During INTEX-A (summer 2004), a North American ozonesonde network was initiated to investigate variability in the vertical structure of tropospheric ozone: IONS-04 (INTEX [Intercontinental Transport Experiment] Ozonesonde Network Study). IONS-04 consisted of 11 stations across the US and eastern Canada [Thompson et al., 2007]. Nearly 300 midday launches were coordinated over a six-week period, 1 July-15 August 2004. Influences on free tropospheric O3 were computed for each sounding, assuming the following components: regional convection and lightning; stratospheric; and advected (i.e., a mixture of recently imported ozone and aged, background ozone). Determination of the budget was based on analysis of persistent layers in each ozone and P-T-U sounding, a method referred to as Laminar Identification (“LID” in Thompson et al., 2008; Yorks et al., 2008). Stratospheric ozone was found to be responsible for 25% of tropospheric ozone (free tropospheric ozone plus boundary layer ozone) for six northeastern North American stations in 2004. During the INTEX-B (Intercontinental Chemical Transport Experiment)/MILAGRO (Megacities Initiative: Local and Global Research Observations) experiments in March-May 2006 and the TEXAQS-II/GOMACCS (Texas Air Quality Study/Gulf of Mexico Atmospheric Chemistry and Climate Study) in August-September 2006, regular IONS-06 (), ozonesonde launches were made over 15 North American sites (spring) and 22 sites (summer), resulting in ~700 profiles. Satellite validation and model comparisons, as well as LID O3 budgets were supported by IONS-06. Over all stations, local and imported O3 intermingles with O3 from regional convection, lightning and stratosphere-troposphere exchange. The day-to-day variability of tropospheric O3 over the Mexico City Basin (MCB; 19N, 99W) and Houston (30N, 95W) was noteworthy. 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. Tropospheric ozone with stratospheric origins were present on 39% (MCB) and 60% (Houston) of summer days. During the 2008 ARC-IONS (ARCTAS = Arctic Research on Composition of the Troposphere with Aircraft and Satellites), coordinated sondes operated from 1-20 April 2008 and 26 June-12 July 2008, for a total of > 400 soundings. During winter-spring, industrial sources contributed to “arctic haze” transport patterns. In summer, eastern North America was subject to typical urban pollution; northwestern and central North America was affected by Asian pollution and emissions from forest fires in the western US and central Canada.