A partnership between Great Rivers Environmental Law Center/USA and Washington University in St. Louis/USA

Project Scope

Ground-level ozone (O3), an ambient air pollutant, mainly originates from reactions with nitrogen oxides (NOx) and volatile organic compounds (VOCs) under sun light. Exposure to high levels of ozone can damage lung cells and respiratory tissues and cause symptoms like coughing and sore throats. Additionally, high ozone levels are a significant risk for children and people with asthma, exacerbating the frequency and severity of asthma attacks. High levels of ozone can also affect ecosystems and the productivity of sensitive vegetation.

Ground-level Ozone Formation
Picture credit: Yan He, Washington University in St. Louis

As compared to thirty years ago, today’s ozone air quality is much better, with certain areas exposed to higher ozone levels than others. As a result, it is clear that further action is needed to continue to improve ozone air quality. Saint Louis City and Saint Louis County have a sparse monitoring network with eight active ozone air quality monitoring sites currently operating in the St. Louis metropolitan area. In this project, coordinating with Great Rivers Environmental Law Center, we set out to develop a denser ozone monitoring network with a total of 20 sampling sites located in three neighborhoods in St. Louis (Dutchtown, Old North, Pine Lawn) to study the small-scale spatial variations in ozone air concentrations. In particular, this project will investigate whether some areas have consistently higher ozone concentrations, and to examine differences across the neighborhoods. We are deploying OGAWA passive samplers at each site with the samplers attached to light poles. Two-week integrated samples will be collected every month for two years. With 80% of our samplers in the Dutchtown area, we will conduct spatial modeling to estimate the ozone concentrations at unmonitored locations in Dutchtown. Since we only have two samplers each in Old North and Pine Lawn, spatial models will not be developed in those two areas. Instead, the data collected in Old North and Pine Lawn will be analyzed for temporal variations in ozone levels.

Installed Samplers
Picture credit: Yan He, Washington University in St. Louis


  • Washington University in St. Louis: Yan He and Jay Turner [WashU Lead]
  • Great Rivers Environmental Law Center
  • Dutchtown South Community Corporation
  • Old North St. Louis Restoration Group
  • A Red Circle
  • Funding provided by Missouri Foundation for Health

Ozone Monitoring Network

Samplers are being installed at a total of 20 locations, with 16 in Dutchtown, 2 in Old North, and 2 in Pine Lawn. The map below shows our sampling locations.

Ozone Monitoring Network
Picture credit: Yan He, Washington University in St. Louis

Ozone Data from EPA in 2022

U.S. Environmental Protection Agency (EPA) mandates the minimum number of air monitor stations required in each urban area. Illinois Environmental Protection Agency and the Missouri Department of Natural Resources (MDNR) operate approximately eight ozone monitoring sites in the St. Louis metropolitan area. Here, we present the ozone data collected by MDNR in 2022 from the three sites located closest to the study area. Blair Street air monitoring site is located in St. Louis City. Maryland Heights air monitoring site is located in St. Louis county. Arnold West air monitoring site is located on the boundary of Jefferson county and St. Louis counties. The map below shows the locations of those three ozone monitoring sites relative to the study area.

Three EPA ozone monitoring site locations relative to our study area
Picture credit: Yan He, Washington University in St. Louis

The federal primary national ambient air quality standard (NAAQS) for ozone is 70 parts per billion (ppb) (expressed as the annual fourth-highest daily maximum 8-hr averaged concentration, averaged over 3 years). The plots below depict ozone concentrations observed at the three identified MDNR ozone monitoring sites in 2022, and are color-coded based on the  Air Quality Index (AQI). The AQI is an index for reporting daily air quality and understanding the potential associated health effects and risks. The AQI is divided into six categories: Green, Yellow, Orange, Red, Purple, and Maroon, to represent areas presenting increasing levels of health concern. Green days represent no health concerns; yellow days may present a moderate health concern for people who are unusually sensitive to ozone pollution; and orange days might cause health effects to members of sensitive groups, including people with lung disease, older adults, and children. Those people should shorten their outdoor activities and make them less intense during orange days.

The top plot presents data from MDNR’s Arnold West ozone monitor; the middle plot presents data from the Blair Street monitor, and the bottom plot presents data from the Maryland Heights monitor. Data is collected by MDNR at the Maryland Heights and Arnold West monitors from March 1st to October 31st, the federal ozone season for ozone monitoring. The Blair Street monitor is part of the National Core (NCore) network, which measures  ozone concentrations year-round to determine compliance with NAAQS and for research purposes. The plots below demonstrate that ozone NAAQS were exceeded two days in June and July at the Arnold West site, one day in June at the Blair street site, and one day in July at Maryland Heights site.

Daily 1st maximum 8-hr average concentration in 2022 at Arnold West (top), Blair Street (middle), and Maryland Heights (bottom)
Picture credit: Yan He, Washington University in St. Louis

Most-Recent Sampling Event

Two-Week Average Ozone Concentrations

(12/21/2023 to 01/04/2024)

updated by the 15th of each month

Since our sampling event in February 2023, we have noted temporal fluctuations in ozone concentrations attributed to changes in temperature and sunlight. The box plot below illustrates the temporal variations of ozone levels monitored within our designated study area. The map below depicts the spatial distribution of the ozone concentration-to-median value ratio. Locations with darker hues and larger circles indicate relatively higher ozone concentrations compared to the other locations. And since May 2023, we added on additional sampling site located at MDNR Blair street air monitoring site to collocate with the EPA reference monitors, by doing this, we will be able to directly compare the concentration at Blair street station to the concentrations monitored within our study area.

Note: our data is collected and averaged over a two-week time period, it cannot be directly compared to the national ambient air quality standard (NAAQS). And maps from previous months are available in historical data.

Temporal variation of two-week average ozone concentrations (ppb)
Picture credit: Yan He, Washington University in St. Louis
Two-week Average Ozone Concentrations-to-median value ratio
Picture credit: Yan He, Washington University in St. Louis

For more information

If you have feedback to share, and are interested in this project, please contact one of Great Rivers’ staff members:

Bruce Morrison, President, bamorrison@greatriverslaw.org

Sarah Rubenstein, Staff Attorney, srubenstein@greatriverslaw.org

Bob Menees, Staff Attorney, bobmenees@greatriverslaw.org

Ethan Thompson, Staff Attorney, ethompson@greatriverslaw.org

Linden Mueller, Development Director, Linden@greatriverslaw.org

Or call us at (314) 231-4181