Air Quality in the Eastern US: A Diagnostic Analysis of Present Day Conditions using various CESM CAM-chem configurations

This study simulates three configuration of the Community Earth System Model (CESM) CAM-Chem model from 1994-2005. These simulations are compared to extant observations for the eastern US in order to diagnose the model's ability to accurately represent present-day climate and atmospheric chemistry conditions. This will serve as a baseline for future simulations CESM CAM-Chem in which there can be no comparison to observations. All three configurations have identical emissions and chemistry, and each configuration is kept as close as possible to its default configuration. The first configuration (fully offline) is an offline simulation that uses MERRA meteorology with forced ocean and ice data. The second configuration (partially online) uses active online simulated meteorology and forced ocean and ice data. The third configuration (fully online) has all active online components. This study compares these three model configurations to available observations of surface and sonde observations in order to provide a baseline for future simulations of present-day and future conditions. Comparisons include: (1) a comparison of the mean value and distribution of various chemical and meteorological variables over land surfaces within the Eastern US with a particular focus on the Northeastern US; (2) a comparison of the seasonality of ozone and major ozone precursors, as well as their distributions, over the Eastern US; (3) a comparison of hourly mixing ratios of ozone and ozone precursors for seasonally averaged diurnal cycles; and (4) a characterization of the regional and seasonal climate change penalty (CCP) , defined as the slope of the best-fit line for daily-maximum 8-hour ozone and daily maximum temperature. Preliminary results show that ozone mixing ratios for the fully offline and partially online configurations are within 5 ppbv of each other for all seasons averaged over the Eastern US. Hourly ozone mixing ratios in the boundary layer in the eastern US show a wider distribution in the fully offline configuration, with values exceeding 200 ppbv in the summertime. The partially online configuration does not exceed 170 ppbv for any time period simulated. This high bias in ozone in the eastern US using MERRA winds has already been noted in the CESM CAM-Chem model. The fully offline configuration also has a significantly shortened planetary boundary layer height when compared to the partially online configuration in the summer and autumn (~250 meters and ~180 meters shorter, respectfully). The simulations show a distinct CCP seasonality in the Northeastern US with a strong positive correlation between DM8H ozone and temperature (r > 0.5) for May through September for the fully offline configuration and from April to October for the partially online configuration. Both configurations show a weak or negative correlation for the other months. For the Northeastern US, the June to September CCP ranges from 2 - 5 ppbv/K and 3 - 5 ppbv/K for the fully offline and the partially online simulations, respectfully. These CCP values show a similar seasonality but slightly smaller values than CCP estimations found in recent studies of other global climate-chemistry models.