Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α‑Pinene Oxidation Products

Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueous-phase OH oxidation chemistry of oxidation products of α-pinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, <i>cis</i>-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToF-CIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (±0.5) × 10⁹ and 3.1 (±0.2) × 10⁸ M^–1 s^–1, respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation

Heterogeneous Chlorination of Squalene and Oleic Acid

Washing with chlorine bleach leads to high mixing ratios of gas-phase HOCl. Using two methods that are sensitive to surface film compositionattenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy and direct analysis in real time mass spectrometry (DART-MS)we present the first study of the chlorination chemistry that occurs when gaseous HOCl reacts with thin films of squalene and oleic acid. At mixing ratios of 600 ppbv, HOCl forms chlorohydrins by adding across carbon–carbon double bonds without breaking the carbon backbone. The initial uptake of one HOCl molecule occurs on the time scale of a few minutes at these mixing ratios. For oleic acid, ester formation proceeds immediately thereafter, leading to dimeric and trimeric chlorinated products. For squalene, subsequent HOCl uptake occurs until all six of its carbon–carbon double bonds become chlorinated within 1–2 h. These results indicate that chlorination of skin oil, which contains substantial carbon unsaturation, is likely to occur rapidly under common cleaning conditions, potentially leading to the irritation associated with chlorinated bleach. This chemistry will likely also proceed with cooking oils, in the human respiratory system which has unsaturated surfactants as important components of lung fluid, and with organic components of the sea surface microlayer

Behavior of Isocyanic Acid and Other Nitrogen-Containing Volatile Organic Compounds in The Indoor Environment

Isocyanic acid (HNCO) and other nitrogen-containing volatile chemicals (organic isocyanates, hydrogen cyanide, nitriles, amines, amides) were measured during the House Observation of Microbial and Environmental Chemistry (HOMEChem) campaign. The indoor HNCO mean mixing ratio was 0.14 ± 0.30 ppb (range 0.012–6.1 ppb), higher than outdoor levels (mean 0.026 ± 0.15 ppb). From the month-long study, cooking and chlorine bleach cleaning are identified as the most important human-related sources of these nitrogen-containing gases. Gas oven cooking emits more isocyanates than stovetop cooking. The emission ratios HNCO/CO (ppb/ppm) during stovetop and oven cooking (mean 0.090 and 0.30) are lower than previously reported values during biomass burning (between 0.76 and 4.6) and cigarette smoking (mean 2.7). Bleach cleaning led to an increase of the HNCO mixing ratio of a factor of 3.5 per liter of cleaning solution used; laboratory studies indicate that isocyanates arise via reaction of nitrogen-containing precursors, such as indoor dust. Partitioned in a temperature-dependent manner to indoor surface reservoirs, HNCO was present at the beginning of HOMEChem, arising from an unidentified source. HNCO levels are higher at the end of the campaign than the beginning, indicative of occupant activities such as cleaning and cooking; however the direct emissions of humans are relatively minor

Exploring Conditions for Ultrafine Particle Formation from Oxidation of Cigarette Smoke in Indoor Environments

Cigarette smoke is an important source of particles and gases in the indoor environment. In this work, aging of side-stream cigarette smoke was studied in an environmental chamber via exposure to ozone (O₃), hydroxyl radicals (OH) and indoor fluorescent lights. Aerosol mass concentrations increased by 13–18% upon exposure to 15 ppb O₃ and by 8–42% upon exposure to 0.45 ppt OH. Ultrafine particle (UFP) formation was observed during all ozone experiments, regardless of the primary smoke aerosol concentration (185–1950 μg m^–3). During OH oxidation, however, UFP formed only when the primary particle concentration was relatively low (<130 μg m^–3) and the OH concentration was high (∼1.1 × 10⁷ molecules cm^–3). Online aerosol composition measurements show that oxygen- and nitrogen- containing species were formed during oxidation. Gas phase oxidation of NO to NO₂ occurred during fluorescent light exposure, but neither primary particle growth nor UFP formation were observed. Overall, exposure of cigarette smoke to ozone will likely lead to UFP formation in indoor environments. On the other hand, UPF formation via OH oxidation will only occur when OH concentrations are high (∼10⁷ molecules cm^–3), and is therefore less likely to have an impact on indoor aerosol associated with cigarette smoke

Sources of Wintertime Atmospheric Organic Pollutants in a Large Canadian City: Insights from Particle and Gas Phase Measurements

Although atmospheric organic pollutants have been extensively studied to elucidate summertime urban photochemical air pollution, uncertainties remain concerning the quality of wintertime air in large northern North American cities. Here, we used online mass spectrometric measurements of volatile organic compounds (VOCs) and organic aerosol (OA), combined with positive matrix factorization (PMF), to identify sources of organic pollutants in downtown Toronto, Canada during February–March 2023. In some cases, comparable PMF factors were identified for both VOCs and OA, such as from traffic, cooking, and background oxygenated sources. However, VOC PMF yielded additional information, such as a factor associated with human-related emissions of VOCs. Additionally, VOC PMF yields two traffic factors: one likely related to gasoline and one to diesel use. Despite cold and relatively dark conditions, the OA and VOC oxygenated factors both grow in intensity during the daytime, indicative of photochemical activity, whereas the traffic and cooking factors were enhanced in the morning and late evening due to the timing of vehicle use, cooking, and boundary layer effects. This study illustrates the benefits that arise from the parallel source–receptor analyses of organic gases and aerosol particles

Dynamic Wood Smoke Aerosol Toxicity during Oxidative Atmospheric Aging

Wildfires are a major source of biomass burning aerosol to the atmosphere, with their incidence and intensity expected to increase in a warmer future climate. However, the toxicity evolution of biomass burning organic aerosol (BBOA) during atmospheric aging remains poorly understood. In this study, we report a unique set of chemical and toxicological metrics of BBOA from pine wood smoldering during multiphase aging by gas-phase hydroxyl radicals (OH). Both the fresh and OH-aged BBOA show activity relevant to adverse health outcomes. The results from two acellular assays (DTT and DCFH) show significant oxidative potential (OP) and reactive oxygen species (ROS) formation in OH-aged BBOA. Also, radical concentrations in the aerosol assessed by electron paramagnetic resonance (EPR) spectroscopy increased by 50% following heterogeneous aging. This enhancement was accompanied by a transition from predominantly carbon-centered radicals (85%) in the fresh aerosol to predominantly oxygen-centered radicals (76%) following aging. Both the fresh and aged biomass burning aerosols trigger prominent antioxidant defense during the in vitro exposure, indicating the induction of oxidative stress by BBOA in the atmosphere. By connecting chemical composition and toxicity using an integrated approach, we show that short-term aging initiated by OH radicals can produce biomass burning particles with a higher particle-bound ROS generation capacity, which are therefore a more relevant exposure hazard for residents in large population centers close to wildfire regions than previously studied fresh biomass burning emissions

Ventilation in a Residential Building Brings Outdoor NO_<i>x</i> Indoors with Limited Implications for VOC Oxidation from NO₃ Radicals

Energy-efficient residential building standards require the use of mechanical ventilation systems that replace indoor air with outdoor air. Transient outdoor pollution events can be transported indoors via the mechanical ventilation system and other outdoor air entry pathways and impact indoor air chemistry. In the spring of 2022, we observed elevated levels of NOx (NO + NO2) that originated outdoors, entering the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility through the mechanical ventilation system. Using measurements of NOx, ozone (O3), and volatile organic compounds (VOCs), we modeled the effect of the outdoor-to-indoor ventilation of NOx pollution on the production of nitrate radical (NO3), a potentially important indoor oxidant. We evaluated how VOC oxidation chemistry was affected by NO3 during NOx pollution events compared to background conditions. We found that nitric oxide (NO) pollution introduced indoors titrated O3 and inhibited the modeled production of NO3. NO ventilated indoors also likely ceased most gas-phase VOC oxidation chemistry during plume events. Only through the artificial introduction of O3 to the ventilation duct during a NOx pollution event (i.e., when O3 and NO2 concentrations were high relative to typical conditions) were we able to measure NO3-initiated VOC oxidation products, indicating that NO3 was impacting VOC oxidation chemistry