The Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS): model description and application to a temperate deciduous forest canopy
Forest canopies are primary emission sources of biogenic volatile organic compounds (BVOCs) and have the potential to significantly influence the formation and distribution of secondary organic aerosol (SOA) mass. Biogenically-derived SOA formed as a result of emissions from the widespread forests across the globe may affect air quality in populated areas, degrade atmospheric visibility, and affect climate through direct and indirect forcings. In an effort to better understand the formation of SOA mass from forest emissions, a 1-D column model of the multiphase physical and chemical processes occurring within and just above a vegetative canopy is being developed. An initial, gas-phase-only version of this model, the Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS), includes processes accounting for the emission of BVOCs from the canopy, turbulent vertical transport within and above the canopy and throughout the height of the planetary boundary layer (PBL), near-explicit representation of chemical transformations, mixing with the background atmosphere and bi-directional exchange between the atmosphere and canopy and the atmosphere and forest floor. The model formulation of ACCESS is described in detail and results are presented for an initial application of the modeling system to Walker Branch Watershed, an isoprene-emission-dominated forest canopy in the southeastern United States which has been the focal point for previous chemical and micrometeorological studies. Model results of isoprene profiles and fluxes are found to be consistent with previous measurements made at the simulated site and with other measurements made in and above mixed deciduous forests in the southeastern United States. Sensitivity experiments are presented which explore how canopy concentrations and fluxes of gas-phase precursors of SOA are affected by background anthropogenic nitrogen oxides (NO x). Results from these experiments suggest that the level of ambient NO x influences the pathways by which SOA is formed by affecting the relative magnitudes and fluxes of isoprene oxidation products emitted from the canopy. Future versions of the ACCESS model are planned to be multiphase, including gas- and aerosol-phase chemical and physical processes, to more fully explore these preliminary results.