Combustion Processes as a Source of High Levels of Indoor Hydroxyl Radicals through the Photolysis of Nitrous Acid

Hydroxyl radicals (OH) are known to control the oxidative capacity of the atmosphere but their influence on reactivity within indoor environments is believed to be of little importance. Atmospheric direct sources of OH include the photolysis of ozone and nitrous acid (HONO) and the ozonolysis of alkenes. It has been argued that the ultraviolet light fraction of the solar spectrum is largely attenuated within indoor environments, thus, limiting the extent of photolytic OH sources. Conversely, the ozonolysis of alkenes has been suggested as the main pathway of OH formation within indoor settings. According to this hypothesis the indoor OH radical concentrations span in the range of only 10(4) to 10(5) cm(-3). However, recent direct OH radical measurements within a school classroom yielded OH radical peak values at moderate light intensity measured at evenings of 1.8 × 10(6) cm(-3) that were attributed to the photolysis of HONO. In this work, we report results from chamber experiments irradiated with varying light intensities in order to mimic realistic indoor lighting conditions. The exhaust of a burning candle was introduced in the chamber as a typical indoor source causing a sharp peak of HONO, but also of nitrogen oxides (NOx). The photolysis of HONO yields peak OH concentration values, that for the range of indoors lightning conditions were estimated in the range 5.7 ×· 10(6) to 1.6 × 10(7) cm(-3). Excellent agreement exists between OH levels determined by a chemical clock and those calculated by a simple PSS model. These findings suggest that significant OH reactivity takes place at our dwellings and the consequences of this reactivity-that is, formation of secondary oxidants-ought to be studied hereafter.

Kinetic and mechanistic study of the reaction of OH radicals with methylated benzenes: 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene

The reaction of OH radicals with a series of methylated benzenes was studied in a temperature range 300-350 K using a flash-photolysis resonance fluorescence technique. Reversible OH additions led to complex OH decays dependent on the number of distinguishable adducts. Except for hexamethylbenzene, triexponential OH decay curves were obtained, consistent with formation of at least two adduct species. For three compounds that can strictly form two adduct isomers for symmetry reasons (1,4-dimethyl-, 1,3,5-trimethyl-, and 1,2,4,5-tetramethylbenzene) with OH bound ortho or ipso with respect to the methyl groups, OH decay curves were analysed in terms of a reaction mechanism in which the two adducts can be formed directly by OH addition or indirectly by isomerization. In all cases one adduct (add1) is dominating the decomposition back to OH. The other (add2) is more elusive and only detectable at elevated temperatures, similar to the single OH adduct of hexamethylbenzene. Two limiting cases of the general reaction mechanism could be examined quantitatively: reversible formation of add2 exclusively in the OH reaction or by isomerization of add1. Total OH rate constants, adduct loss rate constants and products of forward and reverse rate constants of reversible reactions were determined. From these quantities, adduct yields, equilibrium constants, as well as reaction enthalpies and entropies were derived for the three aromatics. Adduct yields strongly depend on the selected reaction model but generally formation of add1 predominates. For both models equilibrium constants of OH reactions lie between those of OH + benzene from the literature and those obtained for OH + hexamethylbenzene. The corresponding reaction enthalpies of add1 and add2 formations are in a range -87 ± 20 kJ mol(-1), less exothermic than for hexamethylbenzene (-101 kJ mol(-1)). Reaction enthalpies of possible add1 → add2 isomerizations are comparatively small. Because results for 1,3,5-trimethylbenzene are partly inconsistent with a direct formation of add2, we promote the existence of isomerization reactions. Moreover, based on available theoretical work in the literature, add1 and add2 are tentatively identified as ortho and ipso adducts, respectively. Total OH rate constants were obtained for all title compounds. They can be described by Arrhenius equations: kOH = A × exp(-B/T). The parameters ln(A/(cm(3) s(-1))) = -25.6 ± 0.3, -25.3 ± 0.6, -27.3 ± 0.3, -24.6 ± 0.3, -26.2 ± 0.4, -26.2 ± 0.4 and -24.5 ± 0.2, and B/K = -160 ± 90, -550 ± 180, -1120 ± 90, -330 ± 100, -820 ± 100, -980 ± 130, and -570 ± 40 were determined for 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene.

Time Resolved Infrared Spectroscopy of Formation and Processing of Secondary Organic Aerosol

An aerosol flow reactor was coupled to an infrared absorption cell to study aerosol formation processes with high temporal resolution. The recorded infrared spectra were referenced using aerosol smog chamber experiments. Evaluation was done by studying the formation of secondary organic aerosol from α-pinene and catechol as precursors and ozone as oxidant. Three main infrared absorptions: ν(O-H), ν(C-H) and ν(C=O) were considered, and humic like properties of the secondary organic aerosol are mainly interpreted according to the formation and variations of carbonyl bands in the region between 1850 and 1600 cm−1, especially the ν(C=O) of aryl carbonyls from catechol oxidation products below 1700 cm−1. The relative intensities of two major ν(C=O) stretching vibrations at 1690 cm−1 and 1755 cm−1 were observed to depend strongly on the available ozone concentration. At high precursor/ozone ratios (2:1 or 1:1) the vibration at 1690 cm−1 predominates, indicating aryl carbonyl vibrations. With increasing ozone concentrations this vibration is replaced by the higher carbonyl vibration at 1755 cm−1 indicating unsaturated carbonyl-containing compounds. This is a strong hint at ring opening processes leading to unsaturated aliphatic compounds in the resulting particle. Aryl carbonyls and aromatic or olefinic ν(C=C) at 1620 cm−1 in aged particles remain visible, as aerosol smog chamber studies exhibit – thus a strong hint at humic like properties of the SOA from the spectroscopic point of view.

OH radical reactivity of pesticides adsorbed on aerosol materials: first results of experiments with filter samples

Preliminary results of a new method to investigate the OH radical reactivity of semi-volatile organic compounds (e.g., pesticides) are presented. Terbuthylazine, simazine, sodium benzoate, and bromoxynil were adsorbed on highly disperse silicon dioxide powder as an unreactive carrier at a thickness well below one monolayer. The coated material was suspended in air as an aerosol, sampled on filters, and exposed in an 840-liter Duran chamber to OH radicals, produced by photolysis of hydrogen peroxide in the gas phase. Sunlamps on top of the chamber were used as cold light sources [T(aerosol) approximately 25 degreesC]. OH radical concentrations (10(5)<cOH (cm-3)<3x10(6)) were monitored in the gas phase using a set of four hydrocarbons with well-known OH reactivities as reference compounds. The triazine terbuthylazine (kOH=1.1x10(-11) cm3 s-1) was used as a reference compound in the filter samples. Simazine and isoproturon were found to react with a comparable OH rate constant with respect to terbuthylazine. Sodium benzoate reacts about a factor of 3 slower. Rapid degradation was observed for bromoxynil, explained mainly by photolysis. Besides the characterization and discussion of the experimental setup used, the rate constants obtained are discussed and compared with estimated values.

Abiotic degradation of halobenzonitriles: investigation of the photolysis in solution

This paper presents first experiments of laboratory investigations of the photodegradation by direct photolysis (lambda>290 nm) of cholorotahlonil, dichlobenil, chloroxynil, bromoxynil, and ioxynil in aqueous, pH-buffered, and organic solutions and the calculation of the quantum yields. The photolysis of chlorothalonil in water is low, with a corresponding low quantum yield (Phi=0.0001). Dichlobenil is photostable under the laboratory conditions used. The photoreactivity of bromoxynil and ioxynil was found to be comparable in aqueous solutions and about three times lower with respect to chloroxynil. The quantum yields obtained in water of chloroxynil, bromoxynil, and ioxynil are Phi=0.0060, 0.0093, and 0.0024, respectively. Half-lives of the pesticides in the environment with respect to direct irradiation are estimated using UV spectra and quantum yields as input variables obtained in the laboratory.

High-Resolution Infrared Fourier-Transform Spectroscopy and Rotational Constants of the nu4 Band of BrNO2

For the first time, high-resolution infrared gas-phase absorption spectra of the BrNO2 molecule were recorded using a Fourier-transform spectrometer. In this paper, the nu4 bands of the 79BrNO2 and 81BrNO2 isotopomers around 1670 cm-1 are investigated. Although the spectra are highly congested, rotational and centrifugal distortion constants for the ground and v4 = 1 states of 79BrNO2 and 81BrNO2 were determined. The results show that BrNO2 is a planar molecule of C2nu symmetry and confirm predictions from a recent ab initio study. Copyright 1998 Academic Press.