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Closing the reactive carbon flux budget: Observations from dual mass spectrometers over a coniferous forest
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  • Michael P Vermeuel,
  • Dylan B. Millet,
  • Delphine Farmer,
  • Matson A. Pothier,
  • Michael F. Link,
  • Mj Riches,
  • Sara Williams,
  • Lauren A. Garofalo
Michael P Vermeuel
University of Minnesota - Twin Cities Department of Soil, Water, and Climate
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Dylan B. Millet
University of Minnesota

Corresponding Author:[email protected]

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Delphine Farmer
Colorado State University
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Matson A. Pothier
Department of Chemistry, Colorado State University
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Michael F. Link
Colorado State University
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Mj Riches
Colorado State University
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Sara Williams
Colorado State University
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Lauren A. Garofalo
Department of Chemistry, Colorado State University
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Abstract

We use observations from dual high-resolution mass spectrometers to characterize ecosystem-atmosphere fluxes of reactive carbon across an extensive range of volatile organic compounds (VOCs) and test how well that exchange is represented in current chemical transport models. Measurements combined proton-transfer reaction mass spectrometry (PTRMS) and iodide chemical ionization mass spectrometry (ICIMS) over a Colorado pine forest; together, these techniques have been shown to capture the majority of ambient VOC abundance and reactivity. Total VOC mass and associated OH reactivity fluxes were dominated by emissions of 2-methyl-3-buten-2-ol, monoterpenes, and small oxygenated VOCs, with a small number of compounds detected by PTRMS driving the majority of both net and upward exchanges. Most of these dominant species are explicitly included in chemical models, and we find here that GEOS-Chem accurately simulates the net and upward VOC mass and OH reactivity fluxes under clear sky conditions. However, large upward terpene fluxes occurred during sustained rainfall, and these are not captured by the model. Far more species contributed to the downward fluxes than are explicitly modeled, leading to a major underestimation of this key sink of atmospheric reactive carbon. This model bias mainly reflects missing and underestimated concentrations of depositing species, though inaccurate deposition velocities also contribute. The deposition underestimate is particularly large for assumed isoprene oxidation products, organic acids, and nitrates—species that are primarily detected by ICIMS. Ecosystem-atmosphere fluxes of ozone reactivity were dominated by sesquiterpenes and monoterpenes, highlighting the importance of these species for predicting near-surface ozone, oxidants, and aerosols.
25 Feb 2023Submitted to ESS Open Archive
27 Feb 2023Published in ESS Open Archive