A comparison of regional oxidant model (ROM) output with observed ozone data

The output from the regional oxidant model (ROM) is compared to observed ozone over northern Illinois for June, July and August 1987. The 8-h daily average ozone at the ozone monitoring stations is interpolated to the ROM grid cells using a spatial statistical method. Differences between the model output and spatial predictions are compared at three levels of spatial averaging (with approximate scales of 19, 100, 400 km) and three levels of temporal averaging (daily, weekly, 3 months). In addition two ozone monitoring stations are paired with weather stations and with ROM cells in order to investigate the performance of ROM as a function of meteorological conditions. For daily values the root mean squared error (RMSE) between the ROM values and those predicted from the monitoring network varies between 14 and 25 ppb with the largest discrepancies occurring near Lake Michigan. Weekly averages reduce the RMSE by approximately 30% but spatial aggregation is not helpful in improving the agreement. The difference between ROM ozone predictions and the observed ozone at two paired sites depends most strongly on temperature and to a lesser extent on dew point temperature. The R2 from linear regressions is approximately 35%. An examination of the synoptic-scale and meso-scale weather patterns during this period indicates that ROM is sensitive to dynamic situations such as a frontal passage.