OH Reaction Research Articles

The spatial distribution of hydrogen and oxygen atoms in a cold atmospheric pressure plasma jet

Cold atmospheric pressure plasma jets (CAPJs) are an emerging technology for the localised treatment of heat sensitive surfaces. Adding humidity to the CAPJ’s feed gas yields an effective production of highly reactive intermediate species, such as hydrogen atoms, oxygen atoms, and hydroxyl radicals, among others, which are key species for biomedical applications. This study focusses on the effluent of the CAPJ kINPen, which was operated with argon feed gas and a humidity admixture of 3000 ppm, while a gas curtain was used to limit the diffusion of ambient air into the effluent. The axial and radial density distribution of O and H atoms is measured by means of picosecond two-photon absorption laser induced fluorescence spectroscopy (ps-TALIF). A maximum O atom density of (3.8 ± 0.7) × 1015 cm−3 and a maximum H atom density of (3.5 ± 0.7) × 1015 cm−3 are found at the nozzle of the plasma jet. The experimental results are compared to a two-dimensional reacting flow model that is coupled with a local zero-dimensional plasma chemical model. With this model, the main H and O atom production mechanisms are determined to be the dissociation of H2O and O2 in the plasma zone of the plasma jet. The latter indicates, that a significant amount of oxygen (1%) was present inside the device. The reaction of OH with O atoms represents the main consumption pathway for O atoms and is at the same time a significant production pathway for H atoms. The main consumption of H atoms is through a three-body reaction including O2 to form HO2, which consumes more H and O atoms to form OH. It is pointed out, that most of the species are produced in the plasma zone, and that O and H atoms, OH and HO2 radicals, and O2 and H2O molecules are strongly connected.

Pressure and Temperature Dependencies of Rate Coefficients for the Reaction OH + NO2 + M → Products.

The OH + NO2 reaction is a critically important process for radical chain termination in the atmosphere with a major impact on the ozone budgets of the troposphere and stratosphere. Rate constants for the reaction of OH + NO2 + M → products have been measured under conditions relevant to the upper troposphere/lower stratosphere with a laser photolysis-laser-induced fluorescence (LP-LIF) technique augmented by in situ optical spectroscopy for quantification of [NO2]. The experiments are carried out over the temperature range of 230-293 K and the pressure range 50-750 Torr of N2 and air and as a function of [O2]. The observed rate coefficients in N2 agree with the newest experimental literature data sets and are within experimental uncertainty of current recommended literature values at 293 K but are systematically higher by up to 22% at 700 Torr and 230 K. The efficacy of different falloff parametrizations has been examined and compared to those in literature sources. The collisional quenching efficiency of O2 was found to be in excellent agreement with current literature sources, and rate coefficients determined in air at 293 and 245 K were observed to be within uncertainty of the rate coefficients measured in N2 bath gas. This work has improved confidence in the literature rate coefficients under conditions of the lower troposphere (∼760 Torr, 280-310 K) toward the stratosphere (10-100 Torr, 220-250 K).

Outside Front Cover Picture: Plasma Process. Polym. 12/2020

Outside Front Cover: Surface microdischarge has shown great potential in practical applications. In our work, the effects of macroscopic parameters on the temporal evolution of OH radicals are investigatedin both plasma layer and afterglow region. The results reveal that OH lifetime dependence on the power is more pronounced compared to the modulation time, and the density decay is mainly ascribed to the transport process and recombination reactions of OH with itself. Further details can be found in the article by Zhiwei Wang, Chunlei Feng, Liang Gao, and Hongbin Ding (e2000122).

Correcting model biases of CO in East Asia: impact on oxidant distributions during KORUS-AQ

Global coupled chemistry-climate models underestimate carbon monoxide (CO) in the Northern Hemisphere, exhibiting a pervasive negative bias against measurements peaking in late winter and early spring. While this bias has been commonly attributed to underestimation of direct anthropogenic and biomass burning emissions, chemical production and loss via OH reaction from emissions of anthropogenic and biogenic volatile organic compounds (VOCs) play an important role. Here we investigate the reasons for this underestimation using aircraft measurements taken in May and June 2016 from the Korea-United States Air Quality (KORUS-AQ) experiment in South Korea and the Air Chemistry Research in Asia (ARIAs) in the North China Plain (NCP). For reference, multispectral CO retrievals (V8J) from the Measurements of Pollution in the Troposphere (MOPITT) are jointly assimilated with meteorological observations using an ensemble adjustment Kalman filter (EAKF) within the global Community Atmosphere Model with Chemistry (CAM-Chem) and the Data Assimilation Research Testbed (DART). With regard to KORUS-AQ data, CO is underestimated by 42% in the control run and by 12% with the MOPITT assimilation run. The inversion suggests an underestimation of anthropogenic CO sources in many regions, by up to 80% for northern China, with large increments over the Liaoning Province and the North China Plain (NCP). Yet, an often-overlooked aspect of these inversions is that correcting the underestimation in anthropogenic CO emissions also improves the comparison with observational O3 datasets and observationally constrained box model simulations of OH and HO2. Running a CAM-Chem simulation with the updated emissions of anthropogenic CO reduces the bias by 29% for CO, 18% for ozone, 11% for HO2, and 27% for OH. Longer-lived anthropogenic VOCs whose model errors are correlated with CO are also improved, while short-lived VOCs, including formaldehyde, are difficult to constrain solely by assimilating satellite retrievals of CO. During an anticyclonic episode, better simulation of O3, with an average underestimation of 5.5 ppbv, and a reduction in the bias of surface formaldehyde and oxygenated VOCs can be achieved by separately increasing by a factor of 2 the modeled biogenic emissions for the plant functional types found in Korea. Results also suggest that controlling VOC and CO emissions, in addition to widespread NO x controls, can improve ozone pollution over East Asia.

Atmospheric reactivity of biogenic volatile organic compounds in a maritime pine forest during the LANDEX episode 1 field campaign

Trace gas measurements were performed during the LANDEX (the LANDes EXperiment) Episode 1 field campaign in the summer 2017, in one of the largest European maritime pine forests (> 95% Pinus pinaster) located in southwestern France. Efforts have been focused on obtaining a good speciation of 20 major biogenic volatile organic compounds (BVOCs, including pinenes, carenes, terpinenes, linalool, camphene, etc.). This was made possible by the development of a new and specific chromatographic method. In order to assess the role of BVOCs in the local gas phase chemistry budget, their reactivity with the main atmospheric oxidants (hydroxyl radicals (OH), ozone (O3) and nitrate radicals (NO3)) and the corresponding consumption rates were determined. When considering the OH reactivity with BVOCs, isoprene and linalool accounted for 10–47% of the OH depletion during daytime, and monoterpenes for 50–65%, whereas monoterpenes were the main contributors during the night (70–85%). Sesquiterpenes and monoterpenes were the main contributors to the ozone reactivity, especially β-caryophyllene (30–70%), with a maximum contribution during nighttime. Nighttime nitrate reactivity was predominantly due to monoterpenes (i.e. 90–95%). Five specific groups have been proposed to classify the 19 BVOCs measured in the forest, according to their reactivity with atmospheric oxidants and their concentrations. The total amount of BVOCs consumed under and above the forest canopy was evaluated for 7 BVOCs (i.e. isoprene, α-pinene, β-pinene, myrcene, limonene + cis-ocimene and Δ3-carene). The reactivity of atmospheric oxidants and BVOCs at a local level are discussed in order to highlight the compounds (BVOCs, other VOCs), the atmospheric oxidants and the main associated reactive processes observed under the canopy of a maritime pine forest.

(Invited) Photoelectrochemical Hydrogen Generation Derived from Water Dissociation Under Supercontinuum Solar Light (25)

We created a two-compartment cell (2CC) separated by a bipolar membrane (BPM1) sandwiched between through-holed electrodes (THEs) assembly (BPMTHEA-2CC) based on the Golden Role for sustainable water splitting derived from water dissociation (WD) with a dual chemical bias (CB).2-4 The BPMTHEA-2CC provides an infinite supply of active reaction species, H+ and OH-, from water with an opposite bidirectional charge separation under appropriate applied voltage conditions. We have achieved hydrogen generation derived from WD with BPMTHEA-2CC using an oxygen evolving reaction active TH dimensionally stable anode (De Nora Permelec Ltd) prepared on Pt gauze. Liberated gas was analyzed using a gas chromatograph equipped with a MS-5A column and a thermal conductive detector (TCD). We succeeded in detecting appropriate amounts of molecular hydrogen generated in the cathode of a large-sized BPMTHEA-2CC, anolyte (water)|TH anode|BPM|TH cathode|catholyte (water), by imposing a voltage with less than the theoretical 1.23 V for water splitting at room temperature. We disprove the theory of water electrolysis established for hundreds of years. However, we still require external voltage for water electrolysis. Instead of external voltage, we have harnessed supercontinuum solar light throughout by designing optical tandem, wavelength multiplexing. Zero-bias photocurrent was observed by applying TH n-type semiconductor photoanodes to BPMTHEA-2CC. WD at the junction of BPM is considered to be originated in second Wien effect. Thus Au nanoparticle (Au-NP) was introduced into the junction of BPM to carry out plasmon-induced (PI) WD. With mixing different sizes of Au NPs, a wide range of absorption wavelengths would be attainable. Preliminary results obtained using Au-NP modified BPMTHEA-2CC were reported with two plausible mechanisms.3 A simple 4-port chamber has been developed to measure hydrogen concentrations down to 7.5 ppm and was used to demonstrate PEC zero bias hydrogen generation with harnessing supercontinuum solar light. V. J. Frilette, J. Phys. Chem., 60, 435 (1956).K. Sakamaki et al., AST2019, June 2-5, 2019, Awaji Yumebutai, Japan, O-24 (2019).K. Sakamaki et al., 236th ECS Meeting, Oct. 13-17, 2019, Atlanta, GA, USA, I04-1896 (2019).K. Sakamaki et al., J. Applied Electrochem. 49, 949-962 (2019). https://doi.org/10.1007/s10800-019-01337-8 Figure 1

Impact of COVID-19 lockdown on ambient levels and sources of volatile organic compounds (VOCs) in Nanjing, China

A lot of restrictive measures were implemented in China during January–February 2020 to control rapid spread of COVID-19. Many studies reported impact of COVID-19 lockdown on air quality, but little research focused on ambient volatile organic compounds (VOCs) till now, which play important roles in production of ozone and secondary organic aerosol. In this study, impact of COVID-19 lockdown on VOCs mixing ratios and sources were assessed based on online measurements of VOCs in Nanjing during December 20, 2019-Feburary 15, 2020 (P1-P2) and April 15–May 13, 2020 (P3). Average VOCs levels during COVID-19 lockdown period (P2) was 26.9 ppb, about half of value for pre-lockdown period (P1). Chemical composition of VOCs also showed significant changes. Aromatics contribution during decreased from 13% during P1 to 9% during P2, whereas alkanes contribution increased from 64% to 68%. Positive matrix factorization (PMF) was then applied for non-methane hydrocarbons (NMHCs) sources apportionment. Five sources were identified, including a source related to transport and background air masses, three sources related to petrochemical industry or chemical industry (petrochemical industry#1-propene/ethene, petrochemical industry#2-C7-C9 aromatics, and chemical industry-benzene), and a source attributed to gasoline evaporation and vehicular emission. During P2, NMHCs levels from petrochemical industry#2-C7-C9 aromatics showed the largest relative decline of 94%, followed by petrochemical industry#1-propene/ethene (67%), and gasoline evaporation and vehicular emission (67%). Furthermore, ratios of OH reactivity of NMHCs versus NO2 level (ROH,NMHCs/NO2) and total oxidant production rate (P (OX)) were calculated to assess potential influences of COVID-19 lockdown on O3 formation.

Contrasting Reactive Organic Carbon Observations in the Southeast United States (SOAS) and Southern California (CalNex).

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 O3 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 O3 and SOA production, than in the forested southeastern U.S.

Photocatalyst-coated carbon microtube electrodes: Preparation and characterization of their properties and photocatalytic degradation of methylene blue

Photocatalysis is a potential technology for removing pollutants from water. As the recombination of the photogenerated electron-hole pairs can hinder the photocatalytic efficiency in the treatment of wastewater, the surface of the carrier is usually coated with a semiconductor. In this study, carbon microtube electrode prepared from corncob was coated with either titanium oxide (TiO2) or bismuth phosphate (BiPO4) and then used as a photocatalyst (C–TiO2 or C–BiPO4) to investigate the photodegradation of methylene blue (MB). The two photocatalysts, C–TiO2 and C–BiPO4, were characterized by phase determination, microstructure observation, water contact angle measurement, and base site analysis. The influences of reaction time, stability, MB concentration, initial pH, and OH radicals quenching on the degradation of MB were also evaluated. The degradation of MB by C–TiO2 and C–BiPO4 was mainly dominated by OH radical oxidation. The carbon microtube increased both the mass transfer rate and the photogenerated electron-hole pairs separation rate, thereby increasing the photocatalysis of both C–TiO2 and C–BiPO4 as revealed by an increase in the rate of MB degradation. The rate constants obtained for the degradation of MB by C–TiO2 and C–BiPO4 at 20 °C were 9.739 × 10−7 mM min−1 and 1.111 × 10−7 mM min−1, respectively. The coating of TiO2 and BiPO4 on the surface of the carbon microtube electrode enhanced their photocatalytic performance, and therefore, C–TiO2 and C–BiPO4 could be developed into a novel material to be used in the photodegradation of dye pollutants.

Technical Note: Effect of varying the <i>λ</i> = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors

Abstract. Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (O3), nitrate (NO3) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added O3 at λ=254 nm (OFR254) or combined photolysis of O2 and H2O at λ=185 nm plus photolysis of O3 at λ=254 nm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [O3], [H2O], and photon flux at λ=254 nm (I254), OFR185 radical generation is influenced by [O2], [H2O], I185, and I254. Because the ratio of photon fluxes, I185:I254, is OFR-specific, OFR185 performance varies between different systems even when constant [H2O] and I254 are maintained. Thus, calibrations and models developed for one OFR185 system may not be applicable to another. To investigate these issues, we conducted a series of experiments in which I185:I254 emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked λ=185 nm radiation. Integrated OH exposure (OHexp) values achieved for each lamp type were obtained using the tracer decay method as a function of UV intensity, humidity, residence time, and external OH reactivity (OHRext). Following previous related studies, a photochemical box model was used to develop a generalized OHexp estimation equation as a function of [H2O], [O3], and OHRext that is applicable for I185:I254≈0.001 to 0.1.

Ambient Non-Methane Hydrocarbons (NMHCs) Measurements in Baoding, China: Sources and Roles in Ozone Formation

Ambient non-methane hydrocarbons (NMHCs) are important precursors of ozone (O3) and secondary organic aerosol (SOA). Online and offline measurements of NMHCs were conducted during September 2015 in Baoding, Hebei province of China, in order to investigate their sources and roles in ozone formation. Average levels of total NMHCs online measured at the urban site were 44.5 ± 26.7 ppb. Aromatics was the largest contributor to NMHCs levels and OH reactivity, with fraction of 27.1% and 35.9%, respectively. Based on offline measurements at eight sites, we found that toluene, ethylbenzene, and m,p-xylene displayed the highest level at the site close to automobile manufacturing factories, followed by downwind receptor sites and other sites. Positive matrix factorization (PMF) model was then used to analyze NMHCs sources. Four factors were identified, including traffic-related emission, automobile manufacturing coating, biogenic emission, and NG/LPG usage and background. Average relative contribution of automobile manufacturing coating to NMHCs levels during the entire online measurement period was 33.4%, and this value increased to 42% during two O3 pollution days. Sensitivity of O3 formation to NMHCs and NOX during an O3 pollution episode were analyzed using a box model based on observations. Relative incremental reactivity (RIR) results suggested that O3 formation was in NOx-titration regime (i.e., highly NMHCs-limited regime). Further scenario analyses on relationship of O3 formation with reduction of NOx and anthropogenic NMHCs (AHC) indicated that AHC and NOx should be reduced by a ratio greater than two and three to achieve 5% and 10% O3 control objectives, respectively. The largest RIR value for anthropogenic NMHC species was from xylenes, which were also an important contributor to SOA formation and dominantly from sources related to automobile manufacturing coating and traffic emission. This means reducing NMHCs emission from automobile manufacturing coating and traffic emission should be given priority for synergetic control of O3 and PM2.5.

Inorganic Sulfur Species Formed upon Heterogeneous OH Oxidation of Organosulfates: A Case Study of Methyl Sulfate

Recent studies reveal that organosulfates at the particle surface can be oxidized by gas-phase OH radicals with significant rates. Inorganic sulfur species, such as the bisulfate ion (HSO4–) and sulfate ion (SO42–), can be formed upon these heterogeneous oxidation processes through the formation and subsequent reactions of sulfate radical anions (SO4•–) in the particle phase. However, the amount of inorganic sulfur species produced in these heterogeneous oxidation reactions is not known. We investigate the heterogeneous OH oxidation of sodium methyl sulfate (CH3SO4Na), the smallest organosulfate detected in atmospheric particles, using an oxidation flow reactor at a relative humidity of 75%. We quantify the kinetics by measuring the decay of CH3SO4Na and the amount of HSO4– and SO42– formed upon oxidation using ion chromatography. Kinetic measurements determine the heterogeneous OH reaction rate to be (5.72 ± 0.14) × 10–13 cm3 molecule–1 s–1, with an effective OH uptake coefficient, γeff, of 0.31 ± 0.06. The molar yield of inorganic sulfur species, defined as the total number of moles of HSO4– and SO42– formed per mole of CH3SO4Na consumed upon oxidation, is found to be significant and has an average value of 0.62 ± 0.18 upon oxidation. A kinetic model is developed to describe the kinetics and inorganic sulfur species formation upon oxidation. Model simulations suggest that CH3SO4Na tends to decompose rapidly into formaldehyde and SO4•–, and the reaction of SO4•– with CH3SO4Na plays a significant role in both governing the kinetics and the formation of inorganic sulfur species.

Ozonation and ozone / UV process for industrial wastewater treatment: comparison between two types of wastewater

This study investigated the degradation of organic and inorganic matters of wastewater by conventional ozonation and ozone/UV AOPs. The experiments performed with two types of wastewater (synthetic and industrial) having the same chemical oxygen demand COD, varying solution pH and employing UV clearly showed that oxidation during ozonation process was dominated by OH reactions. Conventional ozonation with 2 g/hr maximum ozone dose led to 56% COD removal efficiency in synthetic wastewater, while 52% was estimated from industrial wastewater, both having the same initial COD value of almost 50 ppm and contact time of 80 min. In meanwhile, using UV improved the utilization efficiency of oxidation by reducing the OH scavenging contribution of oxidants, 62% and 70% were the efficiencies of industrial and synthetic respectively in ozone/UV applications. Therefore, combined ozone/AOPs shows better removal efficiency of toxic contaminants than using ozone alone.

New Insights into the Radical Chemistry and Product Distribution in the OH-Initiated Oxidation of Benzene.

Emissions of aromatic compounds cause air pollution and detrimental health effects. Here, we explore the reaction kinetics and products of key radicals in benzene photo-oxidation. After initial OH addition and reaction with O2, the effective production rates of phenol and bicyclic peroxy radical (BCP-peroxy) are experimentally constrained at 295 K to be 420 ± 80 and 370 ± 70 s-1, respectively. These rates lead to approximately 53% yield for phenol and 47% yield for BCP-peroxy under atmospheric conditions. The reaction of BCP-peroxy with NO produces bicyclic hydroxy nitrate with a branching ratio <0.2%, indicating efficient NOx recycling. Similarly, the reaction of BCP-peroxy with HO2 largely recycles HOx, producing the corresponding bicyclic alkoxy radical (BCP-oxy). Because of the presence of C-C double bonds and multiple functional groups, the chemistry of BCP-oxy and other alkoxy radicals in the system is diverse. Experimental results suggest the aldehydic H-shift and ring-closure to produce an epoxide functionality could be competitive with classic decomposition of alkoxy radicals. These reactions are potential sources of highly oxygenated molecules. Finally, despite the large number of compounds observed in our study, we are unable to account for ∼20% of the carbon flow.

Investigation of CO recombination in the boundary layer of CH4/O2 rocket engines

In this work, the combustion and thermal recombination in the boundary layer of a single-element methane/oxygen rocket combustor is investigated using large-eddy simulations. The experimental configuration consists of a coaxial injector and an operating point with a nominal pressure of 20 bar and gaseous injection of both propellants are considered. A non-adiabatic flamelet model is utilized with the purpose of examining its capability to predict the wall heat transfer. Good agreement of the simulation results with measurements of heat flux and pressure profiles is obtained using the non-adiabatic model. By comparing results with a frozen flamelet model, the importance of the recombination reactions in the cold boundary layer is investigated. The species profiles of CO and CO2 are examined and the reaction pathways leading to the recombination reactions at the wall were analyzed. Results from this analysis show that the reaction of OH with CO forming CO2 is the main contributor to the additional heat release in the boundary layer.

Effects of stereoisomeric structure and bond location on the ignition and reaction pathways of hexenes

AbstractThe current work presents new experimental autoignition and speciation data on the two cis‐hexene isomers: cis‐2‐hexene and cis‐3‐hexene. The new data provide insights on the effects of carbon‐carbon double bond location and stereoisomeric structures on ignition delay times and reaction pathways for linear hexene isomers. Experiments were performed using the University of Michigan rapid compression facility to determine ignition delay times from pressure‐time histories. Stoichiometric (ϕ = 1.0) mixtures at dilution levels of inert gas to O2 = 7.5:1 (mole basis) were investigated at an average pressure of 11 atm and temperatures from 809 to 1052 K. Speciation experiments were conducted at T = 900 K for the two cis‐hexene isomers, where fast‐gas sampling and gas chromatography were used to identify and quantify the two cis‐hexene isomers and stable intermediate species. The ignition delay time data showed negligible sensitivity to the location of the carbon‐carbon double bond and the stereoisomeric structure (cis‐trans), and the species data showed no correlation with the stereoisomeric structure, but there was a strong correlation of some of the measured species with the location of the double bond in the hexene isomer. In particular, 2‐hexene showed strong selectivity to propene, acetaldehyde, and 1,3‐butadiene, and 3‐hexene showed selectivity to propanal. Model predictions of ignition delay times were in excellent agreement with the experimental data. There was generally good agreement for the model predictions of the species data for 2‐hexene; however, the mechanism overpredicted some of the small aldehyde (C2‐C4) species for 3‐hexene. Reaction pathway analysis indicates the hexenes are almost exclusively consumed by H‐atom abstraction reactions at the conditions studied (P = 11 atm, T &gt; 900 K), and not by C3‐C4 scission as observed in high‐temperature (&gt;1300 K) hexene ignition studies. Improved estimates for 3‐hexene + OH reactions may improve model predictions for the species measured in this work.

OH reactivity from the emissions of different tree species: investigating the missing reactivity in a boreal forest

Abstract. In forested area, a large fraction of total hydroxyl radical (OH) reactivity remains unaccounted for. Very few studies have looked at the variations in total OH reactivity from biogenic emissions. In the present study, we investigate the total OH reactivity from three common boreal tree species (Scots pine, Norway spruce, and downy birch) by comparing it with the calculated reactivity from the chemically identified emissions. Total OH reactivity was measured using the comparative reactivity method (CRM), and the chemical composition of the emissions was quantified with two gas chromatographs coupled with mass spectrometers (GC–MSs). Dynamic branch enclosures were used, and emissions from one branch of a tree at the time were measured by periodically rotating between them. Results show that birch had the highest values of total OH reactivity of the emissions (TOHRE), while pine had the lowest. The main drivers for the known reactivity of pine and spruce were monoterpenes and sesquiterpenes. Birch emissions were dominated by sesquiterpenes, but monoterpenes and green leaf volatiles (GLVs) were present as well. However, calculated reactivity values remained low, leading to the highest missing fraction of reactivity (&gt;96 %), while pine and spruce had similar missing reactivity fractions between 56 % and 82 % (higher in the spring and decreasing as the summer proceeded). The high average values were driven by low-reactivity periods, and the fraction of missing reactivity got smaller for pine and spruce when the TOHRE values increased. Important exceptions were identified for periods when the emission profiles changed from terpenes to GLVs, a family of compounds containing a backbone of six carbon atoms with various functionalities (e.g. alcohols, aldehydes, esters) that indicate that the plant is suffering from stress. Then, very high TOHRE values were measured, and the missing fraction remained high. This study found a different trend in the missing OHRE fraction of the Norway spruce from spring to autumn compared to one previous study (Nölscher et al., 2013), which indicates that additional studies are required to fully understand the complexity of biogenic reactive emissions. Future studies of boreal trees in situ should be conducted to confirm the findings presented.

High temperature branching ratio of acetaldehyde +OH reaction

The reaction of acetaldehyde (CH3CHO) with hydroxyl radicals (OH) plays an important role in atmospheric and combustion chemistry. The low-temperature chemistry of this reaction has been studied widely in the literature. However, the branching of this reaction at high temperatures is not well known. Aiming to deduce the branching ratio of CH3CHO + OH, measurements were carried out in a shock tube by introducing deuterium into the chemical system. Overall rate coefficients for OH reactions with acetaldehyde (CH3CHO), acetaldehyde-2,2,2-d3 (CD3CHO), acetaldehyde-d4 (CD3CDO) were measured over the temperature range of 950-1300 K and 1.5–3.0 bar. In addition, rate coefficients of OH radicals with acetone (CH3C(O)CH3) and acetone-d6 (CD3C(O)CD3) were measured to deduce the kinetic isotopic effect of H-abstraction reaction at the methyl site. The measured rate coefficients can be represented by the following Arrhenius expressions (cm3/molecule/s):k1(CH3CHO+OH)=1.29×10−10exp(−1996.5KT)k2(CD3CHO+OH)=1.06×10−10exp(−2151.9KT)k3(CD3CDO+OH)=1.18×10−10exp(−2554.1KT)k4(CH3COCH3+OH)=7.15×10−11exp(−2695.7KT)k5(CD3COCD3+OH)=6.02×10−11exp(−3130.5KT)In contrast to the low-temperature chemistry, our results indicate that H-abstraction from the methyl site of acetaldehyde is important at high temperatures and the branching fraction of this channel is ∼ 65%. On per H atom basis, however, H-abstraction from the aldehydic group is faster than that of methyl group even at high temperatures.

Effect of hydrogen addition on the ionization of partially premixed methane flame

The flame ionization characteristic is a key point to realize the combustion diagnosis and control of hydrocarbon flame based on electric fields. In this paper the effects of hydrogen addition on the ionization of partially premixed methane flame were investigated experimentally and computationally. The flame current was measured by applying a small electric field across the partially premixed methane flame with different hydrogen addition and the effects of hydrogen addition on the ionization intensity and the flame ionization structure were analyzed. Furthermore, the intrinsic reaction mechanism of hydrogen addition affecting the ionization of partially premixed methane flame was studied based on a comprehensive reaction mechanism, which combines GRI 3.0 for neutral species and an 85-steps flame ionization reaction for 13 kinds of charged species. The results show that hydrogen addition can promote the ionization of partially premixed flame by affecting the chemical ionization process. However, when the hydrogen content reaches 50% of the total fuel amount, the promotion effect will be significantly weakened. Moreover, for partially premixed methane flame, the effect of hydrogen addition on the flame ionization was affected by the amount of primary premixed air. The larger the amount of primary premixed air, the more significant the promotion of hydrogen addition. What’s more, the hydrogen addition mainly affects the concentration of e− in the flame through the reactions H + CH3 (+M) ⇔ CH4 (+M) and H + O2 ⇔ O + OH while affects the concentration of H3O + through the reactions OH + CH3 ⇔ CH2 (S) + H2O and H3O+ + e− ⇔ OH + 2H.

PH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase.

Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid-base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.