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Regional Visibility

Status and Trend

Interpretation and Commentary

Regional Visibility 50th Percentile

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Status: At or Somewhat Better than Target
Trend: Little or no change
Confidence: Moderate

Regional Visibility 90th Percentile

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Status: At or Somewhat Better than Target
Trend: Little or no change
Confidence: Moderate

  • Relevance - This indicator measures regional visibility or 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.
  • Adopted Standards  - TRPA: 1) 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 kilometer (97 miles)), and 2) Achieve an extinction coefficient of 34 Mm-1 at least 90 percent of the time, as calculated from aerosol species concentrations measured at the Bliss State Park monitoring site (visual range of 115 kilometers (71 miles)). Calculations will be made on three year running periods, beginning with the existing 1991-93 monitoring data as the performance standards to be met or exceeded.
  • Indicator - 3-year running average of the reconstructed light-extinction (Mm-1, “inverse mega meters”) from data collected at the Bliss Monitoring Site1.
  • Condition Status – The most recent data for annual average visibility from 2009 shows that “average visibility days” are 18.36 Mm-1, and “worst visibility days” are 27.1 Mm-1 for the Region. The most recent data for the 3-year average visibility from 2007 to 2009 show that “average visibility days” are 22.8 Mm-1, and “worst visibility days” are 31.96 Mm-1. The most recent 3-year running average values for “average visibility days” were 8.8% better than the regional 50th percentile standard of 25 Mm-1, resulting in a determination of “somewhat better than the target.” Current 3-year running average values for “worst days” were 6% better than the regional 90th percentile standard, resulting in a determination of “at or somewhat better than target.” According to the monitoring record, the Region has been in compliance with regional standards for “average days” and “worst days” in all years. Decreases in visibility occurring in 2008 and 2009 running average values, were attributed to wildfires occurring outside of the Lake Tahoe Region; more than 2.3 million acres were consumed by wildfire in California according to Chen et al. (2011) and the CAL Fire incident database.
  • Trend – Trends were calculated using the Thiel regression method which is academically well accepted for air quality analysis.
    • Long Term Trends - There is “little or no change” in the long-term (1991-2009) trend of regional “average visibility days” (50th percentile values; Slope = 0.057, S-value = 26, P =0.154) and for “worst visibility days” (90th percentile; Slope = 0.021, S-value = -11, P=0.358). The uptick in the regional “worst visibility day” trend was attributed to smoke generated as a result of greater than 2.3 million acres of wildfire recorded in 2007 and 2008 throughout California (CALFIRE 2011, Chen et al. 2011).

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  • Confidence
    • Condition Status - There is high confidence in the determination of regional visibility conditions because current bext data were compared with optical measurements from 1999-2003, and showed a good correspondence (Chen et al. 2011). Results of the Lake Tahoe Atmospheric Deposition Study (CARB 2006), and satellite remote sensors, confirmed that the location of the regional monitoring site (D.L. Bliss State Park) was representative of visibility conditions for the Tahoe Basin (DRI 2011a). Bext data are also collected using the IMPROVE national protocol that has been reviewed extensively.
    • Trend - Trend in average visibility was determined to be “moderate” because the data have been consistently collected according to the IMPROVE protocol. The Theil regression analysis indicated no significant trends for both average days and worst days (P=0.358 and P = 0.154, respectively).
    •  Overall Confidence - The overall confidence for both the “average visibility day” and “worst days” status and trend was “moderate” due to the moderate confidence assign to trend.
  • Human and Environmental Drivers - Particulate matter in the atmosphere is the primary driver of visibility impairment because of the optical properties and long retention times in the air (Green et al. 2011). The main sources of particulate matter in the Basin are residential and wildfire smoke, and entrained roadway dust (DRI 2011a). Effective motor vehicle tail pipe emission controls, residential wood combustion controls, appropriately managed prescribed burning, and road dust emission control would aid in improving regional visibility conditions (DRI 2011a). 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. (DRI 2011a).
  • Monitoring Approach – Air samples needed to calculate bext were collected at least every 6 days at D.L. Bliss State Park. This is an appropriate site for monitoring regional conditions because it is not influenced by urban sources (DRI 2011a). Data are collected, analyzed, and reported by the IMPROVE (national Interagency Monitoring of Protected Environments) network using nationally accepted protocols.
  • Monitoring Partners – U.S. Forest Service, UC Davis, US National Park Services and Colorado State University.
1Calculated: bext = bscat+babs = bsg+bsp+bag+bap. Where, bscat is the sum of scattering by gases and scattering by particles, and babs is the sum of absorption by gases and particles. Scattering by gases in the atmosphere, bsg, is described by the Rayleigh scattering theory (a standard value of 9 Mm-1). Scattering by particles, bsp, is caused by both fine and coarse aerosol species. Absorption due to gases, bag, is primarily due to nitrogen dioxide (NO2) and is assumed to be negligible in rural locations. Absorption by particles, bap, is caused primarily by carbon containing particles.


  • Monitoring Plan
  • Conceptual Model


Lake Tahoe Visibility Monitoring Locations

(Bliss State Park was used for Regional measurements and South Lake Tahoe (SOLA) was used for sub-regional measurements).

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Trend Charts

Chart data not available at this time.

Additional Info



Additional Information

  3. Conceptual Model:
  4. Monitoring Plan:
Last Updated on Wednesday, 05 September 2012 10:21