Mechanisms Responsible for the Size-dependence of, and Bi-directionality of, Ultrafine Particle Fluxes Over Forest
Sara C. C Pryor1, Rebecca J Barthelmie
2 and Ryan C Sullivan
1, (1)Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY, United States, (2)Cornell University, Ithaca, NY, United States
Contact First Author: Sara C. C Pryor; sp2279@cornell.edu
Previously Published Material: I presented some very preliminary results in a poster at AGU Fall 2013 meeting
English Abstract:
Given the global importance of forests, there is a specific need to understand aerosol particle removal to, and formation in/above, forests. We present 18 months of size-resolved and total ultrafine particle fluxes (i.e. diameters < 100 nm) over (at 46 m) and in (at 7 m) a deciduous forest and use them to quantify controls on particle uptake. The flux data presented derive from 3 Gill 3-D WindMaster Pro sonic anemometers (deployed at 46 m, in the canopy at 20 m and below the canopy at 7 m), along with data from, an Ultrafine Condensation Particle Counter (UCPC) operated at 10 Hz and a Fast Mobility Particle Sizer (FMPS) operated at 1 Hz. Size-resolved particle profiles during the same period are measured using a separate FMPS scanning at three measurement heights across the canopy (top, middle and bottom). The results are integrated with fluxes of sensible heat, momentum and carbon dioxide derived using a Licor LI-7200. Results for the total number flux concentrations and the size-resolved concentrations derived using micrometeorological flux approaches show a high degree of accord, but relatively high uncertainty. Other results will be described in detail and include the following:
a) Selecting only deposition fluxes, above/below canopy fluxes indicate a median flux ratio of 3 (i.e. total number fluxes above canopy are 3× below canopy) and above versus below canopy deposition velocities implies a median interception fraction of 60%.
b) The net flux of both the total number of ultrafine particles and all size classes in the ultrafine modes are downwards, but a considerable number of the sampling periods are characterized by upward fluxes. Heat fluxes during large magnitude upward particle fluxes do not exhibit different quadrant partitioning than during large downward fluxes. But particle fluxes exhibit an increased role of both ejections and sweeps in upward fluxes that are differentially important in terms of leaf on versus leaf off and phenological stage.
c) Distinctly different flux diurnal profiles for the nucleation versus Aitken mode particles, and a clear link to nucleation events. This indicates differential control mechanisms on fluxes of particles of different sizes and a key role for aerosol dynamics.
