Major products in the photochemistry of perylene adsorbed in models of atmospheric particulate matter

Abstract

The photodegradation of adsorbed perylene is strongly influenced by the environment in which it is found as demonstrated by the nature of the photoproducts characterized on the adsorbed state in comparison to its photochemical behavior in solution. The separation, characterization, and identification of the products were carried out using HPLC equipped with UV–vis diode array and MS detection. Two of the products were identified as 1,12-perylenedione and 3,10-perylenedione. Three additional products were characterized as a perylenedione and two perylenediols based on their m/z ratio. Based on this information, two possible mechanisms of formation were proposed for the identified diones. The experimental data showed that both the radical cation and singlet oxygen participate in the route of photodegradation of perylene which occurs through a mixed Type I and Type II pathways. These results assist in the understanding of complex processes undergone by perylene and other PAHs in the environment.

Introduction

Perylene, Per, is a polycyclic aromatic hydrocarbon (PAH) found ubiquitously as an environmental contaminant. It has been identified in products of incomplete combustion such as cigarette smoke and engine exhaust, as well as smoked food products and in emissions of oil fires, fly ash from municipal waste incinerations, asphalt plants, and cooking plants. Per is also one of the few PAHs found in nature whose source can include soil, dust-rise by wind, sea spray, forest fires, volcanic dust, and some vegetation [1], [2], [3], [4], [5], [6], [7], [8]. Most airborne hydrocarbons are associated with particulate matter [9]. Because of its low vapor pressure, Per exists solely in the particulate phase in the ambient atmosphere.

The persistence of PAHs associated with fine airborne particulate matter is important not only because in the atmosphere they may be transported over long distances, but also because they can undergo chemical reactions. These transformations can lead to their degradation or to transformations into products more toxic than the parent compound [10]. Recent studies link air pollutants released from motor vehicles and power plants to health effects and higher death rates [11]. Because of the risks arising from pollution to human health and to the ecological equilibria, studies of the fate of atmospheric pollution are of great importance. In these terms, studies of the photodegradation of PAHs adsorbed on surfaces are of relevance for our understanding of the fate of these compounds in the atmosphere.

Fly ash produced in electric power plants consists mostly of aluminum–silicon–oxygen compounds, mixed with smaller amounts of sodium, magnesium, potassium, calcium, and titanium [12]. This is called the aluminosilicate matrix. For this reason, silica gel of different pore diameter and alumina has been used as model surfaces in our laboratory, because they resemble the composition of fly ash in particular and the atmospheric particulate matter in general.

Photodegradation studies of Per and other PAHs have focussed on the effect of the surface's chemical and physical properties such as color, carbon content, surface area, particle porosity, and size and surface loading upon the rate of phototransformation [13], [14], [15]. Photolytic half-lives highly dependent on the physical and chemical nature of the substrate have been reported [14]. For Per adsorbed on silica or alumina substrates, half-lives of 4.7 and 1.7 h have been calculated under specific laboratory experimental conditions [14]. Recently, more comprehensive mechanistic photochemical studies of other PAHs have been reported [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27].

McDow et al. [28] reported that PAHs adsorbed onto atmospheric aerosol particles from sources such as diesel soot and wood smoke are found in an organic layer (possibly liquid) around a carbon core. The composition and polarity of this organic layer depends on the emission source, and can influence the decay of the PAHs. For these reasons, it was of interest to compare the effect of surface interactions on the photochemical behavior of perylene with that in solution. By analogy, this could provide information on the photodegradation mechanisms of PAHs when adsorbed in inorganic oxides in ashes versus those in diesel soot or wood smoke particles.

This study reports on the photochemistry of Per adsorbed on the inactivated surfaces of silica gel and alumina, and in some nonpolar and polar solvents as models of atmospheric particulate matter. The objective was to study the effect of the nature of the adsorbent or the solvent, the average pore diameter, coadsorbed gases and water, and surface loading on (1) the photochemical reactions, (2) the participating excited states and reactive intermediates in the photochemical reactions [29], and (3) the nature and yield of the products. Under these controlled laboratory conditions we were able to obtain some understanding of the more complex environment of an atmospheric aerosol. Because photodegradation products can be more toxic than the parent compound, it is important to characterize them, and to establish their possible routes of formation in order to understand their ultimate environmental fate.