The reaction of ozone with unsaturated organic compounds is a fundamental process in chemistry, central to both synthetic transformations and environmental molecular cycling. Here, we reveal a mechanistic branching in the Criegee ozonolysis of squalene, a model poly unsaturated triterpene, induced by gaseous ammonia, a ubiquitous nucleophile. Our experimental findings reveal that NH3 suppresses the ozone uptake by a factor of ∼3, an inhibition traced to competitive adsorption. DFT calculations and kinetic modeling indicate that NH3·H2O complexes outcompete O3 for reactive C═C sites, suggesting a site-blocking suppression mechanism. Real-time ultrahigh-resolution mass spectrometry identifies nearly 40 unique nitrogen-containing organic compounds (NOCs) formed exclusively in the presence of NH3. Gibbs free energy profiles support a thermodynamically feasible mechanism wherein NH3 acts as a nucleophile, intercepting reactive carbonyl oxide intermediates (Criegee intermediates) and downstream carbonyl species. This work elucidates a direct pathway for nitrogen incorporation into unsaturated lipid oxidation matrices, demonstrating how a simple nucleophile can fundamentally rewire a classic organic reaction mechanism simultaneously controlling and steering product distribution toward complex, heteroatom-enriched architectures. These findings provide molecular-scale understanding of nucleophile participation in multiphase ozonolysis, with implications for synthetic and complex organic chemistry.

The first 5 hours of aging in biomass burning plumes can strongly affect ozone photochemistry. We examine how volatile organic compounds (VOCs), nitrogen oxides, and nitrous acid influence hydroxyl radical, ozone, and peroxyacetyl nitrate (PAN) based on three aircraft campaigns over the United States. Our analyses reveal variable, highly elevated hydroxyl radical concentrations in the first 2 hours, resulting in evident fire-to-fire variability in VOCs oxidation and in ozone and PAN production. About 40 to 70% of the variability is explained by chemical aging. Ozone production in the plumes is usually VOC-limited for the first 2 hours and then nitrogen oxide limited downwind. Box model results for hydroxyl radical, ozone, and most VOCs, using the full, explicit Master Chemical Mechanism (MCM) mechanism, suggest no major gaps in the current best knowledge of gas-phase chemistry. However, the MCM sometimes overestimates PAN due to underestimated nitrogen oxide sinks. GEOS-Chem, a widely used chemical transport model with a reduced mechanism, generally underperforms because of incomplete VOC representation. We identify these critical pathways to guide future model development.

We report hourly surface observations of PM2.5, CO, NOx, O3, and 75 speciated VOCs in Missoula, Montana, during a strong smoke event in 2020. This study tests our current understanding of wildfire emissions, chemistry, and health effects as implemented in the GEOS-Chem chemical transport model. Three-or-more-day-old smoke transported from California and the Pacific Northwest increased CO, PM2.5, and total measured VOCs by factors of 2–8, with hourly maxima of 800 ppb, 120 µg m−3, and 85 ppb, respectively. In contrast, NOx levels were not elevated compared to the urban background. O3 showed a non-monotonic response to wildfire smoke: MDA8 O3 increased under light smoke but flattened or declined when PM2.5 exceeded ∼ 30–40 µg m−3, a feature that GEOS-Chem failed to reproduce. A 2020-style wildfire season recurring annually would yield an excess lifetime cancer risk of 100-in-1 million or approximately 7 times the non-smoke baseline. The chronic non-cancer hazard index (HI) would reach 3.0, indicating appreciable potential for chronic non-cancer effects. About 90 % of cancer risks are from PM2.5 whereas non-cancer risks are dominated by formaldehyde, benzene, acrolein, and acetaldehyde. GEOS-Chem captured major smoke intrusions but underestimated CO, PM2.5, and VOCs by 30 %–90 %. These model biases propagate to health metrics, with GEOS-Chem underestimating smoke-attributable cancer risk by ∼ 40 % and chronic HI by ∼ 10 times. We attribute the model errors to underpredicted fire emissions and unrepresented VOC chemistry, which together led to an overestimation of OH and insufficient secondary production.

Hydroperoxides (HPs) are potent oxidants that exert a substantial impact on the atmospheric oxidizing capacity. However, a fundamental understanding of the in situ formation of HPs in atmospheric waters remains incomplete and controversial. Here, we report direct observations of HP (hydrogen peroxide, alkyl HP, and peroxycarboxylic acid) formation with quantum yields 10-4 - 10-3, through photochemistry of α-keto acids [α-KAs; glyoxylic acid, pyruvic acid (PA), and 2-ketobutyric acid]. These quantum yields are comparable to reported HP formation rates in cloud and fog waters, suggesting that α-KA photochemistry could contribute 5 to 15% of the observed atmospheric hydrogen peroxide in aqueous phases, based on typical α-KA concentrations (0.1 to 10 μM). The model results showed that the photochemistry of excited triplet 3PA* contributes ~40 and ~33%, respectively, to the formation of hydrogen peroxide and peroxyacetic acid at pH 2.5. Given the recurrent detection of α-KAs in organic-rich aerosols, this photochemistry represents a notable in situ source of HPs.

Indoor combustion activities, including candle burning, incense use, and cooking, are significant but understudied sources of reactive nitrogen species and oxygenated volatile organic compounds (OVOCs). Using real-time high-resolution mass spectrometry, we comparatively characterize emissions from these sources in a residential setting. The tested unscented candles emitted 46.86 ± 26.27 ppb NOx and 1.63 ± 0.92 ppb HONO which is ten times higher than the specific scented candles while the tested incense combustion released biomass-burning tracers (e.g., methoxyphenols, acetonitrile) and nitrogen-containing heterocycle compounds (e.g., pyrroles). Cooking with vegetable oils generated aldehydes (e.g., hexanal, nonanal) via fatty acid decomposition, with peanut oil producing more oxidized OVOCs than olive oil. By comparing days with different OH level, we show that post-cooking aldehyde rebound occurs only when OH concentration exceed a threshold, providing direct evidence for OH-driven secondary formation. This observation is consistent with OH-initiated photochemical production of secondary aldehydes, though we acknowledge that physical transport from the kitchen to the sampling point cannot be fully separated in this real-world residential setting. The burning of the selected incense sticks emerged as an unanticipated source of isocyanates (∼0.38 ppb isocyanic acid), comparable to biomass burning. Ammonia (NH3), an underreported byproduct,increased by 3-9 ppb above backgroundduring candle/incense combustion, reaching absolute concentrations of 30-38 ppb. Our comparative analysis reveals the distinct chemical complexity of indoor emissions, their dependence on source-specific mechanisms, and the corresponding need for mitigation strategies to reduce human exposure to toxic pollutants.

Surface microlayer at freshwater (rivers, lakes, ponds, streams, and groundwater) and seawater is abundant with organic compounds compared to subsurface water. These organics adsorbed at the air-water interface can interact with the atmospheric oxidants and influence the exchange of organic materials between the water and the atmosphere. Here, we assess the chemical interaction between gaseous NO2 and authentic surface microlayers collected at the lake water (Dianchi Lake) situated in China. The formation of the gas-phase product compounds was evaluated in real time using a novel secondary electrospray ionization ultrahigh-resolution quadrupole Orbitrap mass spectrometer (SESI-UHR-MS) upon exposure of surface microlayer to gaseous NO2 (20 or 50 ppb) in dark and under simulated sunlight irradiation at two different temperatures: 5 degrees C and 25 degrees C. The obtained results revealed that the sampling sites of the lake impacted by human activities (municipal sewage and agricultural activities) significantly impact the number and the composition of the formed gas-phase product compounds. The formation of nitrogen (N)-containing compounds was observed as well, which contain most likely nitro or amino functional groups, or alternatively, they could be aromatic compounds. The observed N-containing compounds may contribute to the "brown carbon" which act as light-absorbing compounds, thus influencing the radiative forcing of aerosols in the atmosphere.

Mineral dust particles are omnipresent in the atmosphere all over the globe. Nitrogen dioxide (NO2) can be adsorbed on the dust surface and converted to nitrous acid (HONO), which in turn represents one of the most important sources of hydroxyl radicals (OH) driving the oxidation capacity of the atmosphere. Here, we evaluate the conversion of NO2 to HONO on mineral dust samples from different regions of the world. We reveal that the synergistic effects of relative humidity (RH), UV-light, titanium dioxide (TiO2), and microbes present on the mineral dust surface are responsible for the observed high HONO yields. The light-induced uptake coefficients of NO2 on mineral dust surface are 1 order of magnitude higher than the uptakes measured in the dark. Intriguingly, the uptakes of NO2 are higher in the absence of water vapor; however, the HONO yields increase with the increase of RH (0-90%), the NO2 concentration (10-50 ppb), and the light intensity (19-50.4 W m-2). Our findings demonstrate that mineral dust contributes to atmospheric HONO through light- and RH-dependent processes with high HONO yields (up to 80.3%) under realistic conditions. Global models must account for both uptake coefficients and HONO yields to accurately quantify this source, particularly in dust-prone regions.

Polybrominated diphenyl ether (PBDE) flame retardants are persistent organic pollutants that are ubiquitous in both indoor and outdoor environments and exhibit adverse health effects. Among all of the PBDEs, decabromodiphenyl ether (BDE-209) is the most abundant due to the increased production of electronic devices. Here we evaluate the uptake coefficients of ozone (O3) on glass plates coated with BDE-209 at different relative humidities (RH) over a range of temperatures. The uptake of O3 slightly increased with the increase of RH from 1.2 x 10-5 at 30% RH to 2.2 x 10-5 at 90% RH, but was independent of the temperature change. Real-time measurements of the gas-phase product compounds formed by the reaction of O3 with BDE-209 were performed with a high-resolution Q Exactive hybrid quadrupole Orbitrap mass spectrometer (UHR-MS) in both positive and negative ionization modes. Interestingly, the molecular-level analysis revealed that the observed gas-phase product compounds in the presence of water vapor did not contain Br atoms and, in most cases, had fewer than 12 C atoms, indicating that both debromination and aromatic ring fragmentation occurred. The developed reaction mechanism suggests that the formation of most CHO compounds occurs by the combination of reductive debromination triggered by HO2 with ring and open-chain fragmentation induced by O3. The molecular-level understanding of the volatile product compounds produced by the ozonolysis of glass coated with BDE-209 provides valuable insights into the reaction mechanism, enabling more accurate characterization in atmospheric model studies.

The chemistry of ozone (O-3) on indoor surfaces leads to secondary pollution, aggravating the air quality in indoor environments. Here, we assess the heterogeneous chemistry of gaseous O-3 with glass plates after being 1 month in two different kitchens where Chinese and Western styles of cooking were applied, respectively. The uptake coefficients of O-3 on the authentic glass plates were measured in the dark and under UV light irradiation typical for indoor environments (320 nm < lambda < 400 nm) at different relative humidities. The gas-phase product compounds formed upon reactions of O-3 with the glass plates were evaluated in real time by a proton-transfer-reaction quadrupole-interface time-of-flight mass spectrometer. We observed typical aldehydes formed by the O-3 reactions with the unsaturated fatty acid constituents of cooking oils. The formation of decanal, 6-methyl-5-hepten-2-one (6-MHO), and 4-oxopentanal (4-OPA) was also observed. The employed dynamic mass balance model shows that the estimated mixing ratios of hexanal, octanal, nonanal, decanal, undecanal, 6-MHO, and 4-OPA due to O-3 chemistry with authentic grime-coated kitchen glass surfaces are higher in the kitchen where Chinese food was cooked compared to that where Western food was cooked. These results show that O-3 chemistry on greasy glass surfaces leads to enhanced VOC levels in indoor environments.

Cleaning detergents are a source of numerous volatile organic compounds (VOCs) which are highly reactive towards ozone leading to the formation of secondary organic aerosols (SOA) in indoor environments. Here we perform real-time measurements of the organic composition of aerosols produced upon ozone reaction with floor cleaning detergent by extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF-MS) coupled to a chamber reactor. The experiments were performed in the absence of light and under light irradiation (320 nm < lambda < 400 nm) simulating the fraction of sunlight that penetrates indoors. The multiple increases in particle number concentrations correspond to rise in the signal intensity of specific species. Notably, the secondary increase in particle mass concentration is mainly contributed by highly oxidized molecules (HOMs), which increased from 16.5% upon ozone oxidation to 19.9% under photo-oxidation reactions. A large fraction of CHON compounds such as imidazole, pyrazine/pyrimidine, and azaindole was observed most likely formed through the reaction of O-3 with benzothiazole (constituent of the cleaning detergent). The difference between the molecular compositions detected in the absence of light and in the presence of light indicates that sunlight penetrating through the windows can affect the SOA produced by the reaction of ozone with the floor cleaning detergent.

We present measurements of volatile organic compounds (VOCs) and other trace gases taken in Salt Lake City, Utah in August and September 2022. As part of the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA), 35 VOCs were measured with two methods: a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) and 2,4-dinitrophenylhydrazine (DNPH) cartridges analyzed by high-performance liquid chromatography (HPLC). Over two months, the total measured VOCs averaged 32 +/- 24 ppb (mean +/- standard deviation) with the hourly maximum at 141 ppb, and the total calculated OH reactivity averaged 3.7 +/- 3.0 s-1 (maximum at 20.7 s-1). Among them, methanol and ethanol were the most abundant VOCs, making up 42% of the ambient mixing ratio. Isoprene and monoterpenes contributed 25% of the OH reactivity from VOCs, while formaldehyde and acetaldehyde made up another 30%. The positive matrix factorization analysis showed 5 major sources of VOCs, with 32% of abundance being attributed to secondary production/biogenic sources, 44% from the combination of traffic and personal care products, 15% from industrial solvent use, and the rest from biomass burning (10%). Moderate smoke-impacted days elevated various hazardous air pollutants (HAPs) on average by 45%-217% compared to smoke-free days. The ratio of OH reactivity from NOx to that from VOCs showed that ozone production was mostly VOC-limited throughout the campaign, consistent with our modeling study. VOCs and NOx both showed increased OH reactivity due to smoke influence. NOx featured increased reactivity on weekdays compared to weekends, an effect not shown for VOC reactivity during SAMOZA. Salt Lake City, Utah has higher concentrations of ozone, a pollutant harmful to human and plant life, in the atmosphere than the standard set by the United States Environmental Protection Agency (US EPA). The reasons for the high levels of ozone remain uncertain. Volatile organic compounds (VOCs) are a class of air pollutants that undergo reactions that produce ozone. Understanding their sources and reactions is important to be able to reduce air pollution. In this study, we measured 35 VOCs in SLC in August and September 2022 and used a model to identify their major sources. Concentrations of hazardous VOCs identified by the US EPA increased by 45%-217% when wildfire smoke was present in the air. Methanol and ethanol were the most important VOCs in terms of total concentration in the air, while isoprene and monoterpenes were the most important in terms of reactions that could create ozone. According to the model results, VOCs are emitted from five major sources including traffic and solvent use. Further measurements are needed to confirm the model results and reduce uncertainty of the important sources of VOCS. Methanol and ethanol dominated the total measured VOC abundance while isoprene dominated total OH reactivity from volatile organic compounds (VOCs) in Salt Lake City Traffic and solvent use are roughly equivalent contributors to anthropogenic VOC emissions in Salt Lake City during SAMOZA Ozone production was limited by VOCs in Salt Lake City in summer 2022

The Northern Wasatch Front area is one of ~ 50 metropolitan regions in the U.S. that do not meet the 2015 O3 standard. To better understand the causes of high O3 days in this region we conducted the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA) in the summer of 2022. The primary goals of SAMOZA were: Measure a suite of VOCs, by Proton Transfer Reaction Mass Spectrometry (PTR-MS) and the 2,4-dinitrophenylhydrazine (DNPH) cartridge method.Evaluate whether the standard UV O3 measurements made in SLC show a positive bias during smoke events, as has been suggested in some recent studies.Use the observations to conduct photochemical modeling and statistical/machine learning analyses to understand photochemistry on both smoke-influenced and non-smoke days.Implications: The Northern Wasatch Front area is one of ~50 metropolitan regions in the U.S. that do not meet the 2015 O3 standard. To better understand the causes of high O3 days in this region we conducted the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA) in the summer of 2022. A number of policy relevant findings are identified in the manuscript including role of smoke and NOx vs VOC sensitivity.

Humic-like substances (HULIS) are a ubiquitous reactive component of atmospheric aerosol. They participate in the formation of secondary organic aerosols via chemical reactions with atmospheric oxidants. Here, we assess the influence of transition metal ions (namely ferric iron, Fe(III)), and nitrate ions (NO3-) on the heterogeneous reaction of gaseous NO2 with an aqueous film containing gallic acid (GA) or tannic acid (TA) as proxies for HULIS. Using a vertical wetted wall flow tube technique, the uptake coefficients of gaseous NO2 on GA and TA increased nonlinearly with increasing [Fe(III)], in dark and under light irradiation. However, the combined effect of both ions, Fe(III) and NO3-, led to a substantial decrease in NO2 uptake in the dark and under simulated near-UV sunlight irradiation (300 < lambda < 400 nm). The lifetime of GA in dilute aqueous phase, which corresponds to cloud water, due to reaction with NO2 would be 6 hr during both nighttime and daytime. However, the lifetime of GA in aerosol particles which contain both ions, that is, Fe(III) and NO3-, would increase to 27 hr during nighttime and 11 days and 6 hr due to light-induced reaction with NO2. Also, we observed, using Fourier transform ion cyclotron resonance mass spectrometry, the formation of nitrocatechols compounds (e.g., methyl-nitrocatechol), which contribute to brown carbon. Compounds with reduced functional groups such as amines were also observed in the presence of iron and nitrate ions in the dark and under irradiation, indicating that Fe(III) and NO3-, can influence the kinetics and product distribution in deliquescent aerosol particles.

Rapid production of formic acid in biomass burning smoke is not captured by the Master Chemical Mechanism (MCM) nor simplified GEOS-Chem chemistry, likely due to missing secondary chemical production.

Using OH reactivity we assess the major daytime OH radical sinks in western U.S. wildfire plumes and other smoke impacted environments, testing their current model representation while providing a roadmap for future model development.

Abstract. The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chem chemical transport model (CTM) to constrain BB emissions in the western USA at ∼ 25 km resolution. Across three BB emission inventories widely used in CTMs, the inventory–inventory comparison suggests that the totals of 14 modeled BB VOC emissions in the western USA agree with each other within 30 %–40 %. However, emissions for individual VOCs can differ by a factor of 1–5, driven by the regionally averaged emission ratios (ERs, reflecting both assigned ERs for specific biome and vegetation classifications) across the three inventories. We further evaluate GEOS-Chem simulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) and FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BB inventories or applying various injection height assumptions, the model–observation comparison suggests that GEOS-Chem simulations underpredict observed vertical profiles by a factor of 3–7. The model shows small to no bias for most species in low-/no-smoke conditions. We thus attribute the negative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed vertical profiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no to less significant improvements for oxygenated VOCs, particularly for formaldehyde, formic acid, acetic acid, and lumped ≥ C3 aldehydes, suggesting the model is missing secondary sources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable to underpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs, as constrained by aircraft and ground measurements. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nested GEOS-Chem which could explain the negative model bias partially, though back-of-the-envelope calculation and evaluation using longer-term ground measurements help support the argument of the dry matter burned underestimation. The total ERs of the 14 BB VOCs implemented in GEOS-Chem only account for half of the total 161 measured VOCs (∼ 75 versus 150 ppb ppm−1). This reveals a significant amount of missing reactive organic carbon in widely used BB emission inventories. Considering both uncertainties in effective dry matter burned (× 3) and unmodeled VOCs (× 2), we infer that BB contributed to 10 % in 2019 and 45 % in 2018 (240 and 2040 Gg C) of the total VOC primary emission flux in the western USA during these two fire seasons, compared to only 1 %–10 % in the standard GEOS-Chem.

Human daily activities such as cooking, and cleaning can affect the indoor air quality by releasing primary emitted volatile organic compounds (VOCs), as well as by the secondary product compounds formed through reactions with ozone (O3) and hydroxyl radicals (OH). However, our knowledge about the formation processes of the secondary VOCs is still incomplete. We performed real-time measurements of primary VOCs released by commercial floor-cleaning detergent and the secondary product compounds formed by heterogeneous reaction of O3 with the constituents of the cleaning agent by use of high-resolution mass spectrometry. We measured the uptake coefficients of O3 on the cleaning detergent at different relative humidities in dark and under different light intensities (320 nm < λ < 400 nm) relevant for the indoor environment. On the basis of the detected compounds we developed tentative reaction mechanisms describing the formation of the secondary VOCs. Intriguingly, under light irradiation the formation of valeraldehyde was observed based on the photosensitized chemistry of acetophenone which is a constituent of the cleaning agent. Finally, we modeled the observed mixing ratios of three aldehydes, glyoxal, methylglyoxal, and 4-oxopentanal with respect to real-life indoor environment. The results suggest that secondary VOCs initiated by ozone chemistry can additionally impact the indoor air pollution.

Nitrogen (N)-containing organic compounds, including "brown carbon" (BrC), represent an important fraction of organic aerosols. However, little is known about the processes of formation of the secondarily formed N-containing organics in the atmosphere. Here, we investigated the formation of gas-phase organic compounds, including N-containing organics, through interfacial oxidation chemistry of gaseous O-3 with an authentic riverine surface microlayer (SML) by using a high-resolution quadrupole Orbitrap mass spectrometer coupled to a commercial secondary electrospray ionization source. The resulting hierarchical cluster diagram obtained for real-time observation for 60 min shows the occurrence of 677 ions in positive mode. The level of N-containing organics, including BrC compounds (e.g., imidazoles), formed during the heterogeneous processing of O-3 on the SML in the dark and under ultraviolet-visible light irradiation, was on average 20.7% among all samples. Many of the detected N-containing compounds comprise a C=N bond, suggesting that they are potentially toxic compounds that also affect urban air quality. Overall, this study provides evidence that interfacial ozone oxidation chemistry at the riverine SML plays an important role as an additional source of air pollution in urban environments, which can affect both human health and the absorption properties of urban aerosols.

The primarily emitted compounds by human presence, e.g., skin and volatile organic compounds (VOCs) in breath, can react with typical indoor air oxidants, ozone (O3), and hydroxyl radicals (OH), leading to secondary organic compounds. Nevertheless, our understanding about the formation processes of the compounds through reactions of indoor air oxidants with primary emitted pollutants is still incomplete. In this study we performed real-time measurements of nitrous acid (HONO), nitrogen oxides (NOx = NO + NO2), O3, and VOCs to investigate the contribution of human presence and human activity, e.g., mopping the floor, to secondary organic compounds. During human occupancy a significant increase was observed of 1-butene, isoprene, and d-limonene exhaled by the four adults in the room and an increase of methyl vinyl ketone/methacrolein, methylglyoxal, and 3-methylfuran, formed as secondary compounds through reactions of OH radicals with isoprene. Intriguingly, the level of some compounds (e.g., m/z 126, 6-methyl-5-hepten-2-one, m/z 152, dihydrocarvone, and m/z 194, geranyl acetone) formed through reactions of O3 with the primary compounds was higher in the presence of four adults than during the period of mopping the floor with commercial detergent. These results indicate that human presence can additionally degrade the indoor air quality.