Megan Drake Willis

To understand the reaction pathways and products that drive secondary organic aerosol (SOA) formation from the oxidation of benzyl alcohol, we use online mass spectrometry to follow the formation and loss kinetics of oxidation products in both gas and particle phases. Over 20 h of equivalent photochemical aging (assuming [OH] = 1.5 × 106 molecules cm–3), we observe the formation of oxygenated aromatic and nitroaromatic compounds and highly oxygenated ring-retaining and ring-opening products. In the gas phase, we detect the formation of hydroxybenzyl alcohol (18–24% molar yield), benzaldehyde (14–47% yield), and phenol (3–7% yield); however, their early-generation oxidation products cannot explain rapid SOA formation with peak aerosol mass yields [20(±8)–37(±15)%] at 2–4 h of photochemical aging. The peroxy radical (RO2) bimolecular lifetime in our chamber is 2–3 s against reaction with NO and HO2, which suggests that RO2 fate is a competition between the unimolecular H-shift (lifetime of ∼0.01–1.1 s) and NO, HO2, and RO2 bimolecular reactions. In the particle phase, bicyclic peroxide, polyol, and carbonyl products peak within the first 10 h of photochemical aging, while highly oxygenated ring-opening products, including decomposition and accretion products, primarily accumulate in the aerosol after 10 h of photochemical aging. Observed product distributions demonstrate that the bicyclic peroxy radical’s fate is central to explaining SOA formation from benzyl alcohol.

Marine volatile methylated sulfur compounds (VMS) including dimethyl sulfide (DMS) and methanethiol (MeSH), are important natural sources of sulfur to the atmosphere and precursors to aerosols and cloud condensation nuclei. However, observations of DMS and MeSH in regions with sea ice remain limited. We present VMS measurements in the Arctic springtime from the ARTofMELT expedition. Hourly averaged DMS and MeSH observations range from below detection limit (1500 ppt _v , and from below detection limit (150 ppt _v respectively, with the highest concentrations observed during open ocean influence. MeSH made an important contribution to Arctic atmospheric VMS (median MeSH:DMS ratio of 0.12). In regional atmospheric chemistry model simulations using an oceanic DMS climatology and no sea‐to‐air flux from grid cells with sea ice concentration (SIC) > 0, the DMS log ₁₀ mean bias (LMB) is −0.82 with a mean bias (MB) of −101 ppt _v ; the MeSH LMB is −0.81 (MB = −3 ppt _v ). Scaling the sea‐to‐air flux by ocean fraction improves the atmospheric VMS model‐observation agreement by better representing VMS emissions during ocean influence from sea‐ice regions. Simulations which scale sea‐to‐air flux by ocean fraction and use different oceanic DMS data sets: the Lana climatology, a machine learning model, and a sea‐ice‐ocean‐biogeochemistry model, have a DMS LMB of 0.22, −0.49, and 0.02, respectively. These analyses provide evidence that MeSH is an important fraction of atmospheric VMS and both partial‐sea‐ice and open‐ocean regions are sources of marine sulfur to the atmosphere in the late‐springtime Arctic, both must be considered in atmospheric models.

To understand the atmospheric fate of dimethylsulfide (DMS) during the Arctic spring-to-summer transition, we measured volatile methylated sulfur (VMS) species, including DMS and DMS-derived oxidation products, dimethyl sulfoxide (DMSO), and dimethylsulfone (DMSO2), in the Fram Strait during May and June of 2023. Elevated DMSO contributions to VMS accompany depleted O3, high surface albedo, and air temperatures < 0 °C in air masses influenced by snow-covered sea-ice regions, suggesting that BrO•-initiated oxidation dominated DMS loss prior to the onset of sea-ice melt. In GEOS-Chem simulations, BrO• is the dominant oxidant for DMS, but modeled BrO• concentrations from sea-salt debromination alone are too low to replicate observed O3, DMSO, and DMSO2, suggesting a missing source of reactive bromine from sunlit saline snow. BrO•-initiated oxidation reduces the DMS lifetime and oxidizes DMS through methanesulfinic acid (MSIA) more quickly in the gas phase and in higher yields compared to •OH-initiated oxidation alone. Once melt onset begins and snowpack-driven halogen activation shuts off, •OH oxidation likely dominates Arctic DMS oxidation. Our observations highlight the seasonal variability of halogen oxidation impacting sulfur species in the Arctic marine region, which are likely to change as the sea-ice extent decreases and the melt season in the Arctic lengthens.

Ultrafine particles and submicron aerosol mass negatively impact human health; while indoor primary sources are known, secondary aerosol formation from oxidation of volatile organic compounds (VOCs) is an important, but less understood, exposure source. Previous studies have suggested that indoor new particle formation (NPF) and growth are ozone-limited. However, we find that in an authentic aged unoccupied office in a region with high outdoor O3 concentrations, sufficient O3 is transported indoors that NPF and growth are instead limited by the availability of VOCs. Mechanical ventilation continuously supplies O3 in the summertime and sustains the subsequent growth so that particle growth rates upon the addition of limonene range from 20 to 100 nm h–1─rates 1–2 orders of magnitude greater than those observed outdoors. Additionally, we observe meaningful mass contributions from the growth of newly formed particles. Household use of limonene-containing products in rooms with elevated ozone─either from ozone-producing remediation and germicidal UV technologies, or during outdoor ozone exceedance events─will produce ultrafine particles and increase particle mass. The effect of ventilated O3 from outdoors on particle formation and growth highlights the value of continued efforts to reduce outdoor O3 pollution for moderating both indoor and outdoor sources of secondary pollutants.

Alkyl substitution on glycol diether structures reduces the yield of semi-volatile to intermediate volatility oxidation products and suppresses SOA formation.

Arctic aerosol, cloud, and surface radiation budgets are highly sensitive to how dimethylsulfide (DMS) is represented in atmospheric models. We investigate the impact of (a) the gas‐phase DMS oxidation mechanism (simple 3‐reaction scheme vs. a complex gas‐phase scheme), (b) the representation of oceanic DMS (global climatology vs. detailed Arctic Ocean biogeochemistry simulation), and (c) the parameterization of new particle formation from marine biogenic precursor gases, represented by methanesulfonic acid (MSA). We find that the more detailed approach in (a) and (b) improves agreement with observed DMS and concentrations, and that activating (c) enables the model to capture the observed summertime maximum in total aerosol number concentrations. Together, these changes result in large‐scale modifications to cloud microphysics and surface energy budget in the Arctic, which lead to a very good agreement with observations of cloud cover and surface radiation. We recommend that climate models consider adopting these updates.

To understand the atmospheric fate of dimethylsulfide (DMS) during the Arctic spring-to-summer transition and the onset of ice melt, we measured volatile methylated sulfur (VMS) species, including DMS and DMS-derived oxidation products, dimethyl sulfoxide (DMSO), and dimethyl sulfone (DMSO ₂ ), in the Fram Strait during May and June of 2023. Elevated DMSO contributions to VMS, depleted O ₃ , high surface albedo, and air temperatures < 0 ^◦ C in air masses influenced by snow-covered sea-ice regions demonstrate that BrO ^• -initiated oxidation dominated DMS loss prior to the onset of sea-ice melt. GEOS-Chem simulations demonstrate that BrO ^• is an important oxidant for DMS, but modeled BrO ^• concentrations from sea salt debromination alone are too low to replicate observed O ₃ , DMSO, and DMSO ₂ , highlighting the importance of bromine activation in sunlit saline snow. BrO ^• -initiated oxidation reduces the DMS lifetime, and oxidizes DMS through methanesulfinic acid (MSIA) more quickly in the gas phase and in higher yields compared to ^• OH-initiated oxidation alone. Once melt onset begins and snowpack-driven halogen activation shuts off, ^• OH oxidation will dominate Arctic DMS oxidation. Our observations highlight the seasonal variability of halogen oxidation impacting sulfur species in the Arctic marine region, which are likely to change as sea ice extent decreases and the melt season in the Arctic lengthens.

To understand the atmospheric fate of dimethylsulfide (DMS) during the Arctic spring-to-summer transition and the onset of ice melt, we measured volatile methylated sulfur (VMS) species, including DMS and DMS-derived oxidation products, dimethyl sulfoxide (DMSO), and dimethyl sulfone (DMSO ₂ ), in the Fram Strait during May and June of 2023. Elevated DMSO contributions to VMS, depleted O ₃ , high surface albedo, and air temperatures < 0 ^◦ C in air masses influenced by snow-covered sea-ice regions demonstrate that BrO ^• -initiated oxidation dominated DMS loss prior to the onset of sea-ice melt. GEOS-Chem simulations demonstrate that BrO ^• is an important oxidant for DMS, but modeled BrO ^• concentrations from sea salt debromination alone are too low to replicate observed O ₃ , DMSO, and DMSO ₂ , highlighting the importance of bromine activation in sunlit saline snow. BrO ^• -initiated oxidation reduces the DMS lifetime, and oxidizes DMS through methanesulfinic acid (MSIA) more quickly in the gas phase and in higher yields compared to ^• OH-initiated oxidation alone. Once melt onset begins and snowpack-driven halogen activation shuts off, ^• OH oxidation will dominate Arctic DMS oxidation. Our observations highlight the seasonal variability of halogen oxidation impacting sulfur species in the Arctic marine region, which are likely to change as sea ice extent decreases and the melt season in the Arctic lengthens.

During the ARTofMELT 2023 expedition (8 May–14 June) aboard the icebreaker Oden, we investigated the High Arctic aerosol system across the transition from late spring into sea-ice melt onset. Submicron aerosol properties were characterized using high-resolution soot particle aerosol mass spectrometry (SP-AMS; chemical composition, organic fingerprints and elemental ratios), complemented by measurements of equivalent black carbon (eBC), particle size distributions, back-trajectory analysis, and size-segregated offline analyses by means of ion chromatography and total carbon analysis. We identified nine regimes, spanning Arctic-confined conditions, warm-air intrusions, and fog sampling.Campaign-average submicron particulate matter (PM1) was sulfate-dominated (66% sulfate, 29% organic aerosol OA, minor nitrate, ammonium and chloride; eBC ~1%), but approaching the melt onset variability shifted from accumulation-mode, sulfate-rich background conditions to Aitken-mode dominated aerosols with higher relative contribution from OA. The analysis of the OA fragmentation patterns showed a persistently oxidized background, repeatedly perturbed by transport and fog, with coherent shifts toward fresher material (lower f44 and O:C, higher H:C) coinciding with Aitken-mode dominance. Size-resolved total carbon (TC) measurements (