NOx emissions reduction programs have translated to improved ozone air quality and forest health in the Eastern United States

Jason Lynch, Clara Funk and Richard Haeuber

U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20006,,


Data from several monitoring programs are examined to assess the effectiveness and environmental benefit of air pollution reduction programs in the United States (U.S.).  Integrated assessment of monitoring data demonstrates that reductions of nitrogen compound emissions, such as NOx, and byproducts like ozone lead to improved air quality and forest ecosystem health.  Three important environmental monitoring programs were used to evaluate the impact of emission reductions and improved ozone concentrations on forest health.  Assessing the impact of ground-level ozone on forests in the eastern U.S. involves understanding the risk to tree species from ambient ozone concentrations and accounting for the prevalence of those species within the forest. As a way to quantify the risk to particular trees, scientists have developed concentration-response (C-R) functions that relate ozone exposure to tree response.  In 2014, regulated pollution sources reduced NOx emissions by 4.7 million tons (73%) from 1990 levels.  Based on regional data from CASTNET from 2000 through 2014, annual mean ambient nitrate concentration (an ambient pollutant resulting from NOX emissions) declined 48% from 3.1 ppb to 1.6 ppb in the eastern U.S. Rural ozone concentrations, calculated as DM8A, were found to decrease from 84 ppb to 66 ppb (22%) from 2000-2002 and 2012-14 in the eastern U.S.  Comparing data from the start of the NOX Budget Trading Program (2000-2002) to 2007-2009, we found that the total land area in the Eastern U.S. with significant biomass loss decreased substantially for all sensitive tree species as NOx emissions and concentrations of ambient nitrogen compounds have declined over this time period.