Dynamics of gaseous semivolatile organic compounds in a terrestrial ecosystem—effects of diurnal and seasonal climate variations

The dynamics of gas-phase semivolatile organic compounds (SOCs) were examined in a forested bog in northern Minnesota. A strong diurnal variation in the gas-phase concentration was observed for over 100 compounds including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other organochlorines. This diurnal variation, whereby afternoon concentrations exceeded early morning levels by factors of 2–10, is a likely result of volatilization and deposition of SOCs to and from local plant surfaces. The variation cannot be explained by air turbulence, wind direction, photoreactivity, or plant stomatal activity. Gas-phase SOC concentrations ([SOCg]) are strongly correlated with both local temperature and relative humidity. The correlation between [SOCg] and temperature was examined separately from the correlation between [SOCg] and relative humidity (RH). Only during the winter sampling period, when snow covered the forest surface, was relative humidity not a significant predictor of [SOCg]. For sampling periods when relative humidity was significant, the following relationship was found: In [SOCg] = a + m/T + c RH, where T is the ambient temperature (K) at 1 m, a, m, and c are constants that vary with compound and season. This study suggests that surface adsorption processes dominate atmospheric SOC cycling, rather than absorptive or stomatal partitioning processes.