Continuous VOC flux measurements on boreal forest floor

Volatile organic compound (VOC) emissions from biogenic sources are important contributors to chemical reactions in the air. Soil/forest floor VOCs contribute significantly to the ecosystem scale emissions, however, these emissions and their temporal and spatial variations are poorly characterised. The below-canopy VOC emissions have been measured mainly in campaigns; continuous measurements over the whole growing season are rare. VOCs were measured from boreal forest floor over the snow-free season 2010 in southern Finland with automated flow-through chambers connected to proton transfer reaction-mass spectrometer (PTR-MS). We measured 10 masses in total, of which five quantitatively (M33, M45, M59, M69, M137). All of the fluxes showed clear diurnal and seasonal variation, being at their highest in early summer. Spatial variation in the fluxes was great and the lowest rates were found in chambers with dense vegetation cover. Also, VOCs deposition was observed regularly. Monoterpene (M137) emissions were one magnitude higher (up to 264 ng m−2 s−1) than other emissions. The VOC fluxes correlated positively with temperature and light, while relative humidity correlated negatively. Results indicated that forest floor plays a substantial role in the boreal forest total VOC emissions. Understanding the processes controlling VOC emissions requires more detailed analysis and longtime measurements with sufficient time resolution and analytical accuracy.