Aerosol observations at Chebogue Point during the 1993 North Atlantic Regional Experiment: Relationships among cloud condensation nuclei, size distribution, and chemistry

Abstract

Observations of aerosol chemistry and microphysics were made at Chebogue Point, Nova Scotia, from August 16 to September 8, 1993 as part of the North Atlantic Regional Experiment (NARE) intensive. Most of the aerosols were classified into two groups according to the geometric mean volume diameter (Dgv) of the particles which contributed the greatest volume (sub‐0.5 μm). The group‐1 aerosols, representing 33% of the data, are characterized by Dgv of 0.18–0.19 μm; the group‐2 aerosols, representing 50% of the data, are characterized by Dgv of 0.20–0.22 μm; and the remaining aerosols bear similarities to either groups 1 or 2 but lie outside the Dgv ranges. The differences between these aerosol groups are consistent with the addition of sulfate to the group‐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 higher Dgv for the group‐2 aerosols consistent with the observed size threshold for activation in these clouds, and the association of non‐sea‐salt SO4= (nssSO4=) with larger particle sizes for the group‐2 aerosols. In general, the masses of the most abundant inorganic and organic ions, nssSO4= and oxalate, were associated with the main volume of the sub‐0.5‐μm particles. Cloud condensation nucleus (CCN) concentrations active at 0.4% supersaturation (CCN0.4) were highly correlated with the concentrations of particles >0.01 μm and oxalate and moderately correlated with nssSO4=. Concentrations of CCN active at 0.06% supersaturation (CCN0.06) correlate very well with the concentrations of particles >0.19 μm diameter. In the case of the recently cloud‐processed aerosols, for which nssSO4= is more strongly associated with particles >0.19 μm, the CCN0.06 also correlate well with nssSO4=. CCN spectra computed using the measured size distributions and aerosol chemistry agree well with the measured CCN.