Visibility in the Northeast. Long-Term Visibility Trends and Visibility/Pollutant Relationships.

摘要

The historical data base pertinent to visibility in the Northeast is analyzed. The data base includes approximately 25years of airport visibility observations and more than 10years of NASN particulate measurements. The investigation covers existing visibility levels,long-term trends in visibility,and visibility/pollutant relationships. Visibility in the Northeast is rather poor,median visual range being on the order of 10miles. Visibility is not now substantially better in nonurban areas than in metropolitan areas of the Northeast. From the middle 1950's to the early 1970's,visibility exhibited only slight trends in large metropolitan areas but decreased on the order of 10to 40% at suburban and nonurban locations. Over the same period,visual range declined remarkably during the third calendar quarter relative to other seasons,making the summer now the worst season for visibility. The decrease in visibility during the summer was especially notable at suburban and nonurban locations,where atmospheric extinction apparently increased on the order of 50to 150% during the third calendar quarter. Regression models based on daily variations in visibility and pollutant concentrations indicate that sulfate aerosol is the single major contributor to haze in the Northeast. Sulfates apparently account for approximately 50% of total extinction.