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On the limit to the accuracy of regional-scale air quality models
Citation:
Rao, S., H. Luo, M. Astitha, C. Hogrefe, Valerie Cover, AND R. Mathur. On the limit to the accuracy of regional-scale air quality models. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 20(3):1627–1639, (2020). https://doi.org/10.5194/acp-20-1627-2020
Impact/Purpose:
Though significant progress has been accomplished in the development of air pollution models and in their evaluation relative to observations, not much thought has been devoted towards incorporating in these assessments the inherent or irreducible uncertainties that arise from our inability to properly characterize stochastic variations in atmospheric dynamics and from the incommensurability associated with comparing the volume-averaged model estimates with point measurements. The objective of this article is to illustrate an observation-based method for identifying the lower bound for the errors to be expected even with perfect models with perfect input data and to spur new research in this direction to increase confidence in the use of models for emerging complex air pollution problems; having this quantitative estimation of practical limits for model’s accuracy would help in objectively assessing the current state of the models, measuring progress in their evolution, and providing meaningful and firm targets for improvements in their accuracy relative to measurements.
Description:
Regional-scale air pollution models are routinely being used worldwide for research, forecasting air quality, and regulatory purposes. It is well recognized that there are both reducible (systematic) and irreducible (unsystematic) errors in the meteorology–atmospheric-chemistry modeling systems. The inherent (random) uncertainty stems from our inability to properly characterize stochastic variations in atmospheric dynamics and chemistry and from the incommensurability associated with comparisons of the volume-averaged model estimates with point measurements. Because stochastic variations are not being explicitly simulated in the current generation of regional-scale meteorology–air quality models, one should expect to find differences between the model estimates and corresponding observations. This paper presents an observation-based methodology to determine the expected errors from current-generation regional air quality models even when the model design, physics, chemistry, and numerical analysis, as well as its input data, were “perfect”. To this end, the short-term synoptic-scale fluctuations embedded in the daily maximum 8 h ozone time series are separated from the longer-term forcing using a simple recursive moving average filter. The inherent uncertainty attributable to the stochastic nature of the atmosphere is determined based on 30+ years of historical ozone time series data measured at various monitoring sites in the contiguous United States (CONUS). The results reveal that the expected root mean square error (RMSE) at the median and 95th percentile is about 2 and 5 ppb, respectively, even for perfect air quality models driven with perfect input data. Quantitative estimation of the limit to the model's accuracy will help in objectively assessing the current state of the science in regional air pollution models, measuring progress in their evolution, and providing meaningful and firm targets for improvements in their accuracy relative to ambient measurements.
