NON-SEA-SALT SULFATE AND METHANESULFONATE AT AMERICAN-SAMOA

High-volume bulk aerosol samples have been collected at American Samoa (14.25-degrees-S, 170.58-degrees-W) on a semicontinuous basis since t he system was erected as part of the Sea/Air Exchange Program (SEAREX) in March 1983. In this report we consider those samples collected thr ough May 6, 1992. For most of this period the sample filters were chan ged once a week. However, during November 1989 and from May 10 to June 10, 1990, in conjunction with the aircraft missions of the NASA Globa l Backscatter Experiment (GLOBE), the filters were changed daily. All of the samples were analyzed for nonsea-salt (nss) SO4= and NO3-. Anal yses for methanesulfonate (MSA) include all of the 53 daily samples, 2 2 weekly samples from March 19, 1983, through April 12, 1984, and 96 w eekly samples from January 3, 1990, through May 6, 1992. The mean conc entrations (in micrograms per cubic meter) were 0.37 for nss SO4=, 0.0 229 for MSA, 0.114 for NO3-, and 5.1 for Na+. Nss SO4= and MSA are str ongly linearly correlated in these 171 samples (r2 = 0.66) and the reg ression intercept does not differ significantly from zero. The geometr ic mean (GM) nss SO4=/MSA ratio, 18.1+/-0.9 (where +/- indicates the 9 5% confidence interval of the GM) is about 7 % higher than had previou sly been reported for this station. The ratio exhibits no significant seasonal variation. Although the ratio appeared to be significantly lo wer in the May-June 1990 daily samples (GM = 15.3 +/- 1.2), a further examination of the results indicated that the variance of the measured ratios from 18.1 (the GM for the whole data set) was attributable alm ost exclusively to the typical random errors in the analyses as determ ined from the 1sigma analytical uncertainties of 5% for MSA and SO4= a nd 2% for Na+.