Trends in the extremes of sulfur concentration distributions

Understanding the response of air quality parameters such as visibility to the implementation of new air quality regulations, population growth and re distribution, and federal land managing practices is essential to the evalu ation of air quality management plans on air quality in federal Class I are as. For instance, the reduction of SO2 emissions from large single point so urces should result in the decrease of extreme sulfate concentrations, whil e population growth in geographic areas outside of urban centers could caus e a slow widespread increase of sulfate and organic concentrations. The cha nge in federal land managing practice of increased prescribed fire on a yea r-round basis in lieu of large naturally occurring wild fires could have th e same effect; that is, the frequency of high sulfur days increase and low sulfur days decrease as the result of the management practice. Therefore, i t is of interest to examine the trends associated with the proportion of da ys during which the concentration of some aerosol species is above or below a certain threshold and decide whether this proportion of days is increasi ng or decreasing or shows a lack of trend. This is a direct indication of w hether the quality of the environment is improving or worsening, or neither .