Abstract
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.
Highlights
A number of multi-national, multi-platform field experiments have been conducted to quantify intra- and intercontinental pollution transport
Basin and Houston during spring and summer 2006 are delineated with sondes taken in IONS-06
Wave influences are prominent in the laminar structure in the lower stratosphere and throughout the troposphere, with more frequent wave activity in spring
Summary
A number of multi-national, multi-platform field experiments have been conducted to quantify intra- and intercontinental pollution transport. The signature of RW activity above 12 km for MCB (Fig. 4) is consistent with the appearance of the very dry, high-O3 layer at ∼160 hPa in virtually all the March soundings (see IONS-06 website). After 8 March, a greater ST O3 amount sets in, driven by a 20–40% RW frequency over the remainder of MILAGRO/INTEX-B Both the MCB and Houston increased in tropospheric O3 column 7–8 DU in the spring-to-summer transition (Table 2). Between the March (MILAGRO, IONS-06 Phase 1) and August–September 2006 sampling periods, RCL and ST fractions over Houston decline ∼30% relative to their March contributions (Table 2) This is likely to be a consequence of a highly convective spring over Houston (Fast et al, 2007).
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