Quality assurance measures for ozone, carbon monoxide, sulfur dioxide and oxides of nitrogen

These are some of the measures APCD staff take to ensure the quality of data from the continuous gas analysis systems used to monitor carbon monoxide, sulfur dioxide and oxides of nitrogen.  A "monitoring system" for a given pollutant at a given site usually consists of an inlet tube (called a "probe"), an automatic electronic analyzer and a data acquisition system (possibly connected to multiple instruments) and/or a strip chart recorder.

  • Traceability of standards:
    • Gaseous pollutant concentration standards (in compressed-gas cylinders or in permeation tubes) are traceable to a NIST-traceable reference material (NTRM) or to a NIST-certified gas manufacturer's internal standard (GMIS).
    • Ultraviolet photometers (for ozone) are certified against a NIST-certified reference UV photometer at EPA.
    • Mass flow meters (inside multigas dilution/gas phase titration calibrators) are certified traceable to NIST standards.
    • Flow calibrators (orifices, bubble meters and/or dry flow calibrators) are certified NIST-traceable.
    • Temperature, pressure, etc. instruments or probes are certified NIST-traceable.
    • Multipoint calibrations: The operator notes the monitoring system's response to multiple known concentrations (usually 6), adjusting the system if necessary to read correctly. This is done quarterly, after maintenance or repair and when a zero/span check is out of range.
    • Zero/span checks: The operator or an automated system records the monitoring system's response to pure air, then to a concentration near its upper range limit (at least once every 2 weeks).
    • Precision checks: The operator or an automated system records the monitoring system's response to a known concentration of a roughly consistent value (every 2 weeks). (In practice, this is usually combined with the zero/span check.)
    • Performance audits:
      • Quarterly audits: An APCD auditor (normally a different person than the regular operator) using a different calibrator and different gas standards (or a different photometer) notes the monitoring system's response to multiple concentrations (usually 4 points). Normally, this is done quarterly by APCD for each monitor and quarterly by the Ky. Division for Air Quality (KyDAQ) for each monitor.
      • Interlab audits (EPA's National Performance Audit Program): This is a "blind audit"; a contractor for the US EPA sends us a "black box" dilution system and gas cylinders whose concentrations are unknown to us.  APCD staff record the system's responses to concentrations produced by that audit system and reports these to EPA. This is done once a year on 25% (but at least one) of the monitors for each pollutant.
    • Data audit:  An APCD auditor (not the operator of the monitoring system in question) verifies the highest and one random hourly reading each day for each parameter, and reviews all the data for anomalies. APCD does this for all data every month; KyDAQ does it for one month's data per quarter.


    Ambient Air Quality Data Management System

    Diagram of the air quality data management system at the APCD. 

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