Indicator
Regional Visibility 50th Percentile ("Average Visibility Days")
2019 Evaluation
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This indicator measures regional visibility on an "average" day in the non-urbanized areas of the Tahoe Basin. Visibility is a measure of the distance at which an object or light can be clearly discerned by the human eye. Visibility can be impaired by airborne pollutants from local, regional, and global sources. Sources of locally generated pollutants include entrained/suspended roadway particles, vehicle emissions, residential wood burning, campfires, prescribed fires, and wildfires. Some particles responsible for the degradation of regional visibility in the Tahoe Basin include dust and other pollutants transported into the Basin from areas as far as Asia, and record-setting catastrophic wildfires throughout California and the western states in the last five years. Regional programs such as decreasing the number of polluting wood stoves and work to improve forest health to prevent or limit severe wildfire aim to increase regional visibility. Regional visibility is monitored at DL Bliss State Park as part of the national IMPROVE monitoring network.

Regional Visibility 50Th Percentile (Average Visibility Days) Bliss State Park.jpg
Status

Regional visibility conditions monitored at the DL Bliss IMPROVE monitoring station.

2019 Evaluation
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Status
Considerably Better Than Target
Trend
Moderate Improvement
Confidence
High
Applicable Standard
AQ5: Achieve an extinction coefficient of 25 Mm-1 at least 50 percent of the time as calculated from aerosol species concentrations measured at the Bliss State Park monitoring site (visual range of 156 km, 97 miles).
Key Points
  • Average visibility days at the non-urban DL Bliss State Park site show a long-term improvement since monitoring began in the early 1990’s and are well within air quality standards.
Evaluation Map
Description

Regional visibility monitoring station at DL Bliss State Park.

About the Threshold
This indicator measures regional visibility and the distance that the human eye can see. It is measured by using a reconstructed light extinction (bext) value, which is derived from an equation that combines measured concentrations of several gasses and particles. The equation is corrected for humidity and natural “background” light scattering. Bext is summarized by “average visibility days” (50th percentile values) and “worst visibility days” (90th percentile values) for each year, followed by calculating the 3-year running average. This threshold standard has been adopted to protect regional visibility and air quality.
Particulate matter in the atmosphere is the primary driver of visibility impairment because of the optical properties and long retention times in the air. The main sources of particulate matter in the basin are residential and wildfire smoke, and entrained roadway dust. Effective motor vehicle tail pipe emission controls, residential wood combustion controls, appropriately managed prescribed burning, and road dust emission control aid in improving regional visibility conditions. Increases in the frequency and intensity of wildfires in the western U.S could threaten gains in regional visibility.
Delivering and Measuring Success

EIP Indicators

Example EIP Projects

Monitoring Programs

  • Regional Visibility Monitoring

    Visibility monitoring data are collected, analyzed, and reported by the IMPROVE (national Interagency Monitoring of Protected Environments) network using nationally accepted protocols.

Rationale Details
The current status was determined based on the 50th percentile "average day" measurement at the DL Bliss monitoring station during the most recent year monitoring data is available. The reading at DL Bliss in 2018 was a coefficient extinction of 17.2, which is 69% of the maximum allowable level of 25. Therefore, the current status is considerably better than target.
Trend was determined by all year readings of 50th percentile "average days" at the DL Bliss monitoring station from 1994-2018 using the Theil-Sen robust regression method. Levels have decreased an average of 0.8% per year relative to the threshold target during that time frame. Therefore, the trend is considered moderate improvement.
Confidence Details
High. Monitoring is conducted using strict protocols of the IMPROVE program.
High. Monitoring is conducted using strict protocols and there is a long term data set to analyze trend.
High.
Additional Figures and Resources

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