The NO(2) abundance in the stratosphere has been determined from ground-based spectra of the rising and setting sun and moon and of the twilight sky near 4500 angstroms. The spectra were taken at the Fritz Peak Observatory, at an altitude of 3 kilometers in the Colorado mountains. Separation of the stratospheric contribution requires observations at a relatively unpolluted site; direct measurement of the tropospheric absorption in the Colorado mountains often yields an upper limit on the tropospheric mixing ratio of 0.1 part per billion. The stratospheric NO(2) abundance is two to three times greater at night than during the day and increases significantly during the course of a sunlit day; these changes are related to photolytic decomposition of NO(2) and N(2)O(5) in the daytime stratosphere. Absorption by NO(3) was sought but not found; the results set an upper limit of 2 percent on the nighttime abundance ratio of NO(3) to NO(2) in the stratosphere.

This paper describes a method for obtaining the abundance and altitude of stratospheric NO• by using ground-based spectroscopy at twilight. The method is then used to study the behavior of NO• over Colorado at 40øN; a companion paper discussed observations elsewhere. Large changes in both the abundance of NO• and in its diurnal variation are commonly seen at mid-latitude and appear to reflect the role of quasi-horizontal transport in the stratosphere on a global scale. stratosphere and its center of mass in altitude. Since the density of NO2 changes during the twilight period, we discuss how this affects the interpretation; the abundance is greater at night than in daytime, since sunlight dissociates NO2 into a mixture of NO and NO2. We then discuss the limits and errors associated with this method for determining the abundance and vertical distribution of NO2 in the stratosphere.As in the earlier work [Noxon, 1975], we employed a scanning spectrometer operated at a resolution of 7 A (not the 5 .• incorrectly stated there); instrumental details are described in the earlier paper. Even a simple spectrometer is adequate for such work owing to the relatively strong intensity of the twilight sky and the lack of any requirement for high spectral resolution. Figure I shows at the top two spectra of the zenith Paper number 9C0466. 5047 5048 NOXON ET AL.: STRATOSPHERIC NO•.

This paper discusses observations of the total daytime stratospheric column abundance of NOs made over the range in latitude from 40øS to the North Pole. The equatorial abundance is less by a factor of 2 than at mid-latitude. A summer maximum in the northern hemisphere exists, and the amplitude of the seasonal variation increases from 2 at mid-latitude to over 6 at 65øN. In winter, when there is not a warming in progress in the stratosphere and there is strong zonal flow, there is an abrupt drop in NOs abundance near 50 ø N. The decrease occurs over only a few degrees of latitude where the temperature near 30 km exhibits a poleward drop as well. During a warming, the 'cliff disappears as a result of rapid meridional transport of mid-latitude stratospheric air into the Arctic.

NO3 has been measured in slightly polluted Colorado mountain air up to a few hundred ppt. There is clear evidence that it is removed by scavenging reactions in less than one hour. The identity of the scavenger is not yet established in mountain air, nor is it yet known whether the reaction removes odd nitrogen from the troposphere. In urban air the scavenging occurs in less than a few hundred seconds and may in part be due to the reaction of NO3 with NO.

Nitrogen fixation in the atmosphere has been observed during a lightning storm by means of absorption spectra associated with NO2. A rough estimate of the efficiency of fixation yields the value of 1026 NO2 molecules per lightning stroke.