We address inconsistent procedures and metrics used to evaluate photochemical model performance, recommend a specific set of statistical metrics, and develop updated quantitative performance benchmarks for those metrics. We promote quantitatively consistent evaluations across different applications, scales, models, inputs, and configurations, thereby (1) improving the user's ability to quantitatively place results in context and guide model improvements, and (2) better informing users, regulators, and stakeholders of model uncertainties and weaknesses prior to using results for policy assessments. While we primarily address U.S. modeling and regulatory settings, these recommendations are relevant to any such applications of state-of-the-science photochemical models.
Guidance for the performance evaluation of three-dimensional air quality modeling systems for particulate matter and IMPLICATIONS The National Ambient Air Quality Standards for particulate matter (PM) and the federal regional haze regulations place some emphasis on the assessment of fine particle (PM 2.5 ) concentrations. Current air quality models need to be improved and evaluated against observations to assess the reliability of model simulations. The guidance presented here provides the necessary framework for conducting rigorous performance evaluations of PM and visibility models. The costs associated with the field programs needed to obtain the data necessary for such performance evaluations are estimated to be $15 million for data collection (1-year program with an intensive program of 15 days, over 200,000 km 2 ) and $10-$20 million for planning, emission inventories, data analysis, and modeling. visibility is presented. Four levels are considered: operational, diagnostic, mechanistic, and probabilistic evaluations. First, a comprehensive model evaluation should be conducted in at least two distinct geographical locations and for several meteorological episodes. Next, streamlined evaluations can be conducted for other similar applications if the comprehensive evaluation is deemed satisfactory. In all cases, the operational evaluation alone is insufficient, and some diagnostic evaluation must always be carried out. Recommendations are provided for designing field measurement programs that can provide the data needed for such model performance evaluations.
International audienceThe CAMx photochemical grid model was used to model ozone (O3) and particulate matter (PM) over a European modeling domain for calendar year 2006 as part of the Air Quality Model Evaluation International Initiative (AQMEII). The CAMx base case utilized input data provided by AQMEII for emissions, meteorology and boundary conditions. Sensitivity of model outputs to input data was investigated by using alternate input data and changing other important modeling assumptions including the schemes to represent photochemistry, dry deposition and vertical mixing. Impacts on model performance were evaluated by comparisons with ambient monitoring data. Base case model performance for January and July 2006 exhibited under-estimation trends for all pollutants both in winter and summer, except for SO2. SO2 generally had little bias although some over-estimation occurred at coastal locations and this was attributed to incorrect vertical distribution of emissions from marine vessels. Performance for NOx and NO2 was better in winter than summer. The tendency to under-predict daytime NOx and O3 in summer may result from insufficient NOx emissions or overstated daytime dilution (e.g., too deep planetary boundary layer) or monitors that are located near sources (e.g., roadside monitors). Winter O3 was biased low and this was attributed to a low bias in the O3 boundary conditions. PM10 was widely under-predicted in both winter and summer. The poor PM10 was influenced by underestimation of coarse PM emissions. Sensitivities of O3 concentrations to precursor emissions are quantified using the decoupled direct method in CAMx. The results suggest that O3 production over the central and southern Europe during summer is mostly NOx-limited but for a more northerly city, London, O3 production can be limited either by NOx or VOC depending upon daily meteorological conditions
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