Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem-atmosphere VOC fluxes across the entire detected mass range (m/z 0-335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem-atmosphere exchange. We introduce the reactivity flux as a measure of how Earth-atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (-VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the -VOC carbon and reactivity fluxes by 40-60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ∼30% of the carbon flux and ∼50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds.
Bibliographical noteFunding Information:
This research was supported by the National Science Foundation (NSF Grants AGS-1428257 and AGS-1148951). GEOS-Chem model development and simulations for this work were supported by NASA (Grant NNX14AP89G). Computing resources were provided by the Minnesota Supercomputing Institute (http://www.msi.umn.edu) at the University of Minnesota. We acknowledge the ECCAD database (http://eccad.sedoo.fr) for hosting emission inventories used in this work. The Indiana University and University of Houston groups acknowledge NSF support (Grants AGS-1440834 and AGS-1552077, respectively). The National Center for Atmospheric Research is likewise supported by NSF. This work was also integrated into the CaPPA project (Chemical and Physical Properties of the Atmosphere), funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement d’Avenir) under contract ANR-11-LABX-0005-01 and by the Regional Council “Nord-Pas de Calais-FEDER”. We thank all PROPHET-AMOS participants for making this work possible and the University of Michigan Biological Station for hosting the field study. Thanks to John Ortega, D.D. Montzka, and Andrew Weinheimer for their work on the NO and NO2 measurements, and to the US-UMB Ameriflux team for providing the PPFD data used here.
© Copyright 2018 American Chemical Society.
- chemical transport model
- eddy covariance
- volatile organic compounds