Lu Xu

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.

Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H⁺_air) to ALWC. Inorganic ions dominate H⁺_air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H⁺_air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM₁ ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH₃–NH₄⁺ and HNO₃–NO₃^-. Predicted ammonia partitioning agreed with observations (R² > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH₄⁺ but only 0.1–0.22 for NO₃^-, giving nitrate less redistribution and pH-buffering capacity than NH₄⁺. Including OA water raised pH during smoke by at most 0.62 units and improved HNO₃–NO₃^- agreement. Summertime PM₁ acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.

Unimolecular reactions of organic peroxy radicals (RO2) play a central role in the atmospheric oxidation of volatile organic compounds (VOCs), yet experimental constraints on their reaction kinetics remain limited. Ring-opened RO2 formed in the OH-initiated oxidation of α-pinene undergoes rapid unimolecular reactions, including 1,5 H-shift, 1,6 H-shift, and endocyclization. However, previously reported experimental rate coefficients represent the overall unimolecular reaction rate from all channels and do not resolve branching among chemically distinct channels. As these channels lead to different oxidation products, channel-specific rate coefficients are needed to constrain product distributions under different atmospheric conditions. Here, we employ site-specific deuterium-labeled α-pinene isotopologues to selectively suppress individual H-shift channels and isolate the contributions of competing unimolecular channels. For each deuterated isotopologue, the overall unimolecular rate coefficient of the ring-opened RO2 was determined at 296 K using hydroxy nitrate (HN)-based kinetic analysis and compared with that of unlabeled α-pinene to derive channel-specific rate coefficients. We obtain 0.7−0.4+0.5 s–1 for the 1,6 H-shift,0.5−0.3+0.5 s–1 for the 1,5 H-shift, and 3.0−1.6+1.7 s–1 for endocyclization. These values are consistent with theoretical calculations and provide direct experimental constraints on individual channel rate coefficients in α-pinene-derived RO2 unimolecular chemistry. These channel-specific rate coefficients provide a quantitative basis for predicting oxidation product distributions under different atmospheric conditions and serve as experimental benchmarks for theoretical calculations of RO2 unimolecular chemistry.

The photochemical oxidation of dimethyl sulfide (DMS) by the hydroxyl radical (OH) contributes to the production and growth of particles and cloud condensation nuclei in the marine boundary layer (MBL). DMS oxidized by OH can go on to form a stable intermediate, hydroperoxymethyl thioformate (HPMTF), which is both globally ubiquitous and efficiently lost to multiphase processes in the marine atmosphere. Recent work has shown that clouds act as a significant and irreversible loss path for HPMTF, sequestering sulfur and greatly reducing the production of long-lived, climate-relevant species (e.g., SO2 and OCS). Until recently, there have been few observational constraints on the role of clouds in the spatiotemporal distribution of HPMTF and other soluble molecules in the MBL. Using a steady-state approximation, we determine a reactive uptake and first-order aqueous loss rate constant of HPMTF to cloud droplets of 1.2 (0.9–1.9) × 10–2 and 1.4 (0.4–2.2) × 10–1 s–1, respectively. The results suggest that hydrolysis and ozonolysis are significant within cloud droplets, and their competition likely impacts the distribution of sulfate and organic acids in the marine environment.

Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H⁺_air) to ALWC. Inorganic ions dominate H⁺_air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H⁺_air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM₁ ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH₃–NH₄⁺ and HNO₃–NO₃^-. Predicted ammonia partitioning agreed with observations (R² > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH₄⁺ but only 0.1–0.22 for NO₃^-, giving nitrate less redistribution and pH-buffering capacity than NH₄⁺. Including OA water raised pH during smoke by at most 0.62 units and improved HNO₃–NO₃^- agreement. Summertime PM₁ acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.

Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H+air) to ALWC. Inorganic ions dominate H+air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H+air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM1 ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH3–NH4+ and HNO3–NO3-. Predicted ammonia partitioning agreed with observations (R2 > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH4+ but only 0.1–0.22 for NO3-, giving nitrate less redistribution and pH-buffering capacity than NH4+. Including OA water raised pH during smoke by at most 0.62 units and improved HNO3–NO3- agreement. Summertime PM1 acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.

Lake breezes can strongly influence the environment in coastal areas. As the most populous city along Lake Michigan, Chicago frequently experiences lake breezes, yet their impact on air quality remains poorly understood. Using one-year measurements from meteorological and the Microsoft Eclipse low-cost sensor networks, we identify 42 lake breeze events in 2022 and assess the net effects of lake breezes on the urban environment in Chicago. We find that lake breezes occurring on weekdays generally reduce ozone (O3) mixing ratios on average up to ∼6 ppb along the northeast shoreline, contrary to many other coastal regions, where sea/lake breezes drive O3 exceedances. Reduced O3 during lake breeze events likely result from a combination of processes, including enhanced NO titration in a shallow boundary layer, especially during the early stage of the event and in areas with heavy emissions, the inflow of cleaner air, and the lower temperatures that suppress the O3 formation by reducing precursor emissions and slowing-down reaction rates. In contrast, on average, lake breezes enhance fine particulate matter (PM2.5) concentrations by up to ∼3 μg m–3, due to the accumulation of local emissions within a shallower boundary layer and enhanced partitioning of semivolatile species at lower temperatures. Additionally, we identify 51 weak events during which lake breezes remained mostly within several kilometers of the lakeshore. These weak lake breeze events show broadly consistent effects on temperature and O3, albeit with smaller magnitudes. These results highlight the interactions of emissions, atmospheric chemistry, and meteorological conditions during the lake breeze events and their impact on the urban environment, underscoring the need for region-specific evaluations of sea/lake breeze effects.

Quantifying oxygenated volatile organic compounds (OVOCs) in the atmospheric gas phase remains a major challenge, particularly in urban environments where these compounds are important contributors to air quality and health impacts. Here, we present a collision-induced dissociation based voltage-scanning method to derive sensitivities for an ammonium-adduct chemical ionization mass spectrometer, enabling quantification of OVOCs beyond those available for direct calibration. The method was validated under laboratory conditions and applied during the 2023 AEROMMA airborne campaign and the 2024 SAPHIR-CHANEL chamber campaign. The approach extended quantification coverage to 98% of the total signal in chamber experiments and 78% in field measurements. Detected species spanned a wide volatility range, with chamber experiments allowing enhanced detection of more oxidized, higher-molecular-weight products compared to the more variable and diluted field observations. Although coverage decreased with increasing molecular size and oxidation state, the method consistently extended into these regimes, particularly under controlled chamber conditions. Sensitivity trends between chamber and field data showed strong agreement, supporting the use of chamber-derived calibrations for ambient quantification. Incorporating these additional quantified species increased estimates of OH reactivity and secondary organic aerosol formation potential by around 5% and around 62%, respectively, contributing to improved chemical closure in urban air.

Global measurements of organic aerosol (OA) concentrations and chemical composition remain limited and unevenly distributed. While monitoring networks, including the globally distributed Surface Particulate Matter Network (SPARTAN), provide an established framework for measurements, their current methodologies do not fully support comprehensive OA characterization. Aerosol Mass Spectrometry (AMS) is widely used for real-time OA composition measurements, but its cost, complexity, and logistical requirements limit long-term, multi-site online deployment, particularly at the global scale. Here we develop and evaluate an offline AMS methodology to characterize OA in particulate matter collected on Teflon filters routinely used by monitoring networks. Using a commercial ultrasonic nebulizer coupled with a syringe pump, this offline method is highly reproducible, requires small extract volumes (2 mL), offers low detection limits (1.7 µg OA and 0.43 µg sulfate per filter), and achieves higher nebulization efficiency than previous methods. We evaluate this offline AMS method by co-located online AMS observations. We find that oxygenated OA is effectively recovered (64 ± 28 %) while the recovery is lower for hydrocarbon-like OA due to its limited water solubility. This approach offers new capability for SPARTAN and is readily adaptable to other monitoring networks. Its application across networks will broaden the spatiotemporal coverage of AMS-based OA measurements and improve methodological and instrumental consistency to support ongoing efforts to build a long-term, globally consistent OA dataset.