Hexachlorocyclohexanes (HCHs) are the most abundant organochlorine pesticides in the world's oceans and large lakes, and knowing their chemical reactivity is important for determining environmental fate. Hydrolysis of a-and y-HCHs was carried out in buffered distilled water poisoned with sodium azide in dark, tightly sealed bottles to avoid biodegradation, photolysis, and volatilization losses. Experiments were run at (a) constant temperature (45 "C) and pH 7-9 and (b) constant pH (9) and 5-45 "C over times of 4-248 d, depending on reaction conditions. At constant pH, breakdown of the HCHs followed pseudofirst-order kinetics. Second-order base rate constants ( k b , M-' min-l) were calculated from pseudo-first-order rate constants (k', min-I), the measured pH, and the ion product of water as a function of temperature. At 20 "C, values of k b were 1.57 (a-HCH) and 1.10 (7-HCH). From the variation in k b with temperature, activation energies of 78.3 and 84.6 kJ/mol were determined for a-HCH and yHCH. At pH 7, reaction with HzO contributed to the breakdown of the HCHs; estimates of the neutral rate constant (kn, min-1) were 1.1 X (a-HCH) and 2.0 X 10-6 (7-HCH). At pH 8 and 5 "C, hydrolytic half-lives of a-HCH and y-HCH were 26 and 42 yr. The relative contribution of hydrolysis to removing HCHs from cold, deep water in oceans and lakes requires that rates of other processes such as sedimentation and microbial attack be better established.

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High-volume air samples were collected from 1993 to 1996 in rural areas of Québec to investigate on the levels and the vapor-particle partitioning of polycyclic aromatic hydrocarbons (PAHs). Ranges for the mean concentrations of total PAHs (ng m(-3)) were as follows: Villeroy, 3.31-18.92; St. Anicet, 7.57-22.84; and Mingan, 0.50-0.53. Lower molecular weight PAHs predominated at all locations. Particle-gas partition coefficients (Kp) were in good correlation with their vapor pressures (r2 = 0.79-0.97) with slopes deviating from the expected value of -1. The curve of the fractions of PAHs on particles in St Anicet in 1995 fell on that of Lake Superior. In St. Anicet, fractions of phenanthrene, fluoranthene, pyrene, and chrysene on particles were close to those calculated from the soot-air partition coefficient (KSA). At all sites the mean ratios of particulate PAH of the same molecular weight but of very different reactivities were similar during the same sampling days, suggesting that particle-bearing PAHs in Villeroy and Mingan were of the same nature as those found in St. Anicet where adsorption onto soot particles was the major mechanism. Furthermore, the enthalpies of desorption for the predominant PAHs were close at all sites.

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