Forest canopy interactions with nucleation mode particles

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.

Dry deposition of particles to canopies—A look back and the road forward

“…Studies of particle fluxes above forests continue [e.g., Pryor et al, 2009;Copeland et al, 2014] with slowly accumulating evidence of upward (and often episodic) small-particle fluxes from both coniferous and deciduous forests and with new understanding of the chemical and biological complexities of the mechanisms involved [Kahlil and Rasmussen, 1992;Tolocka et al, 2006;Pryor et al, 2008Pryor et al, , 2014. The number of chemical species involved continues to grow, but it is now well recognized that the bidirectionality of small-particle fluxes over forests is, at least partially, due to gas-to-particle reactions within the subcanopy airspace and depends on the plant species distribution and the related response to environmental stress.…”

Ecosystem‐scale volatile organic compound fluxes during an extreme drought in a broadleaf temperate forest of the Missouri Ozarks (central USA)

“…BVOCs have ecological functions such as protecting vegetation from abiotic stresses (Singsaas & Sharkey, ; Peñuelas et al ., ; Velikova et al ., ) or acting as communication signals in plant–plant, plant–animal, and multitrophic relationships (Kessler & Baldwin, ; Pichersky & Gershenzon, ; Baldwin et al ., ; Seco et al ., 2011a; Filella et al ., ). In addition, BVOCs can substantially influence atmospheric chemistry and composition (Atkinson, ; Tunved et al ., ; Pryor et al ., ).…”

Ultrafine particle number fluxes over and in a deciduous forest

“…A substantial fraction of the total mean LAI (approximately one quarter; Figure ) is contributed by the understory (associated with a secondary peak in canopy area density at 5 m). The understory is considerably more spatially variable than the forest canopy in terms of both density and height and is dominated by pawpaw ( Asimina triloba ), spicebush ( Lindera benzoin ), and sweet cicely ( Osmorhiza claytonii ) [ Pryor et al ., ]. The aerodynamic roughness length of the site was estimated by using data collected in 1998–1999 as 2.1 ± 1.1 m [ Schmid et al ., ], and the zero‐plane displacement height was estimated as 0.75 times the canopy height ( h c ) and 0.6 × h c for the foliated and leaf‐off seasons using data from sonic anemometers deployed at 46 m and 34 m in near‐neutral conditions (assuming a logarithmic profile) [ Su et al ., ].…”

“…Example particle size distribution (PSD) at 28 m from the FMPS operated on the gradient sampling system over a 15 day period during 14–28 September 2013 (see details of the gradient sampling system given in Pryor et al . []). Also shown (by the magenta blocks) are days that are identified as clear nucleation events by the automated protocol presented in Sullivan and Pryor [].…”