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Objective: Assess impacts to vegetation from acute exposure to ozone (O3) during the growing season (April to September).

Data: Two exposure indices were calculated from hourly O3 data collected by the EPA to track effects from chronic O3 exposure (W126 index) and acute O3 exposure (N100). The W126 index is a weighted measure of chronic (lower level) ozone exposure with an explanation of its daily, monthly, annual, and multi-year average values is available here. For both indices, hourly data was used to calculate monthly values. Monitoring sites were only included if at least 75% of the hourly observations were available, and monthly values were corrected to account for any missing data following the procedures of Lefohn, Knudsen, and Shadwick (2011). Corrected monthly values were totaled across the growing season to generate an annual value. Annual values were averaged over the most recently available 3 year period to generate a value for each site. Site values were then interpolated using inverse distance weighting to attribute values to landscapes within 40 km of a monitoring site. There is typically a 1 year lag between the TCA assessment year and the most recently available for instance, for the 2024 TCA Assessment, ozone data from 2021-2023 were used to compute the 3-year average. For the 2023 TCA Assessment, 2022 ozone source data was not available so the same data from the prior assessment was used. Source data ranges and assessment year in parentheses are: 2017-2019 (TCA Assessment 2020), 2018-2020 (TCA Assessment 2021), 2019-2021 (TCA Assessment 2022), 2019-2021 (TCA Assessment 2023), and 2021-2023 (TCA Assessment 2024).

Data Format: Point (site data) interpolated through inverse distance weighting and filtered to areas only within 40 km of an O3 monitor.

Units: W126 is expressed in parts per million of O3-hours or ppm-hours

Spatial Resolution: 1000m (1km)

Source data: EPA Hourly Ozone data

Additional Resources:

Details on Method Changes and Source Data Versions

Overview of the Terrestrial Condition Assessment: TCA Hubsite or Landfire Office Hour Presentation

Explore the results of the most recent assessment: TCA Interactive Data Viewer

Learn more about the TCA KPI: TCA Dashboard

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