An iodide-adduct high-resolution time-of-flight chemical-ionization mass spectrometer: application to atmospheric inorganic and organic compounds

Chemical Characterization of Highly Functionalized Organonitrates Contributing to Night-Time Organic Aerosol Mass Loadings and Particle Growth

Reactions of volatile organic compounds (VOC) with NO₃ radicals and of reactive intermediates of oxidized VOC with NO _x can lead to the formation of highly functionalized organonitrates (ON). We present quantitative and chemical information on ON contributing to high night-time organic aerosol (OA) mass concentrations measured during July-August 2016 in a rural area in southwest Germany. A filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-HR-ToF-CIMS) was used to analyze the molecular composition of ON in both the gas and particle phase. We find larger contributions of ON to OA mass during the night. Identified ON are highly functionalized, with 4 to 12 oxygen atoms. The diel patterns of ON compounds with 5, 7, 10, or 15 carbon atoms per molecule vary, indicating a corresponding behavior of their potential precursor VOC. The temporal behavior of ON after sunset correlates with that of the number concentration of ultrafine particles, indicating a potential role of ON in night-time new particle formation (NPF) regularly observed at this location. We estimate an ON contribution of 18-25% to the mass increase of newly formed particles after sunset. Our study provides insights into the chemical composition of highly functionalized ON in the rural atmosphere and the role of anthropogenic emissions for night-time SOA formation in an area where biogenic VOC emissions dominate.

Time series analysis of wintertime O3 and NOx formation using vector autoregressions

Concentrations of 11 species are reported from continuous measurements taken during a wintertime field study in Utah. Time series data for measured species generally displayed strong diurnal patterns. Six species show a diurnal pattern of daytime maximums (NO, NOy, O3, H2O2, CH2O2, and Cl2), while five species show a diurnal pattern of night time maximums (NO2, HONO, ClNO2, HNO3, and N2O5). Vector autoregression analyses were completed to better understand important species influencing the formation of O3 and NOx. For the species studied, r2 values of predicted versus measured concentrations ranged from 0.82-0.99. Fitting parameters for the autoregressive matrix, Π, indicated the importance of species precursors. In addition, values of fitting parameters for Π were relatively insensitive to data size, with variations generally <10%. Variable causation was quantified using the Granger causation method. Assuming O3 and NOx behave as chemical products, reactants (in order of importance) are as follows: H2O2, N2O5, HONO, and ClNO2.

Gas-Phase Detection of Fluorotelomer Alcohols and Other Oxygenated Per- and Polyfluoroalkyl Substances by Chemical Ionization Mass Spectrometry

Per- and polyfluoroalkyl substances (PFAS) are incorporated into an ever-increasing number of modern products and inevitably enter the environment and ultimately human bodies. Herein, we show that chemical ionization mass spectrometry with iodide reagent ion chemistry is a useful technique for the detection of fluorotelomer alcohols (FTOHs) and other oxygenated PFAS, including per- and polyfluoro carboxylic acids such as hexafluoropropylene oxide dimer acid. This technique offers direct, high-time resolution measurement capability with parts per trillion by volume (nanograms per cubic meter) gas-phase detection limits. Measurements were taken by direct volatilization of samples without prior processing, allowing for fast measurements and reduced sample treatment compared to established PFAS methods. We demonstrate the utility of this technique by sampling volatile and semivolatile PFAS from fluoro additives and fluoro products to quantify levels of FTOHs and identify additional fluorinated compounds for which standards were unavailable.

Using advanced mass spectrometry techniques to fully characterize atmospheric organic carbon: current capabilities and remaining gaps

Organic compounds in the atmosphere vary widely in their molecular composition and chemical properties, so no single instrument can reasonably measure the entire range of ambient compounds. Over the past decade, a new generation of in situ, field-deployable mass spectrometers has dramatically improved our ability to detect, identify, and quantify these organic compounds, but no systematic approach has been developed to assess the extent to which currently available tools capture the entire space of chemical identity and properties that is expected in the atmosphere. Reduced-parameter frameworks that have been developed to describe atmospheric mixtures are exploited here to characterize the range of chemical properties accessed by a suite of instruments. Multiple chemical spaces (e.g. oxidation state of carbon vs. volatility, and oxygen number vs. carbon number) were populated with ions measured by several mass spectrometers, with gas- and particle-phase α-pinene oxidation products serving as the test mixture of organic compounds. Few gaps are observed in the coverage of the parameter spaces by the instruments employed in this work, though the full extent to which comprehensive measurement was achieved is difficult to assess due to uncertainty in the composition of the mixture. Overlaps between individual ions and regions in parameter space were identified, both between gas- and particle-phase measurements, and within each phase. These overlaps were conservatively found to account for little (<10%) of the measured mass. However, challenges in identifying overlaps and in accurately converting molecular formulas into chemical properties (such as volatility or reactivity) highlight a continued need to incorporate structural information into atmospheric measurements.

Electron Attachment Reaction Ionization of Gas-Phase Methylglyoxal

Methylglyoxal (MGLY) plays a significant role in atmospheric chemistry by serving as a key contributor to the formation of active free radicals, ozone, and secondary organic aerosol. Detection of MGLY by traditional chemical ionization such as proton-transfer reaction has several shortcomings such as parent molecule fragmentation. In this study, an electron attachment reaction (EAR) ionization method has been developed for the effective detection of MGLY. Almost no fragmentation was observed during the EAR. The generation of MGLY^- anion in the EAR was further confirmed by cryogenic photoelectron imaging spectroscopy. The concentration of MGLY can be calibrated by using dibromomethane (CH₂Br₂) as reference gas. The detection sensitivity of MGLY was estimated to be (100 ± 2) mV/ppbv (parts per billion by volume). The O₂, H₂O, CO₂, and trace gases in ambient air have no obvious effects on the detection of MGLY^- anion by the EAR ionization method.

Emission of formic and acetic acids from two Colorado soils

A 'missing source' of atmospheric formic acid is consistently observed during model-measurement comparisons, and evidence from multiple environments suggests a near-surface source. Soil emissions are considered to be a small formic acid source, but estimates are based on a single study from a tropical site. Here, we investigate soil emissions of organic acids from two soils - a ponderosa pine forest (Manitou Experimental Forest Observatory), and a managed lawn (Colorado State University) using a laboratory chamber. Both soils are a source of formic and acetic acids. Under ambient conditions, formic acid emissions are 0.11 (pine forest) and 0.15 (lawn) nmol m-2 s-1, and acetic acid emissions are 0.05 (pine forest) and 0.71 (lawn) nmol m-2 s-1. Only acetic acid emissions from the forest site correlate with CO2 fluxes, but all formic and acetic acid emissions increase exponentially with temperature. Increasing soil moisture only enhances acetic acid emissions from the forest. Considering this temperature and moisture dependence, we hypothesize that while equilibrium partitioning may contribute to the forest emissions, organic acid emissions from the lawn are likely driven by microbial activity. Lactic acid was emitted from both soils, but not quantified. The observed formic acid emissions are higher than previous measurements, but still low enough that soils are unlikely the 'missing source' of atmospheric organic acids, although the variability in the soil source is substantial. We contrast observations to previous parameterizations used in models, and present recommendations for modified parameterizations for formic and acetic acid emission.

Primary Formation of Highly Oxidized Multifunctional Products in the OH-Initiated Oxidation of Isoprene: A Combined Theoretical and Experimental Study

It is generally assumed that isoprene-derived secondary organic aerosol (SOA) precursors are mainly formed from the secondary reactions of intermediate products with OH radicals in the gas phase and multiphase oxidation in particles. In this paper, we predicted a theoretical mechanism for the primary formation of highly oxygenated molecules (HOM) in the gas phase through successive intramolecular H-shifts and O₂ addition in the specific Z-δ isomer of hydroxyl-peroxy radicals and alkoxy radicals. The position of O₂ addition is different from that in forming hydroperoxy aldehydes. The prediction was further supported experimentally by successfully identifying a few highly oxidized peroxy radicals and closed-shell products such as C₅H₉O_7,9, C₅H₁₀O_6,7,8, and C₄H₈O₅ in a flow reactor by chemical ionization mass spectrometry at air pressure. These HOM products could serve as important precursors to isoprene-derived SOA. Further modeling studies on the effect of NO concentration suggested that HOM formation could account for up to ∼11% of the branching ratio (∼9% from the 4-OH channel and ∼2% from the 1-OH channel) in the reaction of isoprene with OH when the lifetimes of peroxy radicals due to bimolecular reactions are ∼100 s, which is typical in forest regions.

Photochemical Cloud Processing of Primary Wildfire Emissions as a Potential Source of Secondary Organic Aerosol

We investigated the gas-phase chemical composition of biomass burning (BB) emissions and their role in aqueous secondary organic aerosol (aqSOA) formation through photochemical cloud processing. A high-resolution time-of-flight chemical ionization mass spectrometer using iodide reagent ion chemistry detected more than 100 gas-phase compounds from the emissions of 30 different controlled burns during the 2016 Fire Influence on Regional and Global Environments Experiment (FIREX) at the Fire Science Laboratory. Compounds likely to partition to cloudwater were selected based on high atomic oxygen-to-carbon ratio and abundance. Water solubility was confirmed by detection of these compounds in water after mist chamber collection during controlled burns and analysis using ion chromatography and electrospray ionization interfaced to high-resolution time-of-flight mass spectrometry. Known precursors of aqSOA were found in the primary gaseous BB emissions (e.g., phenols, acetate, and pyruvate). Aqueous OH oxidation of the complex biomass burning mixtures led to rapid depletion of many compounds (e.g., catechol, levoglucosan, methoxyphenol) and formation of others (e.g., oxalate, malonate, mesoxalate). After 150 min of oxidation (approximatively 1 day of cloud processing), oxalate accounted for 13-16% of total dissolved organic carbon. Formation of known SOA components suggests that cloud processing of primary BB emissions forms SOA.

Experimental and model estimates of the contributions from biogenic monoterpenes and sesquiterpenes to secondary organic aerosol in the southeastern United States

Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are important precursors of secondary organic aerosol (SOA), but the amounts and pathways of SOA generation from these precursors are not well constrained by observations. We propose that the less-oxidized oxygenated organic aerosol (LO-OOA) factor resolved from positive matrix factorization (PMF) analysis on aerosol mass spectrometry (AMS) data can be used as a surrogate for fresh SOA from monoterpenes and sesquiterpenes in the southeastern US. This hypothesis is supported by multiple lines of evidence, including lab-in-the-field perturbation experiments, extensive ambient ground-level measurements, and state-of-the-art modeling. We performed lab-in-the-field experiments in which the ambient air is perturbed by the injection of selected monoterpenes and sesquiterpenes, and the subsequent SOA formation is investigated. PMF analysis on the perturbation experiments provides an objective link between LO-OOA and fresh SOA from monoterpenes and sesquiterpenes as well as insights into the sources of other OA factors. Further, we use an upgraded atmospheric model and show that modeled SOA concentrations from monoterpenes and sesquiterpenes could reproduce both the magnitude and diurnal variation of LO-OOA at multiple sites in the southeastern US, building confidence in our hypothesis. We estimate the annual average concentration of SOA from monoterpenes and sesquiterpenes in the southeastern US to be roughly 2 μg m-3.

Chemical feedbacks weaken the wintertime response of particulate sulfate and nitrate to emissions reductions over the eastern United States

Sulfate ([Formula: see text]) and nitrate ([Formula: see text]) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas-particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter [Formula: see text] and [Formula: see text] GEOS-Chem reproduces observed [Formula: see text]-[Formula: see text]-[Formula: see text] particulate concentrations (2.45 μg [Formula: see text]) and composition ([Formula: see text]: 47%; [Formula: see text]: 32%; [Formula: see text]: 21%) during WINTER. Only 18% of [Formula: see text] emissions were regionally oxidized to [Formula: see text] during WINTER, limited by low [H₂O₂] and [OH]. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter [Formula: see text] emissions from 2007 to 2015 and find that the H₂O₂ limitation on [Formula: see text] oxidation weakened, which increased the fraction of [Formula: see text] emissions oxidizing to [Formula: see text] Simultaneously, NOx emissions decreased by 35%, but the modeled [Formula: see text] particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [[Formula: see text]] and no change in [[Formula: see text]], as observed. Wintertime [[Formula: see text]] and [[Formula: see text]] are expected to change slowly between 2015 and 2023, unless [Formula: see text] and NOx emissions decrease faster in the future than in the recent past.

Online Chemical Characterization of Food-Cooking Organic Aerosols: Implications for Source Apportionment

Food-cooking organic aerosols (COA) are one of the primary sources of submicron particulate matter in urban environments. However, there are still many questions surrounding source apportionment related to instrumentation as well as semivolatile partitioning because COA evolve rapidly in the ambient air, making source apportionment more complex. Online measurements of emissions from cooking different types of food were performed in a laboratory to characterize particles and gases. Aerosol mass spectrometer (AMS) measurements showed that the relative ionization efficiency for OA was higher (1.56-3.06) relative to a typical value of 1.4, concluding that AMS is over-estimating COA and suggesting that previous studies likely over-estimated COA concentrations. Food-cooking mass spectra were generated using AMS, and gas and particle food markers were identified with filter inlets for gases and aerosols-chemical ionization mass spectrometer (CIMS) measurements to be used in future food cooking-source apportionment studies. However, there is a considerable variability in both gas and particle markers, and dilution plays an important role in the particle mass budget, showing the importance of using these markers with caution during receptor modeling. These findings can be used to better understand the chemical composition of COA, and they provides useful information to be used in future source-apportionment studies.

Atmospheric OH Oxidation of Three Chlorinated Aromatic Herbicides

Chlorinated phenoxy acids are a widely used class of herbicides and have been found in remote regions far from sources. However, the atmospheric chemistry of these compounds is poorly understood. We use an oxidative flow reactor coupled to chemical ionization mass spectrometry to investigate OH oxidation of two chlorinated phenoxyacid herbicides (2-methyl-4-chlorophenoxyacetic acid (MCPA) and mecoprop-p) and one chlorinated pyridine herbicide (triclopyr). OH radicals add to the aromatic rings of the three herbicides, produce peroxides via hydrogen abstraction, or fragment at the ether bond. OH oxidation of MCPA produced two potentially toxic compounds: chlorosalicylaldehyde and chlorosalicylic acid. We use standards to validate the detection of these oxidation products by acetate CIMS and quantify the reaction rate. Oxidation of triclopyr produced a known endocrine disruptor, 3,5,6-trichloro-2-pyridinol. Thus, while some OH oxidation products are less toxic than the parent molecules (e.g., C₁-₅ carboxylic acids), others may be as or more toxic than the parent herbicide. We determine effective rate coefficients for OH addition to the aromatic ring ( k_OH) for mecoprop-p of 1.5 (±0.7) × 10^-12 cm³ molecules^-1 s^-1 and for MCPA of 2.6 (±0.3) × 10^-12 cm³ molecules^-1 s^-1. The atmospheric lifetimes with respect to OH are thus long enough that photochemistry may be relevant to the environmental fate of these pesticides.

Mutagenic atmospheres resulting from the photooxidation of aromatic hydrocarbon and NOx mixtures

Although many volatile organic compounds (VOCs) are regulated to limit air pollution and the consequent health effects, the photooxidation products generally are not. Thus, we examined the mutagenicity in Salmonella TA100 of photochemical atmospheres generated in a steady-state atmospheric simulation chamber by irradiating mixtures of single aromatic VOCs, NOx, and ammonium sulfate seed aerosol in air. The 10 VOCs examined were benzene; toluene; ethylbenzene; o-, m-, and p-xylene; 1,2,4- and 1,3,5-trimethylbenzene; m-cresol; and naphthalene. Salmonella were exposed at the air-agar interface to the generated atmospheres for 1, 2, 4, 8, or 16 h. Dark-control exposures produced non-mutagenic atmospheres, illustrating that the gas-phase precursor VOCs were not mutagenic at the concentrations tested. Under irradiation, all but m-cresol and naphthalene produced mutagenic atmospheres, with potencies ranging from 2.0 (p-xylene) to 10.4 (ethylbenzene) revertants m3 mgC-1 h-1. The mutagenicity was due exclusively to direct-acting late-generation products of the photooxidation reactions. Gas-phase chemical analysis showed that a number of oxidized organic chemical species enhanced during the irradiated exposure experiments correlated (r ≥ 0.81) with the mutagenic potencies of the atmospheres. Molecular formulas assigned to these species indicated that they likely contained peroxy acid, aldehyde, alcohol, and other functionalities.

Laser Ablation-Aerosol Mass Spectrometry-Chemical Ionization Mass Spectrometry for Ambient Surface Imaging

Mass spectrometry imaging is becoming an increasingly common analytical technique due to its ability to provide spatially resolved chemical information. Here, we report a novel imaging approach combining laser ablation with two mass spectrometric techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, separately and in parallel. Both mass spectrometric methods provide the fast response, rapid data acquisition, low detection limits, and high-resolution peak separation desirable for imaging complex samples. Additionally, the two techniques provide complementary information with aerosol mass spectrometry providing near universal detection of all aerosol molecules and chemical ionization mass spectrometry with a heated inlet providing molecular-level detail of both gases and aerosols. The two techniques operate with atmospheric pressure interfaces and require no matrix addition for ionization, allowing for samples to be investigated in their native state under ambient pressure conditions. We demonstrate the ability of laser ablation-aerosol mass spectrometry-chemical ionization mass spectrometry (LA-AMS-CIMS) to create 2D images of both standard compounds and complex mixtures. The results suggest that LA-AMS-CIMS, particularly when combined with advanced data analysis methods, could have broad applications in mass spectrometry imaging applications.

Chemical evolution of atmospheric organic carbon over multiple generations of oxidation

The evolution of atmospheric organic carbon as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone and oxidants. However, full characterization of organic carbon over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of α-pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. Although quantification of some early generation products remains elusive, full carbon closure is achieved (within measurement uncertainty) by the end of the experiments. These results provide new insights into the effects of oxidation on organic carbon properties (volatility, oxidation state and reactivity) and the atmospheric lifecycle of organic carbon. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs-volatile oxidized gases and low-volatility particulate matter.

Unified Theory of Vapor-Wall Mass Transport in Teflon-Walled Environmental Chambers

Secondary organic aerosol (SOA) formation is studied in laboratory chambers, in which volatile organic compounds (VOCs) are oxidized to produce low-volatility compounds that condense into the aerosol phase. It has been established that such oxidized low-volatility compounds can partition into the chamber walls, which traditionally consist of Teflon film. Several studies exist in which the rates of uptake of individual vapor compounds to the chamber walls have been measured, but a unified theory capable of describing the range of experimental measurements has been lacking. Here, a two-layer model of observed short and long vapor-wall interaction time scales in Teflon-walled environmental chambers is presented and shown to be consistent with experimental data on the rate of wall deposition of more than 90 compounds. Semiempirical relationships between key parameters in the model and vapor molecular properties are derived, which can be used to predict the fate of gas-phase vapor in the chamber under dry conditions.

Growth Kinetics and Size Distribution Dynamics of Viscous Secondary Organic Aerosol

Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration time scale, but its broader impacts on aerosol growth and size distribution dynamics are poorly understood. Here, we present quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of SOA formed under dry conditions from photooxidation of isoprene in the presence of a bimodal aerosol consisting of Aitken (ammonium sulfate) and accumulation (isoprene or α-pinene SOA) mode particles. Aerosol composition measurements and evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs), with some reversibly reacting to form oligomers. Model analysis shows that liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics of accumulation mode particles but fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, the semisolid scenario successfully reproduces both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion time scales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically active sizes.

Production of N2O5 and ClNO2 through Nocturnal Processing of Biomass-Burning Aerosol

Biomass burning is a source of both particulate chloride and nitrogen oxides, two important precursors for the formation of nitryl chloride (ClNO₂), a source of atmospheric oxidants that is poorly prescribed in atmospheric models. We investigated the ability of biomass burning to produce N₂O₅(g) and ClNO₂(g) through nocturnal chemistry using authentic biomass-burning emissions in a smog chamber. There was a positive relationship between the amount of ClNO₂ formed and the total amount of particulate chloride emitted and with the chloride fraction of nonrefractory particle mass. In every fuel tested, dinitrogen pentoxide (N₂O₅) formed quickly, following the addition of ozone to the smoke aerosol, and ClNO₂(g) production promptly followed. At atmospherically relevant relative humidities, the particulate chloride in the biomass-burning aerosol was rapidly but incompletely displaced, likely by the nitric acid produced largely by the heterogeneous uptake of N₂O₅(g). Despite this chloride acid displacement, the biomass-burning aerosol still converted on the order of 10% of reacted N₂O₅(g) into ClNO₂(g). These experiments directly confirm that biomass burning is a potentially significant source of atmospheric N₂O₅ and ClNO₂ to the atmosphere.

Airborne Observations of Reactive Inorganic Chlorine and Bromine Species in the Exhaust of Coal-Fired Power Plants

We present airborne observations of gaseous reactive halogen species (HCl, Cl₂, ClNO₂, Br₂,BrNO₂, and BrCl), sulfur dioxide (SO₂), and nonrefractory fine particulate chloride (pCl) and sulfate(pSO₄) in power plant exhaust. Measurements were conducted during the Wintertime INvestigation of Transport, Emissions, and Reactivity campaign in February-March of 2015 aboard the NCAR-NSF C-130 aircraft. Fifty air mass encounters were identified in which SO₂ levels were elevated ~5 ppb above ambient background levels and in proximity to operational power plants. Each encounter was attributed to one or more potential emission sources using a simple wind trajectory analysis. In case studies, we compare measured emission ratios to those reported in the 2011 National Emissions Inventory and present evidence of the conversion of HCl emitted from power plants to ClNO₂. Taking into account possible chemical conversion downwind, there was general agreement between the observed and reported HCl: SO₂ emission ratios. Reactive bromine species (Br₂, BrNO₂, and/or BrCl) were detected in the exhaust of some coal-fired power plants, likely related to the absence of wet flue gas desulfurization emission control technology. Levels of bromine species enhanced in some encounters exceeded those expected assuming all of the native bromide in coal was released to the atmosphere, though there was no reported use of bromide salts (as a way to reduce mercury emissions) during Wintertime INvestigation of Transport, Emissions, and Reactivity observations. These measurements represent the first ever in-flight observations of reactive gaseous chlorine and bromine containing compounds present in coal-fired power plant exhaust.

Computational Comparison of Different Reagent Ions in the Chemical Ionization of Oxidized Multifunctional Compounds

High pressure anion chemical ionization is commonly used for the detection of neutral molecules in the gas phase. The detection efficiency in these measurements depends on how strongly the reagent ion binds to the neutral target molecule. We have calculated the binding strength of nitrate (NO₃^-), acetate (CH₃C(O)O^-), lactate (CH₃CH(OH)C(O)O^-), trifluoroacetate (CF₃C(O)O^-), trifluoromethanolate (CF₃O^-), bromide (Br^-), and iodide (I^-) reagent ions to ten different products derived from the OH radical-initiated oxidation of butadiene. We found that the binding of these oxidation products to the reagent ions depends almost linearly on the number of oxygen atoms in the target molecule, with the precise chemical identity of the compound (e.g., the number and relative position of hydroxyl or hydroperoxy groups) playing a more minor role. For acetate, the formation free energy decreases on average by around 4 kcal/mol when the number of oxygen atoms in the sample molecule increases by one. For the other reagent ions the corresponding decrease is around 3 kcal/mol. For all of the molecules studied, acetate forms the most stable clusters and I^- the least stable. We also investigated the effect of humidity on the chemical ionization by calculating how strongly water molecules bind to both the reagent ions and the ion-molecule clusters. Water binds much more strongly to the reagent ion monomers compared to the reagent ion "dimers" (defined here as a cluster of the reagent anion with the corresponding neutral conjugate acid, e.g., HNO₃(NO₃^-)) or the ion-molecule clusters. This likely leads to a stronger humidity dependence when using reagent ions that are not able to form reagent ion dimers (such as CF₃C(O)O^-, CF₃O^-, Br^-, and I^-).

Quantifying the Primary Emissions and Photochemical Formation of Isocyanic Acid Downwind of Oil Sands Operations

Isocyanic acid (HNCO) is a known toxic species and yet the relative importance of primary and secondary sources to regional HNCO and population exposure remains unclear. Off-road diesel fuel combustion has previously been suggested to be an important regional source of HNCO, which implies that major industrial facilities such as the oil sands (OS), which consume large quantities of diesel fuel, can be sources of HNCO. The OS emissions of nontraditional toxic species such as HNCO have not been assessed. Here, airborne measurements of HNCO were used to estimate primary and secondary HNCO for the oil sands. Approximately 6.2 ± 1.1 kg hr^-1 was emitted from off-road diesel activities within oil sands facilities, and an additional 116-186 kg hr^-1 formed from the photochemical oxidation of diesel exhaust. Together, the primary and secondary HNCO from OS operations represent a significant anthropogenic HNCO source in Canada. The secondary HNCO downwind of the OS was enhanced by up to a factor of 20 relative to its primary emission, an enhancement factor significantly greater than previously estimated from laboratory studies. Incorporating HNCO emissions and formation into a regional model demonstrated that the HNCO levels in Fort McMurray (∼10-70 km downwind of the OS) are controlled by OS emissions; > 50% of the monthly mean HNCO arose from the OS. While the mean HNCO levels in Fort McMurray are predicted to be below the 1000 pptv level associated with potential negative health impacts, (∼25 pptv in August-September), an order of magnitude increase in concentration is predicted (250-600 pptv) when the town is directly impacted by OS plumes. The results here highlight the importance of obtaining at-source HNCO emission factors and advancing the understanding of secondary HNCO formation mechanisms, to assess and improve HNCO population exposure predictions.

Observations and impacts of bleach washing on indoor chlorine chemistry

Ambient levels of chlorinated gases and aerosol components were measured by online chemical ionization and aerosol mass spectrometers after an indoor floor were repeatedly washed with a commercial bleach solution. Gaseous chlorine (Cl₂ , 10's of ppbv) and hypochlorous acid (HOCl, 100's of ppbv) arise after floor washing, along with nitryl chloride (ClNO₂ ), dichlorine monoxide (Cl₂ O), and chloramines (NHCl₂ , NCl₃ ). Much higher mixing ratios would prevail in a room with lower and more commonly encountered air exchange rates than that observed in the study (12.7 h^-1 ). Coincident with the formation of gas-phase species, particulate chlorine levels also rise. Cl₂ , ClNO₂ , NHCl₂ , and NCl₃ exist in the headspace of the bleach solution, whereas HOCl was only observed after floor washing. HOCl decays away 1.4 times faster than the air exchange rate, indicative of uptake onto room surfaces, and consistent with the well-known chlorinating ability of HOCl. Photochemical box modeling captures the temporal profiles of Cl₂ and HOCl very well and indicates that the OH, Cl, and ClO gas-phase radical concentrations in the indoor environment could be greatly enhanced (>10⁶ and 10⁵  cm^-3 for OH and Cl, respectively) in such washing conditions, dependent on the amount of indoor illumination.

Direct Measurements of Gas/Particle Partitioning and Mass Accommodation Coefficients in Environmental Chambers

Secondary organic aerosols (SOA) are a major contributor to fine particulate mass and wield substantial influences on the Earth's climate and human health. Despite extensive research in recent years, many of the fundamental processes of SOA formation and evolution remain poorly understood. Most atmospheric aerosol models use gas/particle equilibrium partitioning theory as a default treatment of gas-aerosol transfer, despite questions about potentially large kinetic effects. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel method. A simple chemical system produces a very fast burst of low-volatility gas-phase products, which are competitively taken up by liquid organic seed particles and Teflon chamber walls. Clear changes in the species time evolution with differing amounts of seed allow us to quantify the particle uptake processes. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we quantify the aerosol mass accommodation coefficient (α) as 0.7 on average, with values near unity especially for low volatility species. α appears to decrease as volatility increases. α has historically been a very difficult parameter to measure with reported values varying over 3 orders of magnitude. We use the experimentally constrained model to evaluate the correction factor (Φ) needed for chamber SOA mass yields due to losses of vapors to walls as a function of species volatility and particle condensational sink. Φ ranges from 1-4.

Primary and Secondary Sources of Gas-Phase Organic Acids from Diesel Exhaust

Organic acids have primary and secondary sources in the atmosphere, impact ecosystem health, and are useful metrics for identifying gaps in organic oxidation chemistry through model-measurement comparisons. We photooxidized (OH oxidation) primary emissions from diesel and biodiesel fuel types under two engine loads in an oxidative flow reactor. formic, butyric, and propanoic acids, but not methacrylic acid, have primary and secondary sources. Emission factors for these gas-phase acids varied from 0.3-8.4 mg kg^-1 fuel. Secondary chemistry enhanced these emissions by 1.1 (load) to 4.4 (idle) × after two OH-equivalent days. The relative enhancement in secondary organic acids in idle versus loaded conditions was due to increased precursor emissions, not faster reaction rates. Increased hydrocarbon emissions in idle conditions due to less complete combustion (associated with less oxidized gas-phase molecules) correlated to higher primary organic acid emissions. The lack of correlation between organic aerosol and organic acid concentrations downstream of the flow reactor indicates that the secondary products formed on different oxidation time scales and that despite being photochemical products, organic acids are poor tracers for secondary organic aerosol formation from diesel exhaust. Ignoring secondary chemistry from diesel exhaust would lead to underestimates of both organic aerosol and gas-phase organic acids.

Computational and Experimental Investigation of the Detection of HO2 Radical and the Products of Its Reaction with Cyclohexene Ozonolysis Derived RO2 Radicals by an Iodide-Based Chemical Ionization Mass Spectrometer

The HO₂ radical is an important atmospheric molecule that can potentially influence the termination of autoxidation processes of volatile organic compounds (VOCs) that lead to the formation of highly oxygenated multifunctional compounds (HOMs). In this work, we demonstrate the direct detection of the HO₂ radical using an iodide-based chemical ionization mass spectrometer (iodide-CIMS). Expanding on the previously established correlation between molecule-iodide binding enthalpy and iodide-CIMS instrument sensitivity, the experimental detection of the HO₂ radical was preceded by the quantum chemical calculation of the HO₂*I^- cluster (PBE/aug-cc-pVTZ-PP level), which showed a reasonably strong binding enthalpy of 21.60 kcal/mol. Cyclohexene ozonolysis intermediates and closed-shell products were next detected by the iodide-CIMS. The ozone-initiated cyclohexene oxidation mechanism was perturbed by the introduction of the HO₂ radical, leading to the formation of closed-shell hydroperoxides. The experimental investigation once again followed the initial computational molecule-iodide binding enthalpy calculations. The quantum chemical calculations were performed at the PBE/aug-cc-pVTZ-PP level for radicals and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP level for the closed-shell products. A comparison between the iodide-CIMS and nitrate-CIMS spectra with identical measurement steps revealed that the iodide-CIMS was able to detect the low-oxidized (O/C ratio 0.5 and 0.66) cyclohexene ozonolysis monomer products more efficiently than nitrate-CIMS. Higher-oxidized monomers (O/C ratio 1 to 1.5) were detected equally well by both methods. An investigation of dimers showed that both iodide- and nitrate-CIMS were able to detect the dimer compositions possibly formed from reactions between the peroxy radical monomers considered in this study. Additionally, iodide-CIMS detected organic ions that were formed by a previously suggested mechanism of dehydroxylation of peroxy acids (and deoxygenation of acyl peroxy radicals) by H₂O*I^- clusters. These mechanisms were computationally verified.

Highly Oxygenated Multifunctional Compounds in α-Pinene Secondary Organic Aerosol

Highly oxygenated multifunctional organic compounds (HOMs) originating from biogenic emissions constitute a widespread source of organic aerosols in the pristine atmosphere. However, the molecular forms in which HOMs are present in the condensed phase upon gas-particle partitioning remain unclear. In this study, we show that highly oxygenated molecules that contain multiple peroxide functionalities are readily cationized by the attachment of Na⁺ during electrospray ionization operated in the positive ion mode. With this method, we present the first identification of HOMs characterized as C_8-10H_12-18O_4-9 monomers and C_16-20H_24-36O_8-14 dimers in α-pinene derived secondary organic aerosol (SOA). Simultaneous detection of these molecules in the gas phase provides direct evidence for their gas-to-particle conversion. Molecular properties of particulate HOMs generated from ozonolysis and OH oxidation of unsubstituted (C₁₀H₁₆) and deuterated (C₁₀H₁₃D₃) α-pinene are investigated using coupled ion mobility spectrometry with mass spectrometry. The systematic shift in the mass of monomers in the deuterated system is consistent with the decomposition of isomeric vinylhydroperoxides to release vinoxy radical isotopologues, the precursors to a sequence of autoxidation reactions that ultimately yield HOMs in the gas phase. The remarkable difference observed in the dimer abundance under O₃- versus OH-dominant environments underlines the competition between intramolecular hydrogen migration of peroxy radicals and their bimolecular termination reactions. Our results provide new and direct molecular-level information for a key component needed for achieving carbon mass closure of α-pinene SOA.

Chemical Characterization of Gas- and Particle-Phase Products from the Ozonolysis of α-Pinene in the Presence of Dimethylamine

Amines are recognized as key compounds in new particle formation (NPF) and secondary organic aerosol (SOA) formation. In addition, ozonolysis of α-pinene contributes substantially to the formation of biogenic SOAs in the atmosphere. In the present study, ozonolysis of α-pinene in the presence of dimethylamine (DMA) was investigated in a flow tube reactor. Effects of amines on SOA formation and chemical composition were examined. Enhancement of NPF and SOA formation was observed in the presence of DMA. Chemical characterization of gas- and particle-phase products by high-resolution mass spectrometric techniques revealed the formation of nitrogen containing compounds. Reactions between ozonolysis reaction products of α-pinene, such as pinonaldehyde or pinonic acid, and DMA were observed. Possible reaction pathways are suggested for the formation of the reaction products. Some of the compounds identified in the laboratory study were also observed in aerosol samples (PM₁) collected at the SMEAR II station (Hyytiälä, Finland) suggesting that DMA might affect the ozonolysis of α-pinene in ambient conditions.

Isomerization of Second-Generation Isoprene Peroxy Radicals: Epoxide Formation and Implications for Secondary Organic Aerosol Yields

We report chamber measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation, in which radical concentrations were systematically varied and the molecular composition of semi- to low-volatility gases and SOA were measured online. Using a detailed chemical kinetics box model, we find that to explain the behavior of low-volatility products and SOA mass yields relative to input H₂O₂ concentrations, the second-generation dihydroxy hydroperoxy peroxy radical (C₅H₁₁O₆·) must undergo an intramolecular H-shift with a net forward rate constant of order 0.1 s^-1 or higher. This finding is consistent with quantum chemical calculations that suggest a net forward rate constant of 0.3-0.9 s^-1. Furthermore, these calculations suggest that the dominant product of this isomerization is a dihydroxy hydroperoxy epoxide (C₅H₁₀O₅), which is expected to have a saturation vapor pressure ∼2 orders of magnitude higher, as determined by group-contribution calculations, than the dihydroxy dihydroperoxide, ISOP(OOH)₂(C₅H₁₂O₆), a major product of the peroxy radical reacting with HO₂. These results provide strong constraints on the likely volatility distribution of isoprene oxidation products under atmospheric conditions and, thus, on the importance of nonreactive gas-particle partitioning of isoprene oxidation products as an SOA source.

Cavity enhanced spectroscopy for measurement of nitrogen oxides in the Anthropocene: results from the Seoul tower during MAPS 2015

Cavity enhanced spectroscopy, CES, is a high sensitivity direct absorption method that has seen increasing utility in the last decade, a period also marked by increasing requirements for understanding human impacts on atmospheric composition. This paper describes the current NOAA six channel cavity ring-down spectrometer (CRDS, the most common form of CES) for measurement of nitrogen oxides and O₃. It further describes the results from measurements from a tower 300 m above the urban area of Seoul in late spring of 2015. The campaign demonstrates the performance of the CRDS instrument and provides new data on both photochemistry and nighttime chemistry in a major Asian megacity. The instrument provided accurate, high time resolution data for N₂O₅, NO, NO₂, NO_yand O₃, but suffered from large wall loss in the sampling of NO₃, illustrating the requirement for calibration of the NO₃inlet transmission. Both the photochemistry and nighttime chemistry of nitrogen oxides and O₃were rapid in this megacity. Sustained average rates of O₃buildup of 10 ppbv h⁻¹during recurring morning and early afternoon sea breezes led to a 50 ppbv average daily O₃rise. Nitrate radical production rates,P(NO₃), averaged 3–4 ppbv h⁻¹in late afternoon and early evening, much greater than contemporary data from Los Angeles, a comparable U. S. megacity. TheseP(NO₃) were much smaller than historical data from Los Angeles, however. Nighttime data at 300 m above ground showed considerable variability in high time resolution nitrogen oxide and O₃, likely resulting from sampling within gradients in the nighttime boundary layer structure. Apparent nighttime biogenic VOC oxidation rates of several ppbv h⁻¹were also likely influenced by vertical gradients. Finally, daytime N₂O₅mixing ratios of 3–35 pptv were associated with rapid daytimeP(NO₃) and agreed well with a photochemical steady state calculation.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Gas Phase Oxidation of Campholenic Aldehyde and Solution Phase Reactivity of its Epoxide Derivative

The rate constant for the OH reaction with campholenic aldehyde (CA) was measured using the flow tube-chemical ionization mass spectrometry method with a relative rate kinetics technique and was found to be (6.54 ± 0.52) × 10^-11 cm³ molecule^-1 s^-1 at 100 Torr pressure and 298 K. A mechanism for the formation of the observed products was developed for both NO-free and NO-present conditions. On the basis of measurements of the pressure dependent yields of the products, between 5 and 20% of the CA oxidation at atmospheric pressure is predicted to lead to campholenic aldehyde epoxide (CAE). The aqueous solution reaction rate constants for CAE were determined via NMR spectroscopy and were found to be (2.241 ± 0.036) × 10^-5 s^-1 for neutral conditions and 0.0989 ± 0.0053 M^-1 s^-1 for acid-catalyzed conditions at 298 K. The products of the CAE aqueous solution reaction were identified as an isomer of CAE and the aldehyde group hydrated form of this isomer. Unlike the isoprene-derived epoxide, IEPOX, a nucleophilic addition mechanism was not observed. On the basis of the rate constants determined for CA and CAE, it is likely that these species are reactive on atmospherically relevant time scales in the gas and aerosol phases, respectively. The results of the present study largely support a previous supposition that α-pinene-derived secondary organic aerosol may be influenced by the multiphase processing of various intermediate species, including those with epoxide functionality.

Efficient Isoprene Secondary Organic Aerosol Formation from a Non-IEPOX Pathway

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

Reactions of Atmospheric Particulate Stabilized Criegee Intermediates Lead to High-Molecular-Weight Aerosol Components

Aging of organic aerosol particles is one of the most poorly understood topics in atmospheric aerosol research. Here, we used an aerosol flow tube together with an iodide-adduct high-resolution time-of-flight chemical-ionization mass spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS) to investigate heterogeneous ozonolysis of oleic acid (OL), developing a comprehensive oxidation mechanism with observed products. In addition to the well-known first-generation C9 products including nonanal, nonanoic acid, azelaic acid, and 9-oxononanoic acid, the iodide-adduct chemical ionization permitted unambiguous determination of a large number of high-molecular-weight particulate products up to 670 Da with minimum amounts of fragmentation. These high-molecular-weight products are characterized by a fairly uniform carbon oxidation state but stepwise addition of a carbon backbone moiety, and hence continuous decrease in the volatility. Our results demonstrate that heterogeneous oxidation of organic aerosols has a significant effect on the physiochemical properties of organic aerosols and that reactions of particulate SCIs from ozonolysis of an unsaturated particulate species represent a previously underappreciated mechanism that lead to formation of high-molecular-weight particulate products that are stable under typical atmospheric conditions.

Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ

Partitioning of gas-phase organic compounds to the walls of Teflon environmental chambers is a recently reported phenomenon than can affect the yields of reaction products and secondary organic aerosol (SOA) measured in laboratory experiments. Reported time scales for reaching gas-wall partitioning (GWP) equilibrium (τGWE) differ by up to 3 orders of magnitude, however, leading to predicted effects that vary from substantial to negligible. A new technique is demonstrated here in which semi- and low-volatility oxidized organic compounds (saturation concentration c* < 100 μg m(-3)) were photochemically generated in rapid bursts in situ in an 8 m(3) environmental chamber, and then their decay in the absence of aerosol was measured using a high-resolution chemical ionization mass spectrometer (CIMS) equipped with an "inlet-less" NO3(-) ion source. Measured τGWE were 7-13 min (rel. std. dev. 33%) for all compounds. The fraction of each compound that partitioned to the walls at equilibrium follows absorptive partitioning theory with an equivalent wall mass concentration in the range 0.3-10 mg m(-3). Measurements using a CIMS equipped with a standard ion-molecule reaction region showed large biases due to the contact of compounds with walls. On the basis of these results, a set of parameters is proposed for modeling GWP in chamber experiments.

Molecular Composition and Volatility of Organic Aerosol in the Southeastern U.S.: Implications for IEPOX Derived SOA

We present measurements as part of the Southern Oxidant and Aerosol Study (SOAS) during which atmospheric aerosol particles were comprehensively characterized. We present results utilizing a Filter Inlet for Gases and AEROsol coupled to a chemical ionization mass spectrometer (CIMS). We focus on the volatility and composition of isoprene derived organic aerosol tracers and of the bulk organic aerosol. By utilizing the online volatility and molecular composition information provided by the FIGAERO-CIMS, we show that the vast majority of commonly reported molecular tracers of isoprene epoxydiol (IEPOX) derived secondary organic aerosol (SOA) is derived from thermal decomposition of accretion products or other low volatility organics having effective saturation vapor concentrations <10(-3) μg m(-3). In addition, while accounting for up to 30% of total submicrometer organic aerosol mass, the IEPOX-derived SOA has a higher volatility than the remaining bulk. That IEPOX-SOA, and more generally bulk organic aerosol in the Southeastern U.S. is comprised of effectively nonvolatile material has important implications for modeling SOA derived from isoprene, and for mechanistic interpretations of molecular tracer measurements. Our results show that partitioning theory performs well for 2-methyltetrols, once accretion product decomposition is taken into account. No significant partitioning delays due to aerosol phase or viscosity are observed, and no partitioning to particle-phase water or other unexplained mechanisms are needed to explain our results.

Anthropogenic Sulfur Perturbations on Biogenic Oxidation: SO2 Additions Impact Gas-Phase OH Oxidation Products of α- and β-Pinene

In order to probe how anthropogenic pollutants can impact the atmospheric oxidation of biogenic emissions, we investigated how sulfur dioxide (SO2) perturbations impact the oxidation of two monoterpenes, α-and β-pinene. We used chemical ionization mass spectrometry to examine changes in both individual molecules and gas-phase bulk properties of oxidation products as a function of SO2 addition. SO2 perturbations impacted the oxidation systems of α-and β-pinene, leading to an ensemble of products with a lesser degree of oxygenation than unperturbed systems. These changes may be due to shifts in the OH:HO2 ratio from SO2 oxidation and/or to SO3 reacting directly with organic molecules. Van Krevelen diagrams suggest a shift from gas-phase functionalization by alcohol/peroxide groups to functionalization by carboxylic acid or carbonyl groups, consistent with a decreased OH:HO2 ratio. Increasing relative humidity dampens the impact of the perturbation. This decrease in oxygenation may impact secondary organic aerosol formation in regions dominated by biogenic emissions with nearby SO2 sources. We observed sulfur-containing organic compounds following SO2 perturbations of monoterpene oxidation; whether these are the result of photochemistry or an instrumental artifact from ion-molecule clustering remains uncertain. However, our results demonstrate that the two monoterpene isomers produce unique suites of oxidation products.

Highly functionalized organic nitrates in the southeast United States: Contribution to secondary organic aerosol and reactive nitrogen budgets

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (∼2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.

Instrumentation and Measurement Strategy for the NOAA SENEX Aircraft Campaign as Part of the Southeast Atmosphere Study 2013

Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO₂ measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.

Formation of Low Volatility Organic Compounds and Secondary Organic Aerosol from Isoprene Hydroxyhydroperoxide Low-NO Oxidation

Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m(-3)). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10(-2) to 10 μg m(-3) are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr(-1) of SOA production, or 3.3% of global SOA.

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, cis-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) × 10(9) and 3.1 (± 0.2) × 10(8) 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.

Reactive uptake of an isoprene-derived epoxydiol to submicron aerosol particles

The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γIEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γIEPOX for trans-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γIEPOX is more than 500 times greater on ammonium bisulfate (γ ∼ 0.05) than on ammonium sulfate (γ ≤ 1 × 10(-4)). We could accurately predict γIEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry's law coefficient (1.7 × 10(8) M/atm). Suppression of γIEPOX was observed on particles containing both ammonium bisulfate and poly(ethylene glycol) (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0) but is greater than 25 h on less acidic particles (pH > 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation.