Simulated rainfall on agricultural soil reveals enzymatic regulation of short-term nitrous oxide profiles in soil gas and emissions from the surface

© 2016, Springer International Publishing Switzerland. Many microbial species use nitrate to support respiration under oxygen limiting conditions via the process of denitrification, which generates the gaseous products nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2). Denitrifying bacteria reduce NO to N2O, which is a potent greenhouse gas, to maintain intercellular concentrations below cytotoxic levels. The enzymes that reduce N2O to N2 play a crucial role in restricting N2O emissions from the surface. Laboratory studies have demonstrated that accumulation of N2O results from unbalanced rates of the sequence of denitrification reactions, which is ascribed to enzyme kinetics and sequential gene expression. However, the same enzymatic regulation of N2O accumulation in soil during short periods of anoxia has not been observed in the field. Here, we investigated the role of enzymatic regulation on the accumulation of N2O during a transition from oxic to anoxic conditions that was induced by a simulated rainfall in the field. Distinct regulation regimes for activities of pre- and de novo synthesized denitrification enzymes were observed. The activity of N2O reductase played a crucial role in regulating N2O emissions. Dynamics of N2O mixing ratios in soil gas and emissions from the surface were in excellent agreement with simulations using a one-dimensional, diffusion–reaction equation with explicit representations of denitrification enzyme kinetics. A more explicit representation of the regulatory biology of denitrification in current biogeochemical models, like the approach developed in the subject study, is a promising strategy for improving predictions of episodic emissions of N2O from soil

Using multidimensional gas chromatography to group secondary organic aerosol species by functionality

A carbon number-functionality grid (CNFG) for a complex mixture of secondary organic aerosol (SOA) precursors and oxidation products was developed from the theoretical retention index diagram of a multidimensional gas chromatographic (GC×2GC) analysis of a mixture of SOA precursors and derivatized oxidation products. In the GC×2GC analysis, comprehensive separation of the complex mixture was achieved by diverting the modulated effluent from a polar primary column into 2 polar secondary columns. Column stationary phases spanned the widest range of selectivity of commercially available GC analytic columns. In general, separation of the species by the polar primary column was by the number of carbon atoms in the molecule (when the homologous series of reference compounds was selected to have molecular volumes and functionalities similar to the target analytes) and the polar secondary columns provided additional separation according to functionality. An algebraic transformation of the Abraham solvation parameter model was used to estimate linear retention indices of solutes relative to elution of a homologous series of methyl diesters on the primary and secondary columns to develop the theoretical GC×2GC retention diagram. Retention indices of many of the oxidation products of SOA precursors were estimated for derivatized forms of the solutes. The GC stationary phases selected for the primary column [(50%-Trifluoropropyl)-methylpolysiloxane] and secondary columns (90% Cyanopropyl Polysilphenylene-siloxane and Polyethylene Glycol in a Sol-Gel matrix) provided a theoretical separation of 33 SOA precursors and 98 derivatized oxidation products into 35 groups by molecular volume and functionality. Comprehensive analysis of extracts of vapor and aerosol samples containing semivolatile SOA precursors and oxidation products, respectively, is best accomplished by (1) separating the complex mixture of the vapor and underivatized aerosol extracts with a (50%-Trifluoropropyl)-methylpolysiloxane×90% Cyanopropyl Polysilphenylene-siloxane×Polyethylene Glycol in a Sol-Gel matrix arrangement and (2) derivatizing the aerosol extract and reanalyzing the sample on the GC×2GC column combination. Quantifying groupings and organic molecular species in time series of collections of vapor- and aerosol-phase atmospheric organic matter is a promising analytic technique for measuring production of SOA and evaluating transformations of SOA precursors. © 2014 Elsevier Ltd

Vapor- and aerosol-phase atmospheric organic matter in urban air of the Midwest USA

Vapor- and aerosol-phase atmospheric organic matter were collected in East St. Louis, MO using the high-volume sampling method. Samples were processed by traditional analytical methods and analyzed by multidimensional gas chromatography with time-of-flight mass spectrometric detection. Levels of identified, hydrocarbon-like organic vapor and aerosol species (i.e., HOV and HOA, respectively) were 10–42 ng m−3 and 0.020–3.6 ng m−3, respectively. Concentrations of identified, oxygenated organic vapor and aerosol species (i.e., OOV and OOA, respectively) were 2–66 ng m−3 and 23–310 ng m−3, respectively. The principal feature of the HOV was an unresolved complex mixture that represented 54.5 ± 11.3% of the total HOVs during the field campaign. Levels of n- and branched alkanes, alkyl and cycloalkylbenzenes, polyaromatic hydrocarbons (PAHs), and alkyl-substituted PAHs generally declined from the morning rush hour to the 1000–1400 sampling period, which was similar to the expected trend in reactivity with respect to OH. The OOV included aliphatic mono-carboxylic acids, aliphatic and aromatic alcohols, aldehydes, and ketones, and alicyclic ketones, alcohols, and epoxides (i.e., montoterpenoids). The bulk of the OA species (∼99%) were a complex mixture of OOA, which included multifunctional n-aliphatic, alicyclic, and aromatic hydrocarbons, dicarboxylic and ketocarboxylic aliphatic and dicarboxylic monoaromatic acids, lactones, tetrols, and pentitols. The sampling and analytic techniques provided quantitative molecular information for HOVs in ambient air, which are a missing source of secondary organic aerosol precursors. Molecular characterization and quantitation of HOVs and OOA species will facilitate predictions of SOA formation using molecular-specific models

Estimating terpene and terpenoid emissions from conifer oleoresin composition

© 2015 Elsevier Ltd. The following algorithm, which is based on the thermodynamics of nonelectrolyte partitioning, was developed to predict emission rates of terpenes and terpenoids from specific storage sites in conifers:. E \u3c inf\u3e i =x \u3c inf\u3e or \u3c sup\u3e i γ \u3c inf\u3e or \u3c sup\u3e i p \u3c inf\u3e i {ring operator}where E \u3c inf\u3e i is the emission rate (μg C gdw \u3c sup\u3e -1 h \u3c sup\u3e -1 ) and p \u3c inf\u3e i {ring operator} is the vapor pressure (mm Hg) of the pure liquid terpene or terpenoid, respectively, and x \u3c inf\u3e or \u3c sup\u3e i and γ \u3c inf\u3e or \u3c sup\u3e i are the mole fraction and activity coefficient (on a Raoult\u27s law convention), respectively, of the terpene and terpenoid in the oleoresin. Activity coefficients are calculated with Hansen solubility parameters that account for dispersive, polar, and H-bonding interactions of the solutes with the oleoresin matrix. Estimates of p \u3c inf\u3e i {ring operator} at 25°C and molar enthalpies of vaporization are made with the SIMPOL.1 method and are used to estimate p \u3c inf\u3e i {ring operator} at environmentally relevant temperatures. Estimated mixing ratios of terpenes and terpenols were comparatively higher above resin-acid- and monoterpene-rich oleoresins, respectively. The results indicated a greater affinity of terpenes and terpenols for the non-functionalized and carboxylic acid containing matrix through dispersive and H-bonding interactions, which are expressed in the emission algorithm by the activity coefficient. The correlation between measured emission rates of terpenes and terpenoids for Pinus strobus and emission rates predicted with the algorithm were very good (R=0.95). Standard errors for the range and average of monoterpene emission rates were ±6 - ±86% and ±54%, respectively, and were similar in magnitude to reported standard deviations of monoterpene composition of foliar oils (±38 - ±51% and ±67%, respectively)

Evaluation of multistep derivatization methods for identification and quantification of oxygenated species in organic aerosol

© 2015 Elsevier B.V. Two, 3-step methods for derivatizing mono- and multi-functional species with carbonyl (CO), carboxylic acid (-COOH), and alcohol (-OH) moieties were compared and optimized. In Method 1, the CO, -COOH, and -OH moieties were converted (1) to methyloximes (R-CN-OCH3) with O-methylhydroxylamine hydrochloride (MHA), (2) to methyl esters (OC-R-OCH3) with (trimethylsilyl)diazomethane in methanol (TMSD/MeOH), and (3) to trimethylsilyl ethers [R-OSi(CH3)3] with N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) containing 1% trimethylchlorosilane (TMCS), respectively. Steps 1 and 3 of both methods were identical; however, in Step 2 of Method 2, -COOH moieties were derivatized with 10% (v/v) boron trifluoride (BF3) in MeOH or n-butanol (n-BuOH). The BF3/MeOH and BF3/n-BuOH were ineffective at converting species with more than 2-OH moieties. Average standard deviations for derivatization of 36 model compounds by the 3-step methods using TMSD/MeOH and BF3/(MeOH) were 7.4 and 14.8%, respectively. Average derivatization efficiencies for Methods 1 and 2 were 88.0 and 114%, respectively. Despite the lower average derivatization efficiency of Method 1, distinct advantages included a greater certainty of derivatization yield for the entire suite of mono- and multi-functional species and fewer processing steps for sequential derivatization. Detection limits for Method 1 using GC×GC-ToF-MS were 0.3-54pgm-3. Approximately 100 oxygenated organic species were identified and quantified in aerosol filtered from 39m3 of air in an urban location. Levels of species were 0.013-17ngm-3 and were nearly all above the Method 1 limit of detection

Modeling Nitrous Oxide Production and Reduction in Soil through Explicit Representation of Denitrification Enzyme Kinetics

An enzyme-explicit denitrification model with representations for pre- and <i>de novo</i> synthesized enzymes was developed to improve predictions of nitrous oxide (N₂O) accumulations in soil and emissions from the surface. The metabolic model of denitrification is based on dual-substrate utilization and Monod growth kinetics. Enzyme synthesis/activation was incorporated into each sequential reduction step of denitrification to regulate dynamics of the denitrifier population and the active enzyme pool, which controlled the rate function. Parameterizations were developed from observations of the dynamics of N₂O production and reduction in soil incubation experiments. The model successfully reproduced the dynamics of N₂O and N₂ accumulation in the incubations and revealed an important regulatory effect of denitrification enzyme kinetics on the accumulation of denitrification products. Pre-synthesized denitrification enzymes contributed 20, 13, 43, and 62% of N₂O that accumulated in 48 h incubations of soil collected from depths of 0–5, 5–10, 10–15, and 15–25 cm, respectively. An enzyme activity function (<i>E</i>) was defined to estimate the relative concentration of active enzymes and variation in response to environmental conditions. The value of <i>E</i> allows for activities of pre-synthesized denitrification enzymes to be differentiated from <i>de novo</i> synthesized enzymes. Incorporating explicit representations of denitrification enzyme kinetics into biogeochemical models is a promising approach for accurately simulating dynamics of the production and reduction of N₂O in soils

Mechanistic Insight into the Reactivities of Aqueous-Phase Singlet Oxygen with Organic Compounds

Singlet oxygen (1O2) is a selective reactive oxygen species that plays a key role for the fate of various organic compounds in the aquatic environment under sunlight irradiation, engineered water oxidation systems, atmospheric water droplets, and biomedical systems. While the initial rate-determining charge-transfer reaction mechanisms and kinetics of 1O2 have been studied extensively, no comprehensive studies have been performed to elucidate the reaction mechanisms with organic compounds that have various functional groups. In this study, we use density functional theory calculations to determine elementary reaction mechanisms with a wide variety of organic compounds. The theoretically calculated aqueous-phase free energies of activation of single electron transfer and 1O2 addition reactions are compared to the experimentally determined rate constants in the literature to determine linear free-energy relationships. The theoretically calculated free energies of activation for the groups of phenolates and phenols show excellent correlations with the Hammett constants that accept electron densities by through-resonance. The dominant elementary reaction mechanism is discussed for each group of compounds. As a practical implication, we demonstrate the fate of environmentally relevant organic compounds induced by photochemically produced intermediate species at different pH and evaluate the impact of predicting rate constants to the half-life

Elucidation of the Photochemical Fate of Methionine in the Presence of Surrogate and Standard Dissolved Organic Matter under Sunlight Irradiation

The abiotic fate of dissolved free amino acids considerably contributes to the cycling of dissolved sulfur and nitrogen in natural aquatic environments. However, the roles of the functional groups of chromophoric dissolved organic matter (CDOM) and the fate of free amino acids under sunlight irradiation in fresh waters are not fully understood. This study aims to elucidate the fate of photolabile methionine in the presence of three CDOM surrogate compounds, i.e., 1,4-naphthoquinone, 2-naphthaldehyde, and umbelliferone, and two standard CDOM by coupling experimental measurement, quantum chemical computations, and kinetic modeling. Results indicate that excited triplet-state CDOM and hydroxyl radicals are able to cleave the C-S bond in methionine, resulting in the formation of smaller amino acids and volatile sulfur-containing compounds. Singlet oxygen forms methionine sulfoxide and methionine sulfone. The distribution of phototransformation products offers an improved understanding of the fate of nitrogen- and sulfur-containing compounds and their uptake by microorganisms in natural aquatic environments

Evaluation of novel techniques for measurement of air-water exchange of persistent bioaccumulative toxicants in Lake Superior

We report initial measurements of concentrations and net air-water exchange fluxes of target persistent bioaccumulative toxicants (PBTs) in Lake Superior utilizing techniques not previously applied for this purpose. Gaseous PBTs are collected in diffusion denuders containing sections of commercial chromatography columns and subsequently thermally extracted into the cooled injection inlet of a high-resolution gas chromatograph. The PBT sampling/analytical methods enable accurate determination of gas-phase PBT concentration and micrometeorological measurement of fluxes to be carried out. PBT fluxes are measured by the modified Bowen ratio technique in which sensible heat flux is related to PBT flux, with the assumption of identical transfer velocities of heat and PBTs between two heights in the atmospheric surface layer. Micrometeorological measurement of flux accounts for all sources of resistance to mass transfer, including atmospheric stability effects, surface films, waves, sea spray, and bubbles. The sensible heat flux, PBT concentration, and PBT flux measurements carried out in 14 2- or 3-h periods during seven sampling events in Lake Superior in summer and fall 2002 and spring 2003 demonstrate advantages under the constraints of the techniques. The uncertainty of the flux measurements was typically in the range from 1% to 160%. Gaseous concentrations of α-hexachlorocyclohexane (α-HCH) and hexachlorobenzene (HCB) over Lake Superior were in the range from 6 to 170 and 12-95 pg/m3, respectively. Fluxes out of Lake Superior were measurable in 75% of the cases in which a concentration gradient was measured, and were in the range from -0.17 to +0.064 ng/m 2·h for α-HCH and from -0.60 to -0.093 ng/m 2·h for HCB. © 2005 American Chemical Society