Empirical studies of the relationships between emissions and visibility in the Southwest

Historical emission trends of SOx, NOx, and NMHC are determined year-by-year from 1948 to 1975 for four Southwestern states (Arizona, Colorado, Nevada, and Utah). Trends in visibility levels (medians and other percentiles) are documented for the period 1948-1976 at 12 airports in the Southwest. Two analyses are used to relate emission changes to variations in regional extinction levels. The first analysis examines the air quality changes associated with a 90 percent reduction of Southwestern SOx emissions during a nine month copper strike and estimates the extinction produced by SOx emissions on various spatial scales. The second analysis involves regression studies relating historical extinction levels from 1948-1975. Because of limitations in the analytical methods, there is a high degree of uncertainty in many of the results. However, the studies do provide insights into the effects of aerosol precursor emissions on extinction at various distances from sources. In the case of mesoscale effects of SOx in the Southwest, quantitative coefficients are proposed which link emissions to regional extinction.