Aldehyde and monocyclic aromatic hydrocarbon mixing ratios at an urban site in Las Vegas, Nevada

BTEX (benzene, toluene, ethylbenzene, and xylenes) hydrocarbons (HCs) constitute a significant fraction, (similar to 25% on a carbon basis) of the reactive HCs in urban atmospheres and have predominantly anthropogenic sources.(1-3) These sources may be broadly categorized as either industrial or vehicular in origin(4.5) Motor vehicle sources of BTEX include incompletely combusted fuel, chemical reactions in the combustion process, and evaporation, and account for more than 70% of the total atmospheric emissions of BTEX.(6) Mixing ratios of BTEX HCs in the atmosphere vary widely; in a recent study, m- and p-xylene values ranged from 30 pptv (parts per trillion by volume) in remote areas to 16 ppbv (parts per billion by volume) in urban areas.' BTEX HCs react so slowly with O-3 (k < 1 X 10(-20) cm(3)/ molecule/sec)(8) and NO3 radicals (k = 10(-17) to 10(-16) cm(3)/ molecule/sec)(9) at room temperature that atmospheric removal of BTEX by these reactions is negligible.(10) The major decomposition route for atmospheric BTEX is reaction with OH, which is followed by subsequent reactions of the radicals thus formed (e.g., with O-2 and NOx).(11) Reactions of OH with aromatic HCs may proceed by H atom abstraction from the alkyl substituent (which is of minor importance(10)) or by OH addition to, the aromatic ring. The yields of the ring-retaining products from benzene, toluene, and xylenes are less than 10%, leaving OH addition to the aromatic ring as the major reaction pathway for removal of BTEX. This reaction results in the formation, of a cyclohexyldienyl radical, which subsequently reacts with molecular oxygen in a. complex series of reactions yielding simple aldehydes, organic acids, and a-dicarbonyls. The greater Las Vegas, NV metropolitan area has a population of more than 1 million people and continues to grow rapidly. Based on a tourist economy, the region has relatively few industrial emissions; the primary emission sources are vehicular exhaust and dust from construction activities. (12) Las Vegas lies in the middle of the Mojave Desert (annual precipitation approximate to 10 cm), thus, regional production of biogenic HCs (e.g., isoprene) from vegetation is expected to be small. The Las Vegas area is surroundedOxidation of benzene, toluene, ethylbenzene, and xylenes (BTEX) in air, of significance due to, for example, the potential for O-3 formation, is believed to be initiated by OH attack on the ring (addition) or on the alkyl side chain (H abstraction). A series of ring-breaking reactions follows, with major products predicted to be (x-dicarbonyls, simple aldehydes, and organic acids. To test this prediction, ambient air mixing ratios of aldehydes (formaldehyde, acetaldehyde, benzaldehyde, glyoxal, and pyruvaldehyde), along with some supporting BTEX data, were measured at an urban site in Las Vegas, NV. Samples were collected on sorbents and determined by chromatographic methods; mixing ratios were compared to ambient levels of CO, O-3, and NO,,. A meteorological analysis (temperature, wind speed, and wind direction) was also included. Statistically significant relationships were noted among the BTEX hydrocarbons (HCs) and among the photochemically derived species (e.g., O-3, NO2, and some of the aldehydes), although there was seasonal variation. The observations are consistent with a common primary source (i.e., vehicular exhaust or fuel evaporation) for the BTEX compounds and a common secondary source (e.g., OH attack) for glyoxal and pyruvaldehyde.