as w ell with measurement uncertainties as, e.g., the sizing of the DMPS/APS.