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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 N100 index is the sum of hours over 100 ppb in a given month and captures peak events of high or acute exposure. 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.

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

Units: N100 is expressed in 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: Dashboard link when available