The origin and significance of short-term variability of semivolatile contaminants in air.

Persistent semivolatile contaminants such as polychlorinated biphenyls (PCBs) cycle between air and surface media in the environment. At different locations and times, PCB concentrations in air over a diel (24-hour) period have been observed to have maxima either during the day or at night. These observations have been interpreted as evidence of temperature-mediated air-surface exchange and of degrading reactions with hydroxyl radicals in the atmosphere. However, a general explanation of the processes responsible for the observed diel variability in concentrations has not been provided. Here, we interpret diel monitoring data using a multimedia mass balance model parametrized with local data on temperature, wind speed, atmospheric mixing height, and hydroxyl radical concentrations. We demonstrate that four factors are sufficient to account for the variability of PCB concentrations in air over a diel period; temperature, local atmospheric stability, hydroxyl radical concentration, and source type. We apply the model to re-interpret past diel monitoring studies and find that the observed patterns of concentrations can be rationalized by consideration of these factors. Using insights from this study, future diel monitoring campaigns can be targeted to observe the influence of specific fate and transport processes. Such studies will contribute to more accurate understanding of the processes controlling the short-term local, and long-term global fate of persistent semivolatile contaminants.