Quality Assurance Performance Audits
To ensure that the ambient air monitoring data collected throughout California can be considered good quality data (data-for-record) and complies with procedures and regulations set forth by the U.S. Environmental Protection Agency (U.S. EPA), the California Air Resources Board (CARB) has a robust quality assurance program which includes several types of performance audit activities. Standard Operating Procedures (SOP) ensure that all audits are conducted consistently and in accordance with federal requirements and can be found in Volume V of CARB’s Quality Assurance Manual. When instruments are found to be operating outside CARB's Performance Criteria, a corrective action notification (CAN) or Air Quality Data Action (AQDA) request may be issued. Below are descriptions of CARB’s audit activities:
Through-the-Probe Performance Audits for Continuous Analyzers
Through-the-probe (TTP) performance audits are conducted annually for continuous analyzers (CO, NO2, SO2, O3, and H2S) to verify the accuracy of the automated system and ensure the integrity of the entire sampling system. TTP audits are conducted by introducing National Institute of Standards and Technology (NIST) traceable gases into the sampling probe inlet at various concentrations. The results obtained from the continuous analyzer are compared to the known value for compliance determination.
Flow Rate Performance Audits
Since an accurate measurement of particulate matter is dependent upon flow rate, sampler flow audits are conducted biannually. The accuracy of particulate samplers is determined by comparing the instrument's flow rate to a certified orifice (e.g. PM10 and TSP samplers), or a calibrated mass flow meter (e.g. PM2.5, TEOM, and BAM samplers). These devices are certified against a NIST traceable flow device or calibrator. The audit device is connected in-line with the sampler's flow path and the flow rate is measured while the sampler is operating under normal sampling conditions. The true flow rate is calculated from the audit device's calibration curve and then compared to the sampler's actual flow rate. A percent difference is calculated based on the readings to determine compliance.
Meteorological Sensor Performance Audits
Real-time meteorological data are critical when conducting modeling, characterizing meteorological processes such as transport and diffusion, and when making air quality forecasts and burn-day decisions. Sensor accuracy is determined by conducting annual performance audits and include wind speed, wind direction, ambient temperature, and barometric pressure.p
Laboratory Performance Audits
Laboratory performance audits are designed to assess the accuracy of the methods and instruments used in the laboratory to generate analytical data.
- Mass Analysis - Annual mass analysis performance audits are conducted for both PM10 and PM2.5 laboratory operations. The mass analysis audits include an on-site check of the filter weighing balance, relative humidity and temperature sensors, as well as a review of the documentation records. The audit is conducted to ensure that the programs are operating in accordance with U.S. EPA guidelines as outlined in 40 CFR, Part 50, Appendices J and L.
- Analytical Analyses – Laboratory performance audits are conducted to support the toxics air monitoring program through participation in the U.S. EPA’s National Air Toxics Trends Stations (NATTS) audit program. These annual audits consist of three NATTS performance tests that include ambient volatile organic compounds (VOC), carbonyls, and metals (collected on Teflon filters).
During each audit, basic information regarding each air monitoring station is gathered. The information collected includes maps to each site, GPS coordinates, site photos, pollutants monitored, and site surveys. The site surveys record in-depth monitoring information such as traffic descriptions, calibration dates, distances to trees and obstacles, and residence times to determine compliance with 40 CFR Part 58, Appendix E requirements. Station information is available through either the California Air Basin Map or Site list, which is an alphabetical listing of air monitoring stations in California. More sophisticated queries can be made from the search page. Site webpages are updated as new information becomes available.
In addition to the detailed site information, tables summarizing each agency's ambient air monitoring programs are provided. The tables list parameters that are being measured at each site.