Open AccessSulfate formation in sea‐salt aerosols: Constraints from oxygen isotopes
Author(s) -
Alexander B.,
Park R. J.,
Jacob D. J.,
Li Q. B.,
Yantosca R. M.,
Savarino J.,
Lee C. C. W.,
Thiemens M. H.
Publication year - 2005
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2004jd005659
Subject(s) - sulfate , sea salt , alkalinity , aerosol , environmental chemistry , sea salt aerosol , salt (chemistry) , chemistry , isotopes of oxygen , nuclear chemistry , organic chemistry
We use observations of the mass‐independent oxygen isotopic composition (Δ 17 O) of sulfate in the marine boundary layer (MBL) to quantify the sulfate source from aqueous SO 2 (S(IV)) oxidation by O 3 in alkaline sea‐salt aerosols. Oxidation by O 3 imparts a large Δ 17 O signature to the resulting sulfate (8.8‰) relative to oxidation by H 2 O 2 (0.9‰) or by OH or O 2 (0‰). Ship data from two Indian Ocean Experiment (INDOEX) cruises in the Indian Ocean indicate Δ 17 O values usually <1‰ in the submicron sulfate aerosol but considerable variability in the supermicron sulfate with frequent occurrences above 1‰ and up to 6.7‰. The large Δ 17 O values are associated with high concentrations of sea‐salt aerosols, providing evidence for the S(IV) + O 3 pathway. We use a global chemical transport model (GEOS‐CHEM) to interpret quantitatively the INDOEX observations and to assess the global importance of sulfate production in sea‐salt aerosols. The model accounts for titration of sea‐salt alkalinity in the MBL by uptake of acid gases (SO 2 , H 2 SO 4 , and HNO 3 ), shutting down the S(IV) + O 3 pathway. We find that this titration occurs rapidly over much of the oceans except at high latitudes (strong sea‐salt emission) and is due to both the S(IV) + O 3 reaction and HNO 3 (g) condensation; that is, sulfate formation in sea‐salt aerosols is limited by the alkalinity flux from the ocean and by competition for this alkalinity supply from HNO 3 (g). The model is consistent with the Δ 17 O magnitudes and patterns in the INDOEX data. Titration of alkalinity is critical for the success of the model simulation. Regeneration of sea‐salt aerosol alkalinity by OH uptake is inconsistent with the Δ 17 O observations in INDOEX. Model results indicate that sulfate production in sea‐salt aerosols decreases MBL SO 2 concentrations and gas phase H 2 SO 4 production rates by typically 10–30% (up to >70%) and increases MBL sulfate concentrations by typically >10% (up to 30%). Globally, this mechanism contributes 9% of atmospheric sulfate production and 1% of the sulfate burden. The impact on H 2 SO 4 (g) formation and implications for the potential formation of new particles in the MBL warrants inclusion in models examining the radiative effects of sulfate aerosols.