Atmospheric Simulation Chamber Studies of the Gas-Phase Photolysis of Pyruvic Acid.

Pyruvic acid is an atmospherically abundant α-keto-acid that degrades efficiently from the troposphere via gas-phase photolysis. To explore conditions relevant to the environment, 2-12 ppm pyruvic acid is irradiated by a solar simulator in the environmental simulation chamber, CESAM. The combination of the long path length available in the chamber and its low surface area to volume ratio allows us to quantitatively examine the quantum yield and photochemical products of pyruvic acid. Such details are new to the literature for the low initial concentrations of pyruvic acid employed here. We determined photolysis quantum yields of ϕobsN2 = 0.84 ± 0.1 in nitrogen and ϕobsAir = 3.2 ± 0.5 in air, which are higher than those reported by previous studies that used higher partial pressures of pyruvic acid. The quantum yield greater than unity in air is due to secondary chemistry, driven by O2, that emerges under the conditions in these experiments. The low concentration of pyruvic acid and the resulting oxygen effect also alter the product distribution such that acetic acid, rather than acetaldehyde, is the primary product in air. These results indicate that tropospheric pyruvic acid may degrade in part via photoinduced mechanisms that are different than previously expected.

Quantitative cancer risk assessment and local mortality burden for ambient air pollution in an eastern Mediterranean City.

Health risks posed by ambient air pollutants to the urban Lebanese population have not been well characterized. The aim of this study is to assess cancer risk and mortality burden of non-methane hydrocarbons (NMHCs) and particulates (PM) based on two field-sampling campaigns conducted during summer and winter seasons in Beirut. Seventy NMHCs were analyzed by TD-GC-FID. PM2.5 elemental carbon (EC) components were examined using a Lab OC-EC aerosol Analyzer, and polycyclic aromatic hydrocarbons were analyzed by GC-MS. The US EPA fraction-based approach was used to assess non-cancer hazard and cancer risk for the hydrocarbon mixture, and the UK Committee on Medical Effects of Air Pollutants (COMEAP) guidelines were followed to determine the PM2.5 attributable mortality burden. The average cumulative cancer risk exceeded the US EPA acceptable level (10-6) by 40-fold in the summer and 30-fold in the winter. Benzene was found to be the highest contributor to cancer risk (39-43%), followed by 1,3-butadiene (25-29%), both originating from traffic gasoline evaporation and combustion. The EC attributable average mortality fraction was 7.8-10%, while the average attributable number of deaths (AD) and years of life lost (YLL) were found to be 257-327 and 3086-3923, respectively. Our findings provide a baseline for future air monitoring programs, and for interventions aiming at reducing cancer risk in this population.

Gas-Phase Photolysis of Pyruvic Acid: The Effect of Pressure on Reaction Rates and Products.

In this work, we investigate the impact of pressure and oxygen on the kinetics of and products from the gas-phase photolysis of pyruvic acid. The results reveal a decrease in the photolysis quantum yield as pressure of air or nitrogen is increased, a trend not yet documented in the literature. A Stern-Volmer analysis demonstrates this effect is due to deactivation of the singlet state of pyruvic acid when the photolysis is performed in nitrogen, and from quenching of both the singlet and triplet state in air. Consistent with previous studies, acetaldehyde and CO2 are observed as the major products; however, other products, most notably acetic acid, are also identified in this work. The yield of acetic acid increases with increasing pressure of buffer gas, an effect that is amplified by the presence of oxygen. At least two mechanisms are necessary to explain the acetic acid, including one that requires reaction of photolysis intermediates with O2. These findings extend the fundamental understanding of the gas-phase photochemistry of pyruvic acid, highlighting the importance of pressure on the photolysis quantum yields and products.

Organic aerosol molecular composition and gas-particle partitioning coefficients at a Mediterranean site (Corsica).

Molecular speciation of atmospheric organic matter was investigated during a short summer field campaign performed in a citrus fruit field in northern Corsica (June 2011). Aimed at assessing the performance on the field of newly developed analytical protocols, this work focuses on the molecular composition of both gas and particulate phases and provides an insight into partitioning behavior of the semi-volatile oxygenated fraction. Limonene ozonolysis tracers were specifically searched for, according to gas chromatography-mass spectrometry (GC-MS) data previously recorded for smog chamber experiments. A screening of other oxygenated species present in the field atmosphere was also performed. About sixty polar molecules were positively or tentatively identified in gas and/or particle phases. These molecules comprise a wide range of branched and linear, mono and di-carbonyls (C3-C7), mono and di-carboxylic acids (C3-C18), and compounds bearing up to three functionalities. Among these compounds, some can be specifically attributed to limonene oxidation and others can be related to α- or ß-pinene oxidation. This provides an original snapshot of the organic matter composition at a Mediterranean site in summer. Furthermore, for compounds identified and quantified in both gaseous and particulate phases, an experimental gas/particle partitioning coefficient was determined. Several volatile products, which are not expected in the particulate phase assuming thermodynamic equilibrium, were nonetheless present in significant concentrations. Hypotheses are proposed to explain these observations, such as the possible aerosol viscosity that could hinder the theoretical equilibrium to be rapidly reached.

A new device for formaldehyde and total aldehydes real-time monitoring.

A new sensitive technique for the quantification of formaldehyde (HCHO) and total aldehydes has been developed in order to monitor these compounds, which are known to be involved in air quality issues and to have health impacts. Our approach is based on a colorimetric method where aldehydes are initially stripped from the air into a scrubbing solution by means of a turning coil sampler tube and then derivatised with 3-methylbenzothiazolinone-2-hydrazone in acid media (pH = -0.5). Hence, colourless aldehydes are transformed into blue dyes that are detected by UV-visible spectroscopy at 630 nm. Liquid core waveguide LCW Teflon® AF-2400 tube was used as innovative optical cells providing a HCHO detection limit of 4 pptv for 100 cm optical path with a time resolution of 15 min. This instrument showed good correlation with commonly used techniques for aldehydes analysis such as DNPH derivatisation chromatographic techniques with off-line and on-line samplers, and DOAS techniques (with deviation below 6%) for both indoor and outdoor conditions. This instrument is associated with simplicity and low cost, which is a prerequisite for indoor monitoring.

An experimental study of the gas-phase reactions of NO3 radicals with a series of unsaturated aldehydes: trans-2-hexenal, trans-2-heptenal, and trans-2-octenal.

Rate constants for the gas-phase reactions of the NO(3) radical with a series of unsaturated aldehydes, trans-2-hexenal, trans-2-heptenal, and trans-2-octenal, have been measured using absolute rate method at 294 ± 3 K and atmospheric pressure. This work was performed to clarify discrepancies found in the literature and thus led to a clearer view of the effect of the increasing carbon chain length on the reactivity of trans-2-alkenals. The rate constants were determined to be (4.7 ± 1.5) × 10(-15), (5.3 ± 1.6) × 10(-15), and (5.6 ± 2.3) × 10(-15) cm(3) molecule(-1) s(-1) for trans-2-hexenal, trans-2-heptenal, and trans-2-octenal, respectively. These results clearly indicate that the carbon chain lengthening of the trans-2-alkenals does not significantly affect the rate constant. In addition, the mechanism for the reaction of NO(3) with these unsaturated aldehydes was also investigated. Unsaturated peroxynitrate-type compounds that are exclusively formed through the abstraction channel were observed as the main products.

Prediction of rate constants for gas-phase reactions of nitrate radical with organic compounds: a new structure-activity relationship.

A new structure-activity relationship (SAR), based on parametrization of the molecular structure according to the group-additivity method, is presented. On the basis of existing experimental data for the degradability of approximately 150 organic compounds by the NO(3) radical, this new SAR is developed to estimate the rate constants for reactions with NO(3) radical. At night, nitrate radicals are the most important oxidant of volatile organic compounds. The rate constants for their reactions are therefore essential to the understanding of VOC degradation and atmospheric modelling. The database used for the SAR development includes most classes of compounds such as alkanes, alkenes (acyclic and cyclic), dienes, terpenes and saturated and unsaturated oxygenated compounds (including alcohols, ketones, ethers and esters). The proposed SAR shows good efficiency, as 91% of the rate constants are reproduced within a factor of two. The overall agreement between measured and predicted rate constants is very good for most of the unsaturated and saturated compounds, although for saturated alcohols it is less reliable.

Kinetic and mechanistic study of the gas-phase reactions of a series of vinyl ethers with the nitrate radical.

NO(3) oxidation of methyl, ethyl, propyl, and butyl vinyl ethers has been studied under tropospheric conditions (atmospheric pressure and T = 293 +/- 3 K) in the LISA indoor simulation chamber. NO(3) was produced inside the reactor by thermal decomposition of N(2)O(5) previously added to the air-VOC mixture, and concentrations were monitored using FTIR spectrometry. All the kinetic experiments were carried out by relative rate technique using isoprene as reference compound, leading to the rate constants k(1) = (7.2 +/- 1.5) x 10(-13), k(2) = (13.1 +/- 2.7) x 10(-13), k(3) = (13.3 +/- 3.0) x 10(-13), and k(4) = (17.0 +/- 3.7) x 10(-13) cm(3) molecule(-1) s(-1) for methyl, ethyl, propyl, and butyl vinyl ethers, respectively. Main oxidation products have been identified like being formaldehyde and respectively methyl, ethyl, propyl, and butyl formates. Production yields of oxidation products were close to 50%. Oxygenated nitrates and peroxynitrates were also detected.