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During the First Aerosol Characterization Experiment (ACE 1) field campaign in November and December 1995, the particle analysis by laser mass spectrometry instrument was used to measure the composition of ambient particles in situ at Cape Grim, Tasmania, under various conditions ranging from clean marine air to moderately polluted air. Internal mixtures of sea‐salt compounds and organic species were detected in over half of the negative spectra during clean marine conditions and in about 62% of the negative spectra during polluted conditions. In clean marine air masses, aerosol organics appeared to have two distinct source mechanisms depending on the extent of aerosol aging. Organic peaks had a positive trend with sodium sulfate peaks, indicating that organics and excess sulfate may accumulate in aged marine aerosol particles by similar mechanisms. When the sodium sulfate content was low, iodine had a positive trend with organics, which is consistent with organics and iodine originating from the surface‐active layer of the ocean and becoming incorporated into fresh marine particles by bursting bubbles. Based on limited laboratory calibrations, the average organic mass is estimated to be of the order of 10% of the sea‐salt content and is consistent with some of the missing mass fraction for Cape Grim particles (S. Howell et al., manuscript in preparation, 1997), which is the measured difference between gravimetric and ionic mass.
Middlebrook et al. (Wed,) studied this question.