Size-segregated chemical, gravimetric and number distribution-derived mass closure 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 continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3-10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RW) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity 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 derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically-determined mass with both number distribution-derived (slope=1.23/1.09; R-2 > 0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope = 1.02; R-2 = 0.79) for particles smaller than 3 mu m in diameter. Except for the 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 fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size-segregated mass fractions can be explained by the measurement uncertainties. However, the number distribution-derived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.