The Clean Air Act requires EPA to review the science every five years, and if necessary, revise the National Ambient Air Quality Standards (NAAQS) for six major air pollutants – carbon monoxide, lead, oxides of nitrogen, ozone, particulate matter, and oxides of sulfur. These NAAQS reviews, including the development of the Integrated Science Assessments (ISAs), play a significant role in EPA’s commitment to ensuring a clean and healthy environment for the public.

Here we present updates from the 2^nd draft ISA for oxides of nitrogen, oxides of sulfur, and particulate matter- ecological criteria (NOxSOxPM ISA- ECO). The NOxSOxPM ISA represents the Agency’s latest evaluation of the scientific literature on potential ecological effects associated with these pollutants individually and in combination. The assessment includes over 3000 publications, of which approximately half were published since the last review (2008) and our literature cut-off date (May 2017).

The main findings of the ISA characterize the state of the science on atmospheric chemistry, deposition and ecological effects.  The ISA finds that although Current NO₂ and SO₂ Secondary Standards are based on foliar injury, there is no new evidence that foliar injury occurs at current concentrations in U.S.

Nitrogen (N) enrichment from atmospheric deposition of NOy and particulate ammonium currently alters many ecosystems in the U.S. National N deposition rates have been broadly constant; decreasing NO_Y deposition offset by increased reduced nitrogen (NH_x= NH₃+NH₄⁺) deposition. New quantitative evidence shows current rates of N deposition cause: decreases in lichen biodiversity and herbaceous plant biodiversity, positive and negative effects on tree growth and mortality and increases in algal growth, loss of sensitive aquatic species.  Since 2008 many new thresholds of deposition (critical loads) are available for biological effects. Wetlands, estuaries, and surface waters are less sensitive to N deposition because they also receive N inputs from agricultural and urban sources. Acidification from N and sulfur (S) deposition continues to affect ecosystems across the US. Negative effects on fish, plants, plankton are well-documented. S deposition has greatly declined over the past 25 years; driving decreases in total acidifying deposition observed in the East.  Some geochemical recovery has been documented in Northeast. Lastly, S enrichment from deposition alters aquatic and wetland ecosystems with new evidence that S deposition causes increases in sulfide toxicity and mercury methylation.