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Home Air Greenhouse Gases

Ozone 8 Hour Average Concentration

Status and Trend

Interpretation and Commentary

Somewhat worse than target
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Status: Somewhat worse
than target
Trend: Little or No Change
Confidence: Moderate

  • Relevance - This indicator addresses the highest monitored O3 concentration averaged over every 8 hours for the calendar year. 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.
    Standards, with varying time averaging periods, have been adopted to protect human health.
  • Adopted Standards - TRPA/Nevada: no adopted standard. California: 0.070 ppm, highest 8-hour average concentration not to be exceeded.
  • Indicator - Highest 8-hour average concentration measured within a calendar year at any monitoring site.
  • Condition Status – The highest 8-hour average O3 concentrations measured at the Incline Village and South Lake Tahoe sites for 2009 were 0.068ppm and 0.071ppm respectively1. The South Lake Tahoe measurement for 2009 of 0.071 ppm is equal to 101% of the CA standard of 0.070ppm. A condition status of “slightly worse than target” is designated for 2009. The Region was in attainment with the CA standard in 1984, 2004 and 2005; otherwise, the Region has not attained this standard.
  • Trend – Trends were calculated using the Thiel regression method which is academically well accepted for air quality analysis.
    • Long Term Trends - There is a long-term trend between 1984 and 2009 of “little to no change,” in the highest 8-hour average O3 concentration in the Lake Tahoe Basin. The graph above illustrates the long-term change in concentration of -0.0002 ppm/year. This change is -0.28% per year of the CA standard of 0.070.
    • 5 Year Trend - The trend in the most recent 5 years (2005-2009) is not consistent with the long term trend and indicates an increase in the highest 8 hour average values by +0.001 ppm/year., or +1.4% per year of the standard. Note that short term trends in air quality are typically not reliable due to the indicator’s inter-annual variability, response to meteorology, and the small sample size (5 data points).

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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 were collected continuously to 2009.
    • Long Term Trends - The confidence in the long term trend for the analysis of 26 data points of the highest 8 hr. avg. O3 concentration is moderate with a confidence level of 91%, a S value of -64 and a P value of 0.09.    
    • 5 Year Trend - The confidence in the trend over the most recent 5 years (2005-2009) is moderate with a confidence level of 59%, a S value of 2 and a P value of 0.41.
    •  Overall Confidence - The overall confidence in the status and trend is represented by the border in the reporting icon above and is moderate because the moderate confidence in long-term trend and 5 year trend outweighs the high confidence in the condition 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. Typically, the greater the volume of sources contributing to precursor gas concentration (e.g., increased
    traffic volume) during optimal weather conditions (cloudless days), the higher the concentration of Ozone.
  • 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

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Trend Charts

The highest 8-hour-average concentration measured at any of the monitoring stations in the Lake Tahoe Basin. Concentrations are generally worse than the California standard which was adopted in 2006. The long term trend shows little change during the 1984-2009 period of record.
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 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 10:44