Existing and natural background levels of visibility and fine particles in the rural east

Abstract

An investigation is conducted into existing and natural background levels of visibility and fine particles in nonurban areas of the eastern U.S. An analysis of data for 100 airports nationwide indicates that nonurban areas of the East experience relatively low visibilities; specifically, rural areas east of the Mississippi and south of the Great Lakes generally exhibit annual median visual ranges of 15–25 km. A review of data from eight monitoring programs indicates that ambient fine (</ 2.5 μm) particle concentrations presently average approximately 29 μg m −3 in the rural East. The largest components are water ( ~ 11 μg m −3), sulfates ( ~ 9 μg m −3), and organics ( ~ 4 μg m −3). The majority of the water is thought to be attached to hygroscopic sulfate aerosols. Currently, sulfates and fine particles exhibit a pronounced maximum in the summer quarter, when visibility shows a pronounced minimum. This seasonal pattern is a new phenomenon historically; prior to the 1960s, visibility was distinctly higher during the summer then during the remainder of the year. An investigation of natural background conditions suggests that the natural line aerosol concentrations would average 5 1 2 ± 2 1 2 μg m −3 in the East, with the largest components being organics and water. Natural background visual range for the East is estimated to be 95 ± 45 km. It is not currently possible to check this estimate of natural background visual range through an analysis of historical visibility trends from the 1930s to the 1970s because of limitations in historical emission trend data, uncertainties in airport visibility trend data and the confounding effects of meteorology. The best check would be to collect simultaneous measurements of fine particle mass, fine particle chemical composition, and visibility in remote continental areas of the Southern Hemisphere.