Harvey E. Jeffries

Atmospheric photooxidation of alkylbenzenes—I. Carbonyl product analyses

Abstract Six alkylbenzenes—toluene, p -xylene, m -xylene, o -xylene, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene—were selected to investigate the carbonyl products resulting from OH-initiated oxidation of aromatic compounds. Experiments were conducted in both indoor and outdoor smog chambers under simulated atmospheric conditions. Both batch samples and 30 min interval samples were taken in the outdoor smog chamber experiments using 1 ppmV alkylbenzene, 0.67 ppm NO x and sunlight as the light source. A wide variety of carbonyl products were detected and identified using gas chromatography/mass spectrometric (GC/MS) detection by their O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBHA) derivatives. Among the observed carbonyl products are aromatic aldehydes, quinones, di-unsaturated 1,6-dicarbonyls, unsaturated 1,4-dicarbonyls, saturated dicarbonyls, hydroxy dicarbonyls, glycolaldehyde, hydroxy acetone, and possibly triones and epoxy carbonyls. Quantification was achieved using 13 C 3 -acetone as a gas-phase internal standard. The numerous carbonyl products detected in itself partially explain previous difficulties in balancing the reacted carbon. They also provide additional insight into the oxidation mechanism for aromatic compounds, which will be discussed in this paper.

Sensitivity of ozone to model grid resolution — II. Detailed process analysis for ozone chemistry

Most of the current air quality models used to simulate ozone (O3) formation predict only the concentrations of O3 without the capabilities of understanding and explaining the formation processes of O3. In this paper we present a process analysis method developed to understand and quantify the chemical and physical processes that lead to formation of O3 in Eulerian grid models. In a previous study we used a high-resolution version of regional acid deposition model (HR-RADM) to simulate O3 formation at different grid resolutions. In this study we further applied this detailed process analysis method to the HR-RADM simulations to determine the roles of individual mechanistic processes contributing to O3 formation, as well as to examine the effects of grid resolution on these regulating processes. We first selected several source areas and examined the processes that lead O3 formation in these areas. The “OH-cycle” and “NO-cycle” pathways derived from the process analysis method appear as important measures that can significantly enhance our ability to quantify and explain the formation processes of O3. We also compared O3 processes between two different grid resolutions over an equal source area with nearly equal emissions. The results suggest that (1) the effects of grid resolution on the chemistry of NOx are far more important than that on the chemistry of VOC; (2) grid resolution significantly influences the competing rates of chemistry and vertical transport processes for the emitted NOx, causing the differences in O3 predictions between two different grid resolutions. Because the balance of chemistry and vertical transport controls the model predictions, correct representation is needed for both. This leads to a conclusion that to improve model accuracy in predicting O3 formation, it is not only necessary to have adequate horizontal grid resolution, but also necessary to have adequate vertical grid resolution and accurate representation of the vertical transport process.

A comparison of two photochemical reaction mechanisms using mass balance and process analysis

Abstract Output from air quality models typically consists of model species concentrations given as a function of time or location. When two different photochemical reaction mechanisms are used in the same air quality model, predictions of species concentrations may differ. Explaining why the models predict differently is difficult using only the typical output. We have developed a process analysis method in which a complete mass balance is used to quantify characteristics values of the reacting system. We have modified both Lagrangian and Eulerian air quality models to calculate the integrated rate, i.e. the mass throughout for each chemical reaction and each physical process. The integrated rates are written to an external file, and a post processing program is used to perform a detailed mass balance on the model simulation. Using the mass balance information, we compute characteristic values that describe the reacting system. In this paper we describe the method and give an example analysis comparing two photochemical reaction mechanisms, the Carbon Bond IV and the SAPRC90 mechanism in a Lagrangian model simulation. This analysis method significantly increases our ability to explain the predictions of air quality models. In addition the method reveals several significant reactivity parameters that can be used to compare different reaction mechanisms and that suggest additional measurements that might distinguish between the different representations.

Atmospheric photooxidation of alkylbenzenes-II. Evidence of formation of epoxide intermediates

Abstract Photooxidation experiments of six alkylbenzene compounds were conducted under simulated atmospheric conditions. Carbonyl products with their molecular weights matching a series of epoxide carbonyls were observed. This observation supports the supposition formation of epoxide intermediates as suggested by Bartolotti and Edneys theoretical calculations. These epoxide intermediates are also consistent with recently observed prompt HO2 formation upon hydroxyl radical attack on the ring. An alternative origin of the observed epoxides might be from a second OH attack upon the epoxide/oxepin intermediate proposed by Barnes et al. (1996, Air & Waste Management Association 89th Annual Meeting and Exhibition, Nashville). Because of the potential toxicity and mutagenicity of the epoxide products, the atmospheric chemistry of these epoxide compounds should be studied in greater detail with indoor and outdoor chamber and in field experiments to assess their role in the urban atmosphere.

Sensitivity of ozone to model grid resolution—I. Application of high-resolution regional acid deposition model

Abstract This paper examines the sensitivity of ozone (O3) predictions to grid resolution in Eulerian grid models. A high-resolution version of the regional acid deposition model (HR-RADM) was developed and applied to simulate O3 formation at different grid resolutions. Horizontal grid-cell sizes of 20, 40, and 80 km were selected for this sensitivity study. Individual meteorological and chemical processes that contribute to O3 and its precursors were further separated and analyzed to determine their importance to O3 formation and the effects of grid resolution on these regulating processes. We first examined the model predictions of O3 maxima and minima at different grid resolutions over several major source areas. The results showed that the coarser-grid model tended to underpredict O3 maxima and overpredict O3 minima over the major source areas, because emission strengths were not as well resolved. Process contribution analyses of O3 over these source areas revealed that grid resolution significantly influences the magnitude of O3 formation and loss processes, especially chemistry and vertical transport. We also compared the process contributions between two different grid resolutions over an equal source area with nearly equal emissions to examine the nonlinearities of processes and their interactions with respect to grid resolution. These comparisons showed that for nonreactive species, the average transport applied to a coarse-grid cell is the same as that applied to the same area at higher resolution. For reactive species, however, the average transport is no longer the same between two different grid resolutions because the transport process interacts closely with chemistry, which is nonlinearly related to grid resolution. As a result, over the same source area, the coarser grid tended to predict more O3 but less NO2 from chemistry and to export more O3 and NO but less NO2 by vertical transport than did the finer grid.

Design and Testing of Electrostatic Aerosol In Vitro Exposure System (EAVES): An Alternative Exposure System for Particles

Conventional in vitro exposure methods for cultured human lung cells rely on prior suspension of particles in a liquid medium; these have limitations for exposure intensity and may modify the particle composition. Here electrostatic precipitation was used as an effective method for such in vitro exposures. An obsolete electrostatic aerosol sampler was modified to provide a viable environment within the deposition field for human lung cells grown on membranous support. Particle deposition and particle-induced toxicological effects for a variety of particles including standardized polystyrene latex spheres (PSL) and diesel exhaust emission particle mixtures are reported. The Electrostatic Aerosol in Vitro Exposure System (EAVES) efficiently deposited particles from an air stream directly onto cells. Cells exposed to the electric field of the EAVES in clean air or in the presence of charged PSL spheres exhibited minimal cytotoxicity, and their release of inflammatory cytokines was indistinguishable from that of the controls. For the responses tested here, there are no significant adverse effects caused neither by the electric field alone nor by the mildly charged particles. Exposure to diesel exhaust emissions using the EAVES system induced a threefold increase in cytokines and cytotoxicity as compared to the control. Taken together, these data show that the EAVES can be used to expose human lung cells directly to particles without prior collection in media, thereby providing an efficient and effective alternative to the more conventional particle in vitro exposure methods.

Photochemical Products in Urban Mixtures Enhance Inflammatory Responses in Lung Cells

Complex urban air mixtures that realistically mimic urban smog can be generated for investigating adverse health effects. “Smog chambers” have been used for over 30 yr to conduct experiments for developing and testing photochemical models that predict ambient ozone (O3) concentrations and aerosol chemistry. These chambers were used to generate photochemical and nonirradiated systems, which were interfaced with an in vitro exposure system to compare the inflammatory effects of complex air pollutant mixtures with and without sunlight-driven chemistry. These are preliminary experiments in a new project to study the health effects of particulate matter and associated gaseous copollutants. Briefly, two matched outdoor chambers capable of using real sunlight were utilized to generate two test atmospheres for simultaneous exposures to cultured lung cells. One chamber was used to produce a photochemically active system, which ran from sunrise to sunset, producing O3 and the associated secondary products. A few hours after sunset, NO was added to titrate and remove completely the O3, forming NO2. In the second chamber, an equal amount of NO2 and the same amount of the 55-component hydrocarbon mixture used to setup the photochemical system in the first side were injected. A549 cells, from an alveolar type II-like cell line grown on membranous support, were exposed to the photochemical mixture or the “original” NO2/hydrocarbon mixture for 5 h and analyzed for inflammatory response (IL-8 mRNA levels) 4 h postexposure. In addition, a variation of this experiment was conducted to compare the photochemical system producing O3 and NO2, with a simple mixture of only the O3 and NO2. Our data suggest that the photochemically altered mixtures that produced secondary products induced about two- to threefold more IL-8 mRNA than the mixture of NO2 and hydrocarbons or O3. These results indicate that secondary products generated through the photochemical reactions of NOx and hydrocarbons may significantly contribute to the inflammatory responses induced by exposure to urban smog. From previous experience with relevant experiments, we know that many of these gaseous organic products would contribute to the formation of significant secondary organic particle mass in the presence of seed particles (including road dust or combustion products). In the absence of such particles, these gaseous products remained mostly as gases. These experiments show that photochemically produced gaseous products do influence the toxic responses of the cells in the absence of particles.

Effects of 1,3-Butadiene, Isoprene, and Their Photochemical Degradation Products on Human Lung Cells

Because of potential exposure both in the workplace and from ambient air, the known carcinogen 1,3-butadiene (BD) is considered a priority hazardous air pollutant. BD and its 2-methyl analog, isoprene (ISO), are chemically similar but have very different toxicities, with ISO showing no significant carcinogenesis. Once released into the atmosphere, reactions with species induced by sunlight and nitrogen oxides convert BD and ISO into several photochemical reaction products. In this study, we determined the relative toxicity and inflammatory gene expression induced by exposure of A549 cells to BD, ISO, and their photochemical degradation products in the presence of nitric oxide. Gas chromatography and mass spectrometry analyses indicate the initial and major photochemical products produced during these experiments for BD are acrolein, acetaldehyde, and formaldehyde, and products for ISO are methacrolein, methyl vinyl ketone, and formaldehyde; both formed < 200 ppb of ozone. After exposure the cells were examined for cytotoxicity and interleukin-8 (IL-8) gene expression, as a marker for inflammation. These results indicate that although BD and ISO alone caused similar cytotoxicity and IL-8 responses compared with the air control, their photochemical products significantly enhanced cytotoxicity and IL-8 gene expression. This suggests that once ISO and BD are released into the environment, reactions occurring in the atmosphere transform these hydrocarbons into products that induce potentially greater adverse health effects than the emitted hydrocarbons by themselves. In addition, the data suggest that based on the carbon concentration or per carbon basis, biogenic ISO transforms into products with proinflammatory potential similar to that of BD products.

Deciphering the role of radical precursors during the Second Texas Air Quality Study.

Abstract The Texas Environmental Research Consortium (TERC) funded significant components of the Second Texas Air Quality Study (TexAQS II), including the TexAQS II Radical and Aerosol Measurement Project (TRAMP) and instrumented flights by a Piper Aztec aircraft. These experiments called attention to the role of short-lived radical sources such as formaldehyde (HCHO) and nitrous acid (HONO) in increasing ozone productivity. TRAMP instruments recorded daytime HCHO pulses as large as 32 parts per billion (ppb) originating from upwind industrial activities in the Houston Ship Channel, where in situ surface monitors detected HCHO peaks as large as 52 ppb. Moreover, Ship Channel petrochemical flares were observed to produce plumes of apparent primary HCHO. In one such combustion plume that was depleted of ozone by large emissions of oxides of nitrogen (NOx), the Piper Aztec measured a ratio of HCHO to carbon monoxide (CO) 3 times that of mobile sources. HCHO from uncounted primary sources or ozonolysis of underestimated olefin emissions could significantly increase ozone productivity in Houston beyond previous expectations. Simulations with the CAMx model show that additional emissions of HCHO from industrial flares or mobile sources can increase peak ozone in Houston by up to 30 ppb. Other findings from TexAQS II include significant concentrations of HONO throughout the day, well in excess of current air quality model predictions, with large nocturnal vertical gradients indicating a surface or near-surface source of HONO, and large concentrations of nighttime radicals (∼30 parts per trillion [ppt] HO2). HONO may be formed heterogeneously on urban canopy or particulate matter surfaces and may be enhanced by organic aerosol of industrial or motor vehicular origin, such as through conversion of nitric acid (HNO3). Additional HONO sources may increase daytime ozone by more than 10 ppb. Improving the representation of primary and secondary HCHO and HONO in air quality models could enhance the simulated effectiveness of control strategies.

The use of ambient data to corroborate analyses of ozone control strategies

Abstract Data from environmental-chamber studies and photochemical box-model simulations were used to evaluate and revise a method for developing a qualitative understanding of the sensitivity of ozone formation at a particular time and place to changes in concentrations of volatile organic compounds (VOC) and oxides of nitrogen (NOx). The revised method requires measurements of ozone, NO, and either NOx or NOy. The sensitivities of the method to biases in measurements were evaluated. The method potentially can be used for qualitative assessment of VOC versus NOx limitation, comparison with the predictions of grid-based photochemical air-quality models, and evaluation of trends over time in the relative effectiveness of VOC versus NOx controls.