Functional Group Analysis of α-Pinene Oxidation Products Using Derivatization Reactions and High-Resolution Electrospray Ionization Collision-Induced Dissociation Mass Spectrometry.

Oxidation products of monoterpenes (C10H16) play a significant role as precursors for secondary organic aerosol formation. They contain several structural isomers with multifunctional groups. However, only a few of these isomers have been identified experimentally. We describe a measurement technique for identifying oxygen-containing functional groups (carbonyl, carboxyl, and hydroxyl groups) included in monoterpene oxidation products. This technique involves (i) three derivatization reactions (oximation of carbonyls by O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine, methyl esterification of carboxylic acids by trimethylsilyl diazomethane, and acylation of alcohols by acetic anhydride), (ii) no preseparation high-resolution electrospray ionization mass spectrometry, and (iii) collision-induced dissociation. This technique was applied to functional group analysis of ozonolysis products for α-pinene. Multifunctional groups of known ozonolysis products were accurately identified. Furthermore, we successfully estimated the multifunctional groups of products that have not been previously reported.

Plant Molecular Phenology and Climate Feedbacks Mediated by BVOCs.

Climate change profoundly affects the timing of seasonal activities of organisms, known as phenology. The impact of climate change is not unidirectional; it is also influenced by plant phenology as plants modify atmospheric composition and climatic processes. One important aspect of this interaction is the emission of biogenic volatile organic compounds (BVOCs), which link the Earth's surface, atmosphere, and climate. BVOC emissions exhibit significant diurnal and seasonal variations and are therefore considered essential phenological traits. To understand the dynamic equilibrium arising from the interplay between plant phenology and climate, this review presents recent advances in comprehending the molecular mechanisms underpinning plant phenology and its interaction with climate. We provide an overview of studies investigating molecular phenology, genome-wide gene expression analyses conducted in natural environments, and how these studies revolutionize the concept of phenology, shifting it from observable traits to dynamic molecular responses driven by gene-environment interactions. We explain how this knowledge can be scaled up to encompass plant populations, regions, and even the globe by establishing connections between molecular phenology, changes in plant distribution, species composition, and climate. Expected final online publication date for the Annual Review of Plant Biology, Volume 75 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Analysis of Volatile Esters and Alkanoic Acids by an Atmospheric Pressure Corona Discharge Ionization Collision-Induced Dissociation Mass Spectrometry in Positive-Ion Mode: Ionization, Fragmentation Patterns, and Discrimination between Isomeric Compounds.

The ionization and fragmentation patterns of 24 compounds with elemental composition of CmH2mO2, including isomeric esters and alkanoic acids, were investigated by atmospheric pressure corona discharge ionization collision-induced dissociation (CID) mass spectrometry in the positive-ion mode. All compounds were ionized as protonated molecules and NH4+ adducts. In addition, fragment ions were observed in mass spectra of esters other than methyl esters, which are owing to the dissociation of the alkyl group in the alcohol side from the protonated molecules. In CID spectra, protonated alkanoic acids/methyl esters split off H2O/CH3OH and CO or the alkyl group in the acid side, depending on the carbon chain length. Protonated esters (other than methyl esters) mainly fragmented the alkyl group in the alcohol side. These general rules on ionization and fragmentation patterns can provide relevant information on the discrimination of isomers.

Fuel-Type Independent Parameterization of Volatile Organic Compound Emissions from Western US Wildfires.

Volatile organic compounds (VOCs) emitted from biomass burning impact air quality and climate. Laboratory studies have shown that the variability in VOC speciation is largely driven by changes in combustion conditions and is only modestly impacted by fuel type. Here, we report that emissions of VOCs measured in ambient smoke emitted from western US wildfires can be parameterized by high- and low-temperature pyrolysis VOC profiles and are consistent with previous observations from laboratory simulated fires. This is demonstrated using positive matrix factorization (PMF) constrained by high- and low-temperature factors using VOC measurements obtained with a proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) on board the NASA DC-8 during the FIREX-AQ (Fire Influence on Regional and Global Environments and Air Quality) project in 2019. A linear combination of high- and low-temperature factors described more than 70% of the variability of VOC emissions of long-lived VOCs in all sampled wildfire plumes. An additional factor attributable to atmospheric aging was required to parameterize short-lived and secondarily produced VOCs. The relative contribution of the PMF-derived high-temperature factor for a given fire plume was strongly correlated with the fire radiative power (FRP) at the estimated time of emission detected by satellite measurements. By combining the FRP with the fraction of the high-temperature PMF factor, the emission ratios (ERs) of VOCs to carbon monoxide (CO) in fresh wildfires were estimated and agree well with measured ERs (r2 = 0.80-0.93).

Identification of Negative Ion at m/z 20 Produced by Atmospheric Pressure Corona Discharge Ionization under Ambient Air.

The negative ion at m/z 20 observed at atmospheric pressure corona discharge ionization mass spectra has been identified by supplying the vapors of deuterium oxide (D2O) and H218O. From the mass shifts of the ion at m/z 20 observed with D2O and H218O, it was suggested that the chemical composition of the ion at m/z 20 is to be H4O. Further mass shift from m/z 20 to 22 was observed by supplying the vapor of perfluorokerocene, suggesting the chemical composition of H3F. The chemical compositions of the negative ions H4O- and H3F- were consistence with the dipole-bound complex states between hydrogen H2 and polar molecules such as H2O and hydrogen fluoride (HF) having dipole moments beyond a critical dipole moment of 1.625 D, theoretically proposed by Skurski and Simons. The ionic chemical compositions and structures of H4O- and H3F- obtained with density functional theory calculations implied that both dipole-bound complex H2O-…H2 and HF-…H2 can be formed by exothermic reactions by which H2 molecule is complexing with negative ions H2O- and HF-, respectively.

Ozone chemistry in western U.S. wildfire plumes.

Wildfires are a substantial but poorly quantified source of tropospheric ozone (O3). Here, to investigate the highly variable O3 chemistry in wildfire plumes, we exploit the in situ chemical characterization of western wildfires during the FIREX-AQ flight campaign and show that O3 production can be predicted as a function of experimentally constrained OH exposure, volatile organic compound (VOC) reactivity, and the fate of peroxy radicals. The O3 chemistry exhibits rapid transition in chemical regimes. Within a few daylight hours, the O3 formation substantially slows and is largely limited by the abundance of nitrogen oxides (NOx). This finding supports previous observations that O3 formation is enhanced when VOC-rich wildfire smoke mixes into NOx-rich urban plumes, thereby deteriorating urban air quality. Last, we relate O3 chemistry to the underlying fire characteristics, enabling a more accurate representation of wildfire chemistry in atmospheric models that are used to study air quality and predict climate.

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

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

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

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

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

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

Modern mass spectrometry in atmospheric sciences: Measurement of volatile organic compounds in the troposphere using proton-transfer-reaction mass spectrometry.

Volatile organic compounds (VOCs) in the troposphere are emitted from a wide variety of natural and man-made sources and resulting in environmental issues such as air pollution and climate change. Proton-transfer-reaction mass spectrometry (PTR-MS), a chemical ionization mass spectrometry technique using H3 O+ reagent ions, allows real-time measurements of various VOCs in air with high sensitivity (parts-per-trillion to parts-per-billion level) and fast time response (<1 s). Modern PTR-MS equipped with time-of-flight mass analyzers can measure hundreds of compounds simultaneously. The applications of PTR-MS have greatly promoted understanding VOC sources and their roles in environmental issues, and therefore, PTR-MS has become a well-established analytical technique for in situ VOC measurements in atmospheric sciences. Here, the principles and implementation of PTR-MS are described. We additionally highlight modern PTR-MS techniques for accurate identification and comprehensive quantification of various VOCs. Finally, recent applications for atmospheric science are presented, using examples from research in biomass burning emissions and chemistry.

Oxygenated Aromatic Compounds are Important Precursors of Secondary Organic Aerosol in Biomass-Burning Emissions.

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 (R2 = 0.68) and O/C (R2 = 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.

Accurate identification of dimers from α-pinene oxidation using high-resolution collision-induced dissociation mass spectrometry.

Interest in mass spectrometry of highly oxidized dimers from α-pinene oxidation has increased in the atmospheric chemistry field. Here, we apply high-resolution collision-induced dissociation mass spectrometry (HR-CID-MS) with an atmospheric pressure ionization source to investigate in detail how α-pinene-derived dimers are detected and identified by MS. The resulting HR-CID spectra and specific fragmentation patterns suggest that a large fraction of dimer ions detected in full-scan mass spectra can be hydrogen-bonded artifact clusters and the residual small fraction includes covalently bonded actual dimers. We also show how individual fractions of the artifact clusters and actual dimers are calculated using the HR-CID spectra.

Atmospheric Pressure Dark-Current Argon Discharge Ionization with Comparable Performance to Direct Analysis in Real Time Mass Spectrometry.

Herein, a dark-current discharge state created by combining argon flow with a needle electrode in ambient air is described that has an ionization efficiency and mechanism comparable to those of conventional helium direct analysis in real time (DART), without requiring dopants and DART glow discharge. Using this method, polar compounds such as α-amino acids were ionized in the dark-current argon discharge via (de)protonation, molecular anion formation, fragmentation, (de)protonation with the attachment of oxygen, deprotonation with hydrogen loss and negative ion attachment. In contrast, nonpolar compounds (e.g., n-alkanes) were detected as positive ions via hydride abstraction and oxidation. Major background ions observed were H3O+(H2O) n , O2 ·+, O2 ·-(H2O) n and CO3 ·-. These results indicate that the present dark-current discharge efficiently generates resonance-state argon with an internal energy of ∼14.2 eV, higher than that of the well-known metastable state (∼11.6 eV). It is therefore suggested that ionization reactions occurring there can be attributed to the Penning ionization of atmospheric H2O and O2 by resonance-state argon, analogous to helium DART.

Intramolecular Hydrogen Transfer from the Alpha-Carbon (Cα) and Backbone Amide Nitrogen (Nb) to Form c- and y-Ions in Negative-Ion CID of Peptides.

The source of hydrogen in the formation of c- and y-ions produced by intramolecular hydrogen transfer in negative-ion CID experiments with peptides has been examined using Cα-, Cß-, and backbone amide (Nb)-deuterated peptides AAA(d3)AA, AAG(d2)AA, AAAG(d2)A, and AAAAA-d7, as well as five other peptides. The c- and y-ions produced by deuterium transfer from the deuterated residues were detected and identified by the exact m/z values obtained with a high-resolution orbitrap mass spectrometer. The rate of deuterium transfer obtained indicates that over 50% of the hydrogen was originated from the backbone amide nitrogen, with the residual hydrogen coming from the backbone Cα. It is clear that the hydrogen does not originate from the side chain Cß. It is hypothesized that the intramolecular hydrogen transfer to form negative c- and y-ions takes place via 3-, 4-, 6-, 7-, 8-, and 9-membered ring transition states.

A Library of Proton-Transfer Reactions of H3O+ Ions Used for Trace Gas Detection.

We have collected data on the proton-transfer reactions with H3O+ 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.

Administration of hydrogen-rich water prevents vascular aging of the aorta in LDL receptor-deficient mice.

The main cause of arteriosclerosis is atherosclerosis in the aorta. Atherosclerosis is recognized as a chronic inflammatory condition that begins with the dysfunction or activation of arterial endothelium. Low-density lipoprotein (LDL) and especially its oxidized form play a key role in endothelial dysfunction and atherogenesis. Recent studies showed that senescent cells are involved in the development and progression of atherosclerosis, and eliminating senescent cells suppresses the senescence-associated secretory phenotype. We previously reported that molecular hydrogen-rich water (HW) has antioxidant and anti-inflammatory effects in numerous diseases. Here, we used LDL receptor-deficient mice fed a high-fat diet (HFD) for 13 weeks as a model for atherosclerosis and evaluated the effects of continuous administration of HW. The numbers of endothelial cells in the atheroma expressing the senescence factors p16INK4a and p21 decreased in HFD-fed mice given HW compared with HFD-fed mice given control water. Furthermore, macrophage infiltration and Tnfα expression in the atheroma were also suppressed. These results suggest that vascular aging can be suppressed by HW.

Proton-Transfer-Reaction Mass Spectrometry: Applications in Atmospheric Sciences.

Proton-transfer-reaction mass spectrometry (PTR-MS) has been widely used to study the emissions, distributions, and chemical evolution of volatile organic compounds (VOCs) in the atmosphere. The applications of PTR-MS have greatly promoted understanding of VOC sources and their roles in air-quality issues. In the past two decades, many new mass spectrometric techniques have been applied in PTR-MS instruments, and the performance of PTR-MS has improved significantly. This Review summarizes these developments and recent applications of PTR-MS in the atmospheric sciences. We discuss the latest instrument development and characterization work on PTR-MS instruments, including the use of time-of-flight mass analyzers and new types of ion guiding interfaces. Here we review what has been learned about the specificity of different product ion signals for important atmospheric VOCs. We present some of the recent highlights of VOC research using PTR-MS including new observations in urban air, biomass-burning plumes, forested regions, oil and natural gas production regions, agricultural facilities, the marine environment, laboratory studies, and indoor air. Finally, we will summarize some further instrument developments that are aimed at improving the sensitivity and specificity of PTR-MS and extending its use to other applications in atmospheric sciences, e.g., aerosol measurements and OH reactivity measurements.

Collision-Induced Dissociation Study of Strong Hydrogen-Bonded Cluster Ions Y-(HF) n (Y=F, O2) Using Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry Combined with a HF Generator.

Hydrogen fluoride (HF) was produced by a homemade HF generator in order to investigate the properties of strong hydrogen-bonded clusters such as (HF) n . The HF molecules were ionized in the form of complex ions associated with the negative core ions Y- produced by atmospheric pressure corona discharge ionization (APCDI). The use of APCDI in combination with the homemade HF generator led to the formation of negative-ion HF clusters Y-(HF) n (Y=F, O2), where larger clusters with n≥4 were not detected. The mechanisms for the formation of the HF, F-(HF) n , and O2-(HF) n species were discussed from the standpoints of the HF generator and APCDI MS. By performing energy-resolved collision-induced dissociation (CID) experiments on the cluster ions F-(HF) n (n=1-3), the energies for the loss of HF from F-(HF)3, F-(HF)2, and F-(HF) were evaluated to be 1 eV or lower, 1 eV or higher, and 2 eV, respectively, on the basis of their center-of-mass energy (ECM). These ECM values were consistent with the values of 0.995, 1.308, and 2.048 eV, respectively, obtained by ab initio calculations. The stability of [O2(HF) n ]- (n=1-4) was discussed on the basis of the bond lengths of O2H-F-(HF) n and O2-H-F(HF) n obtained by ab initio calculations. The calculations indicated that [O2(HF)4]- separated into O2H and F-(HF)3.

Improvement in ionization efficiency of direct analysis in real time-mass spectrometry (DART-MS) by corona discharge.

Herein it is shown that a combination of direct analysis in real time (DART) with a corona discharge system consisting of only a needle electrode easily improves DART ionization efficiency. Positive and negative DC corona discharges led to a formation of abundant excited helium atoms as well as the reactant ions H3O(+)(H2O)n and O2˙(-) in the DART analyte ionization area. These phenomena resulted in an increase in the absolute intensities of (de)protonated analytes by a factor of 2-20 over conventional DART. The other analyte ions detected in this corona-DART system (i.e., molecular ions, fragment ions, oxygenated (de)protonated analytes, dehydrogenated deprotonated analytes, and negative ion adducts) were quite similar to those obtained from DART alone. This indicates a lack of side reactions due to the corona discharge. The change in the relative intensities of individual analyte-related ions due to the combination of a corona discharge system with DART suggests that there is no effect of the abundant excited helium in the analyte ionization area on the fragmentation processes or enhancement of oxidation due to hydroxyl radicals HO˙. Furthermore, it was found that the corona-DART combination can be applied to the highly sensitive analysis of n-alkanes, in which the alkanes are ionized as positive ions via hydride abstraction and oxidation, independent of the type of alkane or the mass spectrometer used.

Ionization Capabilities of Hydronium Ions and High Electric Fields Produced by Atmospheric Pressure Corona Discharge.

Atmospheric pressure corona discharge (APCD) was applied to the ionization of volatile organic compounds. The mass spectra of analytes having aromatic, phenolic, anilinic, basic and aliphatic in nature were obtained by using vapor supply and liquid smear supply methods. The vapor supply method mainly gave protonated analytes [A+H]+ caused by proton transfer from hydronium ion H3O+, except for benzene, toluene and n-hexane that have lower proton affinity. The use of the liquid smear supply method resulted in the formation of molecular ion A·+ and/or dehydride analyte [A-H]+, according to the nature of analytes used. The formation of A·+ without fragment ions could be explained by the electron tunneling via high electric fields 108 V/m at the tip of the corona needle. The dehydride analytes [A-H]+ observed in the mass spectra of n-hexane, di- and tributylamines may be explained by the hydride abstraction from the alkyl chains by the hydronium ion. The hydronium ion can play the two-roles for analytes, i.e., the proton donor to form [A+H]+ and the hydride acceptor to form [A-H]+.