Laskin et al. (1) investigated the reaction OH + Cl– → OH–+ Cl at surfaces of deliquesced 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 significant S(IV) oxidation by O3 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 H2SO4 originating from both S(IV) oxidation within aerosol solutions and direct scavenging from the gas phase. However, measurements of size-resolved NO3– and non-sea-salt SO42– in marine air [see (2–6) and references therein] indicate that, with the exception of high-latitude southern-ocean regions, HNO3, not H2SO4, is typically the dominant source of acidity in sea-salt aerosols. In addition, other acids (including HCl, HCOOH, and CH3COOH) are also present at significant concentrations in polluted and remote marine air [e.g., (6–11)] and contribute to efficient alkalinity titration. Because Laskin et al. either did not consider acidification by acids other than H2SO4 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-μm size fractions in most regions range from the mid threes to the upper fives. These modeled values are within the range of sea-salt aerosol pHs estimated from direct acidity measurements (5) and from the measured phase partitioning and thermodynamic properties of HCl in both polluted (15) and remote (6) regions. S(IV) oxidation by O3 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 chemical 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 CH3COOH provide useful diagnostics of aerosol pH. Although these acids are infinitely soluble in alkaline solution, thermodynamic properties (16) indicate that they partition almost exclusively in the gas phase in the presence of acidic aerosols. Measurements over the North Atlantic Ocean indicate that virtually all HCOOH and CH3COOH 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 CH3COO– in size-resolved aerosols were undetectable (< ∼20 pmol m–3). In addition, we have measured these carboxylic species in several 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 concentrations 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 available evidence based on measurements, thermodynamic relationships, and model calculations suggests that acids and precursors 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 acidification during daytime and thereby sustains S(IV) oxidation by O3 enough to affect S cycling significantly, except perhaps in very remote marine regions.