Indicator
Highest 1-Hour Average Concentration of Ozone
2023 Evaluation
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This indicator measures the highest one hour ozone (O3) concentration in the calendar year. Ozone is created through a photochemical reaction between atmospheric oxygen, hydrocarbons and/or carbon monoxide, oxides of nitrogen, and sunlight. At high concentrations at ground level in the lower atmosphere, O3 is an air pollutant that can harm the respiratory systems of people and animals and damage plant tissue. The primary sources of the precursor gases in the Tahoe Basin include on-road motor vehicles, residential fuel combustion, motorized watercraft, off-road equipment, solvent, and fuel evaporation, and off-road recreational vehicles. Ozone can also be transported into the Tahoe Basin from outside sources. Regional programs such as reducing automobile use through transit and bike paths and cleaner-burning wood stoves aim to reduce ozone levels. Ozone is currently monitored at five locations around the Tahoe Basin by TRPA and partners. 

Highest 1-Hour Average Concentration Of Ozone.png
Status

The highest 1-hour ozone concentration in parts per million (ppm) measured at all monitoring sites in the Lake Tahoe Basin. The location of monitoring stations has changed over the years. Ozone is currently monitored in five locations.

Data provided by the Desert Research Institute. Access detailed datasets on Tahoe Open Data, including:: Air Quality Station Location Data. Daily Monitoring Data. Yearly Average Data.

2023 Evaluation
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Status
Considerably Better Than Target
Trend
Little or No Change
Confidence
Moderate
Applicable Standard
AQ3: Maintain ozone concentrations at or below 0.08 parts per million averaged over 1 hour.
Key Points
  • Ozone is a secondary pollutant, primarily produced in the lower atmosphere by reaction of NO2 and atmospheric oxygen in the presence of sunlight.  The release of Nitrogen Oxides from wildfires contributes to the production of ozone.
  • The ozone exceedances coincide with periods of very high PM concentration, indicating that they are likely related to an influx of smoke from regional wildfires.
  • Due to its high elevation and frequent clear skies in summer, ozone levels in the Tahoe Basin are naturally high compared to other locations with comparable pollutant emissions. In addition to car, truck, and boat exhaust from within Tahoe, added ozone may come from large urban centers to the west. (Tahoe Regional Planning Agency Ambient Air Monitoring Program, DRI 2023.)
  • Peak ozone levels have dropped substantially since monitoring began in the 1970’s, due in part to the implementation of emission controls over the past several decades. For the last three threshold evaluations, ozone levels have been at or somewhat better than target. The presence of close proximity wildfires in recent years has put this indicator out of attainment. 
Evaluation Map
Description

A map displaying current air quality monitoring stations within the Tahoe Basin. Ozone is monitored at five of these stations.

About the Threshold
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 have been adopted to protect human health. This indicator addresses the highest monitored O3 concentration averaged over every 1-hour for the calendar year.
Ozone is a secondary pollutant created by photochemical reactions between hydrocarbons (HC) and oxides of nitrogen (NOx) in sunlight. The primary sources of HC and NOx include in-basin mobile sources such as cars, trucks, boats, aircraft and off-road vehicles; biomass burning such as wood stoves, wildfires and prescribed burning; and consumer products such as solvents. Ozone is also transported into the basin to a lesser extent from populated areas surrounding the basin. 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 such as increased traffic contributing to precursor gas concentration during optimal weather conditions, the higher the concentration of ozone.
Delivering and Measuring Success

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Monitoring Programs

  • Ozone Monitoring

    Monitoring of criteria pollutants to assess air quality and visibility in the Lake Tahoe Basin.

Rationale Details
Considerably Better Than Target. The method for status determination follows State guidelines for assessing compliance with 1-hour Ozone standard. Sites are assessed as in attainment if there is not more than one exceedance (1 HR value above 0.08 parts per million) per year averaged over three years.
The analysis was conducted both with and without the inclusion of data from periods influenced by exceptional events. Exceptional events are natural or unusual events that can overwhelm existing strategies designed to control man-made pollution, such as wildfires. If such an event occurs, data that would cause an exceedance of a standard was excluded when determining compliance with that standard.
The data was analyzed with and without days influenced by wildfire. Days where air quality was impacted by wildfire are listed on the “Fire Dates” tab of the spreadsheet below. Ozone is monitored at three sites within the region. Including all days, the standard was exceeded on 24 over the four-year period, or on average twice a year per site, although 22 of the 24 exceedance days were in 2020 and 2021. Removing days influenced by wildfires, just three exceeds were recorded over the four-year period, one at each site. The three-year average number of exceedances for each site was 0.33, relative to a target of one or less. The status relative to the standard was significantly better than the target in all years of the evaluation and at each site.
Little or No Change. Trend was determined using the Theil-Sen robust regression method using the maximum reading each year at any monitoring site in the Region. Trend data is available from 1975-2023. Overall, there is a 0.5% decrease per year in ozone levels, which is considered little or no change.
Confidence Details
High. Monitoring was conducted using strict protocols to collect a long-term, high quality dataset.
Low. Confidence of trend is based on the coefficient of determination and t-test significance. R²=0.3 (low), p-value= 5.30E-09 (high).
Moderate. If one confidence rating is high and the other is low, the overall confidence rating is moderate.
Additional Figures and Resources

No photos available.

Air Quality Exceedances with Exceptional Events
Uploaded On
1/17/2025
File Type
Excel (XLSX)
Description
This document includes sheets for PM2.5, PM10, O3, and exceptional event dates. It shows exceedance values and processed values with and without exceptional events, as well as relevant Threshold Standards . Columns ending in "_exempt" exclude exceptional events, while "_all" includes all dates.