Abstract The U.S. Environmental Protection Agency is promoting the development and application of sampling methods for the semicontinuous determination of fine particulate matter (PM2.5, particles with an aerodynamic diameter <2.5 µm) mass and chemical composition. Data obtained with these methods will significantly improve the understanding of the primary sources, chemical conversion processes, and meteorological atmospheric processes that lead to observed PM2.5 concentrations and will aid in the understanding of the etiology of PM2.5-related health effects. During January and February 2007, several semicontinuous particulate matter (PM) monitoring systems were compared at the Utah State Lindon Air Quality Sampling site. Semicontinuous monitors included instruments to measure total PM2.5 mass (filter dynamic measurement system [FDMS] tapered element oscillating microbalance [TEOM], GRIMM), nonvolatile PM2.5 mass (TEOM), sulfate and nitrate (two PM2.5 and one PM10 [PM with an aerodynamic diameter <10 µm] ionchromatographic-based samplers), and black carbon (aethalometer). PM10 semicontinuous mass measurements were made with GRIMM and TEOM instruments. These measurements were all made on a 1-hr average basis. Source apportionment analysis indicated that sources impacting the site were mainly urban sources and included mobile sources (gasoline and diesel) and residential burning of wood, with some elevated concentrations because of the effect of winter inversions. The FDMS TEOM and GRIMM instruments were in good agreement, but TEOM monitor measurements were low because of the presence of significant semi-volatile material. Semi-volatile mass was present dominantly in the PM2.5 mass.
The formation of sulfuric acid (H(2)SO(4)), nitric acid (HNO(3)), acetic acid (CH(3)C(O)OH), and formic acid (HC(O))H) complexes with ammonia (NH(3)), amidogen radical (NH(2)), and imidogen radical (NH) was studied using natural bond orbital calculations. The equilibrium structures, binding energies, and harmonic frequencies were calculated for each acid-NH(x) complex using hybrid density functional (B3LYP) and second-order Møller-Plesset perturbation approximation methods with the 6-311++G(3df,3pd) basis set. The results presented here suggest that the effect of NH(2) on the formation of new condensation nuclei will be similar to that of NH(3), but to a lesser degree and confined primarily to complexes with H(2)SO(4) and HNO(3). The NH radical is not expected to play a significant role in the formation of new atmospheric condensation nuclei.
Xylose reversion reactions to form xylooligomers represent a potentially important mechanism of sugar loss during dilute acid pretreatment of biomass. We have conducted a study to identify the products that result from these reactions and to determine the kinetics of their formation. A major obstacle is that there are few commercial standards available for xylose disaccharides, which are essential for the identification and quantification of the xylose reversion products formed during these reactions. To overcome this obstacle, we have used GC/MS and NMR analysis of xylose disaccharides isolated by preparative HPLC to identify the reaction products. At the xylose concentration we used (300 g L–1), only xylose disaccharides were observed. As with glucose reversion reactions [Pilath, H. M.; et al. J. Agric. Food Chem. 2010, 58, 6131], the disaccharides contained linkages that involved the anomeric carbon atom of one of the sugar monomers. Eight out of the nine possible disaccharides, including alpha and beta anomers, were observed. Whereas the GC/MS allowed for the identification of the linkages, NMR was needed to distinguish between the α and β isomers of the disaccharides. The kinetics of combined xylose disaccharide formation was measured using HPLC. Arrhenius parameters for the rates of disaccharide formation were calculated by fitting the data to a simple model.
Abstract Rate coefficients k for the OH+Cl 2 O reaction are measured as a function of temperature (230–370 K) and pressure by using pulsed laser photolysis to produce OH radicals and laser‐induced fluorescence to monitor their loss under pseudo‐first‐order conditions in OH. The reaction rate coefficient is found to be independent of pressure, within the precision of our measurements at 30–100 Torr (He) and 100 Torr (N 2 ). The rate coefficients obtained at 100 Torr (He) showed a negative temperature dependence with a weak non‐Arrhenius behavior. A room‐temperature rate coefficient of k 1 (297 K)=(7.5±1.1)×10 −12 cm 3 molecule −1 s −1 is obtained, where the quoted uncertainties are 2 σ and include estimated systematic errors. Theoretical methods are used to examine OH ⋅⋅⋅ OCl 2 and OH ⋅⋅⋅ ClOCl adduct formation and the potential‐energy surfaces leading to the HOCl+ClO (1 a) and Cl+HOOCl (1 d) products in reaction (1) at the hybrid density functional UMPW1K/6‐311++G(2df,p) level of theory. The OH ⋅⋅⋅ OCl 2 and OH ⋅⋅⋅ ClOCl adducts are found to have binding energies of about 0.2 kcal mol −1 . The reaction is calculated to proceed through weak pre‐reactive complexes. Transition‐state energies for channels (1 a) and (1 d) are calculated to be about 1.4 and about 3.3 kcal mol −1 above the energy of the reactants. The results from the present study are compared with previously reported rate coefficients, and the interpretation of the possible non‐Arrhenius behavior is discussed.
ABSTRACT Peroxy radicals can complex with water vapor. These complexes affect tropospheric chemistry. In this study, β‐HEP (hydroxyethyl peroxy radical) serves as a model system for investigating the effect of water vapor on the kinetics and product branching ratio of the self‐reaction of peroxy radicals. The self‐reaction rate coefficient was determined at 274–296 K with water vapor between 1.0 × 10 15 and 2.5 × 10 17 molecules cm −3 at 200 Torr total pressure by slow‐flow laser flash photolysis coupled with UV time‐resolved spectroscopy and long‐path, wavelength modulated, diode‐laser spectroscopy. The overall self‐reaction rate constant expressed as the product of both a temperature‐dependent and water vapor–dependent term is , suggesting formation of a β‐HEP‐H 2 O complex is responsible for the increase in the self‐reaction rate coefficient with increasing water concentration. Complex formation is supported by computational results identifying three local energy minima for the β‐HEP‐H 2 O complex. As the troposphere continues to get warmer and wetter, more of the peroxy radicals present will be complexed with water. Investigating the effect of water vapor on kinetics of atmospherically relevant radicals and determining the effects of these altered kinetics on tropospheric ozone concentrations is thus important.
