Research Program Area: Atmospheric Processes
Topic Areas: Chemistry & Reactivity
Although the atmospheric chemistry of NOx / hydrocarbon systems involved in photochemical smog formation is becoming well understood, little is known today about the environmental fates of airborne toxic organic chemicals. Relevant chemical questions include: What are their ambient levels? How fast do they degrade in "clean" vs. polluted ambient air? What are their reaction products?
Since the study of all volatile toxic organics would be an overwhelming experimental task and prohibitively expensive, we have initiated an investigation of representative compounds relevant to California's polluted ambient and occupational atmospheres. The results of these experimental studies will contribute to a database that will be the foundation for a sound predictive capability to assess the potential risk of many toxic organic chemicals currently in use in California. This new database could also be used to predict the atmospheric lifetimes and fates of new compounds being synthesized for industrial and agricultural use.
Compounds studied, include 1,3-dichloropropene (widely used as an agricultural insecticide fumigant), vinyl chloride (a carcinogen emitted from chemical manufacturing plants as well as waste chemical sites) and tri- and tetrachloroethylene (widely used solvents which are suspected carcinogens). Experiments were carried out in the SAPRC 5800-R evacuable chamber-solar simulator facility using both in situ Fourier transform infrared (FT-IR) spectroscopy and UV / visible differential optical absorption spectroscopy (DOAS) to identify and quantify the reaction products.
The atmospheric half-lives of the cis- and trans-1, 3-dichloropropene isomers toward attack by OH radicals in simulated moderately polluted air were found to be seven hours and 12 hours, respectively, while the half-lives for reaction with O3 were determined to be approximately two weeks and seven weeks, respectively. Major products formed from the reaction of OH radicals with 1,3-dichloropropene included formyl chloride (HCCl) and chloroacetaldehyde (C1CH2CHO).
With respect to the chloroethylenes reaction of vinyl chloride with OH radicals gave an -100% yield of formyl chloride (HCl > C=O), a toxic photolabile species while trichloroethylene formed both phosgene (COCl2) and formyl chloride in each case with ~20-25% yield each. An ~50-55% yield of phosgene was formed from OH attack on tetrachloroethylene.
A major study was completed of the atmospheric chemistry of three aromatic hydrocarbons, toluene and o- and m-xylene, which are present in substantial concentrations in unleaded gasoline. Products of hydroxyl radical-initiated, ring opening photooxidation included the a-dicarbonyl species glyoxal and methylglyoxal. However their yields were much lower than those currently used in chemical computer models describing the atmospheric degradation of aromatic hydrocarbons. Indeed, for toluene the identified reaction pathways following OH attack account for only -50% of the overall reaction. Clearly, in order to improve the validity of widely used kinetic-computer models of photochemical air pollution, more extensive research needs to be carried out on the products and mechanisms of such OH-aromatic hydrocarbon reactions.
In another study, alkyl nitrate (RONO2) yields were determined for a series of NOX-air photooxidations of branched and cyclic alkanes. The latter are major constituents of diesel, gasoline and aviation fuels and the nitrates are important reaction products. Alkanes studied were: neopentane, 2,3-dimethylbutane, 2,2-dimethylbutane, 3-methylpentane and cyclohexane. Since the reaction
RO2 + NO - > RONO2
is a removal process for both NO, and radicals, it has a marked effect in decreasing the photo- chemical reactivity of these longer chain alkanes. The data obtained in this work, together with our previous studies, now allow a-priori predictions of the significance of this reaction for the alkanes emitted into the atmosphere from anthropogenic sources.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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