Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation.

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3-H2SO4-NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.

Rapid growth of new atmospheric particles by nitric acid and ammonia condensation.

A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog1,2, but how it occurs in cities is often puzzling3. If the growth rates of urban particles are similar to those found in cleaner environments (1-10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below -15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid-base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms4,5.

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.

Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity.

The mechanistic pathway by which high relative humidity (RH) affects gas-particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10-20 to 60-80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2-4 in the concentration of C10H16O2-3, while the particle-phase concentrations of low-volatility species, such as C10H16O6-8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth.

High Gas-Phase Methanesulfonic Acid Production in the OH-Initiated Oxidation of Dimethyl Sulfide at Low Temperatures.

Dimethyl sulfide (DMS) influences climate via cloud condensation nuclei (CCN) formation resulting from its oxidation products (mainly methanesulfonic acid, MSA, and sulfuric acid, H2SO4). Despite their importance, accurate prediction of MSA and H2SO4 from DMS oxidation remains challenging. With comprehensive experiments carried out in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, we show that decreasing the temperature from +25 to -10 °C enhances the gas-phase MSA production by an order of magnitude from OH-initiated DMS oxidation, while H2SO4 production is modestly affected. This leads to a gas-phase H2SO4-to-MSA ratio (H2SO4/MSA) smaller than one at low temperatures, consistent with field observations in polar regions. With an updated DMS oxidation mechanism, we find that methanesulfinic acid, CH3S(O)OH, MSIA, forms large amounts of MSA. Overall, our results reveal that MSA yields are a factor of 2-10 higher than those predicted by the widely used Master Chemical Mechanism (MCMv3.3.1), and the NOx effect is less significant than that of temperature. Our updated mechanism explains the high MSA production rates observed in field observations, especially at low temperatures, thus, substantiating the greater importance of MSA in the natural sulfur cycle and natural CCN formation. Our mechanism will improve the interpretation of present-day and historical gas-phase H2SO4/MSA measurements.

Secondary Organic Aerosol Mass Yields from NO3 Oxidation of α-Pinene and Δ-Carene: Effect of RO2 Radical Fate.

Dark chamber experiments were conducted to study the SOA formed from the oxidation of α-pinene and Δ-carene under different peroxy radical (RO2) fate regimes: RO2 + NO3, RO2 + RO2, and RO2 + HO2. SOA mass yields from α-pinene oxidation were <1 to ∼25% and strongly dependent on available OA mass up to ∼100 µg m-3. The strong yield dependence of α-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Δ-carene + NO3 were consistently higher, ranging from ∼10-50% with some dependence on OA for <25 µg m-3. Explicit kinetic modeling including vapor wall losses was conducted to enable comparisons across VOC precursors and RO2 fate regimes and to determine atmospherically relevant yields. Furthermore, SOA yields were similar for each monoterpene across the nominal RO2 + NO3, RO2 + RO2, or RO2 + HO2 regimes; thus, the volatility basis sets (VBS) constructed were independent of the chemical regime. Elemental O/C ratios of ∼0.4-0.6 and nitrate/organic mass ratios of ∼0.15 were observed in the particle phase for both monoterpenes in all regimes, using aerosol mass spectrometer (AMS) measurements. An empirical relationship for estimating particle density using AMS-derived elemental ratios, previously reported in the literature for non-nitrate containing OA, was successfully adapted to organic nitrate-rich SOA. Observations from an NO3- chemical ionization mass spectrometer (NO3-CIMS) suggest that Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semivolatile species, when reacted with NO3, regardless of RO2 regime. The similar Δ-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multistep processes such as alkoxy radical isomerization and decomposition may play a role in the formation of SOA from Δ-carene + NO3. The scarcity of peroxide functional groups (on average, 14% of C10 groups carried a peroxide functional group in one test experiment in the RO2 + RO2 regime) appears to rule out a major role for autoxidation and organic peroxide (ROOH, ROOR) formation. The consistently substantially lower SOA yields observed for α-pinene + NO3 suggest such pathways are less available for this precursor. The marked and robust regime-independent difference in SOA yield from two different precursor monoterpenes suggests that in order to accurately model SOA production in forested regions the chemical mechanism must feature some distinction among different monoterpenes.

Using highly time-resolved online mass spectrometry to examine biogenic and anthropogenic contributions to organic aerosol in Beijing.

Organic aerosols, a major constituent of fine particulate mass in megacities, can be directly emitted or formed from secondary processing of biogenic and anthropogenic volatile organic compound emissions. The complexity of volatile organic compound emission sources, speciation and oxidation pathways leads to uncertainties in the key sources and chemistry leading to formation of organic aerosol in urban areas. Historically, online measurements of organic aerosol composition have been unable to resolve specific markers of volatile organic compound oxidation, while offline analysis of markers focus on a small proportion of organic aerosol and lack the time resolution to carry out detailed statistical analysis required to study the dynamic changes in aerosol sources and chemistry. Here we use data collected as part of the joint UK-China Air Pollution and Human Health (APHH-Beijing) collaboration during a field campaign in urban Beijing in the summer of 2017 alongside laboratory measurements of secondary organic aerosol from oxidation of key aromatic precursors (1,3,5-trimethyl benzene, 1,2,4-trimethyl benzene, propyl benzene, isopropyl benzene and 1-methyl naphthalene) to study the anthropogenic and biogenic contributions to organic aerosol. For the first time in Beijing, this study applies positive matrix factorisation to online measurements of organic aerosol composition from a time-of-flight iodide chemical ionisation mass spectrometer fitted with a filter inlet for gases and aerosols (FIGAERO-ToF-I-CIMS). This approach identifies the real-time variations in sources and oxidation processes influencing aerosol composition at a near-molecular level. We identify eight factors with distinct temporal variability, highlighting episodic differences in OA composition attributed to regional influences and in situ formation. These have average carbon numbers ranging from C5-C9 and can be associated with oxidation of anthropogenic aromatic hydrocarbons alongside biogenic emissions of isoprene, α-pinene and sesquiterpenes.

Yields and Variability of Ozone Reaction Products from Human Skin.

The skin of 20 human participants was exposed to ∼110 ppb O3 and volatile products of the resulting chemistry were quantified in real time. Yields (ppb product emitted/ppb ozone consumed) for 40 products were quantified. Major products of the primary reaction of ozone-squalene included 6-methyl 5-hepten-2-one (6-MHO) and geranyl acetone (GA) with average yields of 0.22 and 0.16, respectively. Other major products included decanal, methacrolein (or methyl vinyl ketone), nonanal, and butanal. Yields varied widely among participants; summed yields ranged from 0.33 to 0.93. The dynamic increase in emission rates during ozone exposure also varied among participants, possibly indicative of differences in the thickness of the skin lipid layer. Factor analysis indicates that much of the variability among participants is due to factors associated with the relative abundance of (1) "fresh" skin lipid constituents (such as squalene and fatty acids), (2) oxidized skin lipids, and (3) exogenous compounds. This last factor appears to be associated with the presence of oleic and linoleic acids and could be accounted for by uptake of cooking oils or personal care products to skin lipids.

Influence of the NO/NO2 Ratio on Oxidation Product Distributions under High-NO Conditions.

Organic oxidation reactions in the atmosphere can be challenging to parse due to the large number of branching points within each molecule's reaction mechanism. This complexity can complicate the attribution of observed effects to a particular chemical pathway. In this study, we simplify the chemistry of atmospherically relevant systems, and particularly the role of NOx, by generating individual alkoxy radicals via alkyl nitrite photolysis (to limit the number of accessible reaction pathways) and measuring their product distributions under different NO/NO2 ratios. Known concentrations of NO in the classically "high-NO" range are maintained in the chamber, thereby constraining first-generation RO2 (peroxy radicals) to react nearly exclusively with NO. Products are measured in both the gas phase (with a proton-transfer reaction mass spectrometer) and the particle phase (with an aerosol mass spectrometer). We observe substantial differences in measured products under varying NO/NO2 ratios (from ∼0.1 to >1); along with modeling simulations using the Master Chemical Mechanism (MCM), these results suggest indirect effects of NOx chemistry beyond the commonly cited RO2 + NO reaction. Specifically, lower-NO/NO2 ratios foster higher concentrations of secondary OH, higher concentrations of peroxyacyl nitrates (PAN, an atmospheric reservoir species), and a more highly oxidized product distribution that results in more secondary organic aerosol (SOA). The impact of NOx concentration beyond simple RO2 branching must be considered when planning laboratory oxidation experiments and applying their results to atmospheric conditions.

Laboratory Investigation of Renoxification from the Photolysis of Inorganic Particulate Nitrate.

Nitrogen oxides (NOx) play a key role in regulating the oxidizing capacity of the atmosphere through controlling the abundance of O3, OH, and other important gas and particle species. Some recent studies have suggested that particulate nitrate, which is conventionally considered as the ultimate oxidation product of NOx, can undergo "renoxification" via photolysis, recycling NOx and HONO back to the gas phase. However, there are large discrepancies in estimates of the importance of this channel, with reported renoxification rate constants spanning three orders of magnitude. In addition, previous laboratory studies derived the rate constant using bulk particle samples collected on substrates instead of suspended particles. In this work, we study renoxification of suspended submicron particulate sodium and ammonium nitrate through controlled laboratory photolysis experiments using an environmental chamber. We find that, under atmospherically relevant wavelengths and relative humidities, particulate inorganic nitrate releases NOx and HONO less than 10 times as rapidly as gaseous nitric acid, putting our measurements on the low end of recently reported renoxification rate constants. To the extent that our laboratory conditions are representative of the real atmosphere, renoxification from the photolysis of inorganic particulate nitrate appears to play a limited role in contributing to the NOx and OH budgets in remote environments. These results are based on simplified model systems; future studies should investigate renoxification of more complex aerosol mixtures that represent a broader spectrum of aerosol properties to better constrain the photolysis of ambient aerosols.

Real-time organic aerosol chemical speciation in the indoor environment using extractive electrospray ionization mass spectrometry.

Understanding the sources and composition of organic aerosol (OA) in indoor environments requires rapid measurements, since many emissions and processes have short timescales. However, real-time molecular-level OA measurements have not been reported indoors. Here, we present quantitative measurements, at a time resolution of five seconds, of molecular ions corresponding to diverse aerosol-phase species, by applying extractive electrospray ionization mass spectrometry (EESI-MS) to indoor air analysis for the first time, as part of the highly instrumented HOMEChem field study. We demonstrate how the complex spectra of EESI-MS are screened in order to extract chemical information and investigate the possibility of interference from gas-phase semivolatile species. During experiments that simulated the Thanksgiving US holiday meal preparation, EESI-MS quantified multiple species, including fatty acids, carbohydrates, siloxanes, and phthalates. Intercomparisons with Aerosol Mass Spectrometer (AMS) and Scanning Mobility Particle Sizer suggest that EESI-MS quantified a large fraction of OA. Comparisons with FIGAERO-CIMS shows similar signal levels and good correlation, with a range of 100 for the relative sensitivities. Comparisons with SV-TAG for phthalates and with SV-TAG and AMS for total siloxanes also show strong correlation. EESI-MS observations can be used with gas-phase measurements to identify co-emitted gas- and aerosol-phase species, and this is demonstrated using complementary gas-phase PTR-MS observations.

Always Lost but Never Forgotten: Gas-Phase Wall Losses Are Important in All Teflon Environmental Chambers.

Yields of secondary organic aerosol (SOA) formation from oxidation of volatile organic compounds are measured in laboratory chambers and then applied in regional and global models. Gas-phase losses to large Teflon-walled environmental chambers have been recently shown to reduce SOA yields. Historically, most chambers have operated in batch mode. Increasingly, however, continuous flow (CF) mode is being used, in which reactants and products are continuously introduced and exhausted from the chamber. Recent literature reports indicate a belief that SOA yields measured in CF chambers are not affected by gas-phase wall losses (GWL). Here, we use an experimentally-constrained box model to show that gas-phase wall losses impact both types of chambers when run under similar conditions. We find CF experiments do mitigate some effects of gas-phase wall losses after long (>2 days) experiment run times, but they have significant losses for typical literature experiment times of 1 day. However, this mitigation phenomenon is an experiment- and mechanism-dependent, and GWL still affects the absolute SOA yield. Finally, we show that at condensation sink values higher than the wall loss rate a lack of change in yield vs seed surface area does not necessarily indicate whether GWL affects the experiment and does not suggest the magnitude.

Budgets of Organic Carbon Composition and Oxidation in Indoor Air.

The chemical composition of indoor air at the University of Colorado, Boulder art museum was measured by a suite of gas- and particle-phase instruments. Over 80% of the total observed organic carbon (TOOC) mass (100 µg m-3) consisted of reduced compounds (carbon oxidation state, OSC < -0.5) with high volatility (log10 C* > 7) and low carbon number (nC < 6). The museum TOOC was compared to other indoor and outdoor locations, which increased according to the following trend: remote < rural ≤ urban < indoor ≤ megacity. The museum TOOC was comparable to a university classroom and 3× less than residential environments. Trends in the total reactive flux were remote < indoor < rural < urban < megacity. High volatile organic compound (VOC) concentrations compensated low oxidant concentrations indoors to result in an appreciable reactive flux. Total hydroxyl radical (OH), ozone (O3), nitrate radical (NO3), and chlorine atom (Cl) reactivities for each location followed a similar trend to TOOC. High human occupancy events increased all oxidant reactivities in the museum by 65-125%. The lifetimes of O3, NO3, OH, and Cl reactivities were 13 h, 15 h, 23 days, and 189 days, respectively, corresponding to over 88% of indoor VOC oxidant reactivity being consumed outdoors after ventilation.

Evaluation of a New Reagent-Ion Source and Focusing Ion-Molecule Reactor for Use in Proton-Transfer-Reaction Mass Spectrometry.

We evaluate the performance of a new chemical ionization source called Vocus, consisting of a discharge reagent-ion source and focusing ion-molecule reactor (FIMR) for use in proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF) measurements of volatile organic compounds (VOCs) in air. The reagent ion source uses a low-pressure discharge. The FIMR consists of a glass tube with a resistive coating, mounted inside a radio frequency (RF) quadrupole. The axial electric field is used to enhance ion collision energies and limit cluster ion formation. The RF field focuses ions to the central axis of the reactor and improves the detection efficiency of product ions. Ion trajectory calculations demonstrate the mass-dependent focusing of ions and enhancement of the ion collision energy by the RF field, in particular for the lighter ions. Product ion signals are increased by a factor of 10 when the RF field is applied (5000-18 000 cps ppbv-1), improving measurement precision and detection limits while operating at very similar reaction conditions as traditional PTR instruments. Because of the high water mixing ratio in the FIMR, we observe no dependence of the sensitivity on ambient sample humidity. In this work, the Vocus is interfaced to a TOF mass analyzer with a mass resolving power up to 12 000, which allows clear separation of isobaric ions, observed at nearly every nominal mass when measuring ambient air. Measurement response times are determined for a range of ketones with saturation vapor concentrations down to 5 × 104 µg m-3 and compare favorably with previously published results for a PTR-MS instrument.

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.

Impact of Thermal Decomposition on Thermal Desorption Instruments: Advantage of Thermogram Analysis for Quantifying Volatility Distributions of Organic Species.

We present results from a high-resolution chemical ionization time-of-flight mass spectrometer (HRToF-CIMS), operated with two different thermal desorption inlets, designed to characterize the gas and aerosol composition. Data from two field campaigns at forested sites are shown. Particle volatility distributions are estimated using three different methods: thermograms, elemental formulas, and measured partitioning. Thermogram-based results are consistent with those from an aerosol mass spectrometer (AMS) with a thermal denuder, implying that thermal desorption is reproducible across very different experimental setups. Estimated volatilities from the detected elemental formulas are much higher than from thermograms since many of the detected species are thermal decomposition products rather than actual SOA molecules. We show that up to 65% of citric acid decomposes substantially in the FIGAERO-CIMS, with ∼20% of its mass detected as gas-phase CO2, CO, and H2O. Once thermal decomposition effects on the detected formulas are taken into account, formula-derived volatilities can be reconciled with the thermogram method. The volatility distribution estimated from partitioning measurements is very narrow, likely due to signal-to-noise limits in the measurements. Our findings indicate that many commonly used thermal desorption methods might lead to inaccurate results when estimating volatilities from observed ion formulas found in SOA. The volatility distributions from the thermogram method are likely the closest to the real distributions.

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.

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.

Increasing Contributions of Temperature-Dependent Oxygenated Organic Aerosol to Summertime Particulate Matter in New York City.

As part of the summer 2022 NYC-METS (New York City metropolitan Measurements of Emissions and TransformationS) campaign and the ASCENT (Atmospheric Science and Chemistry mEasurement NeTwork) observational network, speciated particulate matter was measured in real time in Manhattan and Queens, NY, with additional gas-phase measurements. Largely due to observed reductions in inorganic sulfate aerosol components over the 21st century, summertime aerosol composition in NYC has become predominantly organic (80-83%). Organic aerosol source apportionment via positive matrix factorization showed that this is dominated by secondary production as oxygenated organic aerosol (OOA) source factors comprised 73-76% of OA. Primary factors, including cooking-related organic aerosol (COA) and hydrocarbon-like organic aerosol (HOA) comprised minor fractions of OA, only 13-15% and 10-11%, respectively. The two sites presented considerable spatiotemporal variations in OA source factor concentrations despite similar average PM2.5 concentrations. The less- and more-oxidized OOA factors exhibited clear temperature dependences at both sites with increased concentrations and greater degrees of oxidation at higher temperatures, including during a heatwave. With strong temperature sensitivity and minimal changes in summertime concentrations since 2001, secondary OA poses a particular challenge for air quality policy in NYC that will very likely be exacerbated by continued climate change and extreme heat events.

Quantification of natural gas and other hydrocarbons from production sites in northern West Virginia using tracer flux ratio methodology.

Tracer flux ratio (TFR) methodology performed downwind of 15 active oil and natural gas production sites in Ohio County, West Virginia sought to quantify air pollutant emissions over two weeks in April 2018. In coordination with a production company, sites were randomly selected depending on wind forecasts and nearby road access. Methane (CH4), ethane (C2H6), and tracer gas compounds (acetylene and nitrous oxide) were measured via tunable infrared direct absorption spectroscopy. Ion signals attributed to benzene (C6H6) and other volatile gases (e.g., C7 - C9 aromatics) were measured via proton-transfer reaction time-of-flight mass spectrometry. Short-term whole facility emission rates for 12 sites are reported. Results from TFR were systematically higher than the sum of concurrent on-site full flow sampler measurements, though not all sources were assessed on-site in most cases. In downwind plumes, the mode of the C2H6:CH4 molar ratio distribution for all sites was 0.2, which agreed with spot sample analysis from the site operator. Distribution of C6H6:CH4 ratios was skew but values between 1 and 5 pptv ppbv-1 were common. Additionally, the aromatic profile has been attributed to condensate storage tank emissions. Average ratios of C7 - C9 to C6H6 were similar to other literature values reported for natural gas wells.