ATMOSPHERIC FATE OF TOXIC AROMATIC-COMPOUNDS

Detailed mechanisms are outlined for the chemical reactions that contribute to in-situ formation and atmospheric removal of selected aromatic toxic air contaminants. Xylenes (Part A of this article) have short atmospheric half-lives, 0.5-1.0 day. Their removal from the atmosphere involves reactions initiated by OH addition (major) and OH abstraction (minor) pathways, and lead to tolualdehyde, nitroxylenes, dimethylphenols (and their products nitrodimethyl phenols), formaldehyde, and aliphatic dicarbonyls. Benzyl chloride, nitrobenzene, and the chlorobenzenes are examined in Part B. In-situ formation is of importance only for nitrobenzene. In-situ removal involves reaction with OH for all compounds, although at slow rates that correspond to atmospheric half-lives of 1 week for benzyl chloride, several weeks for the chlorobenzenes, and about 2 months for nitrobenzene. Expected reaction products include benzaldehyde, chlorobenzyl phenol and nitrobenzyl chloride from benzyl chloride, chlorophenols and nitrochlorobenzenes from chlorobenzenes, nitrophenol from nitrobenzene, and aliphatic dicarbonyls, chlorine-substituted or not, in all cases. Part C presents data for phenol and nitrophenols. In-situ formation is important for all compounds and involves the reactions of aromatic precursors with OH and NO3. Removal processes also involve reactions of OH and NO3 with phenol and the nitrophenols. Overall, phenol is predicted to be rapidly removed from the atmosphere (half-life, 0.5 day), while production of nitrophenols, nitrocresols and dinitrocresols is faster than their removal by chemical reactions. For cresols and dimethyl phenols (Part D), in-situ formation is important and involves the reaction of OH with toluene (cresols) and with the xylenes (dimethyl phenols). In turn, cresols and dimethyl phenols react rapidly with the hydroxyl radical (daytime) and with the nitrate radical (at night), leading to nitrocresols, other aromatic products, and aliphatic dicarbonyls and ketoacids.