Factors Controlling O3 in the Southeastern United States during Summer as Constrained by the SEAC4RS Campaign

Wednesday, 17 December 2014
Katherine Travis1, Daniel J. Jacob2, Jenny A Fisher3, Eloise A Marais1, Sungshik Kim4, Lei Zhu1, Karen Yu1, Robert Yantosca1, Melissa Payer Sulprizio1, Fabien Paulot1, Jingqiu Mao5, Paul O Wennberg6, John Crounse7, Thomas B Ryerson8, Armin Wisthaler9, Greg Huey10 and Anne M Thompson11, (1)Harvard University, Cambridge, MA, United States, (2)Harvard University, School of Engineering and Applied Sciences, Cambridge, MA, United States, (3)University of Wollongong, Wollongong, Australia, (4)Harvard--EPS Hoffman, Cambridge, MA, United States, (5)Princeton University, Princeton, NJ, United States, (6)California Institute of Technology, Pasadena, CA, United States, (7)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (8)NOAA Chemical Sciences Divisio, Boulder, CO, United States, (9)University of Oslo, Department of Chemistry, Oslo, Norway, (10)Georgia Institute of Technology Main Campus, School of Earth and Atmospheric Sciences, Atlanta, GA, United States, (11)NASA Goddard Space Flight Center, Greenbelt, MD, United States
The Southeast United States (SEUS) is unique in its atmospheric chemistry and the difficulty of models in reproducing observed ozone (O3) (Fiore et al, 2009). Unlike the Western U.S., O3 variability is more heavily influenced by anthropogenic impacts than background sources such as wildfires, foreign transport, and stratospheric intrusions (Zhang et al, 2011). In addition, the SEUS has biogenic VOC emissions, important O3 precursors, which are among the highest in the world. We use observations from the SEAC4RS campaign over the SEUS in summer 2013, interpreted with the global chemical transport model GEOS-Chem, to evaluate the factors controlling O3 in this region. We use the GEOS-Chem model version v9-02 with significant updates, including improved treatment of isoprene nitrates (Lee et al, 2014), revised yields of MVK and MACR (Liu et al, 2013), improved treatment of isoprene epoxides (Bates et al, 2014), and faster deposition of isoprene oxidation products. The model significantly over predicts the observed O3, particularly in isoprene-rich, low-NOx regions. Properly capturing the fate of the isoprene peroxy radical (RO2) is essential to modeling O3 during the campaign. The amount of NOx in the SEUS is mainly driven by anthropogenic emissions with a smaller contribution from lightning and soil NOx, in addition to the amount of NOx recycled by isoprene nitrates. The variability in the amount of HOx available in the model can be influenced by the recycling of OH assumed in the GEOS-Chem chemical mechanism. We use the ratio of measured isoprene hydroxyperoxide (ISOPOOH) to isoprene nitrates (ISOPN) to constrain the modeled branching between the RO2 + HO2 and RO2 + NO2 pathways. Based on this ratio, we find that the RO2 + HO2 pathway is underestimated in our current chemical mechanism. Moreover, our NOx emissions may be overestimated by comparison with satellite tropospheric NO2 columns. We increase the importance of the RO2 + HO2 pathway with the inclusion of HONO chemistry inferred from Li et al, 2014 and explore improvements to our NOx emissions inventories. Our improved treatement of the processes controlling O3 production in this region, together with our constraints on the fate of isoprene RO2, significantly reduces the high surface bias in modeled ozone against aircraft, sonde, and surface observations.