Ubiquity of quasi-horizontal layers in the troposphere

Abstract

letters to nature 316 NATURE | VOL 398 | 25 MARCH 1999 | www.nature.com in the imprinted sites, whereas predominantly aragonite was formed in solution, as expected. In some cases an occasional needle-like crystal of aragonite could be seen, which had probably been deposited onto the polymer particle during ®ltration and drying (compare Fig. 4e and g). More signi®cantly, examination of the surface of control polymer CP-2 ( Fig. 4h) revealed the presence of deposited aragonite needles as well as a few sharp-edged`solution-type' calcite crystals. A crystal-enriched fraction from PI-1, prepared by grinding and sedimentation in chloroform, was shown conclusively to contain calcite, both by FT-IR (Fig. 3b) and XRD (not shown).We attempted to produce aragonite-speci®c polymers using the same monomer 1 and the needles shown in Fig. 4g as template. But the results of this experiment were inconclusive, as the polymers did not nucleate as well as those imprinted with calcite and neither FT-IR nor powder XRD were sensitive enough to quantify the ratio of aragonite to calcite on the polymer surface. It is not clear why the imprinting of calcite should result in much more ef®cient nucleation than the corresponding aragonite imprints. However, as the arrangement of ions on the crystal planes of each polymorph are different, it is reasonable to expect that a functionality which is suitable for creating complementary surfaces in one case may not necessarily be useful for another type of crystal. We note that the use of acrylic and methacrylic acids in place of the monomer 1 resulted in imprinted polymers with inferior nucleating abilities. The reason for this is not clear, but may be due to the presence of the¯exible spacer which separates the acid head group from the polymerizable double bond of 1, allowing this monomer to match more easily the spacing of ions on the crystal surface and for this ordering to be preserved throughout the entire imprinting process. Better understanding of these processes may enable the design of devices for generating crystals of the desired type, size or shape for applications in materials science. In addition, the ability to generate surfaces speci®c to crystals of an enantiomerically pure organic compoundÐto promote its crystallization from a racemic mixtureÐ may lead to the introduction of new separation methods in the pharmaceutical and ®ne-chemical industries. M

Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons

“…Moreover the ozone monthly average within the AEJ layer (Sauvage et al, 2005) was showing only one ozone peak due to smoothing by the average methodology, in contrast with the three distinct peaks in the present case study. These peaks illustrate the fine scale stratification of the atmosphere (Newell et al, 1999). L3 has a distinct origin, as will be demonstrated in the following section by means of a numerical model.…”

Stratosphere‐troposphere exchange: A review, and what we have learned from STACCATO

“…Typically, the intruding stratospheric air forms filamentary structures, which are observed in various contexts. For instance, they appear as laminae in ozone profiles [Dobson, 1973;Reid and Vaughan, 1991], as filamentary features on water vapor satellite images [Appenzeller and Davies, 1992;Wirth et al, 1997], they are evident by ozone lidar measurements [Stohl and Trickl, 1999] and have been detected by in situ aircraft measurements [Reid and Vaughan, 1993;Balluch and Haynes, 1997;Newell et al, 1999]. These filaments cascade down to ever smaller scales, through the action of ''chaotic advection'' [Ottino, 1989], and interleave with tropospheric air [Appenzeller et al, 1996a].…”

“…[28] Extrusions from both the stratosphere and the troposphere are usually organized within small-scale filamentary structures and laminae, which appear in various tracer (e.g., PV, water vapor, O 3 ) fields [e.g., Dobson, 1973;Reid and Vaughan, 1991;Newell et al, 1999], and are produced by shear in the large-scale wind fields. O 3 layers originating from the stratosphere can persist for 10 days in the troposphere [Bithell et al, 2000], but these structures cascade down to ever-smaller scales until they are mixed with the surrounding air.…”

Analysis of water vapor LIDAR measurements during the MAP campaign: evidence of sub-structures of stratospheric intrusions

“…In fact, irreversible transport depends on the degree of fragmentation in the finer structures of the air transported from the stratosphere down into the troposphere (Vaughan et al, 1994;Appenzeller et al, 1996;Langford and Reid,1998). As a result of the filamentation, laminar distributions along the vertical profiles of the transported chemical species appear with respect to the climatological profiles (Newell et al, 1999), with a rapidly decreasing vertical depth versus time (Methven et al, 2003). Filaments are generated in regions with strong wind shear (i.e.…”