Unexpectedly Rapid Removal of Sodium Methanesulfonate upon Heterogeneous OH Oxidation Revealed by Smog Chamber Experiment

SZE IN MADELEINE NG, Zi Jun Li, Angela Buchholz, Iida Pullinen, Snehitha M. Kommula, Aki Nissinen, Mitchell Alton, Pasi Yli-Pirilä, Merete Bilde, Annele Virtanen, Marianne Glasius, Siegfried Schobesberger, Man Nin Chan, The Chinese University of Hong Kong

     Abstract Number: 300
     Working Group: Aerosol Chemistry

Abstract
The atmospheric level ratio of methanesulfonic acid (MSA) to non-sea-salt sulfate (nss-SO₄^2–) is often used as a tracer for marine biological activity, under the assumption that MSA is relatively long-lived. However, this assumption has been challenged by oxidation flow reactors (OFR) experiments probing the chemical transformation of MSA via heterogeneous oxidation by hydroxyl radical (OH), which produces nss-SO₄^2– and formaldehyde.

To better assess the chemical lifetime of MSA, we employed an environmental chamber at 65 % relative humidity (RH) and 298 K to explore heterogeneous OH oxidation of sodium methanesulfonate (CH₃SO₃Na, NaMS), the sodium salt of MSA abundant in the marine boundary layer. Different from OFR, our chamber allowed for atmospherically relevant levels of reactants ([OH]= (9.99 ± 0.76) x 10⁶ molecules cm^–3) and long reaction time, and therefore provided a more accurate estimation of reaction kinetics and time-resolved products. Composition and mass of aerosol particles were simultaneously monitored by real-time techniques including an aerosol mass spectrometer (AMS), an extractive electrospray ionization mass spectrometer (EESI-MS), and a scanning mobility particle sizer (SMPS). In parallel, particles were collected using a spot sampler for analysis with liquid chromatography-mass spectrometry (LC-MS). Gas-phase species were measured by a proton-transfer-reaction ionization mass spectrometer (PTR-MS).

We observed rapid wall loss-corrected decay of NaMS, with an effective reaction rate constant k and an initial effective OH uptake coefficient γeff respectively calculated as (1.28 ± 0.07) x 10^–12 cm³ molecule^–1 s^–1 and 1.26 ± 0.03, indicative of secondary reactions. This corresponds to a chemical lifetime of around 6 days, dependent on ambient OH level. We speculate that current climate models may have underestimated the conversion of MSA to nss-SO₄^2– via heterogeneous oxidation, and future work is necessary to re-evaluate the robustness of MSA to nss-SO₄^2– ratio as a proxy for marine sulfur production.