Technical Comments

Comment on "Reactions at Interfaces As a Source of Sulfate Formation in Sea-Salt Particles" (II)

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Science  30 Jan 2004:
Vol. 303, Issue 5658, pp. 628
DOI: 10.1126/science.1090971

In the laboratory, Laskin et al. (1) studied the role of the surface reaction OH + Cl → OH + Cl for influencing the pH and oxidation of S(IV) in the marine boundary layer. They concluded that it increases the sulfate production rate in tropospheric sea-salt particles. However, in their extrapolation to atmospheric conditions they neglected to include gas-phase diffusion limitations, which are especially important when assuming a high accommodation coefficient (2). Laskin et al. obtained a mass transfer coefficient (i.e., a first-order rate constant for loss of OH to particles) of kmt = 9.6 × 104 cm–3 s–1/5 × 106 cm–3 = 1.9 × 10–2 s–1. Considering gas-phase diffusion to the particles, the correct value is an order of magnitude lower: kmt = 1.6 × 10–3 s–1 (2).

To test the relevance of the proposed mechanism using the corrected value of kmt, we performed model runs with our box model MOCCA (3, 4). The conditions were set as suggested by Laskin et al. (1): 81% relative humidity and mixing ratios of about 30 nmol/mol O3 and 10 pmol/mol NO2. We adopted their monodisperse (1 μm radius) sea-salt particle concentration of 10 cm–3. To keep the interpretation of the model runs simple, we switched off bromine and iodine chemistry.

We implemented the reaction proposed by Laskin et al. as follows: Once OH reaches the aerosol surface, it is immediately converted to aqueous-phase OH. At the same time, a chloride ion is oxidized to a chlorine atom in the gas phase. This instantaneous reaction is equivalent to assuming that all chloride is on the particle surface, available for reaction with OH. We also performed a reference run, in which the surface reaction of OH was switched off. Salient results are shown in Fig. 1. Even though the surface reaction has increased uptake of OH, the corresponding increase in sulfate is small and there is almost no change in pH and chloride. For comparison, we did a sensitivity study setting kmt = 1.9 × 10–2 s–1, thereby neglecting gas-phase diffusion limitations for OH (red dashed line in Fig. 1). Here, we indeed find an increase of sulfate production by about 60%.

Fig. 1.

MOCCA model results. The reference run (solid black) is compared with a model run in which the proposed surface reaction of Laskin et al. (1) is switched on and, thus, gas-phase diffusion limitations for OH are neglected (red dotted line). (A) OH uptake rate versus time. (B) Non-sea-salt sulfate concentration versus time. (C) pH versus time. (D) Chloride concentration versus time.

Our model calculations show that the proposed surface reaction is of minor importance when we initialize our model using the atmospheric conditions as assumed by Laskin et al. In addition, their assumptions do not represent average conditions, but rather conditions that increase the importance of the proposed reaction, as follows: (i) There is no evidence that the accommodation coefficient of OH is actually as high as unity. (ii) Although the assumed liquid-water content represents typical conditions, a sea-salt number concentration of 10 cm–3 is rather high and the radius of 1 μm is rather low [see, e.g., (5)]. Both assumptions increase the transfer rate of OH to the aerosol. (iii) Daytime global average OH concentrations are 2 × 106 cm–3 [e.g., (6)] and not 5 × 106 cm–3, as adopted by Laskin et al. (1).

We have shown that the extrapolation of the laboratory experiments by Laskin et al. (1) to atmospheric conditions is erroneous, because it neglected gas-phase diffusion to the particles. Analyzing our model results and considering “typical” conditions, we conclude that their proposed reaction is not important for regulating sea-salt aerosol pH and sulfate production in the marine troposphere.


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