Role of canopy‐scale photochemistry in modifying biogenic‐atmosphere exchange of reactive terpene species: Results from the CELTIC field study

Craig Stroud,Andrew Turnipseed,Brad Baker,Paul Makar,Eiko Nemitz,Mark Potosnak,Alex Guenther,Thomas Karl,Jose D Fuentes,Chris Geron

doi:10.1029/2005jd005775

Abstract

A one‐dimensional canopy model was used to quantify the impact of photochemistry in modifying biosphere‐atmosphere exchange of trace gases. Canopy escape efficiencies, defined as the fraction of emission that escapes into the well‐mixed boundary layer, were calculated for reactive terpene species. The modeled processes of emission, photochemistry, diffusive transport, and deposition were highly constrained based on intensive observations collected in a Loblolly Pine plantation at Duke Forest, North Carolina, during the CELTIC field study. Canopy top fluxes for isoprene and α,β‐pinene were not significantly altered by photochemistry as calculated escape efficiencies were greater than 0.90 for both species. β‐caryophyllene emission and photochemistry were added to the canopy model as a surrogate for the reactive sesquiterpene class of species. β‐caryopyllene escape efficiencies of 0.30 were calculated for midday summertime conditions. Urbanization scenarios were also performed to assess the impact of pollution on modifying biosphere‐atmosphere exchange. Modest changes in escape efficiencies were calculated for a wide range of anthropogenic hydrocarbon and NOx mixing ratios suggesting a simple parameterization of escape efficiency in terms of grid cell NOx may be possible for incorporating the impact of canopy scale photochemistry within biogenic emission processing systems for regional air quality and climate models. The inferred magnitude of sesquiterpene ozonolysis reactions has important implications on both daytime and nighttime radical formation in the canopy.

Highlights

[2] Biogenic hydrocarbon emissions are estimated to be a larger source of reduced carbon to the global atmosphere than anthropogenic hydrocarbon emissions and are of comparable magnitude to the global emissions of methane [Guenther et al, 1995]
[3] Despite large uncertainties, recent evidence suggests that forests emit unknown reactive hydrocarbons not currently accounted for in biogenic emissions processing systems within air quality and climate models, in addition to the more commonly studied compounds such as isoprene
Faloona et al [2001] postulated that the missing OH source could be from the ozonolysis of reactive terpenes

Summary

Introduction

[2] Biogenic hydrocarbon emissions are estimated to be a larger source of reduced carbon to the global atmosphere than anthropogenic hydrocarbon emissions and are of comparable magnitude to the global emissions of methane [Guenther et al, 1995]. In a Sierra Nevada pine forest, measurements of volatile organic compounds (VOCs) reveal large quantities of previously unreported oxidation products of reactive biogenic precursors [Holzinger et al, 2004] assuming that the observed species were primarily produced by atmospheric oxidation as opposed to direct emission. The emissions of their biogenic precursors must be larger than the emission of total measured terpenes to account for the observed oxidation product mixing ratios; little is known about potential yields of these species. There is still a great deal to be learned about the identity of reactive biogenic emissions and their interactions with radicals, oxidants and aerosol

Objectives

Methods

Results