AEROSOL OBSERVATIONS AT CHEBOGUE POINT DURING THE 1993 NORTH-ATLANTICREGIONAL EXPERIMENT - RELATIONSHIPS AMONG CLOUD CONDENSATION NUCLEI, SIZE DISTRIBUTION, AND CHEMISTRY

Observations of aerosol chemistry and microphysics were made at Chebog ue Point, Nova Scotia, from August 16 to September 8, 1993 as part of the North Atlantic Regional Experiment (NAPE) intensive. Most of the a erosols were classified into two groups according to the geometric mea n volume diameter (D-gv) of the particles which contributed the greate st volume (sub-0.5 mu m) The group-1 aerosols; representing 33% of the data, are characterized by D-gv of 0.18-0.19 mu m; the group-2 aeroso ls, representing 50% of the data, are characterized by D-gv of 0.20-0. 22 mu m; and the remaining aerosols bear similarities to either groups 1 or 2 but lie outside the D-gv ranges. The differences between these aerosol groups are consistent with the addition of sulfate to the gro up-2 aerosols via recent processing through cloud. Factors supporting this possibility include the presence of low marine stratus upwind of the site only on days when the group-2 aerosol was observed, the highe r D-gv for the group-2 aerosols consistent with the observed size thre shold for activation in these clouds, and the association of non-sea-s alt SO4= (nssSO(4)(=)) with larger particle sizes for the group-2 aero sols. In general, the masses of the most abundant inorganic and organi c ions, nssSO(4)(=) and oxalate, were associated with the main volume of the sub-0.5-mu m particles. Cloud condensation nucleus (CCN) concen trations active at 0.4% supersaturation (CCN0.4) were highly correlate d with the concentrations of particles >0.01 mu m and oxalate and mode rately correlated with nssSO(4)(=). Concentrations of CCN active at 0. 06% supersaturation (CCN0.6) correlate very well with the concentratio ns of particles >0.19 mu m diameter. In the case of the recently cloud -processed aerosols, for which nssSO(4)(=) is more strongly associated with particles >0.19 mu m, the CCN0.06 also correlate well with nssSO (4)(=) CCN spectra computed using the measured size distributions and aerosol chemistry agree well with the measured CCN.