Assessing public perception of visibility for standard setting exercises

Using the photographs and optical measurements taken during a summer field program (REVEAL) designed to examine the chemical and physical characteristics of visibility impairment in the Fraser Valley, southwestern British Columbia, a protocol for gauging public perception of visibility in this region was devised and tested in a pilot survey. This paper details the protocol (which is based on previous studies conducted in the United States of America) and analysis techniques for survey responses. A preliminary assessment of the results of the pilot study is also presented. A public perception survey conducted in Denver (Ely et al., 1991), resolved a visibility standard for Denver of bext (total light extinction) = 0.076 × 10−3 m−1. Assuming a homogeneous atmosphere, this level of bext is approximately equal to a visual range of 50 km. Using a similar protocol, responses from this pilot study were used to extrapolate visibility standards for two suburban locations in the Fraser Valley of bext ≈0.09 × 10−3–0.105 × 10−3 m−1 (bsp (particle light scattering) ≈0.051 × 10−3–0.063 × 10−3 m−1) and bsp ≈0.39 × 10−3 m−1. These levels of light extinction (bsp is the largest component of bext) relate to approximate visual range of between 40 and 60 km in a homogeneous atmosphere. Possible reasons for the apparent discrepancies between locations are discussed and the effect of survey group are addressed.