Particulate Characteristics and Visual Effects of the Atmosphere at Research Triangle Park

Abstract

During a 38-day period in the fall of 1987 a wide range in the visual quality of the atmosphere at Research Triangle Park, North Carolina was observed. Throughout the period, the light scalier coefficient; SO₂, NO₂, and O₃ concentrations; temperature; relative humidity; and radiation Intensity of the atmosphere were continuously monitored. Additionally, 60 filter samples were intermittently collected with two dichotomous (coarse and fine particle) size-selective samplers. The dichotomous samplers were operated concurrently to obtain samples on Teflon and quartz filters for different analyses. Also collected were six impactor samples for sulfate size distribution analysis. At selected times during the study, long-path measurements were made of the atmospheric extinction coefficient and the extinction of contrast by the atmosphere.

The primary results were as follows: (1) The average light scatter coefficient to fine particle mass ratio was 4.55 m²/g, and this ratio varied from a Eow of around 3 ma/g when the scatter coefficient was around 0.05/km to a high around 6 m²/g when the scatter coefficient increased to around 0.4/km. (2) Elemental measurements of the compositions of the paniculate samples showed that sulfur dominated the elemental composition of the fine fraction but was generally below the detection limit in the coarse fraction. Quantifiable amounts of SI, Al, K, Fe, Ca, Mg, Ti, and Cu were found in both size fractions. (3) Carbon measurements of the partlculate samples showed the elemental carbon content of the atmosphere was only around 2 percent of the mass concentration, and the volatile carbon (VOC) content was around 17 percent. Most of the elemental carbon content was in the fine fraction, while the volatile carbon content was generally equally divided between the coarse and fine fractions. (4) Sulfate particle size distribution measurements Indicated a mass distribution peak between 0.5 and 1.0 μm diameter and a large mass fraction around 38 percent below 0.25 μm diameter. (5) Measurements of the light scatter coefficient were on average 22 percent lower than light extinction measurements. The major reasons for this difference at this site appears to be due to elemental carbon absorption and nephelometer error.