High Gas-Phase Methanesulfonic Acid Production in the OH-Initiated Oxidation of Dimethyl Sulfide at Low Temperatures

Dimethyl sulfide (DMS) influences climate via cloud condensation nuclei (CCN) formation resulting from its oxidation products (mainly methanesulfonic acid, MSA, and sulfuric acid, H₂SO₄). Despite their importance, accurate prediction of MSA and H₂SO₄ from DMS oxidation remains challenging. With comprehensive experiments carried out in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, we show that decreasing the temperature from +25 to -10 °C enhances the gas-phase MSA production by an order of magnitude from OH-initiated DMS oxidation, while H₂SO₄ production is modestly affected. This leads to a gas-phase H₂SO₄-to-MSA ratio (H₂SO₄/MSA) smaller than one at low temperatures, consistent with field observations in polar regions. With an updated DMS oxidation mechanism, we find that methanesulfinic acid, CH₃S(O)OH, MSIA, forms large amounts of MSA. Overall, our results reveal that MSA yields are a factor of 2-10 higher than those predicted by the widely used Master Chemical Mechanism (MCMv3.3.1), and the NO_x effect is less significant than that of temperature. Our updated mechanism explains the high MSA production rates observed in field observations, especially at low temperatures, thus, substantiating the greater importance of MSA in the natural sulfur cycle and natural CCN formation. Our mechanism will improve the interpretation of present-day and historical gas-phase H₂SO₄/MSA measurements.

Effect of sea ice retreat on marine aerosol emissions in the Southern Ocean, Antarctica

Sea ice retreat in the polar region is expected to increase the emissions of sea salt aerosols and biogenic gases, which may significantly impact the climate by increasing cloud condensation nuclei (CCN) population and changing solar radiation. In this study, aerosol compositions were measured at high-time-resolution (1 h) with an in-situ gas and aerosol composition monitoring system in polynya regions of the Southern Ocean (SO) to access the effects of sea ice concentrations on the sea salt aerosol (SSA) and secondary biogenic aerosol (SBA) in the SO. SSA emissions increased by more than 30% as sea ice concentration decreased from 85% to 29%. However, SSA emissions did not increase monotonically as the sea ice concentration decreased. The highest SSA concentration occurred in drifting sea ice region. Sea ice melting increased SBA concentrations by enhancing the air-sea exchanges of SBA precursor gases and the release of algae from sea ice. Positive correlations between SSA and wind speed were present in different sea ice regions, while SBA didn't reveal an obvious correlation with wind speed in the SO. The impact of wind speed on the SSA emissions were very different, Higher slope value of 41.83 and 35.81 were present in the DSI and SIF region, while the value was only about 16.74 in the SIC region. The results extended the knowledge of the effect of future sea ice retreat on marine aerosol emissions and potential climate changes in the polar region.

Modeling the impact of marine DMS emissions on summertime air quality over the coastal East China Seas

[1] Biogenic emission of dimethyl sulfide (DMS) from seawater is the major natural source of sulfur into the atmosphere. In this study, we use an advanced air quality model (CMAQv5.2) with DMS chemistry to examine the impact of DMS emissions from seawater on summertime air quality over China. A national scale database of DMS concentration in seawater is established based on a five-year observational record in the East China seas including the Bohai Sea, the Yellow Sea and the East China Sea. We employ a commonly used global database and also the newly developed local database of oceanic DMS concentration, calculate DMS emissions using three different parameterization schemes, and perform five different model simulations for July, 2018. Results indicate that in large coastal areas of China, the average DMS emissions flux obtained with the local database is three times higher than that resulting from the global database, with a mean value of 9.1 μmol m^-2 d^-1 in the Bohai Sea, 8.4 μmol m^-2 d^-1 in the Yellow Sea and 13.4 μmol m^-2 d^-1 in the East China Sea. The total DMS emissions flux calculated with the Nightingale scheme is 42% higher than that obtained with the Liss and Merlivat scheme, but is 15% lower than that obtained with the Wanninkhof scheme. Among the three parameterizations, results of the Liss and Merlivat scheme agree better with the ship-based observations over China's coastal waters. DMS emissions with the Liss and Merlivat parametrization increase atmospheric sulfur dioxide (SO₂) and sulfate (SO₄ ^2-) concentration over the East China seas by 6.4% and 3.3%, respectively. Our results indicate that although the anthropogenic source is still the dominant contributor of atmospheric sulfur burden in China, biogenic DMS emissions source is nonnegligible.

Significant Underestimation of Gaseous Methanesulfonic Acid (MSA) over Southern Ocean

Methanesulfonic acid (MSA), derived from the oxidation of dimethylsulfide (DMS), has a significant impact on biogenic sulfur cycle and climate. Gaseous MSA (MSA_g) has been often ignored in previous studies due to its quick conversion to particulate MSA (MSA_p) and low concentrations. MSA_g, MSA_p, and nss-SO₄^2- were observed simultaneously for the first time with high-time-resolution (1 h) in the Southern Ocean (SO). The mean MSA_g level reached up to 3.3 ± 1.6 pptv, ranging from ∼24.5 pptv in the SO, contributing to 31% ± 3% to the total MSA (MSA_T). A reduction of the MSA to nss-SO₄^2- ratios by about 30% was obtained when MSA_g was not accounted for in the calculation, indicating that MSA_g was very important in the assessment of the biogenic sulfur contributions in the atmosphere. Mass ratios of MSA to nss-SO₄^2- increased first and then decreased with the temperature from -10 to 5 °C, with a maximum value at the temperature of -3 °C. Positive correlations between MSA_g to MSA_T ratios and temperature were presented, when the temperature was higher than 5 °C. This study highlights the importance of MSA_g for understanding the atmospheric DMS oxidation mechanism and extends the knowledge of MSA formation in the marine atmosphere.

Characterization of the water soluble fraction in ultrafine, fine, and coarse atmospheric aerosol

Water soluble organic carbon significantly contributes to aerosol's carbon mass and its chemical composition is poorly characterized due to the huge number of species. In this study, we determined 94 water-soluble compounds: inorganic ions (Cl^-, Br^-, I^-, NO₃^-, SO₄^2-,K⁺, Mg⁺, Na⁺, NH₄⁺, Ca²⁺), organic acids (methanesulfonic acid and C₂-C₇ carboxylic acids), monosaccharides, alcohol-sugars, levoglucosan and its isomers, sucrose, phenolic compounds, free l- and d-amino acids and photo-oxidation products of α-pinene (cis-pinonic acid and pinic acid). The sampling was conducted using a micro-orifice uniform deposit impactor (MOUDI) at the urban area of Mestre-Venice from March to May 2016. The main aim of this work is to identify the source of each detected compound, evaluating its particle size distribution. Clear differences in size distributions were observed for each class of analyzed compounds. The positive matrix factorization (PMF) model was used to identify six factors related to different sources: a) primary biogenic aerosol particles with particle size > 10 μm; b) secondary sulfate contribution; c) biomass burning; d) primary biogenic aerosol particles distributed between 10 and 1 μm; e) an aged sea salt input and f) SOA pinene. Each factor was also characterized by different composition in waters soluble compounds and different particles size distribution.

"Sizing" Heterogeneous Chemistry in the Conversion of Gaseous Dimethyl Sulfide to Atmospheric Particles

The oxidation of biogenic dimethyl sulfide (DMS) emissions is a global source of cloud condensation nuclei. The amounts of the nucleating H2SO4(g) species produced in such process, however, remain uncertain. Hydrophobic DMS is mostly oxidized in the gas phase into H2SO4(g) + DMSO(g) (dimethyl sulfoxide), whereas water-soluble DMSO is oxidized into H2SO4(g) in the gas phase and into SO4(2-) + MeSO3(-) (methanesulfonate) on water surfaces. R = MeSO3(-)/(non-sea-salt SO4(2-)) ratios would therefore gauge both the strength of DMS sources and the extent of DMSO heterogeneous oxidation if Rhet = MeSO3(-)/SO4(2-) for DMSO(aq) + ·OH(g) were known. Here, we report that Rhet = 2.7, a value obtained from online electrospray mass spectra of DMSO(aq) + ·OH(g) reaction products that quantifies the MeSO3(-) produced in DMSO heterogeneous oxidation on aqueous aerosols for the first time. On this basis, the inverse R dependence on particle radius in size-segregated aerosol collected over Syowa station and Southern oceans is shown to be consistent with the competition between DMSO gas-phase oxidation and its mass accommodation followed by oxidation on aqueous droplets. Geographical R variations are thus associated with variable contributions of the heterogeneous pathway to DMSO atmospheric oxidation, which increase with the specific surface area of local aerosols.