Comment on “Reactions at Interfaces As a Source of Sulfate Formation in Sea-Salt Particles” (I) Laskin et al.(1) investigated the reaction OH + Cl – 3 OH – + Cl at surfaces of deli- quesced NaCl particles in the laboratory. They estimate that this source of alkalinity, which has not been described previously, is roughly comparable to that of aerosol acidity during daytime and thereby sustains signifi- cant S(IV) oxidation by O 3 in ambient sea- salt aerosols with implications for the global S cycle and climate. Although the laboratory results are compelling, field observations do not support the inferred importance of this pathway in ambient marine air. The assessment by Laskin et al.(1) is based on similarities between calculated rates of alkalinity production via this pathway and corresponding rates of aerosol acidification by H 2 SO 4 originating from both S(IV) oxi- dation within aerosol solutions and direct scavenging from the gas phase. However, measurements of size-resolved NO 3 – and non-sea-salt SO 4 2– in marine air [see (2–6 ) and references therein] indicate that, with the exception of high-latitude southern-ocean re- gions, HNO 3 , not H 2 SO 4 , is typically the dominant source of acidity in sea-salt aero- sols. In addition, other acids (including HCl, HCOOH, and CH 3 COOH) are also present at significant concentrations in polluted and re- mote marine air [e.g., (6–11)] and contribute to efficient alkalinity titration. Because Laskin et al. either did not consider acidifi- cation by acids other than H 2 SO 4 or assumed it to be negligible [note 26 in (1)], the inferred influence of the pathway on S(VI) production was substantially overestimated. Model calculations that do not consider the proposed mechanism predict that most fresh sea-salt aerosols are acidified within seconds to tens of minutes via incorporation of acids and precursors (6, 8, 12–14 ); rates of acidification increase with decreasing particle size because of larger surface-to-volume ratios. Simulated pH levels for super-m size fractions in most regions range from the mid threes to the upper fives. These modeled val- ues are within the range of sea-salt aerosol pHs estimated from direct acidity measure- ments (5 ) and from the measured phase par- titioning and thermodynamic properties of HCl in both polluted (15) and remote (6 ) regions. S(IV) oxidation by O 3 over this pH range is negligible (8). If, as suggested by Laskin et al.(1), substantial alkalinity were produced in sea salt during daytime by a chem- ical mechanism not considered in the models, simulated daytime pHs would systematically diverge from measurement-based estimates, particularly under very clean conditions [e.g., (6 )]. Such divergence is not evident. Finally, the phase partitionings of HCOOH and CH 3 COOH provide useful di- agnostics of aerosol pH. Although these acids are infinitely soluble in alkaline so- lution, thermodynamic properties (16 ) in- dicate that they partition almost exclusively in the gas phase in the presence of acidic aerosols. Measurements over the North At- lantic Ocean indicate that virtually all HCOOH and CH 3 COOH partitions in the gas phase (11); with the exception of one sample collected during a large Saharan dust event, concentrations of dissociated + undissociated HCOO – and CH 3 COO – in size-resolved aerosols were undetectable (20 pmol m –3 ). In addition, we have measured these carboxylic species in sev- eral hundred sea-salt size fractions sampled over discrete day and night intervals during onshore flow at Bermuda and Hawaii and in coastal air along the eastern United States (17 ). The frequency of detectable concen- trations was indistinguishable from 0%; no diel variability was observed. These results also indicate no evidence for significant alkalinity production in sea-salt aerosols. In conclusion, the weight of the avail- able evidence based on measurements, thermodynamic relationships, and model calculations suggests that acids and precur- sors are present in most marine regions at levels sufficient to rapidly titrate alkalinity and acidify sea-salt aerosols. This evidence is inconsistent with the hypothesis that the proposed OH pathway slows aerosol acid- ification during daytime and thereby sus- tains S(IV) oxidation by O 3 enough to af- fect S cycling significantly, except perhaps in very remote marine regions. William C. Keene Department of Environmental Sciences University of Virginia Charlottesville, VA 22904 – 4123, USA Alexander A. P. Pszenny Institute for the Study of Earth, Oceans, and Space University of New Hampshire Durham, NH 03824, USA and Mount Washington Observatory North Conway, NH 03860, USA References and Notes 1. A. Laskin et al., Science 301, 340 (2003). 2. D. L. Savoie, J. M. Prospero, Geophys. Res. Lett. 9, 1207 (1982). 3. B. J. Huebert et al., J. Geophys. Res. 101, 4413 (1996). 4. M. O. Andreae, W. Elbert, Y. Cai, T. W. Andreae, J. Gras, J. Geophys. Res. 104, 21695 (1999). 5. W. C. Keene, A. A. P. Pszenny, J. R . Maben, R. Sander, Geophys. Res. Lett. 29, 1101 (2002). 6. A.A.P.Pszenny et al ., Atmos. Chem. Phys. Disc. 3, 1 (2003). 7. A. A. P. Pszenny et al., Geophys. Res. Lett. 20, 699 (1993). 8. W. C. Keene et al., J. Aerosol Sci. 29, 339 (1998). 9. J. N. Galloway et al., Tellus 41B, 427 (1989). 10. J. L. Moody et al., J. Geophys. Res. 96, 20769 (1991). 11. J. J. Schultz-Tokos, thesis, University of Rhode Island (1989). 12. W. L. Chameides, A. W. Stelson, J. Geophys. Res. 97, 20565 (1992). 13. D. J. Erickson III, C. Seuzaret, W. C. Keene, S. L. Gong, J. Geophys. Res. 104, 8347 (1999). 14. A. M. Fridlind, M. Z. Jacobson, J. Geophys. Res. 105, 17325 (2000). 15. W. C. Keene, D. L. Savoie, Geophys. Res. Lett. 26, 1315 (1999). 16. W. C. Keene et al., J. Geophys. Res. 100, 9345 (1995). 17. W. C. Keene, unpublished data. 18. Funding for this work was provided by the NSF through award numbers ATM-0207786 to the Uni- versity of Virginia and ATM-0207646 to the Univer- sity of New Hampshire. 22 August 2003; accepted 3 December 2003 TECHNICAL COMMENT www.sciencemag.org SCIENCE VOL 303 30 JANUARY 2004 628b on December 15, 2016 http://science.sciencemag.org/ Downloaded from