Uncertainties in O3 concentrations simulated by CMAQ over Japan using four chemical mechanisms.

Abstract Uncertainty was evaluated in four chemical mechanisms pertaining to O 3 concentrations predicted over Japan by the Community Multiscale Air Quality Model (CMAQ) to investigate factors contributing to model overestimation of O 3 concentration. The model setting and meteorological and emissions input data were obtained from a Japanese model inter-comparison project, Japan's Study for Reference Air Quality Modeling (J-STREAM). The compared gas-phase chemical mechanisms included the Carbon Bond Mechanism (CB05TUCL), Regional Atmospheric Chemical Mechanism (RACM2), and two mechanisms developed by the State Air Pollution Research Center (SAPRC), namely SAPRC07TC and SAPRC99. The O 3 concentrations produced by CB05TUCL were low compared to those from SAPRC07TC. The RACM2 concentrations were similar to those from SAPRC07TC over inland Japan and lower over the sea. The concentrations from SAPRC99 were higher than those from SAPRC07TC in urban areas and lower in other areas. At most of the monitoring sites in Japan, the modeled O 3 concentrations were higher than those from observations. Module overestimation can be ranked in the order of SAPRC99 > SAPRC07TC > RACM2 > CB05TUCL for urban sites and SAPRC07TC > SAPRC99 > RACM2 > CB05TUCL for rural sites. The concentration differences between the chemical mechanisms were within 10 ppb, whereas those between the observed and simulated O 3 concentrations reached 40 ppb. Differences in O 3 concentrations between the chemical mechanisms accounted for only a part of the model overestimation, while the rest remained unexplained. To investigate factors influencing the differences in O 3 concentration between the chemical mechanisms, domain- and 10-vertical-layer-average hourly integrated process rates (IPRs) and integrated reaction rates (IRRs) were calculated using process analysis in CMAQ. The O 3 chemical IPRs from SAPRC07TC were higher than those from CB05TUCL and RACM2. The SAPRC99 IPRs were higher than those from SAPRC07TC in urban areas and lower in other areas. The IRR differences in the chemical mechanisms showed that IRRs for the O 3 and NO reactions were responsible for the differences in the O 3 chemical IPR. The coefficients of determination between the O 3 chemical process IPR and IRR differences in the chemical mechanisms were highest for the HO 2 -NO reaction in CB05TUCL and SAPRC99 and the RO 2 -NO reaction in RACM2. Differences in reaction rate constants and lumped volatile organic compounds may have caused some of the differences in O 3 production between the chemical mechanisms. Highlights • O 3 concentrations from different chemical mechanisms were within 10 ppb. • Of the mechanisms, CB05 O 3 concentrations were lowest and closest to observations. • Whole-model O 3 overestimation was not explained by differences in the mechanisms. [ABSTRACT FROM AUTHOR]