The primary and recycling sources of OH during the NACHTT-2011 campaign: HONO as an important OH primary source in the wintertime

Authors

  • Saewung Kim,

    Corresponding author
    1. Atmospheric Chemistry Division, NCAR Earth System Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA
    2. Now at Department of Earth System Science, School of Physical Sciences, University of California, Irvine, California, USA
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  • Trevor C. VandenBoer,

    1. Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
    2. Now at Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada
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  • Cora J. Young,

    1. Now at Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada
    2. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    3. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • Theran P. Riedel,

    1. Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
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  • Joel A. Thornton,

    1. Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
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  • Bob Swarthout,

    1. Natural Resources and Earth System Science Program, University of New Hampshire, Durham, New Hampshire, USA
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  • Barkley Sive,

    1. Department of Chemistry, Appalachian State University, Boone, North Carolina, USA
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  • Brian Lerner,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • Jessica B. Gilman,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • Carsten Warneke,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • James M. Roberts,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
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  • Alex Guenther,

    1. Atmospheric Chemistry Division, NCAR Earth System Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA
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  • Nicholas L. Wagner,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • William P. Dubé,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
    2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA
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  • Eric Williams,

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
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  • Steven S. Brown

    1. NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
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Abstract

We present OH observations from Nitrogen, Aerosol Composition, and Halogens on a Tall Tower 2011 (NACHTT-11) held at the Boulder Atmospheric Observatory in Weld County, Colorado. Average OH levels at noon were ~ 2.7 × 106 molecules cm−3 at 2 m above ground level. Nitrous acid (HONO) photolysis was the dominant OH source (80.4%) during this campaign, while alkene ozonolysis (4.9%) and ozone photolysis (14.7%) were smaller contributions to OH production. To evaluate recycling sources of OH from HO2 and RO2, an observationally constrained University of Washington Chemical Mechanism (UWCM) box model (version 2.1) was employed to simulate ambient OH levels over several scenarios. For the base run, not constrained by observed HONO, the model significantly underestimated OH by a factor of 5.3 in the morning (9:00–11:00) and by a factor of 3.2 in the afternoon (13:00–15:00). The results suggest that known chemistry cannot constrain HONO and, subsequently, OH during the observational period. When HONO is constrained in the model by observations (< 50 m), the discrepancy between observation and model simulation improves to a factor of 1.3 in the morning and a factor 1.1 in the afternoon, within the 35% estimated instrumental uncertainty. However, the model produces both a morning and afternoon maximum in OH, in contrast to the observations, which show strong evidence for morning OH production but no distinct morning maximum. Two additional OH sources were also considered, although they do not improve the differences in modeled and measured temporal OH profiles. First, the impact of daytime HONO gradients near the ground surface (< 20 m) was evaluated. Strong HONO gradients were observed between 06:00 and 09:00 MST (mountain standard time), especially within 20 m of the surface. When constrained to HONO observed below 20 m (rather than 50 m), the model produced an even larger morning OH maximum, in contrast to the observations. Second, Cl atoms from ClNO2 photolysis producing RO2 from reaction with alkanes, while significant, produced steady state Cl atom levels (~ 103 atoms cm−3) that were too low to significantly perturb measured OH through reactions of organic peroxy radicals produced from Cl reactions with volatile organic compounds.

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