A082-04
Formaldehyde as a Proxy for Hydroxyl Radical Variability in the Remote Troposphere

Thursday, 10 December 2020: 04:12
Virtual
Colleen Baublitz1, Arlene M Fiore2, Julie M Nicely3, Glenn M Wolfe4, Luke Valin5, Melanie B Follette-Cook6, Roisin Commane7, Michael J Prather8, Hannah M Allen9, Ilann Bourgeois10, William H Brune11, Thaopaul V Bui12, John D Crounse13, Bruce C Daube14, Glenn S Diskin15, Samuel R Hall16, Thomas F Hanisco6, Michelle J Kim13, David Owen Miller17, Jeff Peischl18, Thomas B Ryerson19, Jason M St Clair20, Alexander Benjamin Thames11, Chelsea R Thompson21, Kirk Ullmann22, Paul O Wennberg13 and Steven C Wofsy14, (1)Columbia University, Department of Earth and Environmental Sciences, Palisades, NY, United States, (2)Columbia University, Palisades, NY, United States, (3)University of Maryland College Park, College Park, MD, United States, (4)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (5)Environmental Protection Agency Research Triangle Park, Research Triangle Park, NC, United States, (6)NASA GSFC, Greenbelt, MD, United States, (7)Columbia University in the City of New York, New York, NY, United States, (8)Univ California Irvine, Irvine, CA, United States, (9)California Institute of Technology, Pasadena, CA, United States, (10)NOAA Chemical Sciences Division, Boulder, CO, United States, (11)Pennsylvania State University, University Park, PA, United States, (12)NASA Ames Research Center, Moffett Field, CA, United States, (13)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (14)Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA, United States, (15)NASA Langley Research Ctr, Hampton, VA, United States, (16)NCAR, Denver, CO, United States, (17)Pennsylvania State Univ, University Park, PA, United States, (18)CIRES and NOAA ESRL, Chemical Sciences Laboratory, Boulder, CO, United States, (19)NOAA ESRL Chemical Sciences Division, Boulder, CO, United States, (20)University of Maryland Baltimore County, Baltimore, MD, United States, (21)University of Colorado at Boulder, Institute of Arctic and Alpine Research, Boulder, CO, United States, (22)National Center for Atmospheric Research (NCAR), Atmospheric Chemistry Observations and Modeling Laboratory, Boulder, CO, United States
Recorded Presentation
Abstract:
The hydroxyl radical (OH) is the primary sink for tropospheric methane, a potent near-term climate forcer, and a precursor to tropospheric ozone, an air pollutant and greenhouse gas. The low concentration and short lifetime of OH (less than one second) precludes a long-term observational network for its local variability, limiting our capacity to evaluate OH representation in models. We seek insight into OH spatiotemporal variability and its drivers using measurements from the NASA Atmospheric Tomography (ATom) aircraft mission, which sampled the free troposphere over remote ocean basins in each season. Formaldehyde (HCHO) fluctuations will mirror OH variability over these remote regions if HCHO is at steady-state. HCHO is primarily lost through photolysis and produced via volatile organic compound (VOC) oxidation (principally that of methane). In turn, local OH variability–and thus HCHO production and subsequent loss rates–will mainly reflect changes in OH production, which fluctuate more rapidly than OH loss pathways. HCHO may also mirror the OH concentration if the loss of OH is relatively uniform and driven by the oxidation of longer-lived reactive carbon species (e.g. methane and carbon monoxide) in the remote troposphere. ATom measurements offer an unprecedented opportunity to directly estimate both OH and HCHO production and loss rates and to investigate the drivers of local OH variability. As expected from theory, observed HCHO photolysis and OH production rates show strong correlation (m = 4.2 ± 1.2; r2 = 0.68 ± 0.12) across the full ATom dataset, but only when we control for HCHO photolysis rates and nitrogen oxide abundance. This relationship persists and strengthens (r2 = 0.72 ± 0.06) when we segregate the data into broad spatial bins (2 km depth by 20° latitude) across the remote mid-troposphere (between 2 and 8 km depth). HCHO photolysis also correlates with OH concentration (r2 = 0.54 ± 0.12). This relationship is weaker than that with OH production when methane is a smaller contributor to total OH reactivity (e.g. the upper troposphere, where CO dominates the OH sink). We conclude that the quantitative chemical relationships that we identify between HCHO photolysis and OH production offer a novel observational basis for assessing model OH representation.
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