Emission of formic and acetic acids from two Colorado soils

HCOOH in the remote atmosphere: Constraints from Atmospheric Tomography (ATom) airborne observations

Formic acid (HCOOH) is an important component of atmospheric acidity but its budget is poorly understood, with prior observations implying substantial missing sources. Here we combine pole-to-pole airborne observations from the Atmospheric Tomography Mission (ATom) with chemical transport model (GEOS-Chem CTM) and back trajectory analyses to provide the first global in-situ characterization of HCOOH in the remote atmosphere. ATom reveals sub-100 ppt HCOOH concentrations over most of the remote oceans, punctuated by large enhancements associated with continental outflow. Enhancements correlate with known combustion tracers and trajectory-based fire influences. The GEOS-Chem model underpredicts these in-plume HCOOH enhancements, but elsewhere we find no broad indication of a missing HCOOH source in the background free troposphere. We conclude that missing non-fire HCOOH precursors inferred previously are predominantly short-lived. We find indications of a wet scavenging underestimate in the model consistent with a positive HCOOH bias in the tropical upper troposphere. Observations reveal episodic evidence of ocean HCOOH uptake, which is well-captured by GEOS-Chem; however, despite its strong seawater undersaturation HCOOH is not consistently depleted in the remote marine boundary layer. Over fifty fire and mixed plumes were intercepted during ATom with widely varying transit times and source regions. HCOOH:CO normalized excess mixing ratios in these plumes range from 3.4 to >50 ppt/ppb CO and are often over an order of magnitude higher than expected primary emission ratios. HCOOH is thus a major reactive organic carbon reservoir in the aged plumes sampled during ATom, implying important missing pathways for in-plume HCOOH production.

Measuring Biosphere-Atmosphere Exchange of Short-Lived Climate Forcers and Their Precursors

Exchange of reactive trace gases over the biosphere is a key source of reactive organic carbon to the atmosphere and thus influences the formation of both ozone (O₃) and secondary organic aerosol (SOA). Both O₃ and aerosol particles are short-lived climate forcers and impact the radiative balance of the planet, and their sources and sinks are chemically complex. However, the biosphere also acts as a deposition sink for organic and inorganic compounds, including O₃, aerosols, and their precursors. Wet and dry deposition provides a key lever on the lifetime of trace gases and particles in the atmosphere and thus on their potential to influence the radiative balance of the planet. The fluxes of reactive trace gases and particles are part of an atmospheric biogeochemical cycle that includes feedbacks through drought and other climate components.Recent advances in measurement techniques have enabled new field observations of trace gas and particle fluxes. Our method development has focused on the leaf, branch, and forest level, although satellite measurements coupled to modeling also provide promising new approaches to constraining trace gas fluxes. Leaf chamber measurements of volatile organic compound (VOC) emissions highlight leaf-to-leaf and plant-to-plant variability in both photosynthesis and emissions of individual VOCs, in addition to differences in emissions across different isomers of monoterpenes. Isomers obviously have different chemical properties (e.g., reaction rates with OH radicals, SOA yield) and thus hold different potentials as precursors for short-lived climate forcers.The biosphere acts as both sources and sinks of the oxidation products of monoterpenes and other biogenic VOCs. Developments in chemical ionization mass spectrometry have recently enabled measurements of volatile organic acids, which demonstrate a strong temperature-dependent ecosystem source, as well as a source from in-canopy chemistry. In-canopy chemistry also influences particle fluxes, although deposition should dominate their net exchange. Our field observations of chemically resolved particle fluxes demonstrate the simultaneous, competing processes driving forest exchange. To separate out these competing processes, we use black carbon as an inert tracer for particle deposition. Our recent measurements demonstrate the importance of wet deposition in controlling particle lifetime in the atmosphere. Overall, new measurement techniques have enabled both field and laboratory observations to improve our understanding of biosphere-atmosphere interactions and their influence on climate processes.

Oxidation of Volatile Organic Compounds as the Major Source of Formic Acid in a Mixed Forest Canopy

Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above-canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above-canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state-of-science atmospheric model (GEOS-Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below-canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.