Developing background condition nitrogen critical loads and ozone critical levels for U.S. tree species using non-anthropogenic background levels
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Critical loads or levels (CL) are estimates of quantitative thresholds for levels of deposition above which significant harmful ecological effects can occur, according to present knowledge.• Recent research focused on establishing CLs for tree species’ growth and survival probability as it relates to nitrogen deposition and ozone exposure.1,2 Here, we evaluate recently established nitrogen CLs that were derived using a machine learning model (XGBoost).• CLs have historically been established at the point where the response function (e.g., growth) is maximized or as a percent decline (e.g., 1%) from the max, but this could potentially include benefits from anthropogenic influences—notably for nitrogen deposition.• If a return to non-anthropogenic conditions is desired, alternative reference points may be more suitable.• There is a research need to understand whether using a maximum response reference point could alter the CL determination as opposed to when a background condition (e.g., non-anthropogenic) reference point is used.• Multi-scale photochemical modeling (e.g., CAMx) can produce non-anthropogenic outputs to evaluate background conditions.