VISIBILITY IN THE SOUTHWEST - EXPLORATION OF THE HISTORICAL DATA-BASE

This paper investigates visibility in the Southwest United States by analyzing over 25 yr of airport visibility observations and more than 10 yr of NASN pariculate data. The visibility data, when subjected to careful quality checks and plotted as cumulative frequency distributions, yield results that are consistent from airport to airport and that agree with conclusions reached using data from photographic photometers and integrating nephelometers. The airport data indicate that median visibility is 30-55 miles in large urban centers of the Southwest and 65-80 miles at suburban/nonurban locations. Although still quite good, visibility has deteriorated approx 10-30% from the middle 1950's to the early 1970's at almost all of the 12 trend study sites. These decreases in visibility are equivalent to increases in extra extinction (above-and-beyond blue-sky scatter) of 20-70%. Regression models indicate that secondary aerosols (nitrates and especially sulfates) dominate extra extinction. The results of the regression models are consistent with other findings published in the literature and with fundamental principles of aerosol optics. Several of the conclusions in this paper are verified by large scale reductions in sulfates (on the order of 40-70%) and increases in visibility (on the order of 5-25%) during the 9-month, industry-wide copper strike in 1967-1968. © 1979.