Parameterization of Leaf-Scale Biogenic Emissions for Application to Air Quality Models

Biogenic emissions are an important source of pollutant precursors and therefore an essential component of air quality models. These emissions vary with vegetation and meteorology and are typically connected to chemical transport models through a big-leaf approach, with relatively simplistic representation of the underlying canopy vertical structure. It is known, however, that the vertical distribution of leaf area density has substantial impacts on the microclimate, leaf morphology, and leaf physiology in forest canopies, which in turn affects emission properties throughout the canopy sub-layers. We present a parameterization of 3D canopy-resolved biogenic emissions as implemented in NOAA’s stand-alone canopy-app (https://github.com/noaa-oar-arl/canopy-app), designed for offline research and testing and application to gridded air quality models. The parameterizations in canopy-app are currently driven by relatively simple, prescribed shape functions for describing vertical foliage distributions, and produce leaf-scale biogenic emissions for a given set of column conditions globally. The biogenic emission fluxes are adapted from established parameterizations based on large eddy simulation experiments and the Model of Emissions of Gases and Aerosols from Nature (MEGAN) versions 2.1 and 3.1. Meteorological boundary (i.e., above-canopy) variable inputs to offline canopy-app are supplied by NOAA’s Global Forecasting System (GFS) with additional canopy variables derived from satellite data. We first compare the estimates of biogenic emissions from canopy-app to integrated and stand-alone versions of MEGAN (v2.1 and 3.1, respectively). We will further compare canopy-app results with existing in-canopy flux measurements wherever possible, but because these are scarce, we also plan to use emissions from canopy-app to drive a photochemical box model (e.g., F0AM: the Framework for 0-D Atmospheric Modeling) that we will use to determine the volume mixing ratios of emitted species, of which measurements are much more common.