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FIRST RESULTS FROM OPERATIONAL TESTING OF THE U.S. EPA MODELS-3 COMMUNITY MULTISCALE MODEL FOR AIR QUALITY (CMAQ)
Citation:
Arnold, J. R. AND R L. Dennis. FIRST RESULTS FROM OPERATIONAL TESTING OF THE U.S. EPA MODELS-3 COMMUNITY MULTISCALE MODEL FOR AIR QUALITY (CMAQ). Presented at Millennium NATO/CCMS International Technical Meeting on Air Pollution Modeling and Its Application, Boulder, CO, May 15-19, 2000.
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 Models 3 / Community Multiscale Model for Air Quality (CMAQ) has been designed for one-atmosphere assessments for multiple pollutants including ozone (O3), particulate matter (PM10, PM2.5), and acid / nutrient deposition. In this paper we report initial results of our evaluation of the model's operational performance for O3 for the northeast U.S. over the 14 day period 5?18 July 1995 with three one-way nested model resolutions at 36km, 12km, and 4km and using two chemical mechanisms, CB4 and RADM2. We defined subdomains with similar photochemical regimes within the modeled domains by analysis of 1995 surface observations, and within these subdomains we segregated airsheds for the large urban areas. CMAQ model performance was evaluated over a number of specific attributes including the ability to reproduce the 1 hour peak and the 3 hour and 8 hour average daily maximum O3 mixing ratios [O3], and the ability to reproduce diurnal profiles of O3 and oxides of nitrogen (NO+NO2=NOX). Statistical results for the 8 hour average maximum [O3] are quite similar to those for the 1 hour peak and 3 hour average. Normalized bias and gross error statistics show that CMAQ does quite well on high [O3] days, but less-well on lower [O3] days and on days with larger mean-to-maximum ranges. One possible explanation is that this O3 performance difference is tied to the model's inability to predict NOX correctly for low O3 days. We find that the 36km grid is likely too coarse for most city and some regional model applications. The 12km model is an improvement nearly everywhere nearly every day as measured by the bias and error statistics. Statistics for the 4km model demonstrate a tendency for improved fits over the12km but this is not so ubiquitous as for the 12km over the 36km. The improvements across grid resolutions measured with bias and error statistics are not uniformly seen when plotting diurnal time series of the different model resolutions against observations at a given site. This most likely indicates significant problems of monitor siting and representativeness for a model cell. Both bias and error and most diurnal profiles in the [O3] results for the CB4 chemistry are quite similar to those obtained with the RADM2 chemistry for most days in most regions and cities. Comparisons to HCHO observations constitute a special test for the model since the chemical mechanisms were not originally designed or fitted to predict HCHO. CMAQ predictions of [HCHO] at both 36km and 12km with CB4 and with RADM2 demonstrate a fairly good fit to the 3-hour averaged observed [HCHO] reported for several PAMS sites in the northeast. Our overall conclusion for this first operational evaluation of CMAQ is that the model is for the most part functioning in a way we can account for both where model fits to the observations are good and where they are not as good. Furthermore, we find that CMAQ is operating well-within the range of results from other large, ozone-only Eulerian AQMs in the U.S. for the high O3 days where model evaluation has traditionally been focused. Hence we find no reason not to use the model and encourage its wide-spread use.