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Journal Article 
Evaluation of atmospheric nitrogen deposition model performance in the context of US critical load assessments 
Williams, JJ; Chung, SH; Johansen, AM; Lamb, BK; Vaughan, JK; Beutel, M 
Atmospheric Environment
ISSN: 1352-2310
EISSN: 1873-2844 
Air quality models are widely used to estimate pollutant deposition rates and thereby calculate critical loads and critical load exceedances (model deposition > critical load). However, model operational performance is not always quantified specifically to inform these applications, We developed a performance assessment approach designed to inform critical load and exceedance calculations, and applied it to the Pacific Northwest region of the U.S. We quantified wet inorganic N deposition performance of several widely-used air quality models, including five different Community Multiscale Air Quality Model (CMAQ) simulations, the Tdep model, and 'PRISM x NTN' model. Modeled wet inorganic N deposition estimates were compared to wet inorganic N deposition measurements at 16 National Trends Network (NTN) monitoring sites, and to annual bulk inorganic N deposition measurements at Mount Rainier National Park. Model bias (model observed) and error (vertical bar model - observed vertical bar) were expressed as a percentage of regional critical load values for diatoms and lichens. This novel approach demonstrated that wet inorganic N deposition bias in the Pacific Northwest approached or exceeded 100% of regional diatom and lichen critical load values at several individual monitoring sites, and approached or exceeded 50% of critical loads when averaged regionally. Even models that adjusted deposition estimates based on deposition measurements to reduce bias or that spatially-interpolated measurement data, had bias that approached or exceeded critical loads at some locations. While wet inorganic N deposition model bias is only one source of uncertainty that can affect critical load and exceedance calculations, results demonstrate expressing bias as a percentage of critical loads at a spatial scale consistent with calculations may be a useful exercise for those performing calculations. It may help decide if model performance is adequate for a particular calculation, help assess confidence in calculation results, and highlight cases where a non-deterministic approach may be needed. (C) 2016 Elsevier Ltd. All rights reserved. 
Atmospheric deposition; Nitrogen; Model performance; Critical load; Exceedance 
• ISA NOxSOxPM Ecology (2018)
     Cited in the Second Draft
          Chapter 1
          Appendix 2
          Appendix 9