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Abstract. Isoprene and its oxidation products, methyl vinyl ketone (MVK) and mcthacrolein (MACR), were measured in a semirural environment that was occasionally heavily impacted by urban emissions. At this site, isoprenc was the most important hydrocarbon in terms of koH' [hydrocarbon], but the aldehydes HCHO and CH3CHO also appear to be very important. The local isoprene photochemistry appears to be occasionally enhanced in NOx-rich urban plumes that are advected to the site over intermediate forested land. When 03 was being rapidly produced in urban plumes advected to this forested site, isoprene was found to contribute = 28% of the total ozone production. We observe that many of the peaks in isoprene oxidation products at this surface site arise from downward mixing of more photochemically processed air aloft, as the nocturnal inversion breaks up in the morning. We estimate that, in the daytime, t3pically 1-2% of the NOy at this NOx-rich site is composed of isoprene nitrates.

Square hollow waveguides are used to integrate measurement of absorption with chip-based electrophoresis. The 50x50 microm liquid channel and 50x50 microm waveguide are etched as a negative pattern into a silicon master and replicated as a positive in poly-dimethylsiloxane (PDMS). The uniform refractive index of the chip prevents guiding by total internal reflection. Instead, light at 488 nm is guided by reflection at the air-PDMS interface. The waveguide has a 60% efficiency over a distance of 3.2 cm. Separation of fluorescein and the dye BODIPY is demonstrated. A detection limit (S/N=3) of 200 microM fluorescein is obtained using a 50 microm pathlength and a simple photocell detector.

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Propagation losses are determined for 100 microm x 100 microm square, hollow waveguides constructed from glass capillaries. The small size makes it possible to observe optical effects not easily seen with larger waveguides. The depletion of higher-order even modes creates a large, nonlinear loss. Over a distance of a meter the loss approaches the smaller, linear value expected for the fundamental mode. Additionally, the lowest two even modes beat to produce an oscillatory loss with a period of approximately 2 cm. Making the focal radius 0.35 the waveguide width minimizes these two effects. In a related study, 50-microm waveguides embossed in polydimethylsiloxane are shown to have losses similar to glass capillaries.