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Photographs of a 120 [degrees] parhelion and a 22 [degrees] parhelion within persistent contrails are presented. These phenomena result from hexagonal plate-shaped ice crystals oriented horizontally with diameters between 300 [[micro]meter] and 2 mm. From our observations and reinvestigation of previous reports, we conclude that a subset of the population in persistent contrails can consist of highly regular, oriented, hexagonal plates or columns comparable to the most regular crystals in natural cirrus clouds. This is explained by measured ambient humidities below the formation conditions of natural cirrus. The resulting strong azimuthal variability of the scattering phase function impacts the radiative transfer through persistent contrails. Key words: Aircraft, contrail, climatic impact, crystal growth, halo phenomena, ice crystals, optical properties, relative humidity, remote sensing, scattering phase function.

Vibration-rotation absorptions in high-resolution Fourier-transform infrared spectra from a 246-m horizontal path were used to derive local concentrations of trace gases at the Alpine observatory at the Zugspitze summit, Germany (2964 m above sea level). The analysis was performed by using the line-by-line nonlinear least-squares spectral fitting software, SFIT, based on the 1992 HITRAN line parameter compilation. (HITRAN is a high-resolution transmission molecular absorption database.) A comparison to in situ measurements shows an agreement of better than 4.3% for the species CO, C[O.sub.2], and C[H.sub.4]. Using the same spectrometer and analysis software, we obtained the vertical column density of [N.sub.2]O together with an adjusted vertical volume mixing ratio distribution. This translates to a local [N.sub.2]O concentration at the altitude of Zugspitze that agrees with the horizontal path-derived value to within 1%. Key words: Remote sensing, Fourier-transform infrared spectroscopy, open path, in situ, solar absorption, vertical profile.