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Discussion papers
https://doi.org/10.5194/acp-2019-642
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-642
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 30 Aug 2019

Submitted as: research article | 30 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

On the Limit to the Accuracy of Regional-Scale Air Quality Models

S. Trivikrama Rao1,2, Huiying Luo1, Marina Astitha1, Christian Hogrefe3, Valerie Garcia3, and Rohit Mathur3 S. Trivikrama Rao et al.
  • 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC
  • 2Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT
  • 3Computational Exposure Division, U.S. Environmental Protection Agency, Research Triangle Park, NC

Abstract. Regional-scale air pollution models are routinely being used world-wide for research, forecasting air quality, and regulatory purposes. It is well known that there are both reducible and irreducible uncertainties in the meteorology-atmospheric chemistry modeling systems. Inherent or irreducible uncertainties stem from our inability to properly characterize stochastic variations in atmospheric dynamics and from the incommensurability associated with comparisons of the volume-averaged model estimates with point measurements. Because stochastic variations in atmospheric dynamics and emissions forcing influencing the air pollutant concentrations are difficult to explicitly simulate, one can expect to find a percentile value from the distribution of measured concentrations to have much greater variability than that of the model. This paper presents an observation-based methodology to determine the expected errors from regional air quality models even when the model design, physics, chemistry, and numerical analysis techniques as well as its input data were perfect. To this end, the short-term synoptic-scale fluctuations embedded in the daily maximum 8-hr ozone time series are separated from the longer-term forcings using a simple recursive moving average filter. The inherent variability 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. The results reveal that the expected root mean square error at the median and 95th percentile is about 2 ppb 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.

S. Trivikrama Rao et al.
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Short summary
Since stochastic variations in atmospheric dynamics influencing the air pollutant concentrations are difficult to explicitly simulate, one can expect to find a percentile value from the distribution of measured concentrations to have much greater variability than that of the model. This paper presents an observation-based methodology to determine the expected errors from regional air quality models even when the model design, physics, chemistry, numerical analysis, and its input were perfect.
Since stochastic variations in atmospheric dynamics influencing the air pollutant concentrations...
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