Stephen R. McDow

Polynuclear aromatic hydrocarbon degradation by heterogeneous reactions with N2O5 on atmospheric particles

Abstract The degradation of particulate polynuclear aromatic hydrocarbons (PAH) on atmospheric soot particles in the presence of gas phase dinitrogen pentoxide (N2O5) was explored. Dilute diesel and wood soot particles containing PAH were reacted with∼10ppm of N2O5 in a 200 l continuous stirred tank reactor (CSTR). To provide a stable source of particles for reaction in the CSTR, diesel or wood soot particles were injected at night into a 25 m3 Teflon outdoor chamber. The large chamber served as a reservoir for the feed aerosol, and the aerosol could then be introduced at a constant flow rate into the CSTR. PAH-N2O5 heterogeneous rate constants for wood soot at 15°C ranged from2 × 10−18to5 × 10−18 cm3 molecules−1 s−1. For diesel soot the rate constants at 16°C were higher and ranged from5 × 10−18to30 × 10−18 cm3 molecules−1 s−1. Comparisons with other studies suggest that sunlight is the most important factor which influences PAH decay. This is followed by ozone, NO2, N2O5 and nitric acid. The rate constants of nitro-PAH formation from a parent PAH and N2O5 were of the order of1 × 10−19−1 × 10−18 molecules−1s−1. The uncertainty associated with all of these rate constants is± a factor of 3. Given, however, the small magnitude of the rate constants and the low levels of N2O5 present in the atmosphere, we concluded that PAH heterogeneous reactions with gas phase N2O5 degrade particle-bound PAH or to form nitro-PAH from PAH arenot very important. (Direct application of the specific rate constants derived in this study to ambient atmospheres should not be undertaken unless the ambient particle size distributions and chemical composition of the particles are similar to the ones reported in this study.)

Effect of composition and state of organic components on polycyclic aromatic hydrocarbon decay in atmospheric aerosols.

published in Advance ACS Abstracts, September 15, 1994. Envlron. Scl. Technol., Vol. 28, No. 12, 1994 2153

Benz[a]anthracene photodegradation in the presence of known organic constituents of atmospheric aerosols.

Benz[a]anthracene photodegradation rates in toluene solutions containing co-solutes were compared using a photochemical turntable reactor. Each co-solute investigated was found at relatively high concentrations in atmospheric particulate matter. Benz-[a]anthracene photodegradation was accelerated by the presence of 9,10-anthraquinone, xanthone, 2-furaldehyde, 2,4-dimethylbenzaldehyde, 9,10-phenanthrenequinone, 2-acetylfuran, and furfuryl alcohol. Decay was inhibited by 7-benzanthrone. Other compounds had little or no effect on benz[a]-anthracene decay. Possible photochemical reaction mechanisms are discussed, and it is concluded that several competing mechanisms may be responsible, including electronic energytransferfollowed by reaction from the triplet state, singlet oxygen attack, and radical chain reactions initiated by hydrogen abstraction of aerosol constituents. The results suggest that particle organic composition can strongly influence polycyclic aromatic hydrocarbon photodegradation rates in atmospheric aerosols.

An approach to studying the effect of organic composition on atmospheric aerosol photochemistry

Physical and chemical characteristics of atmospheric carbonaceous aerosol particles are reviewed, and their likely effects on the particulate matter as a reaction medium are discussed. An approach to studying the effects of organic components of atmospheric particulate matter on aerosol organic photochemistry based on this discussion is described. Available information suggests that atmospheric aerosols from common combustion sources, such as wood smoke or diesel soot, are almost entirely carbonaceous, consisting of a relatively thick, possibly liquid organic layer coating an elemental carbon core. This implies that organic compounds are fairly mobile and consequently that overall organic composition of an atmospheric aerosol could play an important role in the photochemical behavior of associated reactive organic substances. In accordance with this a photochemical turntable reactor was used to investigate the effects of organic aerosol constituents dissolved in an organic solvent, using benz[a]anthracene as a model photochemically reactive compound. Preliminary results showed that 4 out of 10 major organic compound classes found in atmospheric aerosols include compounds which accelerated benz[a]anthracene photodegradation. These were methoxyphenols, polycyclic aromatic ketones and quinones, substituted benzaldehydes, and substituted furans.

POLYCYCLIC AROMATIC HYDROCARBONS AND COMBUSTION AEROSOL PHOTOEMISSION

Abstract An experimental investigation was conducted to explain the physical chemistry of aerosol photoemission (APE) from combustion sources. Strong correlations between APE and total polycyclic aromatic hydrocarbons (TPAH) in particulate matter were observed for aerosols from both oil stove and automobile exhaust. Sampling and analytical procedures were developed to allow detection of PAHs ranging in molecular weight (MW) from phenanthrene (MW 178) to coronene (MW 300) in oil stove exhaust after short sampling periods. Small variations of the oil stove PAH profile with Bacharach number were observed. Interactions between adsorbed polycyclic aromatic hydrocarbons PAHs and particle surface are proposed as the most likely cause of environmental APE after irradiation at 4.9 eV.

Combustion aerosol water content and its effect on polycyclic aromatic hydrocarbon reactivity

Abstract A linear relationship was observed between particle water uptake and relative humidity for aerosol samples collected on Teflon coated glass fiber filters. Gasoline soot and wood smoke sample weight increases were three to five times greater than diesel soot weight increases at comparable relative humidities. Lower limit estimates of wood smoke water content at 90% relative humidity were in the 5–10% range. Based on these results polycyclic aromatic hydrocarbon photodegradation was investigated in liquid mixtures of actual wood smoke components with water content varied from 0 to 15%. Reaction rates of benz[a]anthracene and benzo[k]fluroanthene were significantly greater in methoxyphenol mixtures with 10% water content than in mixtures which did not contain water. Benzo[a]pyrene photodegradation was not significantly different. The results indicate that combustion particle water content increases with increasing relative humidity and that PAH photodegradation rates are likely to increase with increasing particle water content. This provides an explanation for previously described smog chamber studies in which a correlation between polycyclic aromatic hydrocarbon (PAH) decay in sunlight and water vapor concentration was reported.

Mechanistic and Kinetic Studies of the Photodegradation of Benz[a]anthracene in the Presence of Methoxyphenols.

Kinetic studies were employed to assess an empirical rate law describing the rate of photodegradation of polycyclic aromatic hydrocarbons (PAH) in the presence of substituted methoxyphenols. A solution of benz[a]anthracene (BaA) and vanillin in toluene was chosen as the model system. Further experiments using structure activity relationships were applied to the model system to investigate the mechanism for BaA photodegradation. Data from these experiments suggest that the rate-determining step in the mechanism is hydrogen abstraction of the phenolic hydrogen from vanillin

Water uptake by aerosol particles from automobile exhaust and wood smoke

Abstract Water uptake by combustion aerosol particles from common anthropogenic combustion sources was estimated by weighing filter samples while varying the relative humidity in the weighing chamber of an electronic microbalance. This technique was suitable for samples which weighed more than 3 mg. Significant water uptake was observed for diesel soot, gasoline exhaust, and wood smoke particles. Weights for typical diesel soot samples increased by slightly less than 2% as relative humidity was increased from 40% to 90%. Automobile exhaust sample weights increased by at least three to four times and wood smoke by at least three to more than five times as much as diesel soot sample weights. No effect of temperature was observed. It was concluded that atmospheric mass loadings of particulate matter from these sources increase by a small but significant amount as atmospheric relative humidity increases.

Total Organic Carbon Emissions From Municipal Incinerators

Total carbon (TC), carbonate carbon (CC) and total organic carbon (TOC) were determined in bottom ash, filter dust and flue gas of Swiss municipal solid waste (MSW) incinerators. The highest TOC load was found in the bottom ash (2-3 g kg-1 MSW), followed by the filter dust (0.1-1.0 g kg-1 MSW) and the flue gas (0.05-0.3 g kg-1 MSW). The composition and behaviour of the bulk of TOC in these products is not yet known. In order to minimize the risk of leaching organic substances as well as metals due to biological, chemical and physical reactions of the products of incineration in a landfill, it is suggested the incineration process be optimized towards complete combustion.

Water uptake by sunlight and ozone exposed diesel exhaust particles

Two types of experiments were conducted to investigate the effect of aerosol aging on water uptake by diesel soot particles. First, in three experiments diluted diesel soot was exposed to sunlight for up to 10 hours in a 190 m3 Teflon film smog chamber and filter samples were collected at various time intervals. Second, diesel soot filter samples were exposed to 2 to 4 ppm ozone concentrations for 3 hours in a stainless steel chamber. Water uptake was measured by weighing the filter samples from each type of experiment by varying relative humidity in the weighing chamber of an electronic microbalance. When relative humidity was increased from 40% to 90% weights of fresh diesel soot samples from the smog chamber increased by less than 2% and sample weights of diesel soot aged in the chamber for 9 to 10 h increased by 7 to 8%. Sample weights for diesel soot filter samples aged in the stainless steel chamber in the presence of ozone in the dark increased by less than 2.5% over the same relative humidity range.