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AN ASSESSMENT OF THE ABILITY OF 3-D AIR QUALITY MODELS WITH CURRENT THERMODYNAMIC EQUILIBRIUM MODELS TO PREDICT AEROSOL NO3
Citation:
Yu, S., R L. Dennis, S J. Roselle, A. Nenes, J. Walker, B K. Eder, K L. Schere, J L. Swall, AND W. P. Robarge. AN ASSESSMENT OF THE ABILITY OF 3-D AIR QUALITY MODELS WITH CURRENT THERMODYNAMIC EQUILIBRIUM MODELS TO PREDICT AEROSOL NO3. JOURNAL OF GEOPHYSICAL RESEARCH. American Geophysical Union, Washington, DC, 110(D7):1-22, (2005).
Impact/Purpose:
The goal of this task is to thoroughly characterize the performance of the emissions, meteorological and chemical/transport modeling components of the Models-3 system, with an emphasis on the chemical/transport model, CMAQ. Emissions-based models are composed of highly complex scientific hypotheses concerning natural processes that can be evaluated through comparison with observations, but not validated. Both performance and diagnostic evaluation together with sensitivity analyses are needed to establish credibility and build confidence within the client and scientific community in the simulations results for policy and scientific applications. The characterization of the performance of Models-3/CMAQ is also a tool for the model developers to identify aspects of the modeling system that require further improvement.
Description:
The partitioning of total nitrate (TNO3) and total ammonium (TNH4) between gas and aerosol phases is studied with two thermodynamic equilibrium models, ISORROPIA and AIM, and three datasets: high time-resolution measurement data from the 1999 Atlanta SuperSite Experiment and from the 2002 Pittsburgh Air Quality Study (PAQS) Supersite Experiment, and 12-hour measurement data from the Clinton site, NC, in 1999. The capability of thermodynamic models to reproduce the observed partitioning of TNO3 and TNH4 differed from site to site depending on chemical and meteorological conditions at each site. At the Atlanta site, both models reproduced a large percentage of observed aerosol NH4+ and HNO3 (NH4+: >94% and HNO3: >86%) within a factor of 1.5, whereas neither model reproduced a majority of observed aerosol NO3- and NH3 (NO3-: <48% and NH3: <51%) within a factor of 2. At the Pittsburgh site, both models reproduced more than 76% of observed NO3- within a factor of 2. At the Clinton site, both models performed a little better on aerosol NO3- (47-58% within a factor of 1.5) than at the Atlanta site but worse than at the Pittsburgh site. Sensitivity test of hermodynamic models with Gaussian random errors indicates that in many cases measurement errors in SO42- and TNH4 can explain a major fraction of the discrepancies between the equilibrium model predictions and observations in partitioning of TNO3. Comparison of predictions of the 3-D Community Multiscale Air Quality (CMAQ) model with the observations over the US continent indicates that the performance of the 3-D model for SO42- is fairly reasonable spatially and temporally. However, its performance for NO3-, HNO3, NH4+, and NH3 strongly depends on its performance for TNO3, TNH4 and SO42-. Tests show that errors associated with SO42- and TNH4 predictions of the 3-D model can result in the thermodynamic model calculation replicating only 47% and 60% of base case NO3- within a factor of 2 for summer and winter cases, respectively. It was found that errors in TNH4 are more critical than errors in SO42- to prediction of NO3- and that the responses of the aerosol NO3- predictions are not very sensitive to the errors in temperature and relative humidity under the tested conditions. The ability of 3-D models to simulate aerosol NO3- concentrations is limited by uncertainties in predicted SO42- and TNH4. While there is feedback between partitioning and the levels of predicted TNO3, errors in TNO3 are much less sensitive to these uncertainties and 3-D models are capable of predicting TNO3 with accuracy comparable to that of SO42- or TNH4.
The United States Environmental Protection Agency through its Office of Research and Development funded and managed the research described here. It has been subjected to Agency's administrative review and approved for publication.