Forest Service Class I Wilderness Representative Visibility

The goal of this project was to determine the range of visibility conditions at each U.S. Forest Service Class I Wilderness where visibility monitoring has been done. Visibility is expressed as Standard Visual Range (SVR) – the greatest distance at which an observer can just see a black object viewed against the horizon sky. These figures are a synthesis of Forest Service quantitative camera monitoring and IMPROVE and IMPROVE protocol aerosol and optical monitoring. IMPROVE (Interagency Monitoring of PROtected Visual Environments) is a national network designed to determine existing air quality in federal Class I areas, identify sources of existing manmade impairment, and document long-term trends to track progress towards goal of no manmade impairment of protected areas.

Types of monitoring:

Quantitative Camera Monitoring: These systems function by taking color film pictures along a sight path from the visibility monitoring site to a known target on the horizon. Targets are typically dark in color, are 30 to 70 kilometers from the monitoring site, and are at approximately the same elevation as the monitoring site. Once the film is developed, the relative contrast between the target and the background sky is measured using a "slide scanning densitometer". This contrast measurement is then used to estimate the standard visual range. Typically, 3 photographs are automatically taken per day,

IMPROVE Aerosol Monitoring: Up to four modules measure the composition and concentration of the fine particles that produce extinction and the tracers that identify emission sources. Light extinction is calculated from the mass of these particles, then converted to standard visual range. These monitors typically run for two 24-hour periods per week.

Transmissometers: Comprised of a light transmitter and a receiver, transmissometers measure total light extinction directly along a sight path which is usually 5 to 10 kilometers long. Since the intensity of the light source is known, any loss on the receiving end is attributable to absorption and scattering of light by particles. This data can then be used to calculate standard visual range. Transmissometers typically run 10 minutes every hour, 24 hours/day.

The data is plotted on a rank-order cumulative probability graph. From this graph, the 10th, 50th, and 90th percentile cumulative frequency values are derived. Visibility conditons worse (lower SVR) than the 10th percentile SVR occur on only 10% of days, or 90 % of days have better (higher SVR) visibility.Visibility conditions worse than the 50th percentile SVR occur half the days and, likewise, visibility conditions are better than the 50th percentile half the days. Visibility conditions are better than the 90th percentile on 10% of days and are worse on 90% of days.

*Variable Definitions

B_ext = light extinction coefficient – the loss of image-forming light per unit distance due to scattering and absorption by particles and gases in the atmosphere. (Mm^-1)

r = path length or distance to target (Mm)

R_n = sky radiance ration (unitless)

C_o = inherent contrast of the target against the background sky (unitless)

C_r = apparent contrast o the target against the background sky (unitless)

MF = fine mass (particles <2.5 mm) concentrations in mg/m3

Rbext = reconstructed extinction based on aerosol species measurements (Mm^-1)

B_ray = Rayleigh scattering, scattering by natural gases of the atmosphere. For calculations of SVR, Rayleigh scattering is assumed to be 10 Mm^-1