Articles | Volume 8, issue 11
Geosci. Model Dev., 8, 3765–3784, 2015
Geosci. Model Dev., 8, 3765–3784, 2015

Model description paper 26 Nov 2015

Model description paper | 26 Nov 2015

FORest Canopy Atmosphere Transfer (FORCAsT) 1.0: a 1-D model of biosphere–atmosphere chemical exchange

K. Ashworth1, S. H. Chung2, R. J. Griffin3, J. Chen4, R. Forkel5, A. M. Bryan1,a, and A. L. Steiner1 K. Ashworth et al.
  • 1Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
  • 2Department of Civil and Environmental Engineering, Washington State University, Pullman, WA 99164, USA
  • 3Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005, USA
  • 4California Air Resources Board, Sacramento, CA 95814, USA
  • 5Karlsruher Institut für Technologie (KIT), Institut für Meteorologie und Klimaforschung, Atmosphärische Umweltforschung (IMK-IFU), Kreuzeckbahnstr. 19, Garmisch-Partenkirchen, Germany
  • anow at: DOI Northeast Climate Science Center, University of Massachusetts, Amherst, MA 01003, USA

Abstract. Biosphere–atmosphere interactions play a critical role in governing atmospheric composition, mediating the concentrations of key species such as ozone and aerosol, thereby influencing air quality and climate. The exchange of reactive trace gases and their oxidation products (both gas and particle phase) is of particular importance in this process. The FORCAsT (FORest Canopy Atmosphere Transfer) 1-D model is developed to study the emission, deposition, chemistry and transport of volatile organic compounds (VOCs) and their oxidation products in the atmosphere within and above the forest canopy. We include an equilibrium partitioning scheme, making FORCAsT one of the few canopy models currently capable of simulating the formation of secondary organic aerosols (SOAs) from VOC oxidation in a forest environment. We evaluate the capability of FORCAsT to reproduce observed concentrations of key gas-phase species and report modeled SOA concentrations within and above a mixed forest at the University of Michigan Biological Station (UMBS) during the Community Atmosphere-Biosphere Interactions Experiment (CABINEX) field campaign in the summer of 2009. We examine the impact of two different gas-phase chemical mechanisms on modelled concentrations of short-lived primary emissions, such as isoprene and monoterpenes, and their oxidation products. While the two chemistry schemes perform similarly under high-NOx conditions, they diverge at the low levels of NOx at UMBS. We identify peroxy radical and alkyl nitrate chemistry as the key causes of the differences, highlighting the importance of this chemistry in understanding the fate of biogenic VOCs (bVOCs) for both the modelling and measurement communities.

Short summary
Volatile organic compounds released from forests into the atmosphere play a key role in governing atmospheric concentrations of trace gases and aerosol particles. We describe the development of a 1-D model that simulates the processes occurring within and above the forest canopy that regulate the transfer of these compounds and their products. We evaluate model performance by comparison of modelled concentrations against measurements from a field campaign at a northern Michigan forest site.