ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-6799-2012 An analysis of fast photochemistry over high northern latitudes during spring and summer using in-situ observations from ARCTAS and TOPSE Olson J. R. 1 Crawford J. H. 1 Brune W. 2 Mao J. 3 Ren X. 4 Fried A. 5 12 Anderson B. 1 Apel E. 5 Beaver M. 6 13 Blake D. 7 Chen G. 1 Crounse J. 6 Dibb J. 8 Diskin G. 1 Hall S. R. 5 Huey L. G. 9 Knapp D. 5 Richter D. 5 Riemer D. 10 Clair J. St. 6 Ullmann K. 5 Walega J. 5 Weibring P. 5 Weinheimer A. 5 Wennberg P. 6 Wisthaler A. 11 14 NASA Langley Research Center, Hampton, VA, USA Department of Meteorology, Penn State, University Park, PA, USA Atmospheric and Oceanic Sciences, Department of Geosciences, Princeton University, Princeton, NJ, USA NOAA Air Resources Laboratory, Silver Spring, MD, USA NCAR, Boulder, CO, USA California Institute of Technology, Pasadena, CA, USA School of Physical Sciences, Department of Chemistry, University of California, Irvine, CA, USA School of Physical Sciences, Department of Chemistry, University of New Hampshire, Durham, NH, USA Georgia Institute of Technology, Atlanta, GA, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria now at: Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA now at: Environmental Protection Agency, Research Triangle Park, NC, USA now at: Norwegian Institute of Air Research, Kjeller, Norway 01 08 2012 12 15 6799 6825 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/6799/2012/acp-12-6799-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/6799/2012/acp-12-6799-2012.pdf

Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO<sub>2</sub> from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO<sub>2</sub> using observed CH<sub>2</sub>O and H<sub>2</sub>O<sub>2</sub> as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO<sub>2</sub> to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H<sub>2</sub>O<sub>2</sub> during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HO<sub>x</sub> budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH<sub>2</sub>O and H<sub>2</sub>O<sub>2</sub>; however when the model is constrained with observed CH<sub>2</sub>O, H<sub>2</sub>O<sub>2</sub> predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH<sub>2</sub>O is uncertain. Free tropospheric observations of acetaldehyde (CH<sub>3</sub>CHO) are 2–3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH<sub>2</sub>O. The box model calculates gross O<sub>3</sub> formation during spring to maximize from 1–4 km at 0.8 ppbv d<sup>−1</sup>, in agreement with estimates from TOPSE, and a gross production of 2–4 ppbv d<sup>−1</sup> in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO<sub>2</sub> in place of model predictions decreases the gross production by 25–50%. Net O<sub>3</sub> production is near zero throughout the ARCTAS-A troposphere, and is 1–2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.

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