Evaluation of a passive sampler for volatile organic compounds at ppb concentrations, varying temperatures, and humidities with 24-h exposures. 1. Description and characterization of exposure chamber system

A dynamic exposure chamber was constructed to evaluate the performance of t he 3M 3520 organic vapor monitor (3520 OVM, 3M Co., St Paul, MN) when expos ed during 24 h to combined test atmospheres of benzene, 1,3-butadiene, carb on tetrachloride, chloroform, 1,4-dichlorobenzene, methylene chloride, styr ene, tetrachloroethylene, and toluene at target concentrations of 10, 20, a nd 200 mu g/ m(3) in combination with temperatures of 10, 25, and 40 degree s C and relative humidities of 12 50, and 90%. These conditions are general ly representative of the range of community air environments, both indoor a nd outdoor. The system consists of five distinct units: (i) dilution air de livery, (ii) humidification, (iii) VOC generation and delivery, (iv) mixing chamber, and (v) exposure chamber. High-emission permeation tubes were uti lized to generate the target VOCs. Both the target temperatures and humidit ies were achieved and maintained for multiple consecutive days. The variati on of the temperature in the exposure chamber was controlled within +/-1 de grees C, while relative humidity was controlled within +/-1.5% at 12% RH, 1 2% at 50% RH, and +/-3% at 90% RH. Under constant preset temperatures and s table nitrogen flow through the VOC generation unit, various temporal patte rns of permeation rates were observed over time. The lifetimes and permeati on rates of the tubes differed by compound, length of the tube, and manufac turer. For tubes with a long shelf life, an initial conditioning period of up to 50 days in the VOC generation unit was necessary before permeation ra tes became stable. A minimum of 3 days of reconditioning was required when the tubes were stored in the refrigerator before they were used again. 1,3- Butadiene tubes had a short shelf life, and the permeation rates changed si gnificantly and relatively quickly over time; however, the rates could be e stimated by using a best-fit equation for the tube weight loss data for eac h exposure period. By closely monitoring weight loss over time, the permeat ion tubes could be used for delivering low and stable concentrations of VOC s over multiple months.