Aerosol Measurements on American Samoa

DENNIS L. SAVOIE AND JOSEPH M. PROSPERO

University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami, FL 3314-1098

An aerosol sampling program was established at the CMDL station on American Samoa in 1983 as part of the Sea/Air Exchange Program (SEAREX) network [Savoie et al., 1989a]. Our data record starts in March 1983 and continues to the present. The main focus of these studies is on non-seasalt (nss) SO₄⁼, NO₃^-, methane sulfonate (MSA) and various seasalt components. Measurements of ²¹⁰Pb yield concentrations that are among the lowest reported for temperate and tropical ocean regions [Turekian et al., 1989], suggesting that the island is minimally impacted by continental sources^. Al and Sb concentrations at Samoa are among the lowest reported for tropospheric aerosols [Arimoto et al., 1989; Prospero et al., 1989], and concentrations of Pb are about an order of magnitude lower than those in the North Pacific westerlies [Maring et al., 1989; Rosman et al., 1990]. Consequently, the concentrations of nss-SO₄⁼ and NO₃^- at Samoa should largely reflect the input from natural sources in the region.

SAMPLING AND ANALYSIS

Aerosol samples are collected by drawing air through 20 25-cm Whatman-41 filters at a nominal flow rate of 1.1 m³ min^-1; the filters are mounted at the top of a tower at the eastern edge of the promentory where the CMDL site is located. The sampling pumps are controlled by wind sensors that activate the pumps only when the wind is off the ocean at speeds greater than 1m s^-1. Filters are returned to Miami for analysis; see Savoie et al., 1994 for procedures. Nss-SO₄⁼ is obtained as the difference between total SO₄⁼ and seasalt SO₄⁼, the latter being calculated as total Na⁺ times 0.2516, the SO₄⁼/Na⁺ mass ratio in bulk seawater.

MEAN CONCENTRATIONS AND TEMPORAL VARIATIONS

The monthly mean concentrations of nss SO₄⁼, MSA, NO₃^-, and Na⁺ are presented in Table 1. The monthly averages listed in Table 1 and the plots of the weekly concentrations (Figure 1) show evidence of seasonal cycles. The strongest cycle is exhibited by NO₃^- for which the highest monthly mean (0.152 µg m^-3 in September) is 71% higher than the lowest (0.089 µg·m^-3 in April). The seasonal cycle for nss-SO₄⁼ is similar to that for NO₃^- but the seasonal difference is somewhat less; the October mean (0.458 µg m^-3) is 56% higher than the April mean (0.294 µg m^-3). For both species, the seasonal cycles presented here do not differ significantly from those based on the 1983 through 1987 samples [Savoie et al., 1989b].
 

TABLE 1. Aerosol Concentrations Measured at American Samoa From March 19, 1983, to January 3, 1996

Values in bold are the monthly mean concentration, the standard deviation, and the number of samples in the set. The values in italics are the median concentration and the concentrations of the samples at the 16% and 84% level in the frequency distribution.

Fig. 1. Weekly aerosol concentrations for American Samoa (from March 19, 1983 to January 3, 1996) grouped by month. The data are shown with box plots. The length of each box shows the range within which the central 50% of the values fall, with the box borders at the first and third quartiles; the cross-bar shows the median [for details see Tukey, 1997].

There is no clearly evident pattern in the monthly averages for MSA even though there are significant month-to-month variations. This lack of a clear trend may be due in part to the fact that the data set for MSA is not as large as that for the other species. Nonetheless, the maximum occurs in September (mean, 31.31 ng m^-3) and the minimum in April (17.30 ng m^-3).

There is no evidence of any long-term changes in aerosol concentrations at the site. The means in Table 1 are quite similar to those previously reported by Savoie et al. [1989a,b] for 215 samples, 0.36 µg·m^-3 for nss SO₄⁼ and 0.110 for NO₃^-. Even the comparatively small data set (n = 22) used by Saltzman et al. [1985] provided comparable means: nss-SO₄⁼, 0.41 µg·m^-3; MSA, 26 ng·m^-3; NO₃^-, 0.118 µg·m^-3; and Na⁺, 5.6 µg·m^-3. The relationship between nss-SO₄⁼ and MSA also appears to be quite stable over time. Savoie et al. [1994] investigated the relationships among the various species and found that the geometric mean (GM) ratio nss-SO₄=/MSA (18.1 ± 0.9) is only about 7% higher than the GM ratio of 16.9 in the 22 American Samoa samples originally used by Saltzman et al. [1985]. As reported in Savoie et al. [1994] the ratio obtained at American Samoa is similar to that obtained at other ocean stations in the tropics and sub-tropics. Thus, within this region, MSA should provide reasonable estimates of the biogenic component of the total nss-SO₄⁼.

Acknowledgments. We are indebted to the NOAA Climate Monitoring and Diagnostics Laboratory for permitting us to sample at their station and to the numerous people who have been involved in the actual field work, principally C. Farmer and E. Wilson-Godinet. The aerosol research was supported by National Science Foundation grants ATM8703411 and ATM9013125 and NASA contracts NAG8-621 and NAG8-841 to the University of Miami.
 

REFERENCES

Arimoto,R., R.A. Duce, and B.J. Ray, Concentrations, sources and air-sea exchange of trace elements in the atmosphere over the Pacific Ocean, in Chemical Oceanography, vol. 10, edited by J.P. Riley, R. Chester, and R.A. Duce, pp. 107-149, Academic, San Diego, CA., 1989.

Maring, H., C. Patterson, and D. Settle, Atmospheric input fluxes of industrial and natural Pb from the westerlies to the mid-North Pacific, in Chemical Oceanography, vol. 10, SEAREX: The Sea/Air Exchange Program, edited by J. P. Riley, R. Chester, and R. A. Duce, pp. 84-106, Academic, San Diego, CA, 1989.

Prospero, J.M., M. Uematsu, and D.L. Savoie, Mineral aerosol transport to the Pacific Ocean, in Chemical Oceanography, vol. 10, SEAREX: The Sea/Air Exchange Program, edited by J.P. Riley, R. Chester, and R.A. Duce, pp. 187-218, Academic, San Diego, Calif., 1989.

Rosman, K.J.R., C.C. Patterson, and D.M. Settle, The distribution of lead between seasalt, dust, and lead-rich aerosols in the mid-South Pacific easterlies at American Samoa, J. Geophys. Res., 95, 3687-3691, 1990.

Saltzman, E.S., D.L. Savoie, J.M. Prospero, and R.G. Zika, Atmospheric methanesulfonic acid and non-seasalt sulfate at Fanning and American Samoa, Geophys. Res. Lett., 12, 437-440, 1985.

Savoie, D.L., J.M. Prospero, and E.S. Saltzman, Nitrate, non-seasalt sulfate and methanesulfonate over the Pacific Ocean, in Chemical Oceanography, vol. 10, SEAREX: The Sea/Air Exchange Program, edited by J. P. Riley, R. Chester and R. A. Duce, pp. 219-250, Academic, San Diego, CA, 1989a.

Savoie, D.L., J.M. Prospero, J.T. Merrill, and M. Uematsu, Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport, J. Atmos. Chem., 8, 391-415, 1989b.

Savoie, D.L., J.M. Prospero, R. Arimoto, and R.A. Duce, Non-seasalt sulfate and methanesulfonate at American Samoa, J. Geophys. Res., 99 (D2), 3587-3596, 1994.

Tukey, J.W., Exploratory Data Analysis, 688 pp., Addison-Wesley, Reading, MA, 1977.

Turekian, K. K., W.C. Graustein, and J.K. Cochran, Lead-210 in the SEAREX program: An aerosol tracer across the Pacific, in Chemical Oceanography, vol. 10, SEAREX: The Sea/Air Exchange Program, pp. 51-81, edited by J. P. Riley, R. Chester and R. A. Duce, Academic, San Diego, Calif., 1989.
 

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