edu.ucar.opensky::articles:14171 UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact 2025-12-26T03:27:23.216670 ISO 19115-2 Geographic Information - Metadata - Part 2: Extensions for Imagery and Gridded Data ISO 19115-2:2009(E) https://n2t.org/ark:/85065/d71837f2 The primary and recycling sources of OH during the NACHTT-2011 campaign: HONO as an important OH primary source in the wintertime 2014-06-16T00:00:00Z publication https://n2t.org/ark:/85065/d71837f2 https://doi.org/10.1002/2013JD019784 Kim, S. author VandenBoer, T. author Young, C. author Riedel, T. author Thornton, J. author Swarthout, B. author Sive, B. author Lerner, B. author Gilman, J. author Warneke, C. author Roberts, J. author Guenther, Alex B. author Wagner, N. author Dubé, W. author Williams, E. author Brown, S. author UCAR/NCAR - Library publisher 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. UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact PDF Text theme Resource Type 2016-01-01T00:00:00Z publication Copyright 2014 American Geophysical Union. None eng geoscientificInformation