A larger pool of ozone-forming carbon compounds in urban atmospheres

Development of a net ozone production rate detection system based on dual-channel cavity ring-down spectroscopy

A novel system for measuring net photochemical ozone production rates in the atmosphere based on cavity ring-down spectroscopy (OPR-CRDS) was developed. The system consists of two chambers (a reaction chamber and a reference chamber) and a dual-channel Ox-CRDS detector. To minimize the wall loss of Ox in the chambers, the inner surfaces of both chambers are coated with Teflon film. The performance of the OPR-CRDS system was characterized. It was found that even though the photolysis frequency (J value) decreased by 10%, the decrease in the P(O3) caused by the ultraviolet-blocking film coating was less than 3%. The two chambers had a good consistency in the mean residence time and the measurement of NO2 and Ox under the condition of no sunlight. The detection limit of the OPR-CRDS was determined to be 0.20 ppbv/hr. To further verify the accuracy of the system, the direct measurement values of the OPR-CRDS system were compared with the calculation results based on radical (OH, HO2, and RO2) reactions, and a good correlation was obtained between the measured and calculated values. Finally, the developed instrument was applied to obtain the comprehensive field observations at an urban site in the Yangtze River Delta (China) for 40 days, the time series and change characteristics of the P(O3) were obtained directly, and the good environmental adaptability and stability of the OPR-CRDS system were demonstrated. It is expected that the new instrument will be beneficial to investigations of the relationship between P(O3) and its precursors.

Fabrication of Highly Dispersed Ru Catalysts on CeO2 for Efficient C3H6 Oxidation

Emissions of volatile organic compounds (VOCs) threaten both the environment and human health. To realize the elimination of VOCs, Ru/CeO2 catalysts have been intensively investigated and applied. Although it has been widely acknowledged that the catalytic performance of platinum group metal catalysts was highly determined by their dispersion and coordination environment, the most reactive structures on Ru/CeO2 catalysts for VOCs oxidation are still ambiguous. In this work, starting from Ce-BTC (BTC = 1,3,5-benzenetricarboxylic acid) materials, atomically dispersed Ru catalysts and agglomerated Ru catalysts were successfully created via one-step hydrothermal method (Ru-CeO2-BTC) and conventional incipient wetness impregnation method (Ru/CeO2-BTC), respectively. In a typical model reaction of C3H6 oxidation, atomically dispersed Ruδ+ species with the formation of abundant Ru-O-Ce linkages on Ru-CeO2-BTC were found to perform much better than agglomerated RuOx species on Ru/CeO2-BTC. Further characterizations and mechanism study disclosed that Ru-CeO2-BTC catalyst with atomically dispersed Ru ions and more superior low temperature redox performance compared to Ru/CeO2-BTC could better facilitate the adsorption/activation of C3H6 and the decomposition/desorption of intermediates, thus exhibiting superior C3H6 oxidation activity. This work elucidated the reactive sites on Ru/CeO2 catalysts in the C3H6 oxidation reaction and provided insightful guidance for designing efficient Ru/CeO2 catalysts to eliminate VOCs.

Ultra-highly sensitive dual gases detection based on photoacoustic spectroscopy by exploiting a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser

Photoacoustic spectroscopy (PAS) as a highly sensitive and selective trace gas detection technique has extremely broad application in many fields. However, the laser sources currently used in PAS limit the sensing performance. Compared to diode laser and quantum cascade laser, the solid-state laser has the merits of high optical power, excellent beam quality, and wide tuning range. Here we present a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser used as light source in a PAS sensor for trace gas detection. The self-built solid-state laser had an emission wavelength of ~2 μm with Tm:YAP crystal as the gain material, with an excellent wavelength and optical power stability as well as a high beam quality. The wide wavelength tuning range of 9.44 nm covers the absorption spectra of water and ammonia, with a maximum optical power of ~130 mW, allowing dual gas detection with a single laser source. The solid-state laser was used as light source in three different photoacoustic detection techniques: standard PAS with microphone, and external- and intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS), proving that solid-state laser is an attractive excitation source in photoacoustic spectroscopy.

Preformed Pt Nanoparticles Supported on Nanoshaped CeO2 for Total Propane Oxidation

Pt-based catalysts have been widely used for the removal of short-chain volatile organic compounds (VOCs), such as propane. In this study, we synthesized Pt nanoparticles with a size of ca. 2.4 nm and loaded them on various fine-shaped CeO2 with different facets to investigate the effect of CeO2 morphology on the complete oxidation of propane. The Pt/CeO2-o catalyst with {111} facets exhibited superior catalytic activity compared to the Pt/CeO2-r catalyst with {110} and {100} facets. Specifically, the turnover frequency (TOF) value of Pt/CeO2-o was 1.8 times higher than that of Pt/CeO2-r. Moreover, Pt/CeO2-o showed outstanding long-term stability during 50 h. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that the excellent performance of Pt/CeO2-o is due to the prevalence of metallic Pt species, which promotes C-C bond cleavage and facilitates the rapid removal of surface formate species. In contrast, a stronger metal-support interaction in Pt/CeO2-r leads to easier oxidation of Pt species and the accumulation of intermediates, which is detrimental to the catalytic activity. Our work provides insight into the oxidation of propane on different nanoshaped Pt/CeO2 catalysts.

Upgrading Emission Standards Inadvertently Increased OH Reactivity from Light-Duty Diesel Truck Exhaust in China: Evidence from Direct LP-LIF Measurement

Vehicular exhaust is an important source of reactive gases responsible for the formation of ozone and secondary organic aerosols (SOAs) in the atmosphere. Although significant efforts have been made to characterize the chemical compounds associated with vehicular exhaust, there is still a wealth of compounds that are unable to be detected, posing uncertainties in estimating their contribution to atmospheric reactivity. In this study, by improving laser-induced fluorescence techniques, we achieved the first-ever direct measurement of the total OH reactivity (TOR) from light-duty diesel truck (LDDT) exhaust with different emission standards. We found that the TOR from the LDDT exhaust was 80-130 times the TOR from the gasoline exhaust measured in Japan. Unexpectedly, we discovered increased TOR emissions along with upgrading emission standards, possibly as a collective result of high combustion temperature in the engine and the oxidation catalysts in the exhaust after-treatment that favor production of highly oxidized organics in the stricter emission standard. Most of these oxidized organics are unable to be speciated by routine measurements, resulting in the missing OH reactivity increasing rapidly from 1.91% for China III to 42.0% for China V LDDT. Upgrading the emission standard failed to reduce the TOR from LDDT exhaust, which may inadvertently promote the contribution of LDDT to the formation of ozone and SOA pollution in China.

Multisize particulate matter and volatile organic compounds in arid and semiarid areas of Northwest China

To investigate the chemical components, sources, and interactions of particulate matter (PM) and volatile organic compounds (VOCs), a field campaign was implemented during the spring of 2018 in nine cities in northwestern (NW) China. PM was mainly contributed by organic matter and water-soluble inorganic ions (41% for PM₁₀ and approximately 60% for PM_2.5 and PM₁). Two typical haze patterns were observed: anthropogenic pollution type (AP-type), wherein contributions of sulfate, nitrate, and ammonium (SNA) increased, and dust pollution type (DP-type), wherein contributions of Ca²⁺ increased and SNA decreased. Source appointment suggested that regional sources contributed close to half to PM_2.5 pollution (40% for AP-type and 50% for DP-type). Thus, sources from regional transport are also important for haze and dust pollution. The ranking of VOC concentrations was methanol > acetaldehyde > formic acid + ethanol > acetone. Compared with other cities, there are higher oxygenated VOCs (OVOCs) and lower aromatics in NW China. The relationships between VOCs and PM were discussed. The dominating secondary organic aerosols (SOA) formation potential precursors were C₁₀-aromatics, xylene, and styrene under low-nitrogen oxide (NOx) conditions, and benzene, C₁₀-aromatics, and toluene dominated under high-NOx conditions. The quadratic polynomial was the most suitable fitting model for their correlation, and the results suggested that VOC oxidations explained 6.1-10.8% and 9.9-20.7% of SOA formation under high-NOx and low-NOx conditions, respectively.

Comparisons of Combined Oxidant Capacity and Redox-Weighted Oxidant Capacity in Their Association with Increasing Levels of COVID-19 Infection

Background: Ozone (O3) and nitrogen dioxide (NO2) are substances with oxidizing ability in the atmosphere. Only considering the impact of a single substance is not comprehensive. However, people’s understanding of “total oxidation capacity” (Ox) and “weighted average oxidation” (Oxwt) is limited. Objectives: This investigation aims to assess the impact of Ox and Oxwt on the novel coronavirus disease (COVID-19). We also compared the relationship between the different calculation methods of Ox and Oxwt and the COVID-19 infection rate. Method: We recorded confirmed COVID-19 cases and daily pollutant concentrations (O3 and NO2) in 34 provincial capital cities in China. The generalized additive model (GAM) was used to analyze the nonlinear relationship between confirmed COVID-19 cases and Ox and Oxwt. Result: Our results indicated that the correlation between Ox and COVID-19 was more sensitive than Oxwt. The hysteresis effect of Ox and Oxwt decreased with time. The most obvious statistical data was observed in Central China and South China. A 10 µg m−3 increase in mean Ox concentrations were related to a 23.1% (95%CI: 11.4%, 36.2%) increase, and a 10 µg m−3 increase in average Oxwt concentration was related to 10.7% (95%CI: 5.2%, 16.8%) increase in COVID-19. In conclusion, our research results show that Ox and Oxwt can better replace the single pollutant research on O3 and NO2, which is used as a new idea for future epidemiological research.

Dynamic variations of phthalate esters in PM2.5 during a pollution episode

Phthalate esters (PAEs) as hazardous air pollutants can be easily released during the life cycle of plastic products. In this study, a thermal desorption aerosol comprehensive two-dimensional gas chromatography mass spectrometer coupled with a dual-trap was developed and used to measure the hourly-resolved PAEs characteristics in atmospheric PM_2.5 at an urban site. Dimethyl phthalate (DMP), diethyl (DEP), dibutyl (DnBP), benzyl butyl (BBP), di(2-ethylhexyl) (DEHP), and di-n-octyl phthalate (DnOP) in PM_2.5 were analyzed. The most abundant compounds were DEHP and DMP, followed by DnBP and DEP. The mass concentrations of the detected PAEs are comparable to those at other urban sites measured using offline methods with a lower time resolution. The concentrations of PAEs showed intense change with the variation of PM_2.5 mass concentration. The proportion of DEHP increased while that of DMP decreased with the increase in PM_2.5 pollution. Positive correlations between PAEs and PM_2.5, organic carbon, and elemental carbon were observed, while PAEs had negative correlation with the ambient temperature. Our observation provides evidences on understanding the volatile and semi-volatile PAEs in the ambient aerosols.

Atmospheric insight into the reaction mechanism and kinetics of isopropenyl methyl ether (i-PME) initiated by OH radicals and subsequent oxidation of product radicals

Studies on primary gas-phase reactions of emitted saturated and unsaturated ethers with oxidants and subsequent secondary reactions of product radicals with O₂ in the presence of NO are important in their atmospheric chemical processes. To accomplish these findings, we have examined the chemistry of OH-initiated oxidation of isopropenyl methyl ether (i-PME) CH₃C(CH₂)OCH₃ by electronic structure ca using density functional theory. Our energetic calculations show that OH additions to carbon-carbon double bonds of i-PME are more favorable reaction pathways than H-abstraction reactions from the various CH sites of the titled molecule. The rate constant values which are obtained from the transition state theory also signify that OH-addition reactions have faster reaction rates than H-abstraction reactions. Our calculated total rate constant of the reaction is found 9.90 × 10^-11 cm³ molecule^-1 s^-1. The percentage branching ratio calculations imply that OH-addition reactions have 98.09% contribution in the total rate constant. The atmospheric lifetime of i-PME is found to be 2.8 h. Further, we have identified 2-hydroxy-2-methoxypropanol, methyl acetate, methy-1,2-hydroxyacetate and 1-hydroxypropane-2-one, 1,2-dihydroxypropan-2-yl format, 2-hydroxyacetic acid, acetic acid, and formaldehyde from the secondary oxidation of product radicals.

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

Methane, discovered in 1766 by Alessandro Volta, is an attractive energy source because of its high heat of combustion per mole of carbon dioxide. However, methane is the most abundant hydrocarbon in the atmosphere and is an important greenhouse gas, with a 21-fold greater relative radiative effectiveness than CO2 on a per-molecule basis. To avoid or limit the formation of pollutants that are dangerous for both human health and the atmospheric environment, the catalytic combustion of methane appears to be one of the most promising alternatives to thermal combustion. Total oxidation of methane, which is environmentally friendly at much lower temperatures, is believed to be an efficient and economically feasible way to eliminate pollutants. This work presents a literature review, a statu quo, on catalytic methane oxidation on transition metal oxide-modified ceria catalysts (MOx/CeO2). Methane was used for this study since it is of great interest as a model compound for understanding the mechanisms of oxidation and catalytic combustion on metal oxides. The objective was to evaluate the conceptual ideas of oxygen vacancy formation through doping to increase the catalytic activity for methane oxidation over CeO2. Oxygen vacancies were created through the formation of solid solutions, and their catalytic activities were compared to the catalytic activity of an undoped CeO2 sample. The reaction conditions, the type of catalysts, the morphology and crystallographic facets exposing the role of oxygen vacancies, the deactivation mechanism, the stability of the catalysts, the reaction mechanism and kinetic characteristics are summarized.

An increasing role for solvent emissions and implications for future measurements of volatile organic compounds

Volatile organic compounds (VOCs) are a broad class of air pollutants which act as precursors to tropospheric ozone and secondary organic aerosols. Total UK emissions of anthropogenic VOCs peaked in 1990 at 2,840 kt yr^-1 and then declined to approximately 810 kt yr^-1 in 2017 with large reductions in road transport and fugitive fuel emissions. The atmospheric concentrations of many non-methane hydrocarbons (NMHC) in the UK have been observed to fall over this period in broadly similar proportions. The relative contribution to emissions from solvents and industrial processes is estimated to have increased from approximately 35% in 1990 to approximately 63% in 2017. In 1992, UK national monitoring quantified 19 of the 20 most abundant individual anthropogenic VOCs emitted (all were NMHCs), but by 2017 monitoring captured only 13 of the top 20 emitted VOCs. Ethanol is now estimated to be the most important VOC emitted by mass (in 2017 approx. 136 kt yr^-1 and approx. 16.8% of total emissions) followed by n-butane (52.4 kt yr^-1) and methanol (33.2 kt yr^-1). Alcohols have grown in significance representing approximately 10% of emissions in 1990 rising to approximately 30% in 2017. The increased role of solvent emissions should now be reflected in European monitoring strategies to verify total VOC emission reduction obligations in the National Emissions Ceiling Directive. Adding ethanol, methanol, formaldehyde, acetone, 2-butanone and 2-propanol to the existing NMHC measurements would provide full coverage of the 20 most significant VOCs emitted on an annual mass basis. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Emission characteristics and associated assessment of volatile organic compounds from process units in a refinery

The accuracy and reliability of volatile organic compound (VOC) emission data are essential for assessing emission characteristics and their potential impact on air quality and human health. This paper describes a new method for determining VOC emission data by multipoint sampling from various process units inside a large-scale refinery. We found that the emission characteristics of various production units were related to the raw materials, products, and production processes. Saturated alkanes accounted for the largest fraction in the continuous catalytic reforming and wastewater treatment units (48.0% and 59.2%, respectively). In the propene recovery unit and catalytic cracking unit, alkenes were the most dominant compounds, and propene provided the largest contributions (57.8% and 23.0%, respectively). In addition, n-decane (12.6%), m,p-xylene (12.4%), and n-nonane (8.9%) were the main species in the normal production process of the delayed coking unit. Assessments of photochemical reactivity and carcinogenic risk were carried out, and the results indicate that VOC emissions from the propene recovery unit and catalytic cracking unit should be controlled to reduce the ozone formation potential; in addition, alkenes are precedent-controlled pollutants. The cancer risk assessments reveal that 1,2-dibromoethane, benzene, 1,2-dichloroethane, and chloroform were the dominant risk contributors, and their values were much higher than the standard threshold value of 1.0 × 10^-6 but lower than the significant risk value defined by the US Supreme Court. Based on the VOC composition and a classification algorithm, the samples were classified into eight main groups that corresponded to different process units in the petroleum refinery. In conclusion, this work provides valuable data for investigating process-specific emission characteristics of VOCs and performing associated assessments of photochemical reactivity and carcinogenic risk in petrochemical refineries.

Secondary organic aerosol formation from evaporated biofuels: comparison to gasoline and correction for vapor wall losses

With an ongoing interest in displacing petroleum-based sources of energy with biofuels, there is a need to measure and model the formation and composition of secondary organic aerosol (SOA) from organic compounds present in biofuels. We performed chamber experiments to study SOA formation from four recently identified biofuel molecules and mixtures and commercial gasoline under high NOx conditions: diisobutylene, cyclopentanone, an alkylfuran mixture, and an ethanol-to-hydrocarbon (ETH) mixture. Cyclopentanone and diisobutylene had a significantly lower potential to form SOA compared to commercial gasoline, with SOA mass yields lower than or equal to 0.2%. The alkylfuran mixture had an SOA mass yield (1.6%) roughly equal to that of gasoline (2.0%) but ETH had an average SOA mass yield (11.5%) that was six times higher than that of gasoline. We used a state-of-the-science model to parameterize or simulate the SOA formation in the chamber experiments while accounting for the influence of vapor wall losses. Simulations performed with vapor wall losses turned off and at atmospherically relevant conditions showed that the SOA mass yields were higher than those measured in the chamber at the same photochemical exposure and were also higher than those estimated using a volatility basis set that was fit to the chamber data. The modeled SOA mass yields were higher primarily because they were corrected for vapor wall losses to the Teflon® chamber.

Behaviour of traffic emitted semi-volatile and intermediate volatility organic compounds within the urban atmosphere

Particulate matter originated from traffic has attracted major interest over the last few years. The semi-volatile organic component of the particles may evaporate with dispersion away from the emission source, creating vapour which may oxidise to form secondary organic aerosol. Air samples were collected from a street canyon, the adjacent park and an urban background site during the winter-spring period in central London, UK. Emissions of semi-volatile organic compounds (SVOCs) and intermediate volatility organic compounds (IVOCs) ranging from C₁₀ to C₃₆ in both the gas phase and particle phase were measured by using thermal desorption coupled to comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (TD-GC × GC-ToF-MS). Main compound groups identified and quantified were grouped alkanes (n-alkanes and branched alkanes), monocyclic alkanes, bicyclic alkanes and monocyclic aromatics. The carbon preference index (CPI) of n-alkanes was estimated to distinguish the emission sources. Pearson correlations between I/SVOCs and traffic tracers (black carbon, NO_x and benzene) in different locations were compared to analyse the influence of this emission source. The results indicate that while the major emission source at the roadside site is traffic, the lower correlations at background sites are indicative of other source contributions and/or differential reactivity of compounds. Gas-particle phase partitioning of n-alkanes is evaluated and compared between sites. The potential influence of gas phase I/SVOCs upon OH reactivity and secondary organic aerosol (SOA) formation is estimated and found to be relatively small.

Characterization and sources of volatile organic compounds (VOCs) and their related changes during ozone pollution days in 2016 in Beijing, China

Concentrations of 99 volatile organic compounds (VOCs) were continuously measured online at an urban site in Beijing, China, in January, April, July, and October 2016. Characterization and sources of VOCs and their related changes during days with heavy ozone (O₃) pollution were analysed. The total observed concentration of VOCs (TVOCs) was 44.0 ± 28.9 ppbv. The VOC pollution level has decreased in Beijing but remains higher than in other Chinese cities. Alkanes comprised the highest proportion among seven major sampled VOC groups. The concentrations and sources of ambient VOCs showed obvious temporal variations. Six emission sources were identified by the positive matrix factorization (PMF), including biomass burning, coal combustion, gasoline vehicles, diesel vehicles, solvent usage, and biogenic + secondary emissions. The combustion source was the key control factor for VOC reduction in Beijing. From the potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) model, Beijing, Tianjin, Hebei, Shanxi, Inner Mongolia, Shandong, and Henan were identified as major potential source regions of ambient VOCs. O₃ formation was sensitive to VOCs in Beijing according to the VOC/NOx ratio (ppbC/ppbv, 8:1 threshold). High- and low-O₃ days in July were identified, and high O₃ levels were due to both enhanced VOC emission levels and meteorological conditions favourable to the production of O₃. These findings provide evidence that the fuel combustion and regional transport have a great impact on concentrations and sources of VOCs in urban Beijing.

A Combined Experimental and Theoretical Study on the Gas Phase Reaction of OH Radicals with Ethyl Propyl Ether

The reaction of ethyl propyl ether (EnPE) with OH radicals was studied using proton-transfer-reaction mass spectrum (PTR-MS), and the rate constant was measured at 298 K and atmospheric pressure using the relative rate method: k_exp(OH+EnPE) = (1.13 ± 0.09) × 10^-11 cm³ molecules^-1 s^-1. In addition, a parallel theoretical study was performed using the traditional transition state theory (TST) with a tunnelling effect correction in combination at M05-2X method with two basis sets, 6-311++G(d,p) and aug-cc-pVTZ. According to these calculations, H atom abstraction occurs more favorably from the methylene group adjacent to the -O- bond than from the other groups. The theoretical calculation of the total rate constant of the reaction of EnPE with OH radicals was consistent with the experimental values. The gas-phase products indicated that the major products observed were ethyl formate, ethyl propionate, propionic acid. Combined with the experimental and theoretical results, the possible reaction mechanisms were proposed and discussed. The atmospheric implications of the studied reaction are presented, and the lifetime of EnPE in the presence of OH radicals was evaluated to be approximately 1 day.

Theoretical investigation of the mechanism, kinetics and subsequent degradation products of the NO3 radical initiated oxidation of 4-hydroxy-3-hexanone

The oxidation mechanism of 4-hydroxy-3-hexanone (CH3CH2C(O)CH(OH)CH2CH3) initiated by NO3 radicals in the nighttime is investigated systematically by applying quantum theoretical methods. According to thermodynamic research, the process of H-abstraction on the -CH- group adjacent to the hydroxyl group is the most dominant pathway with the lowest activation energy. The analysis of Mulliken charge charts and molecular electrostatic potential maps illustrate that C-H bonds are the active sites of the reaction, and the calculated C-H bond dissociation energy of the CH3CH2C(O)CH(OH)CH2CH3 molecule further confirms that α-CH is the most easily activated. Individual rate constants for five H-abstraction pathways are calculated by canonical variational theory coupled with small curvature tunneling method over the temperature range of 260-330 K, and the branching ratios are also evaluated. A total rate constant of 1.18 × 10-15 cm3 per molecule per s is obtained at 298 K, which is in good agreement with the reported experimental value. A negative temperature dependence is observed in the titular reaction. The subsequent degradation processes of the advantageous product alkyl radical (CH3CH2C˙(OH)COCH2CH3) are carried out in a NO-rich environment, and propionic acid, NO2 and ozone are obtained as the major final products. The nighttime atmospheric lifetime of 4-hydroxy-3-hexanone is estimated to be around 19 days, indicating that it has impact at night. The titular reaction rate constants are fitted to a three-parameter Arrhenius formula.

Application of thermal desorption methods for airborne polycyclic aromatic hydrocarbon measurement: A critical review

Thermal desorption (TD) is a universal solvent-free pre-concentration technique. It is often used to pre-concentrate semi-volatile and volatile organic compounds in various sample types. Polycyclic aromatic hydrocarbons (PAHs) are widespread contaminants from incomplete combustion of organic matter and fossil fuel, which have carcinogenic effects on human health. Conventional methods for determining PAHs, represented by solvent extraction, are gradually being replaced by solvent-free methods, typically the TD technique, because of TD's many advantages, including time savings and environmentally friendly treatment. This work presents an extensive review of the universal methods used to determine PAHs in the atmosphere based on the TD technique. The methods currently used for collection and detection of both gas- and particle-phase PAHs in the air are critically reviewed. In addition, the operating parameters of the TD unit are summarized and discussed. The design shortcomings of existing studies and the problems that researchers should address are presented, and promising alternatives are suggested. This paper also discusses important parameters, such as reproducibility and limit of detection, that form a crucial part of quality assurance. Finally, the limitations and the future prospects of the TD technique for use in airborne PAH analyses are addressed. This is the first review of the latest developments of the TD technique for analysis of PAHs and their derivatives in the atmosphere.

Characterising Particulate Organic Nitrogen at A Savannah-Grassland Region in South Africa

Although atmospheric organic N compounds are considered to be important, especially in new particle formation and their contribution to brown carbon, these species are not that well understood. This can be partially attributed to their chemical complexity. Therefore, the aim of this study was to assess the characteristics of organic N compounds utilising comprehensive two-dimensional gas chromatography coupled with a time-of-flight mass spectrometer (GCxGC-TOFMS) in aerosol samples that were collected at a savanna-grassland background region and to determine the possible sources. 135 atmospheric organic N compounds were tentatively characterised and semi-quantified, which included amines, nitriles, amides, urea, pyridine derivatives, amino acids, nitro-and nitroso compounds, imines, cyanates and isocyanates, and azo compounds. Amines contributed to 51% of the semi-quantified concentrations, while nitriles, pyridine derivatives, and amides comprised 20%, 11%, and 8%, respectively, of the semi-quantified concentrations. Amines, nitriles, amides, and pyridine derivatives concentrations were higher during the dry season, which were attributed to meteorology and open biomass burning. Anthropogenic sources impacting air masses measured at Welgegund, as well as regional agricultural activities, were considered as the major sources of amines, while the regional influence of household combustion was most likely the main source of nitriles, amides, and pyridine derivatives. The other organic N species were most likely related to the influence of local and regional agricultural activities.

Synthesis of ZSM-5/Siliceous Zeolite Composites for Improvement of Hydrophobic Adsorption of Volatile Organic Compounds

In this research, we investigated the hydrophobicity and dynamic adsorption-desorption behaviors of volatile organic compounds (VOCs) by applying different optimized coating dosage (25, 50, and 75%) on designed novel ZSM-5/MCM-41 and ZSM-5/Silicalite-1 hierarchical composites. The relatively large specific surface area and pore volume of adsorbents ZSM-5/MCM-41 and ZSM-5/Silicalite-1 composites with excellent stability were affirmed by ex-situ XRD, FTIR, BET, SEM, and water contact angle analyses. Regarding, toluene adsorption-desorption investigation, ZSM-5/MCM-41 composite lead a longer stable toluene breakthrough time no matter under dry or 50% humid conditions. However, under different loading dosage condition, the breakthrough time of 75% coating ratio was the longest, which was 1.6 times as long as that of pure ZSM-5 under wet adsorption. Meanwhile, the complete elimination of toluene for ZSM-5/MCM-41-75% was done by largest desorption peak area and the lowest desorption temperature of 101.9°C, while, the largest contact angle of ZSM-5/MCM-41-75% was 17.0° higher than pure ZSM-5 zeolite. Therefore, we believe that the present hydrophobic sorbent will provide new insight with great research potential for removing low concentration of VOCs at industrial scale.

Volatile organic compounds in a typical petrochemical industrialized valley city of northwest China based on high-resolution PTR-MS measurements: Characterization, sources and chemical effects

To scientifically understand the emissions and chemistry of volatile organic compounds (VOCs) in a typical petrochemical industrialized and dust-rich region of Northwest China, VOCs were measured at a receptor site in the Lanzhou Valley using a high-resolution online proton transfer reaction-mass spectrometer (PTR-MS). The ranking of VOC mixing ratios was methanol (32.72 ± 8.94 ppb) > acetaldehyde (5.05 ± 2.4 ppb) > acetic acid (3.42 ± 1.71 ppb). Lanzhou has higher oxygenated VOCs (OVOCs) mixing ratios (methanol and acetaldehyde) and lower aromatics levels (benzene, toluene and C8-aromatics) compared with other cities. The positive matrix factorization (PMF) model showed eight sources of VOCs as follows: (1) mixed industrial process-1 (13.5%), (2) secondary formation (13.2%), (3) mixed industrial process-2 (11.8%), (4) residential biofuel use and waste disposal (13.80%), (5) solvent usage (10.1%), (6) vehicular exhaust (11.8%), (7) biogenic (13.8%) and (8) biomass burning (12.0%). Both the PSCF and the CWT results of mixed industrial process-1 were mainly from the northeast of Lanzhou and the biomass burning was from the southeast; the other four sources (without secondary formation and biogenic) were mainly from the west and northwest of Lanzhou, which were associated with the dust area of the Gobi Desert. A trajectory sector analysis revealed that the local emissions contributed 64.9-71.1% to the VOCs. OVOCs accounted for 43% of the ozone production potential (OFP), and residential biofuel use and waste disposal (25.1%), mixed industrial process-2 (15.3%) and solvent usage (13.4%) appeared to be the dominant sources contributors to O₃ production. The rank of main secondary organic aerosols (SOA) precursors under low-NOx conditions is xylene > toluene > benzene > naphthalene > styrene > C10-aromatics > isoprene, while under high-NOx conditions, it is toluene > naphthalene > xylene > C10-aromatics > styrene > benzene > isoprene. Solvent usage and vehicular exhaust appeared to be the dominant contributors to SOA formation.

Low-cost photoionization sensors as detectors in GC × GC systems designed for ambient VOC measurements

Conventional volatile organic compound (VOC) monitoring based on thermal desorption - gas chromatography-mass spectrometry (TD-GC-MS) or gas chromatography-flame ionization detector (TD-GC-FID) is relatively cumbersome and expensive. In this study commercial off the shelf low-cost and low-power photo-ionization detector (PID) sensors are used as simple detectors in VOC analysis systems based on GC, including a miniaturised GC × GC device with portable, low-cost, and low-energy-consumption features. PID sensors produce a voltage signal positively proportional to VOC concentration, which when incorporated into a TD-GC system gave limit of detection of 0.02 ppbV for isoprene. To test PID performance in real-world applications, PID sensors were deployed as (i) a second alternative detector in a GC-Quadruple Time Of Flight Mass spectrometry (GC-Q-TOF-MS), and (ii) the main detector in a compact two-dimensional gas chromatograph (GC × GC). PID sensors with 10.6 eV and 11.7 eV lamps were used to measure eight toxic chemicals including organic sulfide and organic phosphonates via GC; two species were ionized by a 10.6 eV lamp and four species by the 11.7 eV lamp. Commercially available low-cost PIDs designed for standalone could be straightforwardly and effectively re-used as detectors in compact GC × GC systems, in this work showing excellent VOC sensitivity, fast response and low operational demands compared to comparable field instruments based on GC-FID or MS.

Ru/CeO2 Catalyst with Optimized CeO2 Support Morphology and Surface Facets for Propane Combustion

Tailoring the interfaces between active metal centers and supporting materials is an efficient strategy to obtain a superior catalyst for a certain reaction. Herein, an active interface between Ru and CeO₂ was identified and constructed by adjusting the morphology of CeO₂ support, such as rods (R), cubes (C), and octahedra (O), to optimize both the activity and the stability of Ru/CeO₂ catalyst for propane combustion. We found that the morphology of CeO₂ support does not significantly affect the chemical states of Ru species but controls the interaction between the Ru and CeO₂, leading to the tuning of oxygen vacancy in the CeO₂ surface around the Ru-CeO₂ interface. The Ru/CeO₂ catalyst possesses more oxygen vacancy when CeO₂-R with predominantly exposed CeO₂{110} surface facets is used, providing a higher ability to adsorb and activate oxygen and propane. As a result, the Ru/CeO₂-R catalyst exhibits higher catalytic activity and stability for propane combustion compared with the Ru/CeO₂-C and Ru/CeO₂-O catalysts. This work highlights a new strategy for the design of efficient metal/CeO₂ catalysts by engineering morphology and associated surface facet of CeO₂ support for the elimination of light alkane pollutants and other volatile organic compounds.

Mechanisms and Kinetic Parameters for the Gas-Phase Reactions of 3-Methyl-3-buten-2-one and 3-Methyl-3-penten-2-one with Ozone

Ozonolysis of unsaturated ketones is a common atmospheric chemical process that plays a significant role in controlling the atmospheric budget of OH and O₃, organic acids, and secondary organic aerosols (SOA). In this work, the detailed reaction mechanism and rate coefficients for the reactions of O₃ with two unsaturated ketones, 3-methyl-3-buten-2-one (MBO332) and 3-methyl-3-penten-2-one (MPO332), were investigated by using density functional theory (DFT) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results indicate that the major products are butanedione and formaldehyde for MBO332, and butanedione and acetaldehyde for MPO332. Possible reaction mechanism and thermodynamic parameters of some complex stable Criegee intermediates (SCIs) RR'COO were also be investigated in this study. Some organic peroxides can be regarded as the main products for the further reactions of SCIs. The rate constants calculated with O₃ are 2.59 × 10^-16 cm³ molecule^-1 s^-1 and 2.28 × 10^-16 cm³ molecule^-1 s^-1 for MBO332 and MPO332 at 298 K and 1 atm. The total rate constant is negatively correlated with temperature (200-400 K) and positively correlated with pressure. The atmospheric half-lives of MBO332 and MPO332 based on O₃ are estimated.

Investigation of synergistic effects and high performance of La-Co composite oxides for toluene catalytic oxidation at low temperature

Cobalt oxides have been considered as a kind of highly efficient catalyst for the oxidation of volatile organic compounds (VOCs). In this work, lanthanum-cobalt composite oxides were prepared by using the co-precipitation method, and toluene was used as the model compound. Diversified techniques including XRD, SEM, Raman spectra, XPS, H₂-TPR, and N₂ adsorption-desorption were applied to investigate the physicochemical properties of as-prepared materials. The composite catalysts showed different morphology including larger specific surface area and higher pore volume which would accelerate the adsorption of toluene and improve the amount of active sites on surface. Moreover, the addition of lanthanum could enhance the low-temperature reducibility, and it could be also beneficial to expose more Co³⁺ and adsorbed oxygen species on the surface of catalysts which could accelerate the oxidation of toluene and lower onset oxidation temperature. 0.05La-Co (with a molar ratio of lanthanum against cobalt is 0.05) showed the best catalytic performance. The complete conversion of toluene was achieved at 225 °C under the condition of toluene concentration = 1000 ppm and SV = 20,000 ml·g^-1·h^-1. Stability test over 0.05La-Co was conducted at 225 °C and it could maintain the 100% conversion of toluene for 720 min, indicating the excellent stability of as-prepared catalysts. Undoubtedly, lanthanum-cobalt composite oxide is a kind of promising material for the catalytic oxidation of VOCs.

100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

Leaf-deposited semi-volatile organic compounds (SVOCs): An exploratory study using GCxGC-TOFMS on leaf washing solutions

Airborne particulate matter (PM) includes semi-volatile organic compounds (SVOCs), which can be deposited on vegetation matrices such as plant leaves. In alternative to air-point measurements or artificial passive substrates, leaf monitoring offers a cost-effective, time-integrating means of assessing local air quality. In this study, leaf washing solutions from ivy (Hedera hibernica) leaves exposed during one-month at different land use classes were explored via comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GCxGC-TOFMS). The composition of leaf-deposited SVOCs, corrected for those of unexposed leaves, was compared against routinely monitored pollutants concentrations (PM₁₀, PM_2.5, O₃, NO₂, SO₂) measured at co-located air monitoring stations. The first study on leaf-deposited SVOCs retrieved from washing solutions, herein reported, delivered a total of 911 detected compounds. While no significant land use (rural, urban, industrial, traffic, mixed) effects were observed, increasing exposure time (from one to 28 days) resulted in a higher number and diversity of SVOCs, suggesting cumulative time-integration to be more relevant than local source variations between sites. After one day, leaf-deposited SVOCs were mainly due to alcohols, N-containing compounds, carboxylic acids, esters and lactones, while ketones, diketones and hydrocarbons compounds gained relevance after one week, and phenol compounds after one month. As leaf-deposited SVOCs became overall more oxidized throughout exposure time, SVOCs transformation or degradation at the leaf surface is suggested to be an important phenomenon. This study confirmed the applicability of GCxGC-TOFMS to analyze SVOCs from leaf washing solutions, further research should include validation of the methodology and comparison with atmospheric organic pollutants.

OH-Initiated Oxidation of Acetylacetone: Implications for Ozone and Secondary Organic Aerosol Formation

Acetylacetone (AcAc) is a common atmospheric oxygenated volatile organic compound due to broad industrial applications, but its atmospheric oxidation mechanism is not fully understood. We investigate the mechanism, kinetics, and atmospheric fate of the OH-initiated oxidation for the enolic and ketonic isomers of AcAc using quantum chemical and kinetic rate calculations. OH addition to enol-AcAc is more favorable than addition to keto-AcAc, with the total rate constant of 1.69 × 10^-13 exp(1935/T) cm³ molecule^-1 s^-1 over the temperature range of 200-310 K. For the reaction of the enol-AcAc with OH, the activation energies of H-abstraction are at least 4 kcal mol^-1 higher than those of OH-addition, and the rate constants for OH-addition are by 2-3 orders of magnitude higher than those for H-abstraction. Oxidation of AcAc is predicted to yield significant amounts of acetic acid and methylglyoxal, larger than those are currently recognized. A lifetime of less than a few hours for AcAc is estimated throughout the tropospheric conditions. In addition, we present field measurements in Beijing and Nanjing, China, showing significant concentrations of AcAc in the two urban locations. Our results reveal that the OH-initiated oxidation of AcAc contributes importantly to ozone and SOA formation under polluted environments.

Kinetic study of the gas-phase reaction of O3 with three unsaturated alcohols

Rate constants for the reactions of ozone with 1-octen-3-ol, 1-nonen-3-ol and 1-nonen-4-ol have been determined at 298±1K and atmospheric pressure for the first time. The experiments were performed in a 100-L FEP Teflon film bag using absolute rate method; the rate constants were (1.91±0.19)×10^-17, (1.89±0.20)×10^-17, and (0.83±0.08)×10^-17cm³/(molecule·sec) for 1-octen-3-ol, 1-nonen-3-ol, and 1-nonen-4-ol, respectively. The rate constants have been compared with those of unsaturated alcohols structural homologs, and used to estimate the reaction reactivity. The electronegativity of carbon-carbon double bond was calculated by atomic charges analysis. The calculated results show that the electronic effect of the lone pair electrons of hydroxyl oxygen is the main cause of the difference in rate coefficient. According to the obtained rate constants, the atmospheric lifetimes of studied unsaturated alcohols were also estimated, which indicates that the reaction with ozone is an important loss pathway in the atmosphere, especially in polluted areas.

Bidirectional Ecosystem-Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum: How Many Matter?

Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem-atmosphere VOC fluxes across the entire detected mass range (m/z 0-335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem-atmosphere exchange. We introduce the reactivity flux as a measure of how Earth-atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (∑VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the ∑VOC carbon and reactivity fluxes by 40-60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ~30% of the carbon flux and ~50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds.

Addressing the Challenges to Identification in Gas Chromatography by Increased Resolution and Enhanced Detection Modalities

In the past 15+ years, gas chromatography (GC) has undergone a renaissance in its implementation on the basis of the "disruptive" technology of comprehensive two-dimensional gas chromatography (GC×GC). With a foundation based upon a two-column GC separation approach, GC×GC significantly alters the classical multidimensional gas chromatography (MDGC) method by employing very fast separation on a second dimension (2 D) after a conventional one-dimensional (i.e., single column; 1D) column separation. This allows the experimentalist to apply the advantages of multidimensionality to the total sample rather than to just discrete zones of the 1D separation that characterizes MDGC. This requires a new "language" to describe GC×GC separations, applied to the first dimension (1 D) and 2 D, and consideration of the modulation processes that define the transfer of analyte from the 1 D to the 2 D. The present review is based on the FACS Foundation lectureship of the author given at the 17th Asian Chemical Congress of the Federation of Asian Chemical Societies (FACS). The award lecture and this manuscript is based on material deriving largely from research in the area of MDGC and GC×GC separations of the author.

Urban flux measurements reveal a large pool of oxygenated volatile organic compound emissions

The exchange of nonmethane volatile organic compounds (NMVOC) at the surface–atmosphere interface is a fundamental constraint and important boundary condition for atmospheric chemistry and its effects on climate. Anthropogenic emissions are thought to account for about half of the NMVOC flux into the atmosphere of the Northern Hemisphere, yet their budget is considerably uncertain due to the scarcity of appropriate top-down constraints. Here we present direct flux measurements of NMVOCs based on the eddy covariance technique, showing that the contribution of typical urban emission sources is comprised of a surprisingly large portion of oxygenated NMVOC. These results suggest that typical urban NMVOC emission sources could be significantly higher than currently projected in air chemistry and climate models.

Atmospheric chemistry is fueled by a large annual influx of nonmethane volatile organic compounds (NMVOC). These compounds influence ozone formation, lead to secondary organic aerosol production, and play a significant role for the oxidizing capacity of the atmosphere. The anthropogenic NMVOC budget is considerably uncertain due to the diversity of urban emission sources. Here, we present comprehensive observations of urban NMVOC eddy covariance fluxes using a newly designed proton-transfer-reaction quadrupole interface time-of-flight mass spectrometer. We found emission fluxes of a surprisingly large pool of oxygenated NMVOCs (OVOCs) with an appreciable fraction of higher oxidized OVOCs that cannot be explained by known fast photochemical turnaround or current primary emission estimates. Measured OVOC/NMVOC bulk flux ratios are two to four times higher than inferred from aggregated anthropogenic emission inventories. Extrapolating these results would double the global anthropogenic NMVOC flux. In view of globally accelerating urbanization, our study highlights the need to reevaluate the influence of anthropogenic NMVOC on atmospheric chemistry, human health, and the climate system.

Contribution of biogenic and photochemical sources to ambient VOCs during winter to summer transition at a semi-arid urban site in India

This paper presents the sources and characteristics of ambient volatile organic compounds (VOCs) measured using PTR-TOF-MS instrument in a metropolitan city of India during winter to summer transition period. Mixing ratios of VOCs exhibited strong diurnal, day-to-day and episodic variations. Methanol was the most dominant species with monthly mean values of 18-22 pbbv. The emission ratios of VOCs relative to benzene calculated from nighttime data were used to estimate the relative contributions of vehicle exhaust and other sources. The increasing daytime ratios of oxygenated-VOCs (OVOCs)/benzene and isoprene/benzene from February to March indicates increasing contribution of photo-oxidation and biogenic sources. Daytime fractions of acetone (18%), acetaldehyde (15%) and isoprene (4.5%) to the sum of measured VOCs in March were higher than those in February. Variations of VOCs at lower temperatures (<25 °C) were predominantly controlled by anthropogenic sources. At high temperatures, particularly in the range of 32-40 °C during March, levels of OVOCs and isoprene were influenced by biogenic emissions. The emissions of OVOCs from vehicle exhaust were estimated to be smaller (20-40%) than those from other sources. The contributions of biogenic and secondary sources to OVOCs and isoprene increased by 10-15% from winter to summer. This study provides evidence that the winter-to-summer transition has an impact on sources and composition of VOCs in tropical urban areas.

Atmospheric degradation of 2-chloroethyl vinyl ether, allyl ether and allyl ethyl ether: Kinetics with OH radicals and UV photochemistry

Unsaturated ethers are oxygenated volatile organic compounds (OVOCs) emitted by anthropogenic sources. Potential removal processes in the troposphere are initiated by hydroxyl (OH) radicals and photochemistry. In this work, we report for the first time the rate coefficients of the gas-phase reaction with OH radicals (k_OH) of 2-chloroethyl vinyl ether (2ClEVE), allyl ether (AE), and allyl ethyl ether (AEE) as a function of temperature in the 263-358 K range, measured by the pulsed laser photolysis-laser induced fluorescence technique. No pressure dependence of k_OH was observed in the 50-500 Torr range in He as bath gas, while a slightly negative T-dependence was observed. The temperature dependent expressions for the rate coefficients determined in this work are: The estimated atmospheric lifetimes (τ_OH) assuming k_OH at 288 K were 3, 2, and 4 h for 2ClEVE, AE and AEE, respectively. The kinetic results are discussed in terms of the chemical structure of the unsaturated ethers by comparison with similar compounds. We also report ultraviolet (UV) and infrared (IR) absorption cross sections (σ_λ and σ(ν˜), respectively). We estimate the photolysis rate coefficients in the solar UV actinic region to be less than 10^-7 s^-1, implying that these compounds are not removed from the atmosphere by this process. In addition, from σ(ν˜) and τ_OH, the global warming potential of each unsaturated ether was calculated to be almost zero. A discussion on the atmospheric implications of the titled compounds is presented.

New System for Measuring the Photochemical Ozone Production Rate in the Atmosphere

We have developed a new system for measuring photochemical ozone production rates in the atmosphere. Specifically, the system measures the net photochemical oxidant (O_x: the sum of ozone (O₃) and nitrogen dioxide (NO₂)) production rates (P-L(O_x)). Measuring O_x avoids issues from perturbations to the photostationary states between nitrogen oxides (NO_x) and O₃. This system has "reaction" and "reference" chambers. Ambient air is introduced into both chambers, and O_x is photochemically produced in the reaction chamber and not generated in the reference chamber. Air from the chambers is alternately introduced into an NO-reaction (NO: nitric oxide) tube to convert O₃ to NO₂, and then the O_x concentration is measured as NO₂ using a laser-induced fluorescence technique. P-L(O_x) was obtained by dividing the difference in O_x concentrations between air samples from the two chambers by the mean residence time of the air in the reaction chamber. In this study, the P-L(O_x) measurement system was characterized, and the current detection limit of P-L(O_x) was determined to be 0.54 ppbv h^-1 with an integration time of 60 s (S/N = 2), assuming an ambient O_x concentration of 100 ppbv. Field measurements of P-L(O_x) were conducted using the system at a remote forest location.

Layered Double Hydroxides-Based Materials as Oxidation Catalysts

This chapter is focused on the transition-metal-containing LDHs-based materials having potential applications in both catalytic selective oxidation for obtaining chemicals and intermediates, and complete oxidation as a promising valuable technology for the destruction of Volatile Organic Compounds (VOCs).

Computational study of H-abstraction reactions from CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl by Cl atom and OH radical and fate of alkoxy radicals

Multichannel gas-phase reactions of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl with chlorine atom and hydroxyl radical have been investigated using ab initio method and canonical variational transition-state dynamic computations with the small-curvature tunneling correction. Further energetic information is refined by the coupled-cluster calculations with single and double excitations (CCSD)(T) method. Both hydrogen abstraction and displacement processes are carried out at the same level. Our results reveal that H-abstraction from the -OCH₂- group is the dominant channel for CH₃OCH₂CH₂Cl by OH radical or Cl atom, and from α-CH₂ of the group CH₃CH₂- is predominate for the reaction CH₃CH₂OCH₂CH₂Cl with Cl/OH. The contribution of displacement processes may be unimportant due to the high barriers. The values of the calculated rate constants reproduce remarkably well the available experiment data. Standard enthalpies of formation for reactants and product radicals are calculated by isodesmic reactions. The Arrhenius expressions are given within 220-1200 K. The atmospheric lifetime, ozone depleting potential (ODP), ozone formation potential (OFP), and global warming potential (GWP) of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl are investigated. Meanwhile, the atmospheric fate of the alkoxy radicals are also researched using the same level of theory. To shed light on the atmospheric degradation, a mechanistic study is obtained, which indicates that reaction with O₂ is the dominant path for the decomposition of CH₃OCH(O•)CH₂Cl, the C-C bond scission reaction is the primary reaction path in the consumption of CH₃CH(O•)OCH₂CH₂Cl in the atmosphere.

HIGHLIGHTS

Ab initio method and canonical variational transition-state theory are employed to study the kinetic nature of hydrogen abstraction reactions of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl with Cl atom and OH radical and fate of alkoxy radicals (CH₃OCH(O•)CH₂Cl/CH₃CH(O•)OCH₂CH₂Cl).

Fully Automated Portable Comprehensive 2-Dimensional Gas Chromatography Device

We developed a fully automated portable 2-dimensional (2-D) gas chromatography (GC x GC) device, which had a dimension of 60 cm × 50 cm × 10 cm and weight less than 5 kg. The device incorporated a micropreconcentrator/injector, commercial columns, micro-Deans switches, microthermal injectors, microphotoionization detectors, data acquisition cards, and power supplies, as well as computer control and user interface. It employed multiple channels (4 channels) in the second dimension (²D) to increase the ²D separation time (up to 32 s) and hence ²D peak capacity. In addition, a nondestructive flow-through vapor detector was installed at the end of the ¹D column to monitor the eluent from ¹D and assist in reconstructing ¹D elution peaks. With the information obtained jointly from the ¹D and ²D detectors, ¹D elution peaks could be reconstructed with significantly improved ¹D resolution. In this Article, we first discuss the details of the system operating principle and the algorithm to reconstruct ¹D elution peaks, followed by the description and characterization of each component. Finally, 2-D separation of 50 analytes, including alkane (C₆-C₁₂), alkene, alcohol, aldehyde, ketone, cycloalkane, and aromatic hydrocarbon, in 14 min is demonstrated, showing the peak capacity of 430-530 and the peak capacity production of 40-80/min.

Recent advances in the application of 2-dimensional gas chromatography with soft and hard ionisation time-of-flight mass spectrometry in environmental analysis

Two-dimensional gas chromatography has huge power for separating complex mixtures. The principles of the technique are outlined together with an overview of detection methods applicable to GC × GC column effluent with a focus on selectivity. Applications of GC × GC techniques in the analysis of petroleum-related and airborne particulate matter samples are reviewed. Mass spectrometric detection can be used alongside spectral libraries to identify eluted compounds, but in complex petroleum-related and atmospheric samples, when used conventionally at high ionisation energies, may not allow differentiation of structural isomers. Available low energy ionisation methods are reviewed and an example given of the additional structural information which can be extracted by measuring mass spectra at both low and high ionisation energies, hence greatly enhancing the selectivity of the technique.