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The concentrations of common trace gaseous constituents of the atmosphere have been measured in clean air in rural Illinois and Missouri. Sulfur dioxide (S02), nitrogen dioxide (NO2), ammonia (NH3), and aldehydes (RCHO) were removed from the air by 1 hour of wet scrubbing. Hydrogen sulfide (I-I2S) was collected by passing air through an impregnated filter for I or 2 hours. Background concentrations of SO•, NO2, and RCHO ranged from less than I to 5 ppb. The NI-L concentrations were found to be as high as 10 ppb. The H._,S concentrations varied from less than 0.10 to 0.35 ppb. Comparison with previous measurements and with assumed worldwide values indicates that the concentrations of NO2 and I-LS assumed

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The first high‐resolution, absolutely calibrated spectral intensity measurements of noctilucent clouds were made on the night of July 22–23, 1969, from Fort Nelson, British Columbia, with a scanning spectrophotometer. These spectra have no discernible emission or absorption features attributable to NLC. The measured radiance of a bright band in the moderately bright display was about 6 kR/A at 4800 A, the wavelength of maximum radiance. If the clouds are regarded as a monodisperse aerosol of 1300‐A radius ice particles, interpretation of the observation in terms of the Mie scattering theory shows that the NLC particle number density is of order 1 cm−3.