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Home Air Criteria Pollutants

Ozone 3 Year Average 4th High Concentration

Status and Trend

Interpretation and Commentary

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Status: At or somewhat better than target
Trend: Little or No Change
Confidence: Moderate

  • Relevance - This indicator addresses the 3-year average of the 4th-highest 8-hour average O3 concentration, which is the basis of the federal standard. Ozone (O3), in high concentrations can cause health effects such as lung inflammation and other respiratory illness. Ozone can also cause damage to trees and plants at concentrations lower than the human health based ambient air quality standards. TRPA, and federal and state standards, with varying time averaging periods, have been adopted to
    protect the public from this harmful pollutant.
  • Adopted Standards - Federal: The 3-year average of the 4th-highest daily maximum must not be exceed concentration standard of 0.075 ppm.
  • Indicator - 3-year average of the 4th-highest daily maximum ozone concentration (ppm).
  • Condition Status – The 3-year average, 4th high 8-hour average O3 concentration is calculated based on the 4th high 8-hour data values from the EPA AQS website1 for the Lake Tahoe Air Basin. The 2009 value for the South Lake Tahoe site was 0.068 ppm. The site at Incline Village was not operational in 2006-2007, and therefore, a 3-year average was not
    determined for that site for 2009. The South Lake Tahoe value of 0.068 ppm is 91% of the federal standard of 0.075 ppm. Therefore, a status of “at or somewhat better than target” is designated. The monitoring record indicates that the Region has never violated the adopted standard.
  • Trend – Trends were calculated using the Thiel regression method which is academically well accepted for air quality analysis.
    • Long Term Trends - The long term trend is indicated by the arrow in the reporting icon above. There was a long-term trend between
      1984 and 2009 of “little or no change” in the indicator values. The graph above illustrates the long-term change in concentration of
      <-0.001ppm/yr. This change is -0.0% per year of the federal standard of 0.075, so the trend designation is “little or no change.”
    • 5 Year Trend - The trend in the last 5 years (2005-2009) cannot be determined because there is not enough continuous data at any one site to calculate the value for 2006 and 2007. Note that short term trends in air quality are typically not reliable due to the variability of meteorology from one year to the next.

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  • Confidence
    • Condition Status - There is "moderate" confidence in the condition status because the data is collected using federal reference methods2,that are subject to extensive quality assurance requirements, there are noted data gaps for 1993 and 1994, and 2006 and 2007.
    • Long Term Trends - The confidence in the long-term trend for the analysis of 20 data points of this Ozone indicator is “high”
      because of a confidence level = 98%, a S value = -66, and a P = 0.02 were documented.
    • 5 Year Trend - The trend over the most recent 5 years could not be determined because of data gaps recorded in 2006 and 2007.
    •  Overall Confidence - The overall confidence in the status and trend is “moderate” because of the influence of the status confidence determination of “moderate.”
  • Human and Environmental Drivers - Ozone is considered a secondary pollutant, created by photochemical reactions between hydrocarbons (HC) and oxides of nitrogen (NOx) in sunlight. The sources of HC and NOx include mobile sources (cars, trucks, boats, aircraft, off-road vehicles, etc.), biomass burning (wood stoves, wildfires, prescribed burning), and consumer products such as solvents. Ozone is transported from populated areas around the Lake Tahoe Basin into the Basin, and the ambient concentration of O3 is highly dependent on meteorological conditions such as sunlight, temperature, wind speed and mixing conditions.
  • Monitoring Approach – Ozone has been monitored at a number of locations around the Lake Tahoe Basin over the review period: South Lake Tahoe- Tahoe Blvd.; South Lake Tahoe-Sandy Way; South Lake Tahoe-Airport Rd.; Incline Village; and Cave Rock. In 2009 O3 was monitored at the site in Incline Village by the Washoe County Air Quality Management Division and in South Lake Tahoe on Airport Road by the California Air Resources Board. Data is collected, analyzed and reported by the respective agency3,4. In the summer of 2011, the Placer County Air Pollution Control District (PCAPCD) established a new ozone monitoring site in Tahoe City. Results of the monitoring are not yet available.
  • Monitoring Partners– California Air Resources Board, Washoe County Air Quality Management Division, U.S. Environmental Protection Agency, and Tahoe Regional Planning Agency


  • Monitoring Plan
  • Conceptual Model


Ozone Monitoring Locations in the Lake Tahoe Basin

Carbon Monoxide monitoring locations

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Trend Charts

The highest 3-year average of the 4th highest 8-hour ozone concentration recorded at any monitoring site in the Lake Tahoe Basin. The federal standard changed in May 2008 (note shift in federal line). The long-term trend (red line) has shown no significant change over the period of record from 1986 through 2009.
Source: U.S. Environmental Protection Agency, Air Data
Nevada Division of Environmental Protection

Additional Info


  1. U.S. Environmental Protection Agency, AirData website, April 26, 2011: http://www.epa.gov/aqspubl1/.
  2. EPA methods for air quality monitoring: http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf
  3. Washoe County Air Quality Management Division website, April 26, 2011:http://www.co.washoe.nv.us/health/aqm/home.html
  4. California Air Resources Board website, April 26, 2011:http://www.arb.ca.gov/adam/.

Additional Information

  1. Ozone 1 hr avg. Indicator
  2. Ozone 8 hr. avg. Indicator
  3. CARB fact sheet on health effects: http://www.arb.ca.gov/html/fslist.htm#Health
  4. Conceptual Model: A conceptual model for Air Quality CO does not exist as of June 2011 and is anticipated to be developed in 2011.
  5. Monitoring Plan: A monitoring plan for Air Quality CO does not exist as of June 2011 and is anticipated to be developed in 2012
Last Updated on Monday, 05 November 2012 10:44