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Sub-Regional Visibility

Status and Trend

Interpretation and Commentary

SUB-Regional Visibility 50th Percentile


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Status: Unknown
Trend: Unknown
Confidence: N/A

SUB-Regional Visibility 90th Percentile


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Status: Unknown
Trend: Unknown
Confidence: N/A

  • Relevance - This indicator measures sub-regional visibility in South Lake Tahoe, 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 sub-regional visibility and air quality.
  • Adopted Standards  - TRPA: 1) Achieve an extinction coefficient of 50 Mm-1 at least 50 percent of the time as calculated from aerosol species concentrations measured at the South Lake Tahoe monitoring site (visual range of 78 kilometers, 48 miles), and 2) 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 kilometers, 19 miles).
  • Indicator - 3-year running average of the reconstructed light-extinction (Mm-1, “inverse mega meters”) from data collected at the SOLA monitoring site6.
  • Condition Status – Due to insufficient data the current status is “unknown” for both “average visibility days” and “worst visibility days” at the sub-regional scale. Historical annual average data from 2003 showed that “average visibility days” were 42.62 Mm-1 and “worst visibility days” were 72.73 Mm-1 at the sub-regional scale. The 3-year running average for 2003 showed that “average visibility days” were 43.55 Mm-1 and “worst visibility days” were 64.89 Mm-1 at the sub-regional scale. The most recent 3-year running average values (2003) for “average visibility days” were 12.9% better than the regional 50th percentile standard of 50 Mm-1 resulting in a determination of “somewhat better than the target.” The most recent 3-year running average values (2003) for “worst days” were 48% better than the regional 90th percentile standard resulting in a determination of “considerably better than target.” According to the monitoring record, the Region has been in compliance with regional standards for “average days” and “worst days” since 1996.
  • Trend – Due to insufficient data, a current trend determination was not possible and therefore classified as “unknown.” A Theil regression analysis was used to determine trends for the sub-regional 3-year running average visibility indicator prior to 2004. The estimated trend for the 3-year running average of the historical “average visibility days” data (1991-2003) for reconstructed light extinction was improving (less light extinction) at a rate of 4.47%/year (P<0.01) indicating a “rapid improvement.” The trend for the 3-year running average estimated for “worst visibility days” based on historical data (1991-2003) was improving at a rate of 6%/year also indicating a “rapid improvement” (P<0.01).

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  • Confidence - Because of insufficient data for 2004 to 2010, confidence in the determination of status and trends was “N/A” (not applicable).
  • 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 smoke and entrained roadway dust (DRI 2011a). Improving visibility trends are attributable to effective controls over motor vehicle, residential wood combustion, regulatory controls over prescribe burn (burn days) and road dust emissions (DRI 2011a). The most substantial risk and uncertainty related to visibility is the increased 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 a South Lake Tahoe site. Data were collected, analyzed and reported by the IMPROVE (national Interagency Monitoring of Protected Environments) network using nationally accepted protocols. Re-establishing a sub-regional site in South Lake Tahoe is the top priority of a future visibility monitoring program.
  • 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.

Links

 
  • Monitoring Plan
  • Conceptual Model

Map

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

References

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Additional Information

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  3. Conceptual Model:
  4. Monitoring Plan:
Last Updated on Wednesday, 05 September 2012 11:05