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Bidirectional Ecosystem–Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum: How Many Matter?
journal contribution
posted on 2018-06-14, 00:00 authored by Dylan B. Millet, Hariprasad D. Alwe, Xin ChenXin Chen, Malte Julian Deventer, Timothy J. Griffis, Rupert Holzinger, Steven B. Bertman, Pamela S. Rickly, Philip S. Stevens, Thierry Léonardis, Nadine Locoge, Sébastien Dusanter, Geoffrey S. Tyndall, Sergio L. Alvarez, Matthew H. Erickson, James H. FlynnTerrestrial
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.