Abstract
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.
Highlights
Exchanges of energy and mass between the biosphere and atmosphere play a crucial role in the Earth system
Sensitivity studies were conducted for high-NOx conditions, in which the performance of CACM0.0 was found to be closely comparable to that of the Regional Atmopsheric Chemistry Mechanism (RACM) scheme, indicating that the discrepancies shown in Fig. 2 were due to the low levels of NOx at University of Michigan Biological Station (UMBS)
The 1-D Canopy Atmospheric CHemistry Emission (CACHE) canopy model (Forkel et al, 2006; Bryan et al, 2012) has been updated to include a modified version of the CACM gas-phase chemistry scheme (Griffin et al, 2002; Chen et al, 2005) and MPMPO aerosol partitioning mechanism (Griffin et al, 2003; Chen et al, 2005). This new model, FORCAsT 1.0, is one of the few canopy exchange models that incorporate both the gas-phase oxidation of volatile organic compounds (VOCs) and the production of condensable products that can lead to secondary organic aerosols (SOAs) formation
Summary
Exchanges of energy and mass between the biosphere and atmosphere play a crucial role in the Earth system. The potential importance of the individual processes occurring in this space on both the atmosphere and the land surface has prompted a recent focus on the development and application of small-scale or single-point models that explicitly consider the canopy space and its processes (e.g. CACHE, Forkel et al, 2006; Bryan et al, 2012; SOSA(A), Boy et al, 2011; Zhou et al, 2014; CAFE, Wolfe and Thornton, 2011; MLC-Chem, Ganzeveld et al, 2002; ACCESS, Saylor, 2013) These models range in complexity in terms of both vertical resolution and the chemical and physical mechanisms that are included. We evaluate FORCAsT’s performance against its predecessor, the CACHE model, and observations from the CABINEX intensive field campaign, conducted at the University of Michigan Biological Station (UMBS) during the summer of 2009
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