Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation.

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.

Correction: Ring-opening yields and auto-oxidation rates of the resulting peroxy radicals from OH-oxidation of α-pinene and ß-pinene.

[This corrects the article DOI: 10.1039/D2EA00133K.].

Midlatitude Ozone Depletion and Air Quality Impacts from Industrial Halogen Emissions in the Great Salt Lake Basin.

We report aircraft observations of extreme levels of HCl and the dihalogens Cl2, Br2, and BrCl in an industrial plume near the Great Salt Lake, Utah. Complete depletion of O3 was observed concurrently with halogen enhancements as a direct result of photochemically produced halogen radicals. Observed fluxes for Cl2, HCl, and NOx agreed with facility-reported emissions inventories. Bromine emissions are not required to be reported in the inventory, but are estimated as 173 Mg year-1 Br2 and 949 Mg year-1 BrCl, representing a major uncounted oxidant source. A zero-dimensional photochemical box model reproduced the observed O3 depletions and demonstrated that bromine radical cycling was principally responsible for the rapid O3 depletion. Inclusion of observed halogen emissions in both the box model and a 3D chemical model showed significant increases in oxidants and particulate matter (PM2.5) in the populated regions of the Great Salt Lake Basin, where winter PM2.5 is among the most severe air quality issues in the U.S. The model shows regional PM2.5 increases of 10%-25% attributable to this single industrial halogen source, demonstrating the impact of underreported industrial bromine emissions on oxidation sources and air quality within a major urban area of the western U.S.

Evaluation of iodide chemical ionization mass spectrometry for gas and aerosol-phase per- and polyfluoroalkyl substances (PFAS) analysis.

Per- and polyfluoroalkyl substances (PFAS) are a class of ultra-persistent anthropogenic contaminants. PFAS are ubiquitous in environmental and built systems, but very few online methods exist for their characterization in atmospheric gases and aerosols. Iodide time-of-flight chemical ionization mass spectrometry (iodide-ToF-CIMS) is a promising technology for online characterization of PFAS in the atmosphere. Previous work using iodide-ToF-CIMS was successful in measuring gas-phase perfluoroalkyl carboxylic acids and fluorotelomer alcohols, but those are just two of the myriad classes of PFAS that are atmospherically relevant. Therefore, our first objective was to test other sample introduction methods coupled to iodide-TOF-CIMS to evaluate its ability to measure a wider suite of PFAS in both gas and aerosol phases. Using a variety of sample introduction techniques, we successfully measured gas-phase fluorotelomer alcohols (FTOHs), gas and aerosol-phase perfluoroalkyl carboxylic acids (PFCAs), and aerosol-phase perfluoroalkyl sulfonic acids and polyfluoroalkyl phosphoric acid diesters (PFSAs and diPAPs). We also determined iodide-ToF-CIMS response factors for these compounds by introducing known quantities using a Filter Inlet for Gases and AEROsols (FIGAERO). These response factors ranged from 400 to 6 × 104 ions per nanogram, demonstrating low limits of detection. Furthermore, PFAS are a poorly understood diverse class of molecules that exhibit unusual and often unexpected physicochemical properties due to their highly fluorinated nature. Since detection of PFAS with iodide-ToF-CIMS relies on the analyte molecule to either undergo proton transfer or adduct formation with iodide, understanding PFAS behavior during chemical ionization gives rise to a more fundamental understanding of these compounds. Through voltage scanning experiments and DFT calculations, we found that PFCAs and FTOHs readily form iodide adducts, while PFSAs and diPAPs preferentially undergo proton transfer to iodide. Generally, binding energy increased with increasing linear chain length, and PFCAs had stronger binding than FTOHs. Overall, our results suggest that iodide-ToF-CIMS can be used to measure even nonvolatile PFAS such as PFSAs and diPAPs in the aerosol phase in a semi-continuous online fashion.

Monoterpene Photooxidation in a Continuous-Flow Chamber: SOA Yields and Impacts of Oxidants, NOx, and VOC Precursors.

Monoterpene photooxidation plays an important role in secondary organic aerosol (SOA) formation in the atmosphere. The low-volatility products can enhance new particle formation and particle growth and thus influence climate feedback. Here, we present the results of α-pinene and Δ-3-carene photooxidation experiments conducted in continuous-flow mode in an environmental chamber under several reaction conditions. The roles of oxidants, addition of NO, and VOC molecular structure in influencing SOA yield are illustrated. SOA yield from α-pinene photooxidation shows a weak dependence on H2O2 concentration, which is a proxy for HO2 concentration. The high O/C ratios observed in the α-pinene photooxidation products suggest the production of highly oxygenated organic molecules (HOM). Addition of ozone to the chamber during low-NOx photooxidation experiments leads to higher SOA yield. With the addition of NO, the production of N-containing HOMs is enhanced and the SOA yield shows a modest, nonlinear dependence on the input NO concentration. Carene photooxidation leads to higher SOA yield than α-pinene under similar reaction conditions, which agrees with the lower volatility retrieved from evaporation kinetics experiments. These results improve the understanding of SOA formation from monoterpene photooxidation and could be applied to refine the representation of biogenic SOA formation in models.

Coarse sea spray inhibits lightning.

The known effects of thermodynamics and aerosols can well explain the thunderstorm activity over land, but fail over oceans. Here, tracking the full lifecycle of tropical deep convective cloud clusters shows that adding fine aerosols significantly increases the lightning density for a given rainfall amount over both ocean and land. In contrast, adding coarse sea salt (dry radius > 1 µm), known as sea spray, weakens the cloud vigor and lightning by producing fewer but larger cloud drops, which accelerate warm rain at the expense of mixed-phase precipitation. Adding coarse sea spray can reduce the lightning by 90% regardless of fine aerosol loading. These findings reconcile long outstanding questions about the differences between continental and marine thunderstorms, and help to understand lightning and underlying aerosol-cloud-precipitation interaction mechanisms and their climatic effects.

A Four Carbon Organonitrate as a Significant Product of Secondary Isoprene Chemistry.

Oxidation of isoprene by nitrate radicals (NO3) or by hydroxyl radicals (OH) under high NOx conditions forms a substantial amount of organonitrates (ONs). ONs impact NOx concentrations and consequently ozone formation while also contributing to secondary organic aerosol. Here we show that the ONs with the chemical formula C4H7NO5 are a significant fraction of isoprene-derived ONs, based on chamber experiments and ambient measurements from different sites around the globe. From chamber experiments we found that C4H7NO5 isomers contribute 5%-17% of all measured ONs formed during nighttime and constitute more than 40% of the measured ONs after further daytime oxidation. In ambient measurements C4H7NO5 isomers usually dominate both nighttime and daytime, implying a long residence time compared to C5 ONs which are removed more rapidly. We propose potential nighttime sources and secondary formation pathways, and test them using a box model with an updated isoprene oxidation scheme.

Pathways to Highly Oxidized Products in the Δ3-Carene + OH System.

Oxidation of the monoterpene Δ3-carene (C10H16) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Δ3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C7-C10 species. We then use computational methods to develop the first stages of a Δ3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the α-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Δ3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.

Heterogeneous Nucleation Drives Particle Size Segregation in Sequential Ozone and Nitrate Radical Oxidation of Catechol.

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O3 and NO3 in the presence of (NH4)2SO4 seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO3 oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.

Novel Analysis to Quantify Plume Crosswind Heterogeneity Applied to Biomass Burning Smoke.

We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width (wi/wCO) and center (bi/wCO). To validate GOMECH, we investigate OH and NO3 oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO (wHONO/wCO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges (wmaleicanhydride/wCO = 1.06-1.12 ± 0.01). By contrast, NO3 production [P(NO3)] occurs mainly at the plume center (wP(NO3)/wCO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO3, are narrower than CO (wphenol/wCO = 0.96 ± 0.03, wcatechol/wCO = 0.91 ± 0.01, and wmethylcatechol/wCO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center (wnitrophenolicaerosol/wCO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO3). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO3 in our case study.

Rapid cloud removal of dimethyl sulfide oxidation products limits SO2 and cloud condensation nuclei production in the marine atmosphere.

Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth's radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO2) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τHPMTF < 2 h) and terminates DMS oxidation to SO2 When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO2 production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO2 concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate.

Variability and Time of Day Dependence of Ozone Photochemistry in Western Wildfire Plumes.

Understanding the efficiency and variability of photochemical ozone (O3) production from western wildfire plumes is important to accurately estimate their influence on North American air quality. A set of photochemical measurements were made from the NOAA Twin Otter research aircraft as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment. We use a zero-dimensional (0-D) box model to investigate the chemistry driving O3 production in modeled plumes. Modeled afternoon plumes reached a maximum O3 mixing ratio of 140 ± 50 ppbv (average ± standard deviation) within 20 ± 10 min of emission compared to 76 ± 12 ppbv in 60 ± 30 min in evening plumes. Afternoon and evening maximum O3 isopleths indicate that plumes were near their peak in NOx efficiency. A radical budget describes the NOx volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NOx-sensitive chemistry, driven by HOx (=OH + HO2) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HOx) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NOx efficiency to VOC-sensitive O3 production caused by a reduction in photolysis rates and fire emissions. HOx production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).

Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes.

The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.

Predicting secondary organic aerosol phase state and viscosity and its effect on multiphase chemistry in a regional-scale air quality model.

Atmospheric aerosols are a significant public health hazard and have substantial impacts on the climate. Secondary organic aerosols (SOAs) have been shown to phase separate into a highly viscous organic outer layer surrounding an aqueous core. This phase separation can decrease the partitioning of semi-volatile and low-volatile species to the organic phase and alter the extent of acid-catalyzed reactions in the aqueous core. A new algorithm that can determine SOA phase separation based on their glass transition temperature (T g), oxygen to carbon (O : C) ratio and organic mass to sulfate ratio, and meteorological conditions was implemented into the Community Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 and was used to simulate the conditions in the continental United States for the summer of 2013. SOA formed at the ground/surface level was predicted to be phase separated with core-shell morphology, i.e., aqueous inorganic core surrounded by organic coating 65.4 % of the time during the 2013 Southern Oxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeastern United States. Our estimate is in proximity to the previously reported ~ 70 % in literature. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell occurring with lower aerosol liquid water content (western United States and at higher altitudes) has a viscosity that was predicted to be 102-1012 Pa s, which resulted in organic mass being decreased due to diffusion limitation. Organic aerosol was primarily liquid where aerosol liquid water was dominant (eastern United States and at the surface), with a viscosity < 102 Pa s. Phase separation while in a liquid phase state, i.e., liquid-liquid phase separation (LLPS), also reduces reactive uptake rates relative to homogeneous internally mixed liquid morphology but was lower than aerosols with a thick viscous organic shell. The sensitivity cases performed with different phase-separation parameterization and dissolution rate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can have varying impact on fine particulate matter (PM2.5) organic mass, in terms of bias and error compared to field data collected during the 2013 SOAS. This highlights the need to better constrain the parameters that govern phase state and morphology of SOA, as well as expand mechanistic representation of multiphase chemistry for non-IEPOX SOA formation in models aided by novel experimental insights.

Evaluating Organic Aerosol Sources and Evolution with a Combined Molecular Composition and Volatility Framework Using the Filter Inlet for Gases and Aerosols (FIGAERO).

ConspectusThe complex array of sources and transformations of organic carbonaceous material that comprises an important fraction of atmospheric fine particle mass, known as organic aerosol, has presented a long running challenge for accurate predictions of its abundance, distribution, and sensitivity to anthropogenic activities. Uncertainties about changes in atmospheric aerosol particle sources and abundance over time translate to uncertainties in their impact on Earth's climate and their response to changes in air quality policy. One limitation in our understanding of organic aerosol has been a lack of comprehensive measurements of its molecular composition and volatility, which can elucidate sources and processes affecting its abundance. Herein we describe advances in the development and application of the Filter Inlet for Gases and Aerosols (FIGAERO) coupled to field-deployable High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometers (HRToF-CIMS). The FIGAERO HRToFCIMS combination broadly probes gas and particulate OA molecular composition by using programmed thermal desorption of particles collected on a Teflon filter with subsequent detection and speciation of desorbed vapors using inherently quantitative selected-ion chemical ionization. The thermal desorption provides a means to obtain quantitative insights into the volatility of particle components and thus the physicochemical nature of the organic material that will govern its evolution in the atmosphere.In this Account, we discuss the design and operation of the FIGAERO, when coupled to the HRToF-CIMS, for quantitative characterization of the molecular-level composition and effective volatility of organic aerosol in the laboratory and field. We provide example insights gleaned from its deployment, which improve our understanding of organic aerosol sources and evolution. Specifically, we connect thermal desorption profiles to the effective equilibrium saturation vapor concentration and enthalpy of vaporization of detected components. We also show how application of the FIGAERO HRToF-CIMS to environmental simulation chamber experiments and the field provide new insights and constraints on the chemical mechanisms governing secondary organic aerosol formation and dynamic evolution. We discuss the associated challenges of thermal decomposition during desorption and calibration of both the volatility axis and signal. We also illustrate how the FIGAERO HRToF-CIMS can provide additional insights into organic aerosol through isothermal evaporation experiments as well as for detection of ultrafine particulate composition. We conclude with a description of future opportunities and needs for its ability to further organic aerosol science.

Resolving ambient organic aerosol formation and aging pathways with simultaneous molecular composition and volatility observations.

Organic aerosol (OA) constitutes a significant fraction of atmospheric fine particle mass. However, the precursors and chemical processes responsible for a majority of OA are rarely conclusively identified. We use online observations of hundreds of simultaneously measured molecular components obtained from 15 laboratory OA formation experiments with constraints on their effective saturation vapor concentrations to attribute the VOC precursors and subsequent chemical pathways giving rise to the vast majority of OA mass measured in two forested regions. We find that precursors and chemical pathways regulating OA composition and volatility are dynamic over hours to days, with their variations driven by coupled interactions between multiple oxidants. The extent of physical and photochemical aging, and its modulation by NOx, were key to a uniquely comprehensive combined composition-volatility description of OA. Our findings thus provide some of the most complete mechanistic-level guidance to the development of OA descriptions in air quality and Earth system models.

HONO Emissions from Western U.S. Wildfires Provide Dominant Radical Source in Fresh Wildfire Smoke.

Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv-1 using a maximum enhancement method, with the median similar to previous observations of temperate forest fire plumes. Measured HONO to NOx enhancement ratios were generally factors of 2, or higher, at early plume ages than previous studies. Enhancement ratios scale with modified combustion efficiency and certain nitrogenous trace gases, which may be useful to estimate HONO release when HONO observations are lacking or plumes have photochemical exposures exceeding an hour as emitted HONO is rapidly photolyzed. We find that HONO photolysis is the dominant contributor to hydrogen oxide radicals (HOx = OH + HO2) in early stage (<3 h) wildfire plume evolution. These results highlight the role of HONO as a major component of reactive nitrogen emissions from wildfires and the main driver of initial photochemical oxidation.

Photolysis Controls Atmospheric Budgets of Biogenic Secondary Organic Aerosol.

Secondary organic aerosol (SOA) accounts for a large fraction of the tropospheric particulate matter. Although SOA production rates and mechanisms have been extensively investigated, loss pathways remain uncertain. Most large-scale chemistry and transport models account for mechanical deposition of SOA but not chemical losses such as photolysis. There is also a paucity of laboratory measurements of SOA photolysis, which limits how well photolytic losses can be modeled. Here, we show, through a combined experimental and modeling approach, that photolytic loss of SOA mass significantly alters SOA budget predictions. Using environmental chamber experiments at variable relative humidity between 0 and 60%, we find that SOA produced from several biogenic volatile organic compounds undergoes photolysis-induced mass loss at rates between 0 and 2.2 ± 0.4% of nitrogen dioxide (NO2) photolysis, equivalent to average atmospheric lifetimes as short as 10 h. We incorporate our photolysis rates into a regional chemical transport model to test the sensitivity of predicted SOA mass concentrations to photolytic losses. The addition of photolysis causes a ∼50% reduction in biogenic SOA loadings over the Amazon, indicating that photolysis exerts a substantial control over the atmospheric SOA lifetime, with a likely dependence upon the SOA molecular composition and thus production mechanisms.

Thermalized Epoxide Formation in the Atmosphere.

Epoxide formation was established a decade ago as a possible reaction pathway for ß-hydroperoxy alkyl radicals in the atmosphere. This epoxide-forming pathway required excess energy to compete with O2 addition, as the thermal reaction rate coefficient is many orders of magnitude too slow. However, recently, a thermal epoxide-forming reaction was discovered in the ISOPOOH + OH oxidation pathway. Here, we computationally investigate the effect of substituents on the epoxide formation rate coefficient of a series of substituted ß-hydroperoxy alkyl radicals. We find that the thermal reaction is likely to be competitive with O2 addition when the alkyl radical carbon has a OH group, which is able to form a hydrogen bond to a substituent on the other carbon atom in the epoxide ring being formed. Reactants fulfilling these requirements can be formed in the OH-initiated oxidation of many biogenic hydrocarbons. Further, we find that ß-OOR alkyl radicals react similarly to ß-OOH alkyl radicals, making epoxide formation a possible decomposition pathway in the oxidation of ROOR peroxides. GEOS-Chem modeling shows that the total annual production of isoprene dihydroxy hydroperoxy epoxide is 23 Tg, making it by far the most abundant C5-tetrafunctional species from isoprene oxidation.