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Home Air Criteria Pollutants

Ozone 1 Hour Concentration

Status and Trend

Interpretation and Commentary

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

  • Relevance - 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, federal and state standards, with varying time averaging periods, have been adopted to protect the public from this harmful pollutant. This indicator addresses the highest monitored O3 concentration averaged over every 1 hour for the calendar year.
  • Adopted Standards - TRPA: Maintain ozone concentrations at or below 0.08 parts per million averaged over 1 hour; California: 0.09 ppm not to be exceeded; Nevada: 0.10 ppm, not to be exceeded.
  • Indicators - Highest one-hour average concentration of Ozone measured at any monitoring station within a calendar year in the Lake Tahoe Air Basin (ppm).
  • Condition Status – The highest 1-hour O3 concentrations measured at the Incline Village and South Lake Tahoe sites for 2009 were 0.073ppm and 0.077ppm respectively.1 These measurements are 9% and 4% below the most stringent (TRPA) standard of 0.08 ppm. A condition status of “at or better than target” is designated for 2009. The Region has only been in attainment with this standard in 2003, 2004, and most recently in 2009.
  • 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 is a long term trend between 1984 and 2009 of no statistical change in the highest 1-hour average O3 concentrations. Change in concentration is - 0.0003 ppm/yr. This change is -0.4% per year of the most stringent (TRPA) standard of 0.08 ppm, so the long-term designation is “little or no change.”
    • 5 Year Trend - The trend over the most recent 5 years (2005-2009) is not consistent with the long term trend and indicates an increase of 0.002 ppm/year in the highest 1 hour O3 concentration, or +2.5% per year of the standard. Note that short-term trends in air quality are typically not reliable due to the high inter-annual variability of meteorology and small sample size (n=5).

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  • Confidence
    • Condition Status - There is high confidence in the condition status because the data is collected using federal reference methods2, is subject to extensive quality assurance requirements and have been collected continuously in the Region since 1984.
    • Long Term Trends - The confidence in the trend for the highest 1 hour O3 values is moderate for the long range analysis of 26 data points with a confidence level of 81%, a S value of -41 and a P value of 0.19.   
    • 5 Year Trend - The confidence in the trend over the most recent 5 years (2005-2009) is moderate with a confidence level of 76%, a S value of 4 and a P value of 0.24.
    •  Overall Confidence - The overall confidence in the status and trend is “moderate” because of the moderate confidence in long-term trend outweighs the high confidence in status.
  • 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. The data presented in the graph below is the highest monitored concentration at all sites for each year.
  • 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

click map for larger image

Trend Charts

Chart data not available at this time.
The highest 1-hour ozone concentration measured at any of the monitoring sites in the Lake Tahoe Air Basin. The 2009 concentration is better than the most conservative TRPA Threshold Standard. However, historical values have exceeded Threshold Standards. Although the trend line indicates a slightly improving trend, it is not statistically significant due to the high inter-annual variability in indicator values.
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 8 hr avg. Indicator
  2. Ozone 3 yr 4th high 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 10:41