1The Pennsylvania State University, Department of Meteorology, 503 Walker Building, University Park, PA 16802-5013, USA
2SSAI of Lanham, MD 20706 USA
3Valparaiso University, Dept of Physics and Astronomy, Valparaiso, IN 46383, USA
4UNAM (Autonomous University of Mexico), CCA – Center for Chemistry of the Atmosphere, Mexico City, DF, Mexico
5University of Houston, Geosciences Department, Old Sciences Bldg, Houston, Texas 77204-5007, USA
*also at: NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
Abstract. During the INTEX-B (Intercontinental Chemical Transport Experiment)/MILAGRO (Megacities Initiative: Local and Global Research Observations) experiments in March 2006 and the associated IONS-06 (INTEX Ozonesonde Network Study; http://croc.gsfc.nasa.gov/intexb/ions06.html), regular ozonesonde launches were made over 15 North American sites. The soundings were strategically positioned to study inter-regional flows and meteorological interactions with a mixture of tropospheric O3 sources: local pollution; O3 associated with convection and lightning; stratosphere-troposphere exchange. The variability of tropospheric O3 over the Mexico City Basin (MCB; 19 N, 99 W) and Houston (30 N, 95 W) is reported here. MCB and Houston profiles displayed a double tropopause in most soundings and a subtropical tropopause layer with frequent wave disturbances, identified through O3 laminae as gravity-wave induced. Ozonesondes launched over both cities in August and September~2006 (IONS-06, Phase 3) displayed a thicker tropospheric column O3 (~7 DU or 15–20%) than in March 2006; nearly all of the increase was in the free troposphere. In spring and summer, O3 laminar structure manifested mixed influences from the stratosphere, convective redistribution of O3 and precursors, and O3 from lightning NO. Stratospheric O3 origins were present in 39% (MCB) and 60% (Houston) of the summer sondes. Comparison of summer 2006 O3 structure with summer 2004 sondes (IONS-04) over Houston showed 7% less tropospheric O3 in 2006. This may reflect a sampling contrast, August to mid-September 2006 instead of July–mid August 2004.