Indicator
Sub-Regional Visibility 90th Percentile ("Worst Visibility Days")
2019 Evaluation
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This indicator measures visibility on low visibility days in the urbanized area of South Lake Tahoe. Visibility measures the distance at which an object or light can be clearly discerned by the human eye. Sources of locally generated haze pollutants include entrained/suspended roadway particles, vehicle emissions, residential wood burning, campfires, prescribed fires, and wildfires. Decreases in visibility on the worst days is largely attributed to smoke from 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. Regional programs such as increased street sweeping and work to improve forest health aim to increase sub-regional visibility. Sub-regional visibility is monitored at Lake Tahoe Community College as part of the national IMPROVE monitoring network. 

Sub-Regional Visibility 90Th Percentile (Worst Visibility Days) South Lake.jpg
Status

Sub-regional visibility conditions at the old South Lake Tahoe site and current Lake Tahoe Community College site. 

2019 Evaluation
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Status
Considerably Better Than Target
Trend
Insufficient Data to Determine Trend
Confidence
Moderate
Applicable Standard
AQ8: Achieve an extinction coefficient of 125 Mm-1 at least 90 percent of the time as calculated from aerosol species concentrations measured at the South Lake Tahoe monitoring site (visual range of 31 km, 19 miles).
Key Points
  • The South Lake Tahoe monitoring station was recently re-established. Compared to data from the 1990s, sub-regional visibility appears to have greatly improved.
  • Visibility on average days in the urbanized area of South Lake Tahoe is well within air quality standards.
  • Decreases in visibility on the worst days are largely attributed to smoke from severe wildfires. While the fires were bad at the times, there were not enough worst days to offset the good days.
Evaluation Map
Description

Sub-regional visibility monitoring station at the Lake Tahoe Community College.

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. There is uncertainty related to visibility condition in the future due to predicted increases in frequency and intensity of wildfires in the western U.S.
Delivering and Measuring Success

EIP Indicators

  • Miles of Street Sweeping

    Since 2009 EIP partners have swept more than 61,000 miles of roadways within the Tahoe Basin reducing fine sediment particles from roadways entering the air and water.

  • Non-Compliant Wood Stoves Removed or Retrofitted

    This EIP performance measure tracks removal of polluting wood stoves to reduce local air pollution for human and ecosystem health.

Example EIP Projects

Monitoring Programs

Rationale Details
The current status was determined based on the 90th percentile "worst day" measurement at the Lake Tahoe Community College monitoring station during the most recent year monitoring data is available, 2018. The reading for 2018 was an extinction coefficient of 45.7, 37% of the maximum allowable level. Therefore, the current status was determined to be considerably better than target.
Trend was determined by all year readings of 90th percentile "worst days" at the Lake Tahoe Community College monitoring station. There is not yet enough data to determine a trend.
Confidence Details
High. Monitoring was conducted using strict protocols.
Low. Monitoring was conducted using strict protocols but there is not a long data set to analyze trend.
Medium.
Additional Figures and Resources

No photos available.

No documents available.