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We have analyzed Sentinel‐5 Precursor TROPOspheric Monitoring Instrument (TROPOMI) data over the Copperbelt mining region (Democratic Republic of Congo and Zambia). Despite high background values, annual 2019–2022 means of TROPOMI NO2 (nitrogen dioxide) show local enhancements consistent with six point sources (four copper/cobalt mines, two cities) where high‐emission industrial activities take place. We have quantified annual NOx (nitrogen oxides) emissions from these point sources, identified temporal trends in emissions, and found strong correlations with production data from colocated mines and one oil refinery. The Copernicus Atmosphere Monitoring Service Global Anthropogenic (CAMS‐GLOB‐ANT) version 5 inventory underpredicts TROPOMI‐derived emissions and lacks the temporal trends observed in TROPOMI and mine/refinery production. These results demonstrate the potential for satellite monitoring of mining and other industrial activities, often unreported or underestimated, which impact the air quality of local communities. This is particularly important for Africa, where mining is increasing aggressively. Plain Language Summary: We show for the first time that annual NOx gas pollution emitted by individual copper/cobalt mines can be measured with TROPOMI satellite data, even in the presence of high background pollution from biomass burning and other sources. This is important for monitoring the air quality of local communities, particularly when these industrial activities proliferate in close proximity to population centers (as is the case in the Copperbelt mining region and in other African regions) and without sufficient ground measurements of air pollution levels. Additionally, we show for the first time that the annual amount of NOx pollution emitted by these single point sources is strongly correlated with annual production from individual, colocated copper/cobalt mines and one oil refinery. Studies like this can be used to estimate mine/oil refinery production before companies release their annual reports or (for non‐publicly traded companies) in the absence of such reports. Insufficient emissions from mines claiming high production could indicate production from a different source. Joint analysis of satellite‐derived emissions and mine production reports could be useful in improving the traceability of minerals extracted in conflict areas. Key Points: We quantified annual TROPOspheric Monitoring Instrument (TROPOMI)‐derived NOx emissions from point sources corresponding to copper/cobalt mines, despite high background valuesAnnual emissions from individual point sources are strongly correlated with annual production from colocated single mines, one oil refineryTROPOMI is relevant to monitoring air quality and mining/industrial production in remote regions where these activities are growing rapidly [ABSTRACT FROM AUTHOR]

Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution. Plain Language Summary: Particulate matter (PM) is dangerous to human health and impacts visibility and climate. In the United States, Europe, and Asia, PM is severe in urban areas in the winter when ammonium nitrate, NH4NO3, comprises an appreciable fraction of the total PM mass. A key control strategy is to reduce emissions of the limiting reagent. Using measurements from a recent field campaign in the Salt Lake Valley, Utah, which experiences high PM levels in winter, we demonstrate that emission control strategies can be evaluated using the same framework commonly used to control ozone, another common pollutant that occurs at high levels in urban areas in the summer. We show that initial control of the NOx precursor is ineffective at reducing NH4NO3 aerosol in the Salt Lake Valley, while initial control of volatile organic compounds, which are not a direct precursor for either nitrate or ammonium, is effective due to their influence on oxidation cycles. This finding differs from many mitigation strategies in the western United States and may also be relevant to other regions in Europe and Asia which experience high wintertime PM. Key Points: Wintertime ammonium nitrate aerosol pollution is closely tied to photochemical ozone production through a common parameter, Ox,totalBox modeling reveals ammonium nitrate formation in the Salt Lake Valley is nitrate‐limited but NOx‐saturatedMitigation strategies that focus on NOx control in some wintertime‐polluted layers may initially increase ammonium nitrate [ABSTRACT FROM AUTHOR]

The richness of measurements obtained by Proton-Transfer Reactions Mass Spectrometry (PTR-MS) has opened a new paradigm for the quantification of volatile organic compounds (VOCs). A wide range of compounds can be monitored, however, each detected signal is subject to a compound assignment instead of actual identification because PTR techniques are mass-selective and isomers cannot be separately measured. Thus, rapid development in the field requests continued community efforts to identify compounds. In this study we have reviewed the available literature and created a master compound assignment guide called GLOVOCS that can be referred to by PTR-MS practitioners. GLOVOCS is aimed to help in advancing science of VOCs by facilitating the research of multiple groups using PTR-MS to monitor VOCs and to disentangle the physical, chemical and biological mechanisms underlying their production, emission and impact on environment and organisms from bacteria to humans. The guide is freely accessible at http://glovocs.creaf.cat as a collaborative tool, where users can both consult and contribute to the identification of VOCs by providing possible candidates for all chemical formulas from 18 to 330 atomic mass units. When available, we indicate if there is evidence for biogenic or anthropogenic VOC origin, as well as grouping the compounds based on the Classyfire chemotaxonomic classification (Djoumbou Feunang et al., 2016). While GLOVOCS aims to facilitate the first assessment and consistent classification of compounds, we highly recommend further cross-validation for verifying compounds when using PTR-MS techniques. • We have created GLOVOCS,a master compound assignment guide that can be referred to by PTR-MS practitioners. • GLOVOCS is aimed to help in advancing science of VOCs by facilitating the research of multiple groups using PTR-MS. • GLOVOCS, accessible at http://glovocs.creaf.cat , is a collaborative tool where users can both consult and contribute to. [ABSTRACT FROM AUTHOR]

Oxygenated volatile organic compounds (OVOCs) significantly modulate atmospheric chemistry, but the sources and air quality impacts of OVOCs in aged urban outflows remain to be elucidated. At a background site in South China, the ozone formation potential of six nonformaldehyde OVOCs studied was equivalent to that of 3.56 ppbv of formaldehyde, more than half of which was contributed by acetaldehyde. Source apportionment incorporating photochemical age revealed that considerable fractions (52.7%-62.6%) of the OVOCs were of biogenic origin, except for ethanol, which was primarily derived from anthropogenic emissions. The oxidation of cis -/ trans -2-butene explained 71.1% of the in situ acetaldehyde formation. In contrast, α/β-pinenes and isoprene contributed 73.8% and 28.4% to acetone and methylglyoxal formation, respectively. An average of 12.4% of net in situ ozone (O 3 ) production rate was attributed to the OVOCs studied, where the biogenic fractions accounted for 59%. The changes in the O 3 production rate and hydroxyl radical (OH) concentration caused by OVOCs were mainly affected by ozone formation sensitivity. The effects of primary acetaldehyde and acetaldehyde-led O 3 on secondary acetaldehyde formation were weak at this background site; however, they cannot be ignored in polluted areas. This study provides a scientific basis for mitigating O 3 pollution driven by biogenic emissions and OVOCs.

Source attribution of volatile organic compounds (VOCs) can be challenging in urban areas, which have many point sources. Mobile laboratories using time-of-flight mass spectrometers (TOF-MS) can take measurements throughout areas of concern, resulting in data with high spatial resolution that can be used to more easily identify these sources. However, emissions in heavily polluted areas still undergo significant mixing over short distances, making source attribution of some compounds challenging. Positive matrix factorization (PMF) has been widely used for attributing pollutants to different sources when taking stationary measurements due to its ability to process large amounts of data into generally interpretable results. However, some limitations of PMF can impact its usefulness to mobile data; PMF is a computationally intensive process, requires some user choices in attributing factors to emissions sources, and results can be significantly impacted by chemical transformations after emission. Here, both PMF and a simpler comparative analysis method are evaluated in analyzing measurements taken in the Elyria Swansea neighborhood of Commerce City, CO. This neighborhood is located near an oil refinery, a wastewater treatment plant, local industrial shops, and major highways. PMF failed to differentiate between oil refinery emissions and traffic emissions, and had difficulties recognizing other key sources. A simpler comparative analysis showed that the refinery contributed significantly to VOC concentrations throughout the neighborhood, including air toxics such as benzene. A wastewater treatment plant contributed to methanethiol and dimethyl sulfide. Finally, a small woodshop was identified as a hyperlocal VOC source, and contributed high amounts of some VOCs, such as toluene and other solvents, in its immediate surroundings. Implications : This work discusses mobile measurements of VOCs around Commerce City, CO, a heavily polluted urban area north of Denver, using a PTR-TOF-MS. Two different source attribution methods, positive matrix factorization (PMF) and comparative analysis, were evaluated in the context of mobile measurements. The results show that an oil refinery and a woodshop contributed greatly to many VOC concentrations in the Elyria Swansea residential area of Commerce City. Additional sources, such as a wastewater treatment plant, also contributed to some odorous VOCs. PMF was unable to fully describe sources based on the mobile data. Comparative analysis was useful in attributing more VOCs to different sources, but quantitative results were influenced by how the analysis is set up. These findings are relevant to the residents of Denver and regulatory bodies to better understand Denver air pollution, as well as to other mobile studies doing source attribution of VOCs.

Emissions of methane (CH 4 ) in the Permian basin (USA) have been derived for 2019 and 2020 from satellite observations of the Tropospheric Monitoring Instrument (TROPOMI) using the divergence method, in combination with a data driven method to estimate the background column densities. The resulting CH 4 emission data, which have been verified using model data with known emissions, have a spatial resolution of approximately 10 km. The CH 4 emissions show moderate spatial correlation with the locations of oil and gas production and drilling activities in the Permian basin, as well as with emissions of nitrogen oxides (NO x ). Analysis of the emission maps and time series indicates that a significant fraction of methane emissions in the Permian basin is from frequent widespread emissions sources, rather than from a few infrequent very large unplanned releases, which is important considering possible CH 4 emission mitigation strategies. In addition to providing spatially resolved emissions, the divergence method also provides the total emissions of the Permian basin and its main sub-basins. The total CH 4 emission of the Permian is estimated as 3.0 ± 0.7 Tg yr -1 for 2019, which agrees with other independent estimates based on TROPOMI data. For the Delaware sub-basin, it is estimated as 1.4 ± 0.3 Tg yr -1 for 2019, and for the Midland sub-basin 1.2 ± 0.3 Tg yr -1 . In 2020 the emissions are 9% lower compared to 2019 in the entire Permian basin, and respectively 19% and 27% for the Delaware and Midland sub-basins., (© 2023. The Authors.)

The COVID-19 pandemic has posed a challenge for maintaining an engaging learning environment while using remote laboratory formats. In this work, we describe a Student Choice Project (SCP) in an undergraduate instrumental analysis course that was adapted for remote learning without sacrificing research-based learning goals. We discuss the implementation and assessment of this SCP, selected student results, and student feedback. Students were provided handheld carbon dioxide monitors and charged with designing and implementing an investigation centered on COVID-19 airborne transmission. The real-time monitors provided experience with a new analytical tool that demanded considerations and analysis not common to other methods discussed in the course. Students were motivated by the ability to design their own projects and by the real-world implications of their findings. They performed well for all assessments, reported a positive experience, and recommended these monitors be added to the typical repertoire of instrumentation for the course., Competing Interests: The authors declare no competing financial interest., (© 2022 American Chemical Society and Division of Chemical Education, Inc.)

The COVID-19 global pandemic and associated government lockdowns dramatically altered human activity, providing a window into how changes in individual behavior, enacted en masse, impact atmospheric composition. The resulting reductions in anthropogenic activity represent an unprecedented event that yields a glimpse into a future where emissions to the atmosphere are reduced. Furthermore, the abrupt reduction in emissions during the lockdown periods led to clearly observable changes in atmospheric composition, which provide direct insight into feedbacks between the Earth system and human activity. While air pollutants and greenhouse gases share many common anthropogenic sources, there is a sharp difference in the response of their atmospheric concentrations to COVID-19 emissions changes, due in large part to their different lifetimes. Here, we discuss several key takeaways from modeling and observational studies. First, despite dramatic declines in mobility and associated vehicular emissions, the atmospheric growth rates of greenhouse gases were not slowed, in part due to decreased ocean uptake of CO 2 and a likely increase in CH 4 lifetime from reduced NO x emissions. Second, the response of O 3 to decreased NO x emissions showed significant spatial and temporal variability, due to differing chemical regimes around the world. Finally, the overall response of atmospheric composition to emissions changes is heavily modulated by factors including carbon-cycle feedbacks to CH 4 and CO 2 , background pollutant levels, the timing and location of emissions changes, and climate feedbacks on air quality, such as wildfires and the ozone climate penalty., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

The COVID-19 outbreak in 2020 prompted strict lockdowns, reduced human activity, and reduced emissions of air pollutants. We measured volatile organic compounds (VOCs) using a proton-transfer-reaction mass spectrometry instrument in Changzhou, China from 8 January through 27 March, including periods of pre-lockdown, strict measures (level 1), and more relaxed measures (level 2). We analyze the data using positive matrix factorization and resolve four factors: textile industrial emissions (62 ± 10% average reduction during level 1 relative to pre-lockdown), pharmaceutical industrial emissions (40 ± 20%), traffic emissions (71 ± 10%), and secondary chemistry (20 ± 20%). The two industrial sources showed different responses to the lockdown, so emissions from the industrial sector should not be scaled uniformly. The quantified changes in VOCs due to the lockdowns constrain emission inventories and inform chemistry-transport models, particularly for sectors where activity data are sparse, as the effects of lockdowns on air quality are explored., (© 2021. American Geophysical Union. All Rights Reserved.)

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models.

Despite the central role of reactive organic carbon (ROC) in the formation of secondary species that impact global air quality and climate, our assessment of ROC abundance and impacts is challenged by the diversity of species that contribute to it. We revisit measurements of ROC species made during two field campaigns in the United States: the 2013 SOAS campaign in forested Centreville, AL, and the 2010 CalNex campaign in urban Pasadena, CA. We find that average measured ROC concentrations are about twice as high in Pasadena (73.8 μgCsm -3 ) than in Centreville (36.5 μgCsm -3 ). However, the OH reactivity (OHR) measured at these sites is similar (20.1 and 19.3 s -1 ). The shortfall in OHR when summing up measured contributions is 31%, at Pasadena and 14% at Centreville, suggesting that there may be a larger reservoir of unmeasured ROC at the former site. Estimated O 3 production and SOA potential (defined as concentration × yield) are both higher during CalNex than SOAS. This analysis suggests that the ROC in urban California is less reactive, but due to higher concentrations of oxides of nitrogen and hydroxyl radicals, is more efficient in terms of O 3 and SOA production, than in the forested southeastern U.S.

Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH <20%) environmental chamber experiments. The modeling is supported by (i) new oxidation chamber measurements, (ii) detailed concurrent measurements of SOA precursors in biomass-burning emissions, and (iii) development of SOA parameters for heterocyclic and oxygenated aromatic compounds based on historical chamber experiments. We find that oxygenated aromatic compounds, including phenols and methoxyphenols, account for slightly less than 60% of the SOA formed and help our model explain the variability in the organic aerosol mass ( R 2 = 0.68) and O/C ( R 2 = 0.69) enhancement ratios observed across 11 chamber experiments. Despite abundant emissions, heterocyclic compounds that included furans contribute to ∼20% of the total SOA. The use of pyrolysis-temperature-based or averaged emission profiles to represent SOA precursors, rather than those specific to each fire, provide similar results to within 20%. Our findings demonstrate the necessity of accounting for oxygenated aromatics from biomass-burning emissions and their SOA formation in chemical mechanisms.

In 2017, western Canadian wildfires injected smoke into the stratosphere that was detectable by satellites for more than 8 months. The smoke plume rose from 12 to 23 kilometers within 2 months owing to solar heating of black carbon, extending the lifetime and latitudinal spread. Comparisons of model simulations to the rate of observed lofting indicate that 2% of the smoke mass was black carbon. The observed smoke lifetime in the stratosphere was 40% shorter than calculated with a standard model that does not consider photochemical loss of organic carbon. Photochemistry is represented by using an empirical ozone-organics reaction probability that matches the observed smoke decay. The observed rapid plume rise, latitudinal spread, and photochemical reactions provide new insights into potential global climate impacts from nuclear war., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

We have collected data on the proton-transfer reactions with H 3 O + ions for trace gas detection into an online and publicly available library. The library allows users of proton-transfer-reaction mass spectrometry (PTR-MS) and selected-ion flow-tube mass spectrometry (SIFT-MS) to look up at which m/z a trace gas of interest is detected. Vice versa, the library also allows looking up what trace gas may have been responsible for a product ion detected in PTR-MS and SIFT-MS. Finally, the library may serve as a dataset for further research on calculating instrument sensitivity and product-ion fragmentation, improving identification and quantification of newly detectable compounds as advances in instrumentation continue. To demonstrate the utility of the library, we present a brief analysis of product-ion fragmentation. We show that oxygenated organic compounds exhibit trends in neutral loss according to their functionality, and that on average neutral losses decrease the carbon number and increase the extent of unsaturation of product ions. Graphical Abstract.

A 6-week study was conducted at the University of Colorado Art Museum, during which volatile organic compounds (VOCs), carbon dioxide (CO 2 ), ozone (O 3 ), nitric oxide (NO), nitrogen dioxide (NO 2 ), other trace gases, and submicron aerosol were measured continuously. These measurements were then analyzed using a box model to quantify the rates of major processes that transformed the composition of the air. VOC emission factors were quantified for museum occupants and their activities. The deposition of VOCs to surfaces was quantified across a range of VOC saturation vapor concentrations ( C*) and Henry's Law constants ( H) and determined to be a major sink for VOCs with C* < 10 8 μg m -3 and H > 10 2 M atm -1 . The reaction rates of VOCs with O 3 , OH radicals, and nitrate (NO 3 ) radicals were quantified, with unsaturated and saturated VOCs having oxidation lifetimes of >5 and >15 h, making deposition to surfaces and ventilation the dominant VOC sinks in the museum. O 3 loss rates were quantified inside a museum gallery, where reactions with surfaces, NO, occupants, and NO 2 accounted for 62%, 31%, 5%, and 2% of the O 3 sink. The measured concentrations of acetic acid, formic acid, NO 2 , O 3 , particulate matter, sulfur dioxide, and total VOCs were below the guidelines for museums.

Biomass burning (BB) is a large source of reactive compounds in the atmosphere. While the daytime photochemistry of BB emissions has been studied in some detail, there has been little focus on nighttime reactions despite the potential for substantial oxidative and heterogeneous chemistry. Here, we present the first analysis of nighttime aircraft intercepts of agricultural BB plumes using observations from the NOAA WP-3D aircraft during the 2013 Southeast Nexus (SENEX) campaign. We use these observations in conjunction with detailed chemical box modeling to investigate the formation and fate of oxidants (NO 3 , N 2 O 5 , O 3 , and OH) and BB volatile organic compounds (BBVOCs), using emissions representative of agricultural burns (rice straw) and western wildfires (ponderosa pine). Field observations suggest NO 3 production was approximately 1 ppbv hr -1 , while NO 3 and N 2 O 5 were at or below 3 pptv, indicating rapid NO 3 /N 2 O 5 reactivity. Model analysis shows that >99% of NO 3 /N 2 O 5 loss is due to BBVOC + NO 3 reactions rather than aerosol uptake of N 2 O 5 . Nighttime BBVOC oxidation for rice straw and ponderosa pine fires is dominated by NO 3 (72, 53%, respectively) but O 3 oxidation is significant (25, 43%), leading to roughly 55% overnight depletion of the most reactive BBVOCs and NO 2 .

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 × 10 4 μg m -3 and compare favorably with previously published results for a PTR-MS instrument.

The chemical complexity of atmospheric organic aerosol (OA) has caused substantial uncertainties in understanding its origins and environmental impacts. Here, we provide constraints on OA origins through compositional characterization with molecular-level details. Our results suggest that secondary OA (SOA) from monoterpene oxidation accounts for approximately half of summertime fine OA in Centreville, AL, a forested area in the southeastern United States influenced by anthropogenic pollution. We find that different chemical processes involving nitrogen oxides, during days and nights, play a central role in determining the mass of monoterpene SOA produced. These findings elucidate the strong anthropogenic-biogenic interaction affecting ambient aerosol in the southeastern United States and point out the importance of reducing anthropogenic emissions, especially under a changing climate, where biogenic emissions will likely keep increasing., Competing Interests: The authors declare no conflict of interest.

Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations, but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods, during which collocated measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013 provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales. This paper is aimed at discussing progress in evaluating, diagnosing and improving air quality and climate modeling using comparisons to SAS observations as a guide to thinking about improvements to mechanisms and parameterizations in models. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosol (SOA) and other trace species in the troposphere, with the ultimate goal of understanding the radiative impacts of these species in the southeast and elsewhere. Here we address questions surrounding four key themes: gas-phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. We expect this review to serve as a guidance for future modeling efforts., Competing Interests: Competing interests. The authors declare that they have no conflict of interest.

Carbonaceous particulate matter (PM), comprising black carbon (BC), primary organic aerosol (POA) and secondary organic aerosol (SOA, from atmospheric aging of precursors), is a highly toxic vehicle exhaust component. Therefore, understanding vehicle pollution requires knowledge of both primary emissions, and how these emissions age in the atmosphere. We provide a systematic examination of carbonaceous PM emissions and parameterisation of SOA formation from modern diesel and gasoline cars at different temperatures (22, -7 °C) during controlled laboratory experiments. Carbonaceous PM emission and SOA formation is markedly higher from gasoline than diesel particle filter (DPF) and catalyst-equipped diesel cars, more so at -7 °C, contrasting with nitrogen oxides (NO X ). Higher SOA formation from gasoline cars and primary emission reductions for diesels implies gasoline cars will increasingly dominate vehicular total carbonaceous PM, though older non-DPF-equipped diesels will continue to dominate the primary fraction for some time. Supported by state-of-the-art source apportionment of ambient fossil fuel derived PM, our results show that whether gasoline or diesel cars are more polluting depends on the pollutant in question, i.e. that diesel cars are not necessarily worse polluters than gasoline cars.

Secondary organic aerosol (SOA) is formed from the atmospheric oxidation of gas-phase organic compounds leading to the formation of particle mass. Gasoline- and diesel-powered motor vehicles, both on/off-road, are important sources of SOA precursors. They emit complex mixtures of gas-phase organic compounds that vary in volatility and molecular structure-factors that influence their contributions to urban SOA. However, the relative importance of each vehicle type with respect to SOA formation remains unclear due to conflicting evidence from recent laboratory, field, and modeling studies. Both are likely important, with evolving contributions that vary with location and over short time scales. This review summarizes evidence, research needs, and discrepancies between top-down and bottom-up approaches used to estimate SOA from motor vehicles, focusing on inconsistencies between molecular-level understanding and regional observations. The effect of emission controls (e.g., exhaust aftertreatment technologies, fuel formulation) on SOA precursor emissions needs comprehensive evaluation, especially with international perspective given heterogeneity in regulations and technology penetration. Novel studies are needed to identify and quantify "missing" emissions that appear to contribute substantially to SOA production, especially in gasoline vehicles with the most advanced aftertreatment. Initial evidence suggests catalyzed diesel particulate filters greatly reduce emissions of SOA precursors along with primary aerosol.

National-scale emissions of carbon tetrachloride (CCl4) are derived based on inverse modeling of atmospheric observations at multiple sites across the United States from the National Oceanic and Atmospheric Administration's flask air sampling network. We estimate an annual average US emission of 4.0 (2.0-6.5) Gg CCl4 y(-1) during 2008-2012, which is almost two orders of magnitude larger than reported to the US Environmental Protection Agency (EPA) Toxics Release Inventory (TRI) (mean of 0.06 Gg y(-1)) but only 8% (3-22%) of global CCl4 emissions during these years. Emissive regions identified by the observations and consistently shown in all inversion results include the Gulf Coast states, the San Francisco Bay Area in California, and the Denver area in Colorado. Both the observation-derived emissions and the US EPA TRI identified Texas and Louisiana as the largest contributors, accounting for one- to two-thirds of the US national total CCl4 emission during 2008-2012. These results are qualitatively consistent with multiple aircraft and ship surveys conducted in earlier years, which suggested significant enhancements in atmospheric mole fractions measured near Houston and surrounding areas. Furthermore, the emission distribution derived for CCl4 throughout the United States is more consistent with the distribution of industrial activities included in the TRI than with the distribution of other potential CCl4 sources such as uncapped landfills or activities related to population density (e.g., use of chlorine-containing bleach).

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.

The chemical link between isoprene and formaldehyde (HCHO) is a strong, non-linear function of NO x (= NO + NO 2 ). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall photochemical mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the Southeast U.S., we quantify HCHO production across the urban-rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly-emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NO x values (roughly 0.1 - 2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv -1 ), while background HCHO increases by a factor of 2 (from 1.6 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D steady state box model. Both models reproduce the NO x dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models under-estimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or under-estimated hydroxyl radical concentrations. Detailed process rates from the box model simulation demonstrate a 3-fold increase in HCHO production across the range of observed NO x values, driven by a 100% increase in OH and a 40% increase in branching of organic peroxy radical reactions to produce HCHO.

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 NO 2 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.

Secondary organic aerosol (SOA) constitutes a substantial fraction of fine particulate matter and has important impacts on climate and human health. The extent to which human activities alter SOA formation from biogenic emissions in the atmosphere is largely undetermined. Here, we present direct observational evidence on the magnitude of anthropogenic influence on biogenic SOA formation based on comprehensive ambient measurements in the southeastern United States (US). Multiple high-time-resolution mass spectrometry organic aerosol measurements were made during different seasons at various locations, including urban and rural sites in the greater Atlanta area and Centreville in rural Alabama. Our results provide a quantitative understanding of the roles of anthropogenic SO2 and NOx in ambient SOA formation. We show that isoprene-derived SOA is directly mediated by the abundance of sulfate, instead of the particle water content and/or particle acidity as suggested by prior laboratory studies. Anthropogenic NOx is shown to enhance nighttime SOA formation via nitrate radical oxidation of monoterpenes, resulting in the formation of condensable organic nitrates. Together, anthropogenic sulfate and NOx can mediate 43-70% of total measured organic aerosol (29-49% of submicron particulate matter, PM1) in the southeastern US during summer. These measurements imply that future reduction in SO2 and NOx emissions can considerably reduce the SOA burden in the southeastern US. Updating current modeling frameworks with these observational constraints will also lead to more accurate treatment of aerosol formation for regions with substantial anthropogenic-biogenic interactions and consequently improve air quality and climate simulations.

Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F(MAE formation)). The strong temperature dependence of F(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m(-3)) and MAE-derived organosulfates (MAE-OS; ~1 ng m(-3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m(-3)) relative to MAE-OS (<0.0005 ng m(-3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10-100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.

Detailed airborne, surface, and subsurface chemical measurements, primarily obtained in May and June 2010, are used to quantify initial hydrocarbon compositions along different transport pathways (i.e., in deep subsurface plumes, in the initial surface slick, and in the atmosphere) during the Deepwater Horizon oil spill. Atmospheric measurements are consistent with a limited area of surfacing oil, with implications for leaked hydrocarbon mass transport and oil drop size distributions. The chemical data further suggest relatively little variation in leaking hydrocarbon composition over time. Although readily soluble hydrocarbons made up ∼25% of the leaking mixture by mass, subsurface chemical data show these compounds made up ∼69% of the deep plume mass; only ∼31% of the deep plume mass was initially transported in the form of trapped oil droplets. Mass flows along individual transport pathways are also derived from atmospheric and subsurface chemical data. Subsurface hydrocarbon composition, dissolved oxygen, and dispersant data are used to assess release of hydrocarbons from the leaking well. We use the chemical measurements to estimate that (7.8 ± 1.9) × 10(6) kg of hydrocarbons leaked on June 10, 2010, directly accounting for roughly three-quarters of the total leaked mass on that day. The average environmental release rate of (10.1 ± 2.0) × 10(6) kg/d derived using atmospheric and subsurface chemical data agrees within uncertainties with the official average leak rate of (10.2 ± 1.0) × 10(6) kg/d derived using physical and optical methods.

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

The magnitude and sources of chlorine atoms in marine air remain highly uncertain but have potentially important consequences for air quality in polluted coastal regions. We made continuous measurements of ambient ClNO(2) and Cl(2) concentrations from May 15 to June 8 aboard the Research Vessel Atlantis during the CalNex 2010 field study. In the Los Angeles region, ClNO(2) was more ubiquitous than Cl(2) during most nights of the study period. ClNO(2) and Cl(2) ranged from detection limits at midday to campaign maximum values at night reaching 2100 and 200 pptv, respectively. The maxima were observed in Santa Monica Bay when sampling the Los Angeles urban plume. Cl(2) at times appeared well correlated with ClNO(2), but at other times, there was little to no correlation implying distinct and varying sources. Well-confined Cl(2) plumes were observed, largely independent of ClNO(2), providing support for localized industrial emissions of reactive chlorine. Observations of ClNO(2), Cl(2), and HCl are used to constrain a simple box model that predicts their relative importance as chlorine atom sources in the polluted marine boundary layer. In contrast to the emphasis in previous studies, ClNO(2) and HCl are dominant primary chlorine atom sources for the Los Angeles basin.

Plantations of hybrid poplar are used in temperate regions to produce woody biomass for forestry-related industries and are likely to become more prevalent if they are used as a source of cellulose for second-generation biofuels. Species in the genus Populus are known to emit great quantities of the volatile organic compounds (VOCs) isoprene and methanol, and lesser quantities of terpene VOCs, giving poplar plantations the potential to significantly influence regional atmospheric chemistry. The goals of this study were to quantify the differences in isoprene, methanol, and monoterpene emissions from 30 hybrid poplar genotypes, determine how well VOC emissions could be explained by growth, photosynthesis, and stomatal conductance, determine whether the parental crosses that created a genotype could be used to predict its emissions, and determine whether VOC emissions from different genotypes exhibit different responses to elevated CO2. We found that 40-50% of the variation in isoprene emissions across genotypes could be explained by a combination of instantaneous photosynthesis rate and seasonal aboveground growth and 30-35% of methanol emissions could be explained by stomatal conductance. We observed a threefold range in isoprene emissions across all 30 genotypes. Both genotype and parental cross were significant predictors of isoprene and monoterpene emissions. Genotypes from P. tricocarpa X P. deltoides (T x D) crosses generally had higher isoprene emissions and lower monoterpene emissions than those from P. deltoides x P. nigra (D x N) crosses. While isoprene and monoterpene emissions generally decreased under elevated CO2 and methanol emissions generally increased, the responses varied among genotypes. Our findings suggest that genotypes with greater productivity tend to have higher isoprene emissions. Additionally, the genotypes with the lowest isoprene emissions under current CO2 are not necessarily the ones with the lowest emissions under elevated CO2.

2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C(5)H(12)O(6)S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM(2.5)) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM(2.5) collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.

We measured isocyanic acid (HNCO) in laboratory biomass fires at levels up to 600 parts per billion by volume (ppbv), demonstrating that it has a significant source from pyrolysis/combustion of biomass. We also measured HNCO at mixing ratios up to 200 pptv (parts-per-trillion by volume) in ambient air in urban Los Angeles, CA, and in Boulder, CO, during the recent 2010 Fourmile Canyon fire. Further, our measurements of aqueous solubility show that HNCO is highly soluble, as it dissociates at physiological pH. Exposure levels > 1 ppbv provide a direct source of isocyanic acid and cyanate ion (NCO(-)) to humans at levels that have recognized health effects: atherosclerosis, cataracts, and rheumatoid arthritis, through the mechanism of protein carbamylation. In addition to the wildland fire and urban sources, we observed HNCO in tobacco smoke, HNCO has been reported from the low-temperature combustion of coal, and as a by-product of urea-selective catalytic reduction (SCR) systems that are being phased-in to control on-road diesel NO(x) emissions in the United States and the European Union. Given the current levels of exposure in populations that burn biomass or use tobacco, the expected growth in biomass burning emissions with warmer, drier regional climates, and planned increase in diesel SCR controls, it is imperative that we understand the extent and effects of this HNCO exposure.

Inhibitors of phosphodiesterase 5 (PDE5) are widely used for the treatment of erectile dysfunction and pulmonary hypertension. The commercially available inhibitors are effective, well-tolerated drugs, but differ in their phosphodiesterase specificity. To explore and manipulate the specificity of PDE5 inhibitors, a small library of four inhibitors was synthesized using the structure of known PDE5 inhibitors as a scaffold. Their inhibitory potency towards PDE5 and related family members was evaluated. Next, they were immobilized on a matrix to perform affinity pull-down assays in rat testis tissue, followed by mass spectrometric (MS) analysis. By using unique peptide spectral counts of identified proteins in the MS analysis, we were able to assess the relative binding of these inhibitors to a large set of proteins, allowing the determination of their selectivity profiles in vitro. For selected proteins of interest, the results were verified using quantitative isotopic dimethyl labeling and immunoblotting, and isothermal titration calorimetry (ITC). For the PDE5 inhibitors, our data reveal that even slight chemical modifications can bias their selectivity significantly towards other interacting proteins, opening up the potential of these compounds to be used as scaffolds for the development of inhibitors for new protein targets. In a broad sense, we demonstrate that the combination of chemical proteomics and unique peptide spectral counting allows for the confident and facile analysis of the differential interactome of bioactive small molecules.

The coordinated movement of cells is indispensable for normal vertebrate gastrulation. Several important players and signaling pathways have been identified in convergence and extension (CE) cell movements during gastrulation, including non-canonical Wnt signaling. Fyn and Yes, members of the Src family of kinases, are key regulators of CE movements as well. Here we investigated signaling pathways in early development by comparison of the phosphoproteome of wild type zebrafish embryos with Fyn/Yes knockdown embryos that display specific CE cell movement defects. For quantitation we used differential stable isotope labeling by reductive amination of peptides. Equal amounts of labeled peptides from wild type and Fyn/Yes knockdown embryos were mixed and analyzed by on-line reversed phase TiO(2)-reversed phase LC-MS/MS. Phosphorylated and non-phosphorylated peptides were quantified, and significant changes in protein expression and/or phosphorylation were detected. We identified 348 phosphoproteins of which 69 showed a decrease in phosphorylation in Fyn/Yes knockdown embryos and 72 showed an increase in phosphorylation. Among these phosphoproteins were known regulators of cell movements, including Adducin and PDLIM5. Our results indicate that quantitative phosphoproteomics combined with morpholino-mediated knockdowns can be used to identify novel signaling pathways that act in zebrafish development in vivo.

Proton-transfer-reaction mass spectrometry (PTR-MS) allows real-time measurements of volatile organic compounds (VOCs) in air with a high sensitivity and a fast time response. The use of PTR-MS in atmospheric research has expanded rapidly in recent years, and much has been learned about the instrument response and specificity of the technique in the analysis of air from different regions of the atmosphere. This paper aims to review the progress that has been made. The theory of operation is described and allows the response of the instrument to be described for different operating conditions. More accurate determinations of the instrument response involve calibrations using standard mixtures, and some results are shown. Much has been learned about the specificity of PTR-MS from inter-comparison studies as well the coupling of PTR-MS with a gas chromatographic interface. The literature on this issue is reviewed and summarized for many VOCs of atmospheric interest. Some highlights of airborne measurements by PTR-MS are presented, including the results obtained in fresh and aged forest-fire and urban plumes. Finally, the recent work that is focused on improving the technique is discussed., ((c) 2006 Wiley Periodicals, Inc.)

Plant roots release about 5% to 20% of all photosynthetically-fixed carbon, and as a result create a carbon-rich environment for numerous rhizosphere organisms, including plant pathogens and symbiotic microbes. Although some characterization of root exudates has been achieved, especially of secondary metabolites and proteins, much less is known about volatile organic compounds (VOCs) released by roots. In this communication, we describe a novel approach to exploring these rhizosphere VOCs and their induction by biotic stresses. The VOC formation of Arabidopsis roots was analyzed using proton-transfer-reaction mass spectrometry (PTR-MS), a new technology that allows rapid and real time analysis of most biogenic VOCs without preconcentration or chromatography. Our studies revealed that the major VOCs released and identified by both PTR-MS and gas chromatography-mass spectrometry were either simple metabolites, ethanol, acetaldehyde, acetic acid, ethyl acetate, 2-butanone, 2,3,-butanedione, and acetone, or the monoterpene, 1,8-cineole. Some VOCs were found to be produced constitutively regardless of the treatment; other VOCs were induced specifically as a result of different compatible and noncompatible interactions between microbes and insects and Arabidopsis roots. Compatible interactions of Pseudomonas syringae DC3000 and Diuraphis noxia with Arabidopsis roots resulted in the rapid release of 1,8-cineole, a monoterpene that has not been previously reported in Arabidopsis. Mechanical injuries to Arabidopsis roots did not produce 1,8-cineole nor any C6 wound-VOCs; compatible interactions between Arabidopsis roots and Diuraphis noxia did not produce any wound compounds. This suggests that Arabidopsis roots respond to wounding differently from above-ground plant organs. Trials with incompatible interactions did not reveal a set of compounds that was significantly different compared to the noninfected roots. The PTR-MS method may open the way for functional root VOC analysis that will complement genomic investigations in Arabidopsis.