Investigation of a long-term sampling period for monitoring volatile organic compounds in ambient air

The potential health effect, induced by exposure to volatile organic compounds (VOCs) in ambient air for long periods, is an important problem. We have investigated a long-term monitoring method for measurement of VOCs in ambient air. Long-term, nonstop sampling periods of 4-weeks, 7-days, and 24-h were evaluated simultaneously, using mass flow controllers set to 0.5, 2.0, and 14 mL/ min, respectively. Ambient air was drawn through a multisorbent sampling tube packed with Carbotrap C, Carbotrap B, and Carboxen 1000. Adsorbates were introduced into a GC/MS using a thermal desorption cold trap injector and determined according to the EPA Method TO-14 list. Concentrations of almost all VOCs estimated by the 7-day or 4-week sampling method were approximately equal to the mean value calculated from the 24-h sampling method for each term, except for compounds bearing a vinyl group such as chloroethene, 1,3-butadiene, and styrene. Concentration variations of CFCs, tetra chloromethane, or 1,1,1-trichloroethane were small, but the other VOCs exhibited significant day-to-day fluctuation reflecting socioeconomic activities and the direction of wind. As an example, daily concentrations of benzene in February and March 1999 ranged from 2 to 10 mug/m(3) and 1.5-8.6 mug/m(3), respectively. This wide variation indicates that long-term, nonstop sampling is necessary for measuring accurate average concentrations of VOCs in ambient atmospheres.