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

Atmospheric Intermediates at the Air-Water Interface

The air-water interface (AWI) is a ubiquitous reaction field different from the bulk phase where unexpected reactions and physical processes often occur. The AWI is a region where air contacts cloud droplets, aerosol particles, the ocean surface, and biological surfaces such as fluids that line human epithelia. In Earth's atmosphere, short-lived intermediates are expected to be generated at the AWI during multiphase reactions. Recent experimental developments have enabled the direct detection of atmospherically relevant, short-lived intermediates at the AWI. For example, spray ionization mass spectrometric analysis of water microjets exposed to a gaseous mixture of ozone and water vapor combined with a 266 nm laser flash photolysis system (LFP-SIMS) has been used to directly probe organic peroxyl radicals (RO2·) produced by interfacial hydroxyl radicals (OH·) + organic compound reactions. OH· emitted immediately after the laser flash photolysis of carboxylic acid at the gas-liquid interface have been directly detected by time-resolved, laser-induced florescence techniques that can be used to study atmospheric multiphase photoreactions. In this Featured Article, we show some recent experimental advances in the detection of atmospherically important intermediates at the AWI and the associated reaction mechanisms. We also discuss current challenges and future prospects for atmospheric multiphase chemistry.

Spatiotemporal Variation and Influencing Factors of TSP and Anions in Coastal Atmosphere of Zhanjiang City, China

Water-soluble anions and suspended fine particles have negative impacts on ecosystems and human health, which is a current research hotspot. In this study, coastal suburb, coastal urban area, coastal tourist area, and coastal industrial area were explored to study the spatiotemporal variation and influencing factors of water-soluble anions and total suspended particles (TSP) in Zhanjiang atmosphere. In addition, on-site monitoring, laboratory testing, and analysis were used to identify the difference of each pollutant component at the sampling stations. The results showed that the average concentrations of Cl⁻, NO₃⁻, SO₄²⁻, PO₄³⁻, and TSP were 29.8 μg/m³, 19.6 μg/m³, 45.6 μg/m³, 13.5 μg/m³, and 0.28 mg/m³, respectively. The concentration of Cl⁻, NO₃⁻, PO₄³⁻, and atmospheric TSP were the highest in coastal urban area, while the concentration of SO₄²⁻ was the highest in coastal industrial area. Moreover, there were significantly seasonal differences in the concentration of various pollutants (p < 0.05). Cl⁻ and SO₄²⁻ were high in summer, and NO₃⁻ and TSP were high in winter. Cl⁻, SO₄²⁻, PO₄³⁻, and TSP had significant correlations with meteorological elements (temperature, relative humidity, atmospheric pressure, and wind speed). Besides, the results showed the areas with the most serious air pollution were coastal urban area and coastal industrial area. Moreover, the exhaust emissions from vehicles, urban enterprise emissions, and seawater evaporation were responsible for the serious air pollution in coastal urban area. It provided baseline information for the coastal atmospheric environment quality in Zhanjiang coastal city, which was critical to the mitigation strategies for the emission sources of air pollutants in the future.

Role of Ferryl Ion Intermediates in Fast Fenton Chemistry on Aqueous Microdroplets

In the aqueous environment, Fe^II ions enhance the oxidative potential of ozone and hydrogen peroxide by generating the reactive oxoiron species (ferryl ion, Fe^IVO²⁺) and hydroxyl radical (·OH) via Fenton chemistry. Herein, we investigate factors that control the pathways of these reactive intermediates in the oxidation of dimethyl sulfoxide (Me₂SO) in Fe^II solutions reacting with O₃ in both bulk-phase water and on the surfaces of aqueous microdroplets. Electrospray ionization mass spectrometry is used to quantify the formation of dimethyl sulfone (Me₂SO₂, from Fe^IVO²⁺ + Me₂SO) and methanesulfonate (MeSO₃^-, from ·OH + Me₂SO) over a wide range of Fe^II and O₃ concentrations and pH. In addition, the role of environmentally relevant organic ligands on the reaction kinetics was also explored. The experimental results show that Fenton chemistry proceeds at a rate ∼10⁴ times faster on microdroplets than that in bulk-phase water. Since the production of MeSO₃^- is initiated by ·OH radicals at diffusion-controlled rates, experimental ratios of Me₂SO₂/MeSO₃^- > 10² suggest that Fe^IVO²⁺ is the dominant intermediate under all conditions. Me₂SO₂ yields in the presence of ligands, L, vary as volcano-plot functions of E⁰(LFe^IVO²⁺+ O₂/LFe²⁺ + O₃) reduction potentials calculated by DFT with a maximum achieved in the case of L≡oxalate. Our findings underscore the key role of ferryl Fe^IVO²⁺ intermediates in Fenton chemistry taking place on aqueous microdroplets.

Effects of Metal Ions on Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides Derived from Terpene Alcohols

We report the results of a mass spectrometric study of the effects of atmospherically relevant metal ions on the decomposition of α-hydroxyalkyl-hydroperoxides (α-HHs) derived from ozonolysis of α-terpineol in aqueous solutions. By direct mass spectrometric detection of chloride adducts of α-HHs, we assessed the temporal profiles of α-HHs and other products in the presence of metal ions. In addition, reactions between α-HHs and FeCl₂ in the presence of excess DMSO showed that the amount of hydroxyl radicals formed in a mixture of α-terpineol, O₃, and FeCl₂ was 5.7 ± 0.8% of the amount formed in a mixture of H₂O₂ and FeCl₂. The first-order rate constants for the decay of α-HHs produced by ozonolysis of α-terpineol in the presence of 5 mM acetate buffer at a pH of 5.1 ± 0.1 were determined to be (4.5 ± 0.1) × 10^-4 s^-1 (no metal ions), (4.7 ± 0.2) × 10^-4 s^-1 (with 0.05 mM Fe²⁺), (4.7 ± 0.1) × 10^-4 s^-1 (with 0.05 mM Zn²⁺), and (4.8 ± 0.2) × 10^-4 s^-1 (with 0.05 mM Cu²⁺). We propose that in acidic aqueous media, the reaction of α-HHs with Fe²⁺ is outcompeted by H⁺-catalyzed decomposition of α-HHs, which produces the corresponding aldehydes and H₂O₂, which can in turn react with Fe²⁺ to form hydroxyl radicals.

The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate

Surface ocean biogeochemistry and photochemistry regulate ocean-atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO2) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N2O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Understanding the properties of methanesulfinic acid at the air-water interface

Methanesulfinic acid (MSIA), an organic sulfur compound, is mainly produced in the oxidation process of dimethyl sulfide in the atmosphere. The properties of MSIA at the air-water interface were studied using molecular dynamics (MD) simulations. The result shows that the lowest system free energy is located at the interface. Because the free energy difference between the interface and water phase is 3.2 kJ mol^-1, the MSIA molecule can easily get out of the free energy well and travel to water phase by the thermal motion, leading to only a 21% probability of its occurrence at the interface. The MSIA molecule tends to tilt at the interface with the sulfino group (-S(O)-OH) pointing toward the water phase. The feature of hydration status at the air-water interface may be favorable to the heterogeneous oxidation of MSIA.

Understanding the oxidation mechanism of methanesulfinic acid by ozone in the atmosphere

Methanesulfinic acid (MSIA) is an important intermediate in the oxidation of dimethyl sulfide (DMS) in the marine boundary layer. The oxidation of MSIA by ozone in the gas phase to form methanesulfonic acid (MSA) was investigated using theoretical calculations in this paper. Three pathways can be found for the reaction of MSIA with ozone. The highest energy barrier is 13.02 kcal mol⁻¹ in the most favorable pathway. By comparing the reaction rate of MSIA + O₃ with that of MSIA + OH, it can be concluded that the oxidation of MSIA by O₃ to form MSA is of minor significance relative to its oxidation by OH radical in the gas phase. This study can provide some information for the theoretical and experimental studies in the significantly heterogeneous and aqueous-phase oxidation of MSIA by O₃.

OH-Radical Oxidation of Lung Surfactant Protein B on Aqueous Surfaces

Air pollutants generate reactive oxygen species on lung surfaces. Here we report how hydroxyl radicals (·OH) injected on the surface of water react with SP-B_1-25, a 25-residue polypeptide surrogate of human lung surfactant protein B. Our experiments consist of intersecting microjets of aqueous SP-B_1-25 solutions with O₃/O₂/H₂O/N₂(g) gas streams that are photolyzed into ·OH(g) in situ by 266 nm laser nanosecond pulses. Surface-sensitive mass spectrometry enables us to monitor the prompt (<10 μs) and simultaneous formation of primary O _n -containing products/intermediates (n≤5) triggered by the reaction of ·OH with interfacial SP-B_1-25. We found that O-atoms from both O₃ and ·OH are incorporated into the reactive cysteine Cys₈ and Cys₁₁ and tryptophan Trp₉ components of the hydrophobic N-terminus of SP-B_1-25 that lies at the topmost layers of the air-liquid interface. Remarkably, these processes are initiated by ·OH additions rather than by H-atom abstractions from S-H, C-H, or N-H groups. By increasing the hydrophilicity of the N-terminus region of SP-B_1-25, these transformations will impair its role as a surfactant.

Investigation of reactive oxygen species produced by microwave electrodeless discharge lamp on oxidation of dimethyl sulfide

Microwave electrodeless discharge lamp (MEDL) has been regarded as a powerful light source of photoreaction. Four kinds of chemicals, nitrogen (N₂), oxygen (O₂), water (H₂O) and dimethyl sulfide (DMS), were used as molecular probes to explore the generation process of reactive oxygen species (ROS) and their photo-oxidation performances on the photodegradation of organic pollutants with application of an exterior MEDL system. ROS such as O (³P), O₃, O (¹D) and ¹O₂ were generated via irradiation of O₂ and H₂O except dry N₂ by MEDL. They were transformed to other ROS including ·OH and H₂O₂ with increase of relative humidity. The ROS productivity was inhibited evidently by humidity and ·OH became the major active species at high humidity. An optimal mineralization rate of 23.6% for DMS photodegradation was reached in dry air compared with 8.74% at high humidity, which indicated that O (¹D) and ¹O₂ were more powerful oxidants than O₃ and OH. The results showed that the higher mineralization rate of organic pollutants was obtained by increasing the generation efficiency of ROS of O (¹D) and ¹O₂. Furthermore, the results provided an alternative to develop intensification technology on photodegadation of organic pollutants with MEDL system and an optimal operation process including photocatalyst and humidity.

Monitoring variations of dimethyl sulfide and dimethylsulfoniopropionate in seawater and the atmosphere based on sequential vapor generation and ion molecule reaction mass spectrometry

To monitor the fluctuations of dimethyl sulfur compounds at the seawater/atmosphere interface, an automated system was developed based on sequential injection analysis coupled with vapor generation-ion molecule reaction mass spectrometry (SIA-VG-IMRMS). Using this analytical system, dissolved dimethyl sulfide (DMS(aq)) and dimethylsulfoniopropionate (DMSP), a precursor to DMS in seawater, were monitored together sequentially with atmospheric dimethyl sulfide (DMS(g)). A shift from the equilibrium point between DMS(aq) and DMS(g) results in the emission of DMS to the atmosphere. Atmospheric DMS emitted from seawater plays an important role as a source of cloud condensation nuclei, which influences the oceanic climate. Water samples were taken periodically and dissolved DMS(aq) was vaporized for analysis by IMRMS. After that, DMSP was hydrolyzed to DMS and acrylic acid, and analyzed in the same manner as DMS(aq). The vaporization behavior and hydrolysis of DMSP to DMS were investigated to optimize these conditions. Frequent (every 30 min) determination of the three components, DMS(aq)/DMSP (nanomolar) and DMS(g) (ppbv), was carried out by SIA-VG-IMRMS. Field analysis of the dimethyl sulfur compounds was undertaken at a coastal station, which succeeded in showing detailed variations of the compounds in a natural setting. Observed concentrations of the dimethyl sulfur compounds both in the atmosphere and seawater largely changed with time and similar variations were repeatedly observed over several days, suggesting diurnal variations in the DMS flux at the seawater/atmosphere interface.