Oxidation of S(IV) in sea‐salt aerosol at high pH: Ozone versus aerobic reaction

Abstract

[1] The oxidation of dissolved SO 2 (as S(IV)) by dissolved O 3 is known to increase rapidly with pH and is generally thought to be the mechanism responsible for the oxidation of S(IV) in sea-salt aerosol at high pH. Recently, Hoppel et al. (2001) have shown that an aerobic reaction studied by Zhang and Millero (1991) increases even more rapidly with pH than O 3 oxidation and exceeds that of O 3 at pH of about 7.5 to 8.5, depending on the ambient SO 2 concentrations. At high pH, both of these oxidation mechanisms proceed so rapidly that the actual rate is constrained by both gas-phase and aqueous-phase transport processes. A method of analysis is developed to calculate the transport-limited reaction rates for both of these mechanisms in sea-salt aerosol. Even when the unconstrained O 3 -S(IV) rate is faster than the aerobic mechanism, aqueousphase transport limitations on ozone can slow the constrained O 3 -S(IV) rate to the degree that it is slower than the aerobic reaction. In general, the O 3 -S(IV) rate is favored over the aerobic mechanism at low SO 2 and high O 3 concentrations, whereas the aerobic mechanism is favored at high SO 2 concentrations. The analysis indicates that the O 3 -S(IV) mechanism is more important in the remote regions where the SO 2 concentration can be extremely low. When the SO 2 concentration approaches 1 ppb or greater the aerobic mechanism is likely to be more important.Citation: Hoppel, W. A., and P. F. Caffrey (2005), Oxidation of S(IV) in sea-salt aerosol at high pH: Ozone versus aerobic reaction,

Fractionation of sulfur isotopes during heterogeneous oxidation of SO<sub>2</sub> on sea salt aerosol: a new tool to investigate non-sea salt sulfate production in the marine boundary layer

“…In a chamber study with sea salt and pure NaCl aerosols, Hoppel et al (2001) saw production of sulfate but no uptake of ozone. They proposed that oxidation catalysed by Cl·, which is second order in [S(IV)], is the dominant oxidation pathway in the MBL at high SO 2 concentrations (Hoppel and Caffrey, 2005). However, oxidation by Cl· catalysis, like oxidation by O 3 , is strongly pH dependent and limited by alkalinity.…”

“…However, oxidation by ozone is strongly pH dependent and self-limiting as aerosol becomes acidified following sulfate production. The amount of sulfate generated by this pathway is therefore constrained by the alkalinity of the aerosol and the concentration of other gases, such as HNO 3 , which also titrate alkalinity (Chameides and Stelson, 1992;Zhang and Millero, 1991;von Glasow and Sander, 2001;Hoppel and Caffrey, 2005). Thus, O 3 can only efficiently oxidise SO 2 in sea salt aerosol in the first 10-20 min following emission, and oxidation by O 3 occurs mainly in the lowest 50-100 m of the MBL which leads to rapid deposition of the sulfate produced (Chameides and Stelson, 1992;von Glasow and Sander, 2001;.…”

Trace gas and aerosol interactions in the fully coupled model of aerosol‐chemistry‐climate ECHAM5‐HAMMOZ: 1. Model description and insights from the spring 2001 TRACE‐P experiment

“…Thus we only consider sulfate formation on fresh sea salt particles by oxidation with O 3 at a constant pH = 8 (fresh sea salt particles were diagnosed as the particles being only composed of sea salt and water). We do not take into account the sea salt alkalinity titration by HNO 3 (which can be important in tropical regions where NOx emissions dominate over SO 2 emissions [Alexander et al, 2005]), and the competing aerobic S(IV) oxidation proposed by Hoppel and Caffrey [2005].…”

A new approach to studying aqueous reactions using diffuse reflectance infrared Fourier transform spectrometry: application to the uptake and oxidation of SO2 on OH-processed model sea salt aerosol

“…This observation is consistent with the studies of Hoppel et al who observed the uptake and oxidation of gaseous SO 2 in seawater aerosols but not NaCl aerosols. [93][94][95] They attributed this to the rapid acidification of the unbuffered NaCl aerosols which restricted the dissolved S(IV) concentration to B10 mM compared to concentrations that were three orders of magnitude larger in the buffered, alkaline seasalt particles. The spectra taken in the aerosol chamber are those of aqueous liquid particles.…”

“…Dissolved S(IV) is oxidized in the aqueous phase and on the surfaces of particles by a number of oxidants, such as H 2 O 2 , O 3 , HOCl, HOBr and O 2 catalyzed by metals and perhaps by chloride ions. [50][51][52][53][54][55]57,60,71,[81][82][83][84][85][86][87][88][89][90][91][92][93][94][95] SO 2 can also be oxidized in the gas phase, mainly by OH, 96 to produce sulfuric acid which is then taken up by cloud droplets and particles; however, this is generally a minor pathway compared to aqueous oxidation. 16 In this work, we demonstrate the application of DRIFTS to studying the uptake and oxidation of gaseous SO 2 at 50% relative humidity (RH) on NaCl-MgCl 2 Á 6H 2 O salt mixtures that have been previously reacted with OH formed by the photolysis of O 3 in the presence of water vapor.…”