Abstract
Abstract. Ultrafine particle size distributions through a deciduous forest canopy indicate that nucleation mode particle concentrations decline with depth into the canopy, such that number concentrations at the bottom of the canopy are an average of 16% lower than those at the top. However, growth rates of nucleation mode particles (diameters 6–30 nm) are invariant with height within the canopy, which implies that the semi-volatile gases contributing to their growth are comparatively well-mixed through the canopy. Growth rates of nucleation mode particles during a meteorological drought year (2012) were substantially lower than during a meteorologically normal year with high soil water potential (2013). This may reflect suppression of actual biogenic volatile organic compound (BVOC) emissions by drought and thus a reduction in the production of condensable products during the drought-affected vegetation season. This hypothesis is supported by evidence that growth rates during the normal year exhibit a positive correlation with emissions of BVOC modeled on observed forest composition, leaf area index, temperature and photosynthetically active radiation (PAR), but particle growth rates during the drought-affected vegetation season are not correlated with modeled BVOC emissions. These data thus provide indirect evidence that drought stress in forests may reduce BVOC emissions and limit growth of nucleation mode particles to climate-relevant sizes.
Highlights
Introduction and objectivesDeciduous forest canopies are both a source of compounds that may facilitate growth of nucleation mode particles to climate-relevant sizes, and a strong sink for nucleation mode particles due to the large surface area, high foliar uptake efficiencies and generally high turbulence intensity
New particle formation is an important source of climate relevant aerosol particles and biogenic VOC emissions from forests have been identified as playing a key role in the growth of nucleated particles to climate relevant sizes
Uncertainty remains regarding how important biogenic volatile organic compound (BVOC) are to particle growth, how BVOC emissions might alter in the future, and what fraction of nucleation mode particles are removed by forest canopies and play no role in regional climate forcing
Summary
Introduction and objectivesDeciduous forest canopies are both a source of compounds that may facilitate growth of nucleation mode particles to climate-relevant sizes (e.g., oxidation products of biogenic volatile organic compounds, BVOCs), and a strong sink for nucleation mode particles due to the large surface area, high foliar uptake efficiencies and generally high turbulence intensity. Two years of particle size distribution (PSD) measurements at three heights within and above a deciduous forest canopy are analyzed to quantify both the canopy uptake of ultrafine particles and the role of the canopy in particle growth, with a specific focus on differences in a drought-affected and non-drought year. BVOC emissions are estimated to release ∼ 1–2 % of photosynthetically fixed carbon (C) back to the atmosphere over temperate deciduous forests (Kesselmeier et al, 2002). Emissions of monoterpenes and sesquiterpenes exhibit an exponential dependence on air temperature (T ), scale with leaf area index (LAI) and vary with plant functional type (Guenther et al, 1993). Isoprene emissions exhibit an additional dependence on photosynthetically active radiation (PAR) (Guenther et al, 2006). There is evidence that BVOC emissions are controlled by the presence and severity of physiologic stressors, and that prolonged drought may suppress BVOC emissions (Niinemets, 2010)
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