Identifying Worst Case Persistence Factors For Carbon-Monoxide Modeling Near Intersections In Orlando, Florida

At-grade intersections of busy roadways are places where carbon monoxide (CO) concentrations tend to accumulate. Modeling the 8-hour worst-case CO concentration (C8WC) often is done by multiplying a worst-case 1-hour CO concentration (C1WC) by a total persistence factor (TPF). The TPF reflects the fact that both emissions and meteorology vary (and result in lower CO concentrations) over an eight-hour period as compared with the peak 1-hour modeled conditions. A computer tape with hourly meteorological observations and stability classes for Orlando, Florida, was obtained from the National Climatic Data Center. The data were grouped into 18-hour days (6:00 a.m. to midnight) for a ten-year period. Eight-hour turning movement data from several intersections were analyzed and a composite, 18-hour turning movement traffic scenario was developed for this study. The intersection model, TEXIN2, was run with the traffic scenario and historical hourly meteorological data to develop (modeled) daily peak 1-hour CO concentrations (C1MAX) and 8-hour CO concentrations (C8MAX). Dally TPFs were calculated. Comparisons were made between C8MAX and C8WC for various values of TPF. A TPF of 0.55 is recommended for use in Orlando, Florida, based on a comprehensive statistical analysis of 10 years of meteorological data, coupled with detailed hourly modeling results. Both the probability of occurrence of an exceedance, and the acceptable risk of exceeding ambient air quality standards were considered