Meteorologically adjusted air quality trends: Visibility

Abstract

If trends in air quality are to be interpreted in terms of changes in pollutant emissions, the impact of meteorology on those trends needs to be removed. There is good reason to suspect that changes in the weather in the mideastern U.S. over the last 30 years may have contributed to the observed reduction in visual air quality. This report represents an attempt to extract changes in air quality which were not the result of changes in local meteorology. This analysis focuses on changes in visibility under meteorological conditions ‘typical’ for each locale and season over the period 1948–1981. Trends in visual air quality are summarized in terms of a weighted linear least-squares estimate of the percentage change in visibility over the entire 34 year period within each season and at each of 15 representative sites. Confidence limits associated with these percentage changes are evaluated. This meteorologically-adjusted analysis, which focuses on midrange visibility (60th percentile), is more optimistic than those which do not consider meteorological factors or an error analysis. At the metropolitan sites and in the more rural areas, declines in the summertime visibility appear moderate, and significant improvements in visual air quality are seen in the first and fourth quarters. In contrast, the fast growing, medium-sized urban areas do not show a significant improvement in the fall or winter, and the decline in spring and summer visibility levels at these sites is evident even after adjustment for changes in local meteorology.