Matthew B. Goss

The atmospheric oxidation of dimethyl sulfide (DMS) represents a large natural source of sulfate particles and thus is a major contributor to the global radiative effect of aerosols; however, its underlying chemical mechanism remains poorly constrained. In particular, DMS oxidation generates a variety of intermediate organic sulfur species, whose fate and kinetics govern the ultimate amount and distribution of sulfate aerosol. There is thus a need to understand the production and chemistry of such intermediates, including the recently discovered hydroperoxymethyl thioformate, formed from the isomerization of the methylthiomethylperoxy radical (CH3SCH2OO). Here, chamber experiments were performed to measure product formation from the OH-initiated oxidation of DMS. Three real-time mass spectrometers were used to measure the formation and evolution of a broad suite of gas- and aerosol-phase sulfur-containing compounds, including nearly all the closed-shell organic sulfur species included in current mechanisms; some additional species not predicted by such mechanisms are detected as well. The rapid decay of many of the more oxidized organic sulfur species suggests that aerosol uptake and loss to surfaces can be important processes under the conditions of this study. In addition, the isomerization rate constant of the CH3SCH2OO radical was experimentally determined to be 0.09 s–1 (0.03–0.3 s–1, 1σg), in broad agreement with results from other studies.

One strategy for mitigating the indoor transmission of airborne pathogens, including the SARS-CoV-2 virus, is irradiation by germicidal UV light (GUV). A particularly promising approach is 222 nm light from KrCl excimer lamps (GUV222); this inactivates airborne pathogens and is thought to be relatively safe for human skin and eye exposure. However, the impact of GUV222 on the composition of indoor air has received little experimental study. Here, we conduct laboratory experiments in a 150 L Teflon chamber to examine the formation of secondary species by GUV222. We show that GUV222 generates ozone (O3) and hydroxyl radicals (OH), both of which can react with volatile organic compounds to form oxidized volatile organic compounds and secondary organic aerosol particles. Results are consistent with a box model based on the known photochemistry. We use this model to simulate GUV222 irradiation under more realistic indoor air scenarios and demonstrate that under some conditions, GUV222 irradiation can lead to levels of O3, OH, and secondary organic products that are substantially elevated relative to normal indoor conditions. The results suggest that GUV222 should be used at low intensities and in concert with ventilation, decreasing levels of airborne pathogens while mitigating the formation of air pollutants.

Abstract. The atmospheric oxidation of dimethyl sulfide (DMS) represents a major natural source of atmospheric sulfate aerosols. However, there remain large uncertainties in our understanding of the underlying chemistry that governs the product distribution and sulfate yield from DMS oxidation. Here, chamber experiments were conducted to simulate gas-phase OH-initiated oxidation of DMS under a range of reaction conditions. Most importantly, the bimolecular lifetime (τbi) of the peroxy radical CH3SCH2OO was varied over several orders of magnitude, enabling the examination of the role of peroxy radical isomerization reactions on product formation. An array of analytical instruments was used to measure nearly all sulfur-containing species in the reaction mixture, and results were compared with a near-explicit chemical mechanism. When relative humidity was low, “sulfur closure” was achieved under both high-NO (τbi<0.1 s) and low-NO (τbi>10 s) conditions, though product distributions were substantially different in the two cases. Under high-NO conditions, approximately half the product sulfur was in the particle phase, as methane sulfonic acid (MSA) and sulfate, with most of the remainder as SO2 (which in the atmosphere would eventually oxidize to sulfate or be lost to deposition). Under low-NO conditions, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), formed from CH3SCH2OO isomerization, dominates the sulfur budget over the course of the experiment, suppressing or delaying the formation of SO2 and particulate matter. The isomerization rate constant of CH3SCH2OO at 295 K is found to be 0.13±0.03 s−1, in broad agreement with other recent laboratory measurements. The rate constants for the OH oxidation of key first-generation oxidation products (HPMTF and methyl thioformate, MTF) were also determined (kOH+HPMTF=2.1×10-11 cm3 molec.−1 s−1, kOH+MTF=1.35×10-11 cm3 molec.−1 s−1). Product measurements agree reasonably well with mechanistic predictions in terms of total sulfur distribution and concentrations of most individual species, though the mechanism overpredicts sulfate and underpredicts MSA under high-NO conditions. Lastly, results from high-relative-humidity conditions suggest efficient heterogenous loss of at least some gas-phase products.

Levels of volatile organic compounds (VOCs) in the indoor environment can be decreased by the use of “air cleaners”, devices that remove VOCs by sorption and/or oxidative degradation. However, efficacies of these technologies for removing VOCs tend to be poorly constrained, as does the formation of oxidation byproducts. Here, we examine the influence of several oxidation-based air cleaners, specifically ones marketed as consumer-grade products, on the amounts and composition of VOCs. Experiments were conducted in an environmental chamber, with a suite of real-time analytical instruments to measure direct emissions, VOC removal efficacies (by the addition of either limonene and toluene), and byproduct formation. We find that the air cleaners themselves can be a source of organic gases, that removal efficacy can be exceedingly variable, and that VOC loss is primarily driven by physical removal in some cleaners. When oxidative degradation of VOCs was observed, it was accompanied by the formation of a range of oxidation byproducts, including formaldehyde and other oxygenates. These results indicate that some consumer-grade portable air cleaners can be ineffective in removing VOCs and that the air delivered may contain a range of organic compounds, due to direct emission and/or byproduct formation.

Secondary organic aerosol (SOA), atmospheric particulate matter formed from low-volatility products of volatile organic compound (VOC) oxidation, affects both air quality and climate. Current 3D models, however, cannot reproduce the observed variability in atmospheric organic aerosol. Because many SOA model descriptions are derived from environmental chamber experiments, our ability to represent atmospheric conditions in chambers directly affects our ability to assess the air quality and climate impacts of SOA. Here, we develop an approach that leverages global modeling and detailed mechanisms to design chamber experiments that mimic the atmospheric chemistry of organic peroxy radicals (RO 2 ), a key intermediate in VOC oxidation. Drawing on decades of laboratory experiments, we develop a framework for quantitatively describing RO 2 chemistry and show that no previous experimental approaches to studying SOA formation have accessed the relevant atmospheric RO 2 fate distribution. We show proof-of-concept experiments that demonstrate how SOA experiments can access a range of atmospheric chemical environments and propose several directions for future studies.