Ecological Indicators of Effects of Changing Air Quality
Robin L. Dennis
NERL Atmospheric Modeling Division,
Research Triangle Park, NC USA
An emerging critical use of network data is the assessment of total deposition and changes in total deposition relative to target levels or critical loads and the tracking of air management effectiveness. Important reductions in both sulfur and nitrogen deposition are expected between now and 2020. An assessment of total, wet plus dry, deposition is required to assess current levels and expected trends. Key end points are acidity (involving S and N) and nutrient enrichment (involving N). The issue regarding uncertainty is dry deposition. For sulfur (S) we are fine because we measure all species important to the sulfur budget. For reactive nitrogen (Nr = oxidized N + reduced N) we are not o.k. for dry deposition estimates because we are not measuring all of the species important to the nitrogen budget. In particular, the oxidized-N portion that is missing is associated with NO, NO2, PAN, and N2O5 and the reduced-N portion that is missing is associated with NH3. The Community Multiscale Air Quality model, CMAQ, is used as a tool to gain insights into the budget gaps: where they are and how big they are. Simulations for a 12-km grid for the eastern United States are used for the analysis. For oxidized N, CASTNet measures only HNO3 and aNO3-. The oxidized-N budget fraction captured by dry deposition of these two species is 60-80% in rural areas (missing 20-40%) and 30-70% in urban areas (missing 30-70%). For reduced N, CASTNet measures only aNH4+. The reduced-N budget fraction captured by dry deposition of this specie is 2-20% in high emission areas (missing 80-98%) and 50-80% in low emission areas (missing 20-50%). The spatial patterns of these gaps are presented for the eastern U.S. CMAQ is also used to address the question of whether total deposition can be “faked” by using a simple multiplier of wet deposition. The answer is tricky for the east and likely “no” for the west. CMAQ answers are expected to change for reduced N when ammonia bi-directional exchange is fully incorporated in the model.