Are your DAHS PMA Calculations Accurate?
Tuesday, April 20, 2021
The percent monitor availability (PMA) plays an important role in determining hourly concentrations, rates, and stack gas flow values reported during missing data periods. Is your DAHS calculating the PMA correctly?
In general, as detailed in § 75.33, when the missing data period is longer and/or the PMA is lower, substitute data values are more conservative (higher). For example, when the PMA is below 80%, the maximum potential concentration (MPC) of SO2 and CO2 must be reported for the missing data period, regardless of the length of the missing data period. Refer to § 75.33 Tables 1 and 2 for a summary of standard missing data procedures.
During recent reviews of PMA calculations, EPA identified several instances where the reported PMA did not match an EPA-calculated PMA. In most of cases, the reported PMA was lower than the EPA-calculated PMA, indicating that any reported substitute data values could be higher than required under Part 75. EPA recommends that you check the PMA calculations by calculating the PMA for a particular hour (e.g., the last hour of a quarter) and comparing the result with the DAHS-calculated value.
In accordance with the PMA procedures of § 75.32, calculating the PMA is required following initial certification, however, reporting PMA is not required until either the completion of the first 720 quality-assured monitor operating hours following initial certification for SO2, CO2, O2, or moisture monitoring systems or the completion of the first 2,160 quality-assured monitor operating hours following initial certification for NOX-diluent, NOX concentration, or flow monitoring systems at a unit or stack location. For ozone season only reporters, only the hours within the ozone season apply as the first 720 or 2,160 quality-assured monitor operating.
Prior to completion of 8,760 unit (or stack) operating hours following initial certification, in accordance with the PMA procedures of § 75.32, calculate the PMA for a particular hour by dividing the total unit operating hours for which quality-assured data were recorded since certification by the total unit operating hours since certification. For units that report only during the ozone season, use 3,672 unit (or stack) operating hours within the ozone season and following initial certification.
Upon completion of 8,760 unit (or stack) operating hours following initial certification and thereafter, in accordance with the PMA procedures of § 75.32, calculate the PMA for a particular hour by dividing the total unit operating hours for which quality-assured data were recorded during the previous 8,760 unit operating hours by 8,760. If 8,760 unit operating hours are not available in the previous three years (26,280 clock hours), the number of quality-assured hours and the total unit operating hours in the previous 26,280 clock hours should be used instead. For units that report only during the ozone season, use 3,672 unit (or stack) operating hours within the ozone season instead of 8,760 unit (or stack) operating hours.
If the unit reports only during the ozone season, refer to the PMA procedures of § 75.74(c)(7)(iii)(D) through (H) for more information.
The PMA should be calculated as:
Where the appropriate number of hours is either:
- For units that report year-round emissions, 8,760 hours or the actual number of operating hours within a 26,280 clock hour lookback period if 8,760 hours are not available within the lookback period.
- For units that report ozone-season emissions only, 3,672 hours or the actual number of operating hours within a 26,280 clock hour lookback period if 3,672 hours are not available within the lookback period.
- For units with less than 8,760 hours (3,672 for ozone-season only reporters) of operation following monitoring system certification, the actual number of reported operating hours (limited to ozone-season operating hours for ozone-season only reporters) since certification.
For the purpose of calculating PMA, the method of determination codes (MODCs) 01, 02, 03, 04, 14, 16, 17, 19, 20, 21, 22, 26, 45, 47, 53, and 54 are considered quality assured hours. See Tables 19 and 26 in the ECMPS Emissions Reporting Instructions
for more details on MODCs.
Because the use of substitute data comprises a very small portion of hourly emissions data in Part 75 reporting, as detailed in the recently released Substitute data in EPA CAMD’s Power Sector Emissions Data
monitoring insight paper, discrepancies in the PMA may not affect annual emission totals, but could result in differences between reported and recalculated unit level hourly emissions.
Labels: Technical Support Tip