Lauren A. Garofalo

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.

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.

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 NO_x (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 NO_x, ozone (O₃), and volatile organic compounds (VOCs), we modeled the effect of the outdoor-to-indoor ventilation of NO_x pollution on the production of nitrate radical (NO₃), a potentially important indoor oxidant. We evaluated how VOC oxidation chemistry was affected by NO₃ during NO_x 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 NO_x 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.

Wildfires are increasing in frequency, raising concerns that smoke can permeate indoor environments and expose people to chemical air contaminants. To study smoke transformations in indoor environments and evaluate mitigation strategies, we added smoke to a test house. Many volatile organic compounds (VOCs) persisted days following the smoke injection, providing a longer-term exposure pathway for humans. Two time scales control smoke VOC partitioning: a faster one (1.0 to 5.2 hours) that describes the time to reach equilibrium between adsorption and desorption processes and a slower one (4.8 to 21.2 hours) that describes the time for indoor ventilation to overtake adsorption-desorption equilibria in controlling the air concentration. These rates imply that vapor pressure controls partitioning behavior and that house ventilation plays a minor role in removing smoke VOCs. However, surface cleaning activities (vacuuming, mopping, and dusting) physically removed surface reservoirs and thus reduced indoor smoke VOC concentrations more effectively than portable air cleaners and more persistently than window opening.

We use observations from dual high‐resolution mass spectrometers to characterize ecosystem‐atmosphere fluxes of reactive carbon across an extensive range of volatile organic compounds (VOCs) and test how well that exchange is represented in current chemical transport models. Measurements combined proton‐transfer reaction mass spectrometry (PTRMS) and iodide chemical ionization mass spectrometry (ICIMS) over a Colorado pine forest; together, these techniques have been shown to capture the majority of ambient VOC abundance and reactivity. Total VOC mass and associated OH reactivity fluxes were dominated by emissions of 2‐methyl‐3‐buten‐2‐ol, monoterpenes, and small oxygenated VOCs, with a small number of compounds detected by PTRMS driving the majority of both net and upward exchanges. Most of these dominant species are explicitly included in chemical models, and we find here that GEOS‐Chem accurately simulates the net and upward VOC mass and OH reactivity fluxes under clear sky conditions. However, large upward terpene fluxes occurred during sustained rainfall, and these are not captured by the model. Far more species contributed to the downward fluxes than are explicitly modeled, leading to a major underestimation of this key sink of atmospheric reactive carbon. This model bias mainly reflects missing and underestimated concentrations of depositing species, though inaccurate deposition velocities also contribute. The deposition underestimate is particularly large for assumed isoprene oxidation products, organic acids, and nitrates—species that are primarily detected by ICIMS. Net ecosystem‐atmosphere fluxes of ozone reactivity were dominated by sesquiterpenes and monoterpenes, highlighting the importance of these species for predicting near‐surface ozone, oxidants, and aerosols.

We use observations from dual high-resolution mass spectrometers to characterize ecosystem-atmosphere fluxes of reactive carbon across an extensive range of volatile organic compounds (VOCs) and test how well that exchange is represented in current chemical transport models. Measurements combined proton-transfer reaction mass spectrometry (PTRMS) and iodide chemical ionization mass spectrometry (ICIMS) over a Colorado pine forest; together, these techniques have been shown to capture the majority of ambient VOC abundance and reactivity. Total VOC mass and associated OH reactivity fluxes were dominated by emissions of 2-methyl-3-buten-2-ol, monoterpenes, and small oxygenated VOCs, with a small number of compounds detected by PTRMS driving the majority of both net and upward exchanges. Most of these dominant species are explicitly included in chemical models, and we find here that GEOS-Chem accurately simulates the net and upward VOC mass and OH reactivity fluxes under clear sky conditions. However, large upward terpene fluxes occurred during sustained rainfall, and these are not captured by the model. Far more species contributed to the downward fluxes than are explicitly modeled, leading to a major underestimation of this key sink of atmospheric reactive carbon. This model bias mainly reflects missing and underestimated concentrations of depositing species, though inaccurate deposition velocities also contribute. The deposition underestimate is particularly large for assumed isoprene oxidation products, organic acids, and nitrates—species that are primarily detected by ICIMS. Ecosystem-atmosphere fluxes of ozone reactivity were dominated by sesquiterpenes and monoterpenes, highlighting the importance of these species for predicting near-surface ozone, oxidants, and aerosols.

Nitrous acid (HONO) mixing ratios measured in aged wildfire smoke plumes were higher than expected from known homogeneous chemical reactions. In a representative smoke plume, intercepted hours to days downwind of the source, the missing HONO source was highly correlated to particulate nitrate photolysis and NO₂ reactive uptake to particles. Using a multilinear regression involving these two sources, we could explain the missing HONO production in this plume (R² = 0.77). The resulting fit parameters from this plume had good explanatory power (R² = 0.64) for missing HONO production in other fire plumes. The mean enhancement factor for particulate nitrate photolysis relative to gas‐phase nitric acid photolysis was 63 and the mean NO₂ reactive uptake coefficient to submicron aerosol surface area forming HONO was 4.9 × 10⁻⁴. Given the likelihood of other neglected secondary HONO sources, these values are upper‐limits, suggesting a need to revisit HONO formation mechanisms in aged wildfire smoke.