While proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is widely used for ambient volatile organic compounds (VOCs) quantification, its accuracy is limited by isomeric speciation and ionization byproducts. Here, gas chromatography (GC) coupled with PTR-ToF-MS was deployed at a suburban site in Hong Kong to resolve isomers and quantify interferences for ambient VOCs measurement. We identified 48 compounds using GC-PTR and resolved their isomer profiles in direct-PTR data. Our analysis revealed that direct-PTR measurements substantially underestimated long-chain aldehydes (C5-C8) due to extensive fragmentation, while overestimating isoprene, benzene, styrene, and phenol by ∼14-60% because of interference from other species. The biogenic VOCs' OH reactivity was overestimated by up to 31% for monoterpenes and 241% for isoprene, depending on the seasonality of biogenic emissions. Propagating these biases into the photochemical model resulted in a 46% overestimation in daytime net O3 production under VOC-limited and low-isoprene conditions, whereas the overestimation decreased to 19% under VOC-saturated, high-isoprene conditions. Correcting isomer distributions and interference effects reduced modeled ozone production rates and altered precursor sensitivities, revealing a larger role for oxygenated VOCs in ozone formation than previously recognized. Our results highlight the necessity for isomer-resolved measurements and interference-aware calibration to improve VOC-based assessments of photochemical air pollution.

Communities in southeastern Louisiana are subject to disproportionate environmental health burdens, including elevated risk for cancer, from emissions of industrial hazardous air pollutants (HAPs). However, there are few ambient measurements (or none) of various HAPs in the heavily industrialized corridor between Baton Rouge and New Orleans (BR-NO). We measured 17 carcinogenic volatile organic compounds using fast-response in situ instrumentation aboard a mobile laboratory. Using spatially resolved concentrations, we estimate cancer risk in 15 census tracts along an 81 km-long stretch of the BR-NO corridor. In 14 of 15 tracts, our estimates of total cancer risk were higher (range: 0.9 to 11.6[Formula: see text]; median: 5[Formula: see text]) than those from the U.S. Environmental Protection Agency's (USEPA) 2020 Air Toxics Screening Assessment (AirToxScreen). Our maximum estimate for total tract-level cancer risk was 560-in-one million excess cancer cases, far exceeding the upper limit of USEPA's acceptable risk range (100-in-one million). This discrepancy is largely explained by differences between measured vs. modeled ethylene oxide concentrations, though there are important contributions from a number of additional HAPs. Our risk estimates are dominated by ethylene oxide, which contributes between 39.5 and 92.2% of total cancer risk across all tracts; chloroprene (0.2 to 36.8%); and formaldehyde (4.1 to 14.6%). AirToxScreen also identifies these three compounds as primary drivers of risk in this location. Together, these three compounds account for between 63 and 96.9% of total cancer risk. There is substantial spatial variability in total cancer risk and the relative contribution of each HAP, both between and within tracts. These data substantiate claims that the region has high HAPs-related cancer risk and quantify which individual HAPs are of highest concern.

Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based datasets and provides insights into key photochemical and depositional loss processes that control the concentrations and spatiotemporal variability in ozone as well as the physicochemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational datasets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall dataset consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The dataset, consisting of more than 630 000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatiotemporal distribution and long-term trends over the 11 defined ocean/polar regions. The datasets from ships, buoys, and aircraft are complemented by ozonesonde data from 29 launch sites or field campaigns and by 21 non-polar and 17 polar ground-based station datasets. The datasets contain information on how long the observed air masses were isolated from land, as estimated by backward trajectories from the individual observation points. To extract observations representative of oceanic conditions, we recommend using a subset of the data with an isolation time of 72 h or longer, from the analysis with coincident radon observations. These filtered oceanic and polar data showed typically flat diurnal cycles at high latitudes, whereas daytime decreases in ozone (11 %-16 %) were observed at lower latitudes. The ship/buoy- and aircraft-based datasets presented here will supplement the land-based ones in the TOAR-II (Tropospheric Ozone Assessment Report Phase II) database to provide a fully global assessment of tropospheric ozone. The described dataset is available at 10.17596/0004044 (Kanaya et al., 2025).

Non-traditional VOC emissions, including emerging pollutants, air toxics, and volatile chemical products (VPCs) span a range of volatilities and molecular structures that impact their reactivity in the atmosphere and eventual fate. However, little is known their source apportionment, temporal behavior, and relative importance to health impacts along with ozone and SOA formation. With the need to characterize these emissions, comprehensive measurement techniques that capture the unexpected but are also highly specific to detail molecular structure and provide compound quantification are needed. The work presented in this study uses chemical ionization (CI) techniques (H3O+, NH4+, NO+, O2+) for the direct detection and quantification of VOCs considered to be hazardous air pollutants (HAPs). With our work, we show that the ionization patterns for these classes of compounds within each ionization scheme can be used to expand these methods to interpret unknown signals in complex environments. This detailed characterization was conducted by coupling in-situ gas chromatography (GC) to the CI-TOF-MS for pre-separation of the complex mixture. Our results show how the speciated data can be used to deconvolve the complexities of chemical ionization detection (including the presence of fragmentation, cluster formation, and mixed ionization schemes e.g. proton transfer, charge transfer, dehydration). To apply these methods to ambient atmospheric measurements we need to reconcile the need for both continuous isomer specific quantification and high time resolution data. To accomplish this, we simultaneously coupled the in-situ GC with both CI and electron ionization (EI) TOF-MS. Resulting in the generation of three data sets (GC-EI, GC-CI, and direct-CI data) that offer continuous GC quantification, universal detection of speciated organics (EI), speciated CI data to constrain interferences, and direct-CI data for high time resolution data. This instrument combination was deployed in Spring 2023 for a 4-week mobile laboratory campaign in a region of southeast Louisiana, US that is dense with petrochemical production and industrial activity, to quantify hazardous air pollutants to gauge exposure for the local population. The combination of the in-situ GC, EI-TOF-MS, PTR-TOF-MS was used to provide highly specific, quantitative data on VOCs considered to be air toxics in the area, while also acquiring high time resolution PTR-TOF data that allowed the characterization of different point sources and their variability over time-of-day and day-of-week.

Abstract. Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based data sets and provides insights into key photochemical and depositional loss processes that control the concentrations, spatio-temporal variability of ozone, and the physico-chemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational data sets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall data set consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The data set, consisting of more than 630,000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatio-temporal distribution and long-term trends over the defined 11 ocean/polar regions. The data sets from ships, buoys, and aircrafts are complemented with an ozonesonde data set from 29 launch sites or field campaigns, and by 21 non-polar and 17 polar ground-based stations data sets. The data were filtered by using backward trajectories calculated with the HYSPLIT model from the individual observation points to extract essentially oceanic observations, defined as air masses that have travelled over oceans for 72 hours or more, which were further tested with the coincident Radon observations. The oceanic and polar data thus selected showed typically flat diurnal patterns at high latitudes and daytime decreases (11–16 %) at low latitudes, indicating the adequacy of the data collection and processing procedures, as well as the potential for further studies of processes with statistical robustness and coverage. The ship/buoy- and aircraft-based data sets presented here will supplement the land-based ones in the TOAR-II database to provide a fully global assessment of tropospheric ozone.

Volatile organic compounds (VOCs) are important atmospheric components that contribute to air pollution, but their accurate quantification by proton-transfer-reaction mass spectrometry (PTR-MS) remains challenging. In this work, we coupled a gas chromatograph (GC) prior to PTR-MS and analyzed complex ambient air in urban Shanghai to speciate the PTR signal to identify the VOC species that were responsible for the generation of the ions detected by PTR. We classified 176 individual PTR signals with associated compounds resolved by the GC based on whether they could be used to quantify a VOC species without pre-separation. In this classification, Category I includes 45 decent signal ions that were produced from a single VOC species and thus can be used for reliable quantification, although some of the Category I ions are not the conventionally used protonated quasi-molecular ions (MH+). Category II includes 39 signal ions that were produced from a group of isomers and can be used to quantify the isomeric sum, but with an increased uncertainty if a single calibration factor for one specific isomer is used to represent all structures. Category III includes 92 signal ions that were generated from more than one non-isomeric species (e.g., through protonation, fragmentation, cluster formation) and thus merely gave an upper limit of VOC concentrations. In addition, we propose taking aromatic compounds, for instance, the quantification of selected VOCs, utilizing either non-MH+ or non-Category I ions. Our results help to achieve more comprehensive species identification and reliable VOC quantification in PTR measurements.

Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is a technique commonly used to measure ambient volatile organic compounds (VOCs) in urban, rural, and remote environments. PTR-ToF-MS is known to produce artifacts from ion fragmentation, which complicates the interpretation and quantification of key atmospheric VOCs. This study evaluates the extent to which fragmentation and other ionization processes impact urban measurements of the PTR-ToF-MS ions typically assigned to isoprene (m/z 69, C5H8H+), acetaldehyde (m/z 45, CH3CHO+), and benzene (m/z 79, C6H6H+). Interferences from fragmentation are identified using gas chromatography (GC) pre-separation, and the impact of these interferences is quantified using ground-based and airborne measurements in a number of US cities, including Las Vegas, Los Angeles, New York City, and Detroit. In urban regions with low biogenic isoprene emissions (e.g., Las Vegas), fragmentation from higher-carbon aldehydes and cycloalkanes emitted from anthropogenic sources may contribute to m/z 69 by as much as 50 % during the day, while the majority of the signal at m/z 69 is attributed to fragmentation during the night. Interferences are a higher fraction of m/z 69 during airborne studies, which likely results from differences in the reactivity between isoprene and the interfering species along with the subsequent changes to the VOC mixture at higher altitudes. For other PTR masses, including m/z 45 and m/z 79, interferences are observed due to fragmentation and O2+ ionization of VOCs typically used in solvents, which are becoming a more important source of anthropogenic VOCs in urban areas. We present methods to correct these interferences, which provide better agreement with GC measurements of isomer-specific molecules. These observations show the utility of deploying GC pre-separation for the interpretation PTR-ToF-MS spectra.

Dry deposition of ozone (O3) to the ocean surface and the ozonolysis of organics in the sea surface microlayer (SSML) are potential sources of volatile organic compounds (VOCs) to the marine atmosphere. We use a gas chromatography system coupled to a Vocus proton-transfer-reaction time-of-flight mass spectrometer to determine the chemical composition and product yield of select VOCs formed from ozonolysis of coastal seawater collected from Scripps Pier in La Jolla, California. Laboratory-derived results are interpreted in the context of direct VOC vertical flux measurements made at Scripps Pier. The dominant products of laboratory ozonolysis experiments and the largest non-sulfur emission fluxes measured in the field correspond to Vocus CxHy+ and CxHyOz+ ions. Gas chromatography (GC) analysis suggests that C5–C11 oxygenated VOCs, primarily aldehydes, are the largest contributors to these ion signals. In the laboratory, using a flow reactor experiment, we determine a VOC yield of 0.43–0.62. In the field at Scripps Pier, we determine a maximum VOC yield of 0.04–0.06. Scaling the field and lab VOC yields for an average O3 deposition flux and an average VOC structure results in an emission source of 10.7 to 167 Tg C yr−1, competitive with the DMS source of approximately 20.3 Tg C yr−1. This study reveals that O3 reactivity to dissolved organic carbon can be a significant carbon source to the marine atmosphere and warrants further investigation into the speciated VOC composition from different seawater samples and the reactivities and secondary organic aerosol (SOA) yields of these molecules in marine-relevant, low NOx conditions.

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.

The exchange of trace gases between the biosphere and the atmosphere is an important process that controls both chemical and physical properties of the atmosphere with implications for air quality and climate change. The terrestrial biosphere is a major source of reactive biogenic volatile organic compounds (BVOCs) that govern atmospheric concentrations of the hydroxy radical (OH) and ozone (O3) and control the formation and growth of secondary organic aerosol (SOA). Common simulations of BVOC surface–atmosphere exchange in chemical transport models use parameterizations derived from the growing season and do not consider potential changes in emissions during seasonal transitions. Here, we use observations of BVOCs over a mixed temperate forest in northern Wisconsin during broadleaf senescence to better understand the effects of the seasonal changes in canopy conditions (e.g., temperature, sunlight, leaf area, and leaf stage) on net BVOC exchange. The BVOCs investigated here include the terpenoids isoprene (C5H8), monoterpenes (MTs; C10H16), a monoterpene oxide (C10H16O), and sesquiterpenes (SQTs; C15H24), as well as a subset of other monoterpene oxides and dimethyl sulfide (DMS). During this period, MTs were primarily composed of α-pinene, β-pinene, and camphene, with α-pinene and camphene dominant during the first half of September and β-pinene thereafter. We observed enhanced MT and monoterpene oxide emissions following the onset of leaf senescence and suggest that senescence has the potential to be a significant control on late-season MT emissions in this ecosystem. We show that common parameterizations of BVOC emissions cannot reproduce the fluxes of MT, C10H16O, and SQT during the onset and continuation of senescence but can correctly simulate isoprene flux. We also describe the impact of the MT emission enhancement on the potential to form highly oxygenated organic molecules (HOMs). The calculated production rates of HOMs and H2SO4, constrained by terpene and DMS concentrations, suggest that biogenic aerosol formation and growth in this region should be dominated by secondary organics rather than sulfate. Further, we show that models using parameterized MT emissions likely underestimate HOM production, and thus aerosol growth and formation, during early autumn in this region. Further measurements of forest–atmosphere BVOC exchange during seasonal transitions as well as measurements of DMS in temperate regions are needed to effectively predict the effects of canopy changes on reactive carbon cycling and aerosol production.

Industrial emissions of trace gases and VOCs can be an important contributor to air quality in cities. Disentangling different point sources from each other and characterizing their emissions can be particularly challenging in dense industrial areas, such as Detroit, Dearborn and surrounding areas in Southeast Michigan (SEMI). Here, we leverage mobile measurements of trace gases and speciated volatile organic compounds (VOCs) to identify emitting sites. We characterize their complicated emissions fingerprints based on a core set of chemical ratios. We report chemical ratios for 7 source types including automakers, steel manufacturers, chemical refineries, industrial chemical use (cleaning; coatings; etc.), chemical waste sites, compressor stations, and more. The source dataset includes visits to over 85 distinct point sources. As expected, we find similarities between the different types of facilities, but observe variability between them and even at individual facilities day-to-day. Certain larger sites are better thought of as a collection of individual point sources. These results demonstrate the power of mobile laboratories over stationary sampling in dense industrial areas.

Ethylene oxide (EtO) is a human carcinogen whose release from sterilization facilities to ambient air has gained recent attention. Measurements have typically relied on canister samples collected over minutes to hours and analyzed using gas chromatography/mass spectrometry (GC/MS). A novel application of tunable infrared laser direct absorption spectrometry (TILDAS) was recently deployed aboard a mobile air quality laboratory in Toronto, Canada, to measure EtO in near real-time. Detection limits comparable to canister-GC/MS methods were achieved while stationary at averaging times of only 100 s, and high frequency (1-s) sampling detected EtO at several locations. EtO was observed consistently near the only local facility reporting on-site releases to Canada's National Pollutant Release Inventory. The maximum 1-s mixing ratio observed nearby was 18 ppb (33 & mu;g m  3), and a mean 1-s mixing ratio of 0.43 ppb (0.78 & mu;g m  3) was observed in the vicinity of the detectable plumes that covered industrial, commercial and residential land uses up to 900 m downwind. EtO in those plumes was not related to common air pollutants such as nitric oxide (NO), carbon monoxide (CO), or methane (CH4), whereas it was paired with elevated NO or CO and CH4 at some other locations. EtO was less abundant elsewhere in the city, and no detectable EtO was found near potential sources including hospitals, spice distributors and vehicle exhaust. The EtO background measured in wintertime Toronto air was indistinguishable from zero and substantially lower than that reported in studies using canister-GC/MS methods. The multi-pollutant mobile method described herein marks a significant step forward in the capability to characterize atmospherically relevant concentrations of EtO at high spatiotemporal resolution. Future applications of high-resolution techniques will allow for comprehensive investigations of source impacts and characterization of ambient levels for which deployment of conventional canister/GC-MS methods would be unsuitable or prohibitive.

Ambient ozone (O 3 ) concentrations in Southeast Michigan (SEMI) can exceed the U.S. National Ambient Air Quality Standard. Despite past efforts to measure O 3 precursors and elucidate reaction mechanisms, changing emission patterns and atmospheric composition in SEMI warrant new measurements and updated mechanisms to understand the causes of observed O 3 exceedances. In this study, we examine the chemical drivers of O 3 exceedances in SEMI, based on the Phase I MOOSE (Michigan‐Ontario Ozone Source Experiment) field study performed during May to June 2021. A zero‐dimensional (0‐D) box model is constrained with measurement data of meteorology and trace gas concentrations. Box model sensitivity simulations suggest that the formaldehyde to nitrogen dioxide ratio (HCHO/NO 2 ) for the transition between the volatile organic compounds (VOCs)‐ and nitrogen oxides (NO x )‐limited O 3 production regimes is 3.0 ± 0.3 in SEMI. The midday (12:00–16:00) averaged HCHO/NO 2 ratio during the MOOSE Phase I study is 1.62 ± 1.03, suggesting that O 3 production in SEMI is limited by VOC emissions. This finding implies that imposing stricter regulations on VOC emissions should be prioritized for the SEMI O 3 nonattainment area. This study, through its use of ground‐based HCHO/NO 2 ratios and box modeling to assess O 3 ‐VOC‐NO x sensitivities, has significant implications for air quality policy and the design of effective O 3 pollution control strategies, especially in O 3 nonattainment areas.

We analyzed observational and model data to study the sources of formaldehyde over oil and gas production regions and to investigate how these observations may be used to constrain oil and gas volatile organic compound (VOC) emissions. The analysis of aircraft and satellite data consistently found that formaldehyde over oil and gas production regions during spring and summer is mostly formed by the photooxidation of precursor VOCs. Formaldehyde columns over the Permian Basin, one of the largest oil- and gas-producing regions in the United States, are correlated with the production locations. Formaldehyde simulations by the atmospheric chemistry and transport model WRF-Chem, which included oil and gas NOx and VOC emissions from the fuel-based oil and gas inventory, were in very good agreement with TROPOMI satellite measurements. Sensitivity studies illustrated that VOCs released from oil and gas activities are important precursors to formaldehyde, but other sources of VOCs contribute as well and that the formation of secondary formaldehyde is highly sensitive to NOx . We also investigated the ability of the chemical mechanism used in WRF-Chem to represent formaldehyde formation from oil and gas hydrocarbons by comparing against the Master Chemical Mechanism. Further, our work provides estimates of primary formaldehyde emissions from oil and gas production activities, with per basin averages ranging from 0.07 to 2.2 kg h(-1) in 2018. A separate estimate for natural gas flaring found that flaring emissions could contribute 5 to 12% to the total primary formaldehyde emissions for the Permian Basin in 2018.

Abstract. Environmental samples typically contain hundreds or thousands of unique organic compounds, and even minor components may provide valuable insight into their sources and transformations. To understand atmospheric processes, individual components are frequently identified and quantified using gas chromatography/mass spectrometry. However, due to the complexity and frequently variable nature of such data, data reduction is a significant bottleneck in analysis. Consequently, only a subset of known analytes is often reported for a dataset, and a large amount of potentially useful data are discarded. We present here an automated approach of cataloging and potentially identifying all analytes in a large chromatographic dataset and demonstrate the utility of our approach in an analysis of ambient aerosols. We use a coupled factor analysis/decision tree approach to de-convolute peaks and comprehensively catalog nearly all analytes in a dataset. Positive Matrix Factorization (PMF) of small sub-sections of multiple chromatograms is applied to extract factors that represent chromatographic profiles and mass spectra of potential analytes, in which peaks are detected. A decision tree based on peak shape, noise, retention time, and mass spectrum is applied to discard erroneous peaks and combine peaks determined to represent the same analyte. With our approach, all analytes within the small section of the chromatogram are cataloged, and the process is repeated for overlapping sections across the chromatogram, generating a complete list of the retention times and estimated mass spectra of all peaks in a dataset. We validate this approach using samples of known compounds and demonstrate the separation of poorly resolved peaks with similar mass spectra and the resolution of peaks that appear in only a fraction of chromatograms. As a case study, this method is applied to a complex real-world dataset of the composition of atmospheric particles, in which more than 1100 analytes are resolved.

Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by their elemental formula. Unfortunately, calibration of these instruments is challenging due to a lack of commercially available standards for many compounds, which has led to the development of methods for estimating CIMS sensitivity. By coupling a thermal desorption aerosol gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we use the individual particle-phase analytes, quantified by the FID, to examine the sensitivity of the CIMS and its variability between isomers of the same elemental formula. Iodide CIMS sensitivities of isomers within a formula are found to generally vary by 1 order of magnitude with a maximum deviation of 2 orders of magnitude. Furthermore, we compare directly measured sensitivity to a method of estimating sensitivity based on declustering voltage (i.e., “voltage scanning”). This approach is found to carry high uncertainties for individual analytes (0.5 to 1 order of magnitude) but represents a central tendency that can be used to estimate the sum of analytes with reasonable error (∼30 % differences between predicted and measured moles). Finally, gas chromatography (GC) retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlate with iodide CIMS sensitivity, but the relationship is not close enough to be quantitatively useful and could be explored further in the future as a potential calibration approach.

Atmospheric oxidation products of volatile organic compounds consist of thousands of unique chemicals that have distinctly different physical and chemical properties depending on their detailed structures and functional groups. Measurement techniques that can achieve molecular characterizations with details down to functional groups (i.e., isomer-resolved resolution) are consequently necessary to provide understandings of differences of fate and transport within isomers produced in the oxidation process. We demonstrate a new instrument coupling the thermal desorption aerosol gas chromatograph (TAG), which enables the separation of isomers, with the high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), which has the capability of classifying unknown compounds by their molecular formulas, and the flame ionization detector (FID), which provides a near-universal response to organic compounds. The TAG-CIMS/FID is used to provide isomer-resolved measurements of samples from liquid standard injections and particle-phase organics generated in oxidation flow reactors. By coupling a TAG to a CIMS, the CIMS is enhanced with an additional dimension of information (resolution of individual molecules) at the cost of time resolution (i.e., one sample per hour instead of per minute). We found that isomers are prevalent in sample matrix with an average number of three to five isomers per formula depending on the precursors in the oxidation experiments. Additionally, a multi-reagent ionization mode is investigated in which both zero air and iodide are introduced as reagent ions, to examine the feasibility of extending the use of an individual CIMS to a broader range of analytes with still selective reagent ions. While this approach reduces iodide-adduct ions by a factor of 2, [M − H]− and [M + O2]− ions produced from lower-polarity compounds increase by a factor of 5 to 10, improving their detection by CIMS. The method expands the range of detected chemical species by using two chemical ionization reagents simultaneously, which is enabled by the pre-separation of analyte molecules before ionization.