Photochemistry in the Arctic Free Troposphere: Ozone Budget and Its Dependence on Nitrogen Oxides and the Production Rate of Free Radicals |
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| Authors: | Craig Stroud Sasha Madronich Elliot Atlas Christopher Cantrell Alan Fried Brian Wert Brian Ridley Fred Eisele Lee Mauldin Richard Shetter Barry Lefer Frank Flocke Andy Weinheimer Mike Coffey Brian Heikes Robert Talbot Donald Blake |
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| Institution: | (1) Atmospheric Chemistry Division, National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO, 80303, U.S.A., E-mail;(2) Earth and Atmospheric Science Department, Georgia Institute of Technology, Atlanta, GA, U.S.A;(3) School of Oceanography, University of Rhode Island, Narragansett, RI, U.S.A;(4) Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH, U.S.A;(5) Department of Chemistry, University of California, Irvine, CA, U.S.A |
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| Abstract: | Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NOx mixing ratiosup to  300 pptv in February and for NOx mixing ratios up to 500 pptv in May. These NOx limits are an order of magnitude higher thanmedian NOx levels observed, illustrating the strong dependence ofgross ozone production rates on NOx mixing ratios for the majority of theobservations. The threshold NOx mixing ratio needed for netpositive ozone production was also calculated to increase from NOx 10pptv in February to 25 pptv in May, suggesting that the NOx levels needed to sustain net ozone production are lower in winter than spring. This lower NOx threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NOx neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign. |
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| Keywords: | global atmospheric chemistry TOPSE arctic photochemistry ozone production peroxide radical chain length |
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