In-canopy gas-phase chemistry during CABINEX 2009: sensitivity of a 1-D canopy model to vertical mixing and isoprene chemistry

A M Bryan,R F Hansen,P S Stevens,B T Jobson,R Forkel,A L Steiner,B L Lefer,A B Guenther,M A Carroll,S Dusanter,S Griffith,D Helmig,P B Shepson,F N Keutsch,G D Edwards,S N Pressley,S B Bertman

doi:10.5194/acp-12-8829-2012

Abstract

Abstract. Vegetation emits large quantities of biogenic volatile organic compounds (BVOC). At remote sites, these compounds are the dominant precursors to ozone and secondary organic aerosol (SOA) production, yet current field studies show that atmospheric models have difficulty in capturing the observed HOx cycle and concentrations of BVOC oxidation products. In this manuscript, we simulate BVOC chemistry within a forest canopy using a one-dimensional canopy-chemistry model (Canopy Atmospheric CHemistry Emission model; CACHE) for a mixed deciduous forest in northern Michigan during the CABINEX 2009 campaign. We find that the base-case model, using fully-parameterized mixing and the simplified biogenic chemistry of the Regional Atmospheric Chemistry Model (RACM), underestimates daytime in-canopy vertical mixing by 50–70% and by an order of magnitude at night, leading to discrepancies in the diurnal evolution of HOx, BVOC, and BVOC oxidation products. Implementing observed micrometeorological data from above and within the canopy substantially improves the diurnal cycle of modeled BVOC, particularly at the end of the day, and also improves the observation-model agreement for some BVOC oxidation products and OH reactivity. We compare the RACM mechanism to a version that includes the Mainz isoprene mechanism (RACM-MIM) to test the model sensitivity to enhanced isoprene degradation. RACM-MIM simulates higher concentrations of both primary BVOC (isoprene and monoterpenes) and oxidation products (HCHO, MACR+MVK) compared with RACM simulations. Additionally, the revised mechanism alters the OH concentrations and increases HO2. These changes generally improve agreement with HOx observations yet overestimate BVOC oxidation products, indicating that this isoprene mechanism does not improve the representation of local chemistry at the site. Overall, the revised mechanism yields smaller changes in BVOC and BVOC oxidation product concentrations and gradients than improving the parameterization of vertical mixing with observations, suggesting that uncertainties in vertical mixing parameterizations are an important component in understanding observed BVOC chemistry.

Highlights

There is increasing evidence of the important role of forest canopies and biogenic volatile organic compound (VOC) emissions on tropospheric composition and atmospheric chemistry (Goldstein and Galbally, 2007; Lelieveld et al, 2008)
CACHE calculates vertical mixing within and above the forest canopy using K-theory, a parameterization used by many 1-D and 3-D models despite its limitations in the canopy roughness layer
Chemical transformation is modeled using Regional Atmospheric Chemistry Model (RACM), a condensed mechanism that can cover a broad range of chemical situations but with limited biogenic VOC (BVOC) chemistry under low-NOx conditions

Summary

Introduction

There is increasing evidence of the important role of forest canopies and biogenic volatile organic compound (VOC) emissions on tropospheric composition and atmospheric chemistry (Goldstein and Galbally, 2007; Lelieveld et al, 2008). Biogenic VOC (BVOC) emissions and their oxidation products must be mixed effectively out of the forest canopy. This forest-atmosphere exchange is highly sensitive to turbulent mixing and chemistry because BVOC oxidation and transport occur on similar timescales (Molemaker and VilaGuerau de Arellano, 1998; Krol et al, 2000; Pugh et al, 2010)

Methods

Results

Conclusion