Background continental ozone levels in the rural U.S. southwest desert

From 1969 through 1978 an extensive ambient air quality monitoring program, including the measurement of atmospheric concentrations of ozone, has been conducted in a remote section of the desert in the southwestern U.S. Using a Monitor Labs chemiluminescent ozone instrument, these levels were measured atop a small mountain 500 feet above the valley floor of the Colorado River. During the winter months, when the prevailing low level winds are northerly in this river valley, rural continental background levels of ozone are attainable. From this continuous ozone data base, 13 of these representative high pressure periods have been selected for analysis. Results indicate a very distinct diurnal trend with an ozone minimum occurring from 0800-1000 hours Mountain Standard Time (MST) and an ozone maximum from 1600-1900 hours MST. Ozone concentrations range from a low of 19 ppb to a high of 44 ppb with the diurnal pattern exhibiting marked repeatability with respect to time of minima and maxima, seasonal changes and concentration levels.