Laser-Induced Phosphorescence for the in Situ Detection of Glyoxal at Part per Trillion Mixing Ratios

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.

Decomposition of the simplest ketohydroperoxide in the ozonolysis of ethylene

Hydroperoxides from the ozonolysis of alkenes, in addition to Criegee intermediates, have been proposed as an atmospheric source of OH radicals in the absence of sunlight, but have remained largely elusive due to their reactivity. A weak peroxide bond enables facile OH elimination, and subsequent β-scission can lead to a variety of decomposition products depending on the nature of the peroxide. In this paper we explore this process theoretically for the simplest ketohydroperoxide, hydroperoxyacetaldehyde (HPA), which is believed to be formed in the ozonolysis of ethylene. Despite it being the most stable C₂H₄O₃ species in this reaction scheme, lower in energy than the starting materials by around 100 kcal mol^-1, HPA has only been directly observed once in the ozonolysis of ethylene by photoionization mass spectrometry appearance energy. Here we report predictions of the rotational spectrum of HPA conducted in support of microwave spectroscopy experiments. We suggest a new dissociation path from HPA to glyoxal [HOOCH₂CHO → HCOCH₂O + OH → CHOCHO + H], supported by thermochemical calculations. We encourage the search for glyoxal using complementary experimental methods, and suggest possible future experimental directions. Evidence of glyoxal formation from ethylene ozonolysis might provide evidence of this underappreciated path in an important and long studied reaction system.

Potential interferences in photolytic nitrogen dioxide converters for ambient air monitoring: Evaluation of a prototype

Mixing ratios of the criteria air contaminant nitrogen dioxide (NO₂) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O₃ chemiluminescence (CL). In this work, the performance of a photolytic NO₂ converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO₂ and NO_x (= NO + NO₂) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO₂ photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 s^-1. A maximum 96% conversion of NO₂ to NO over a large dynamic range was achieved at a residence time of (1.5 ± 0.3) s, independent of relative humidity. Interferences from odd nitrogen (NO_y) species such as peroxyacyl nitrates (PAN; RC(O)O₂NO₂), alkyl nitrates (AN; RONO₂), nitrous acid (HONO), and nitric acid (HNO₃) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified as follows: direct photolysis, foremost of HONO at a rate constant of 6% that of NO₂; thermal decomposition, primarily of PAN; surface promoted photochemistry; and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connecting tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO₂ in ambient air.

IMPLICATIONS

A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO2) in ambient air by NO-O3 chemiluminescence (CL) was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of nitrous acid, which occurred at a rate constant 6% that of NO2, thermal decomposition of PAN and N2O5, surface promoted chemistry involving nitric acid, and secondary chemistry involving NO in the tubing connecting the converter and CL analyzer. These interferences are predicted to occur in all NO2 P-CL systems but can be avoided by appropriate thermal management and operating at high flow rates.

Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditions

Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevant polluted conditions (NO x ~ 10ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH 4 + CIMS and I- CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O : C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring-scission products are also detected in both the gas and particle phases, and their yields and speciation generally agree with the kinetic model prediction.

Electron Attachment Reaction Ionization of Gas-Phase Methylglyoxal

Methylglyoxal (MGLY) plays a significant role in atmospheric chemistry by serving as a key contributor to the formation of active free radicals, ozone, and secondary organic aerosol. Detection of MGLY by traditional chemical ionization such as proton-transfer reaction has several shortcomings such as parent molecule fragmentation. In this study, an electron attachment reaction (EAR) ionization method has been developed for the effective detection of MGLY. Almost no fragmentation was observed during the EAR. The generation of MGLY^- anion in the EAR was further confirmed by cryogenic photoelectron imaging spectroscopy. The concentration of MGLY can be calibrated by using dibromomethane (CH₂Br₂) as reference gas. The detection sensitivity of MGLY was estimated to be (100 ± 2) mV/ppbv (parts per billion by volume). The O₂, H₂O, CO₂, and trace gases in ambient air have no obvious effects on the detection of MGLY^- anion by the EAR ionization method.

Clair JM, Wolfe GM, Hanisco TF, Keutsch FN, Ryerson TB, Brown SS. Emissions of Glyoxal and Other Carbonyl Compounds from Agricultural Biomass Burning Plumes Sampled by Aircraft

We report enhancements of glyoxal and methylglyoxal relative to carbon monoxide and formaldehyde in agricultural biomass burning plumes intercepted by the NOAA WP-3D aircraft during the 2013 Southeast Nexus and 2015 Shale Oil and Natural Gas Nexus campaigns. Glyoxal and methylglyoxal were measured using broadband cavity enhanced spectroscopy, which for glyoxal provides a highly selective and sensitive measurement. While enhancement ratios of other species such as methane and formaldehyde were consistent with previous measurements, glyoxal enhancements relative to carbon monoxide averaged 0.0016 ± 0.0009, a factor of 4 lower than values used in global models. Glyoxal enhancements relative to formaldehyde were 30 times lower than previously reported, averaging 0.038 ± 0.02. Several glyoxal loss processes such as photolysis, reactions with hydroxyl radicals, and aerosol uptake were found to be insufficient to explain the lower measured values of glyoxal relative to other biomass burning trace gases, indicating that glyoxal emissions from agricultural biomass burning may be significantly overestimated. Methylglyoxal enhancements were three to six times higher than reported in other recent studies, but spectral interferences from other substituted dicarbyonyls introduce an estimated correction factor of 2 and at least a 25% uncertainty, such that accurate measurements of the enhancements are difficult.

Aerosol liquid water driven by anthropogenic nitrate: implications for lifetimes of water-soluble organic gases and potential for secondary organic aerosol formation

Aerosol liquid water (ALW) influences aerosol radiative properties and the partitioning of gas-phase water-soluble organic compounds (WSOCg) to the condensed phase. A recent modeling study drew attention to the anthropogenic nature of ALW in the southeastern United States, where predicted ALW is driven by regional sulfate. Herein, we demonstrate that ALW in the Po Valley, Italy, is also anthropogenic but is driven by locally formed nitrate, illustrating regional differences in the aerosol components responsible for ALW. We present field evidence for the influence of controllable ALW on the lifetimes and atmospheric budgets of reactive organic gases and note the role of ALW in the formation of secondary organic aerosol (SOA). Nitrate is expected to increase in importance due to increased emissions of nitrate precursors, as well as policies aimed at reducing sulfur emissions. We argue that the impacts of increased particulate nitrate in future climate and air quality scenarios may be under predicted because they do not account for the increased potential for SOA formation in nitrate-derived ALW, nor do they account for the impacts of this ALW on reactive gas budgets and gas-phase photochemistry.

Quantitative and qualitative sensing techniques for biogenic volatile organic compounds and their oxidation products

The physiological production mechanisms of some of the organics in plants, commonly known as biogenic volatile organic compounds (BVOCs), have been known for more than a century. Some BVOCs are emitted to the atmosphere and play a significant role in tropospheric photochemistry especially in ozone and secondary organic aerosol (SOA) productions as a result of interplays between BVOCs and atmospheric radicals such as hydroxyl radical (OH), ozone (O3) and NOX (NO + NO2). These findings have been drawn from comprehensive analysis of numerous field and laboratory studies that have characterized the ambient distribution of BVOCs and their oxidation products, and reaction kinetics between BVOCs and atmospheric oxidants. These investigations are limited by the capacity for identifying and quantifying these compounds. This review highlights the major analytical techniques that have been used to observe BVOCs and their oxidation products such as gas chromatography, mass spectrometry with hard and soft ionization methods, and optical techniques from laser induced fluorescence (LIF) to remote sensing. In addition, we discuss how new analytical techniques can advance our understanding of BVOC photochemical processes. The principles, advantages, and drawbacks of the analytical techniques are discussed along with specific examples of how the techniques were applied in field and laboratory measurements. Since a number of thorough review papers for each specific analytical technique are available, readers are referred to these publications rather than providing thorough descriptions of each technique. Therefore, the aim of this review is for readers to grasp the advantages and disadvantages of various sensing techniques for BVOCs and their oxidation products and to provide guidance for choosing the optimal technique for a specific research task.

Microfluidic lab-on-a-chip derivatization for gaseous carbonyl analysis

We present a microfluidic lab-on-a-chip derivatization technique for the analysis of gaseous carbonyl compounds using O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) as the derivatizing reagent. The novel microfluidic lab-on-a-chip derivatization technique has been developed to measure nmol per mole (ppbv) mixing ratios of gaseous carbonyl compounds, which are of particular importance to atmospheric chemistry. The technique utilised a planar glass microreactor comprising three inlets and one outlet, gas and fluid splitting and combining channels, mixing junctions, and a 2.0m long, 620μm internal diameter reaction microchannel. The microreactor integrated three functions, providing: (1) a gas and liquid mixer and reactor, (2) reagent heating, and (3) sample pre-concentration. The concentration of derivatization solution, the volumetric flow rates of the incoming gas sample and PFBHA solution, and the temperature of the microreactor were optimised to achieve a near real-time measurement. The enhanced phase contact area-to-volume ratio and the high heat transfer rate in the microreactor resulted in a fast and high efficiency derivatization reaction, generating an effluent stream which was ready for direct introduction to GC-MS. Good linearity was observed for eight carbonyl compounds over the measurement ranges of 1-500ppbv when they were derivatized under optimal reaction conditions. The method detection limits (MDLs) were below 0.10nmolmol(-1) for most carbonyls in this study, which is below or close to their typical concentrations in clean ambient air. The performance of the technique was assessed by applying the methodology to the quantification of glyoxal (GLY) and methylglyoxal (MGLY) formed during isoprene photo-oxidation in an outdoor photoreactor chamber (EUPHORE). Good agreements between GLY and MGLY measurements were obtained comparing this new technique with Fourier Transform InfraRed (FTIR), which provides support for the potential effectiveness of the microfluidic technique for gaseous measurements.

Quantitative infrared intensity studies of vapor-phase glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) with vibrational assignments

Glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) are all known biomass burning effluents and suspected aerosol precursors. Pressure-broadened quantitative infrared spectra of glyoxal, methylglyoxal, and diacetyl vapors covering the 520-6500 cm(-1) range are reported at 0.112 cm(-1) resolution, each with a composite spectrum derived from a minimum of 10 different sample pressures for the compound, representing some of the first quantitative intensity data for these analytes. Many vibrational assignments for methylglyoxal are reported for the first time, as are some near-IR and far-IR bands of glyoxal and diacetyl. To complete the vibrational assignments, the far-infrared spectra (25-600 cm(-1)) of all three vapors are also reported, those of methylglyoxal for the first time. Density functional theory and ab initio MP2 theory are used to help assign vibrational modes. Potential bands for atmospheric monitoring are discussed.

Characterization of vapor wall loss in laboratory chambers

Laboratory chambers used to study atmospheric chemistry and aerosol formation are subject to wall loss of vapors and particles that must be accounted for in calculating aerosol yields. While particle wall loss in chambers is relatively well-understood and routinely accounted for, that of vapor is less so. Here we address experimental measurement and modeling of vapor losses in environmental chambers. We identify two compounds that exhibit wall loss: 2,3-epoxy-1,4-butanediol (BEPOX), an analog of an important isoprene oxidation product; and glyoxal, a common volatile organic compound oxidation product. Dilution experiments show that BEPOX wall loss is irreversible on short time scales but is reversible on long time scales, and glyoxal wall loss is reversible for all time scales. BEPOX exhibits minimal uptake onto clean chamber walls under dry conditions, with increasing rates of uptake over the life of an in-use chamber. By performing periodic BEPOX wall loss experiments, it is possible to assess quantitatively the aging of chamber walls.

Measurement of airborne carbonyls using an automated sampling and analysis system

Based upon the well established method of derivitization with 2,4-dinitrophenylhydrazine, an instrument was developed for ambient measurement of carbonyls with significantly improved temporal resolution and detection limits through automation, direct injection, and continuous use of a single microsilica DNPH cartridge. Kinetic experiments indicate that the derivitization reaction on the cartridge is fast enough for continuous measurements with 50 min air sampling. Reaction efficiencies measured on the cartridge were 100% for the carbonyls tested, including formaldehyde, acetaldehyde, propanal, acetone, and benzaldehyde. Transmission of the carbonyls through an ozone scrubber (KI) were in the range of 97-101%. Blank levels and detection limits were lower than those obtainable with conventional DNPH methods by an order of magnitude or greater. Mixing ratio detection limits of carbonyls in ambient air were 38-73 ppt for a 50 min air sample (2.5 L). The instrument made continuous measurements of carbonyls on a 2 h cycle over a period of 10 days during a field study in southwestern Ontario. Median mixing ratios were 0.58 ppb formaldehyde; 0.29 ppb acetaldehyde; 1.14 ppb acetone; and 0.45 ppb glyoxal. Glyoxal shows a significant correlation with ozone and zero intercept, consistent with a secondary source and minor direct source to the atmosphere. The method should easily be extendable to the detection of other low molecular weight carbonyls that have been previously reported using the DNPH technique.

A sensitive and versatile detector for atmospheric NO2 and NOx based on blue diode laser cavity ring-down spectroscopy

A sensitive, small detector was developed for atmospheric NO2 and NOx concentration measurements. NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 404 nm. The sum of NO and NO2 (=NOx) is simultaneously measured in a second cavity by quantitative conversion of ambient NO to NO2 in excess ozone. Interferences due to absorption by other trace gases at 404 nm, such as ozone and water vapor, are either negligible or small and are easily quantified. The limit of detection is 22 pptv (2sigma precision) for NO2 at 1 s time resolution. The conversion efficiency of NO to NO2 is 99% in excess O3. The accuracy of the NO2 measurement is mainly limited by the NO2 absorption cross section to +/-3%. Because of the formation of undetectable higher nitrogen oxides in subsequent reactions of NO2 with ozone in the NOx channel, the (1sigma) accuracy of the NOx measurement is increased to approximately +/-5% depending on the level of NOx. The new instrument was designed to be easily deployed in the field with respect to size, weight and consumables. Measurements were validated against a photolysis/chemiluminescence detector during six days of sampling ambient air with colocated inlets. The data sets for NO2, NO and NOx exhibit high correlation and good agreement within the combined accuracies of both methods. Linear fits for all three species give similar slopes of 0.99 in ambient air.

Photooxidation of 2-methyl-3-Buten-2-ol (MBO) as a potential source of secondary organic aerosol

2-Methyl-3-buten-2-ol (MBO) is an important biogenic hydrocarbon emitted in large quantities by pine forests. Atmospheric photooxidation of MBO is known to lead to oxygenated compounds, such as glycolaldehyde, which is the precursor to glyoxal. Recent studies have shown that the reactive uptake of glyoxal onto aqueous particles can lead to formation of secondary organic aerosol (SOA). In this work, MBO photooxidation under high- and low-NO(x) conditions was performed in dual laboratory chambers to quantify the yield of glyoxal and investigate the potential for SOA formation. The yields of glycolaldehyde and 2-hydroxy-2-methylpropanal (HMPR), fragmentation products of MBO photooxidation, were observed to be lower at lower NO(x) concentrations. Overall, the glyoxal yield from MBO photooxidation was 25% under high-NO(x) and 4% under low-NO(x) conditions. In the presence of wet ammonium sulfate seed and under high-NO(x) conditions, glyoxal uptake and SOA formation were not observed conclusively, due to relatively low (< 30 ppb) glyoxal concentrations. Slight aerosol formation was observed under low-NO(x) and dry conditions, with aerosol mass yields on the order of 0.1%. The small amount of SOA was not related to glyoxal uptake, but is likely a result of reactions similar to those that generate isoprene SOA under low-NO(x) conditions. The difference in aerosol yields between MBO and isoprene photooxidation under low-NO(x) conditions is consistent with the difference in vapor pressures between triols (from MBO) and tetrols (from isoprene). Despite its structural similarity to isoprene, photooxidation of MBO is not expected to make a significant contribution to SOA formation.

Measurement of atmospheric hydroxyacetone, glycolaldehyde, and formaldehyde

A method has been modified and optimized for the measurements of hydroxyacetone as well as formaldehyde and glycolaldehyde, based on aqueous scrubbing using a coil sampler followed by DNPH derivatization and HPLC analysis. Derivatization equilibrium and kinetics were studied to optimize the hydroxyacetone-DNPH derivative yield. It was found that the low sensitivity of hydroxyacetone by this method is due to a relatively small equilibrium constant for the hydroxyacetone-DNPH derivatization reaction, and thus it can be improved by increasing DNPH reagent concentration. In a medium containing 500 microM DNPH and 50 mM HCl, the derivatization reaches equilibrium within 30 min. An online reagent purification procedure using a DNPH-saturated Sep-Pak C-18 cartridge effectively removed hydrazone impurities in the DNPH reagent solution, and a sample preconcentration procedure using a C-18 guard column greatly enhanced the sensitivity and lowered the detection limits. The lower detection limits of the system under optimized conditions are 30, 9, and 36 pptv for hydroxyacetone, glycolaldehyde, and formaldehyde, respectively, with a sampling/analysis cycle time of 30 min. The method has been successfully deployed at a rural site in Pinnacle State Park in Addison, NY, for a 5 week period during the summer of 1998. The ambient concentration means (medians) were 372 (332), 301 (323), and 2040 (2030) pptv for hydroxyacetone, glycolaldehyde, and formaldehyde, respectively. A late-afternoon maximum and an early morning minimum were observed in the diurnal concentration distributions of all three carbonyl compounds. Good correlations among the three carbonyl compounds suggest that they originated from a common source, i.e., photochemical oxidation of biogenic hydrocarbons. Formaldehyde photolysis accounted for about 23% of the total radical photoproduction, whereas contributionsfrom hydroxyacetone and glycolaldehyde photolysis were insignificant because of the much slower photolysis and lower concentrations of these compounds.

A laser induced fluorescence-based instrument for in-situ measurements of atmospheric formaldehyde

Direct, in situ detection of gas phase formaldehyde (HCHO) via laser induced fluorescence in a White-type multipass cell is demonstrated with a (3sigma) limit of detection of approximately 0.051 parts per billion by volume in a 1 s sampling time. Calibration is performed in two ways: using permeation tubes and with air bubbled through an aqueous solution of HCHO. The concentration of HCHO output from the bubbler is measured by cavity ring-down spectroscopy. Measurement of ambient HCHO is carried out at the University of Wisconsin, Madison for a period of several days.