Background Concentrations Sample Clauses

Background Concentrations. To develop a realistic nationwide CO background concentration estimate, observed data for 8- hour and 1-hour average CO concentrations were extracted from EPA’s AirData for each of the three most recent years (2014-2016). AirData is a database of air monitoring data. Datasets representing the 1-hour average, 8-hour average, and annual summary statistics from all reporting sites are available.16 To determine a nationally representative background concentration, we chose a form consistent with the CO design value (the 2nd highest non-overlapping observed CO concentrations) and the form of the CO national ambient air quality standards (NAAQS) from each of the nation’s CO monitoring stations. We determined the range of reported DVs from all stations in AirData, excluding those in Mexico and Puerto Rico, from 2014-2016. Table 8 shows the 90th, 95th, and 99th percentiles from the reported DVs from each station in the record meeting a 75% data completeness threshold. To accommodate the recent trend and be consistent with the form of the DV, Table 8 also shows the maximum from the three most recent years for each of those percentages. Based on this review, it was determined that a reasonably conservative value, applicable to almost any location nationwide, is the highest 95th percentile CO concentration from the past three years (Table 8). Using this value, the representative 1-hour background concentration was determined to be 4.4 ppm and the representative 8-hour background concentration was determined to be 2.4 ppm. Percentile 2011 2012 2013 Highest 99th 3.4 2.9 3.7 3.7 95th 2.4 2.3 2.2 2.4 90th 2.0 2.0 1.8 2.0 99th 6.5 5.7 7.2 7.2 95th 4.3 4.4 4.2 4.4 90th 3.2 3.1 2.9 3.2 Source: USEPA AIRData (2018) 16 This analysis was based on tabular pre-generated data files available at: ▇▇▇▇▇://▇▇▇.▇▇▇.▇▇▇/aqsweb/airdata/download_files.html For future years mobile sources are expected to remain the primary source of CO emissions nationwide. EPA provides for the option to adjust for future CO concentrations as a result of emissions rate changes in the mobile source fleet. Continued fleet turnover at a national level to vehicles constructed to more stringent EPA emission standards may reasonably be expected to result in reduced emission rates in the future. However, to preserve the conservative, worst-case approach, no reductions in future emission rates and background levels were assumed for this study. Thus, the results presented in the following Section are representati...