A multi-model assessment for the 2006 and 2010 simulations under the Air Quality Model Evaluation International Initiative (AQMEII) phase 2 over North America: Part I. Indicators of the sensitivity of O₃ and PM_2.5 formation regimes

Under the Air Quality Model Evaluation International Initiative, Phase 2 (AQMEII-2), three online-coupled air quality model simulations, with six different configurations, are analyzed for their performance, inter-model agreement, and responses to emission and meteorological changes between 2006 and 2010. In this Part I paper, we focus on evaluating O₃ and PM_2.5 indicator-based analyses, which are important in the development of applicable control strategies of O₃ and PM_2.5 pollution in different regions worldwide. The O₃ indicators agree on widespread NO_x-limited and localized VOC-limited conditions in the U.S. The NO_y and O₃/NO_y indicators overpredict the extent of the VOC-limited chemistry in southeast U.S., but are more robust than the H₂O₂/HNO₃, HCHO/NO_y, and HCHO/NO₂ indicators at the surface, which exhibit relatively more inter-model variability. The column HCHO/NO₂ indicator is underpredicted in the O₃ and non-O₃ seasons, but there is regional variability. For surface PM_2.5 indicators, there is good inter-model agreement for the degree of sulfate neutralization; however there are systematic underpredictions in the southeast U.S. There is relatively poor inter-model agreement for the less robust adjusted gas ratio indicator, which is largely overpredicted in the summer and both under- and overpredicted in winter in the southeast U.S. There is good inter-model agreement for the O₃ indicator sensitivities, indicating a predominant shift to more NO_x-limited conditions in 2010 relative to 2006. There is less agreement for PM_2.5 indicator sensitivities, which are less robust, while indicating shifts to either regime due to different responses of aerosol treatments to changes in emissions and meteorology.