Quality Assurance Performance Audits

This page last reviewed August 23, 2012


The Quality Assurance Section (QAS) conducts various activities to ensure that data collected comply with procedures and regulations set forth by the United States Environmental Protection Agency (U.S. EPA) and can be considered good quality data and data-for-record. Standard Operating Procedures (SOPs) have been written to ensure all audits are conducted consistently and according to regulations. They are contained in Volume V of the Quality Assurance Manual.In addition, whenever audits are conducted and instruments are found operating outside ARB's allowable range, an Air Quality Data Action (AQDA) report is issued. This document identifies possible questionable data and is a mechanism to resolve the validity of the data.


As part of the annual audit at each air monitoring station, QAS conducts siting evaluations. Physical measurements and observations, including probe/sensor height above ground level, distance from trees, type of ground cover, residence time, obstructions to air flow, and distance to local sources, topography, vehicle counts, predominant wind direction, probe material, etc., are taken to determine compliance with 40 CFR Part 58, Appendix E requirements. A description of the site survey is available in a technical paper titled "Comprehensive Quality Assurance Site Survey."


Through-the-Probe Audits for Continuous Analyzers
Annually, QAS conducts through-the-probe (TTP) audits for continuous analyzers.  The audits 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. QAS compares the results obtained from the continuous analyzer to the known values. A complete description of the audit procedure is available in a technical paper titled, "Through the Probe Performance Audits of Continuous Ambient Air Analyzers."

TTP audits are conducted for the following gaseous pollutants:

  • Carbon Monoxide (CO)
  • Nitrogen Dioxide (NO2)
  • Sulfur Dioxide (SO2)
  • Ozone (O3)
  • Hydrogen Sulfide (H2S) 

Flow Audits

The accuracy of particulate samplers is determined by comparing the instrument's flow rate to a certified orifice (PM10, TSP, and PM2.5 samplers), or a calibrated mass flow meter (TEOM and BAM samplers). These devices are certified against a NIST traceable flow device or calibrator. Since an accurate measurement of particulate matter is dependent upon flow rate, biannual flow audits are conducted. The audit device is connected in-line with the sampler's flow path. The flow rate is measured while the sampler is operating under normal sampling conditions. The true flow is calculated from the audit device's calibration curve.  The sampler's flow is then compared to the true flow, and a percent difference is determined.

Flow audits are conducted for the following samplers:

  • PM10
  • PM2.5
  • Xontech 920/924
  • TEOM
  • TSP
  • BAM 


Meteorological Sensor Audits

QAS conducts annual performance audits for wind speed, wind direction, ambient temperature, and barometric pressure. Real-time meteorological data are generated to characterize meteorological processes such as transport and diffusion, and to make air quality forecasts and burn-day decisions. The data are also used for control strategy modeling and urban airshed modeling.



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.Laboratory audits are conducted on a quarterly or annual basis.


  • Gaseous- Performance audits of the analytical methods for toxic compounds and motor vehicle exhaust are conducted annually to assess the accuracy of the laboratory methods used to measure ambient concentrations of toxic air contaminants and motor vehicle exhaust. Audits are conducted by supplying the laboratory with a cylinder containing a mixture of target compounds prepared by the National Institute of Standards and Technology (NIST).The laboratory analyzes the contents of the cylinder in accordance with their standard operating procedures and reports the results to QAS. The laboratory’s analytical results are compared to the true concentrations for each compound and a percent difference is calculated. Laboratory audit results must be within ARB’s established control limits of ±20% of the true or certified value.If results are found to exceed ARB’s control limits, the laboratory is requested to investigate the cause and implement appropriate corrective action.

  • Media Based- Media based laboratory audits are conducted for methods incorporating a solid sample collection media. Audit samples for PM10 ions, Hexavalent Chromium, Aldehydes and Ketones, and Lead are prepared by accredited vendors using traceable standard materials. Audits are conducted by supplying the laboratory with a spiked filter or cartridge media, which is analyzed in accordance with their standard operating procedures.Analytical results are reported to QAS for comparison to the certified concentration. Laboratory audit results must be within ARB’s established control limits of ±20% of the true or certified value.If results are found to exceed ARB’s control limits, the laboratory is requested to investigate the cause and implement appropriate corrective action.

  • Mass Analysis-Annual mass analysis performance audits are conducted for 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 district documentation records. The audit is conducted to ensure that the districts’ program are operating in accordance with U.S. EPA guidelines as outlined in 40 CFR, Part 50, Appendices J and L, and that the data generated are good quality and can be considered data-for-record.

  • Toxics Whole Air Comparison Check - Whole air comparison checks are conducted annually to compare the analytical methods used by the laboratories measuring the concentrations of toxic compounds in ambient air. Samples are generated using a sampling unit which simultaneously fills up to twelve summa canisters with ambient air over a three hour sampling period. Canister samples are sent to each participating laboratory for analysis in accordance with their standard operating procedures. The analytical results are reported to QAS for comparison with results from the other participating laboratories. Statistical evaluations are performed on each of the reported values in order to develop an accurate depiction of the canisters’ contents. The purpose of the laboratory comparison is to determine the level of agreement across the participating laboratories, not necessarily to determine the accuracy of the methodologies.


Site Information

Information regarding each air monitoring station that is audited by QAS. Included are maps to each site, GPS coordinates, site photos, pollutants monitored, and site surveys. The site surveys list in-depth monitoring information such as traffic descriptions, calibration dates, distances to trees and obstacles, and residence times. Also included are the most recent data from certain sites operated by ARB. Station information is browsable through either the California Air Basin Map or Site list, which is an alphabetical listing of air monitoring stations. More sophisticated queries can be made from the search page. Site webpages will be updated as new information is gathered.


In addition to the detailed site information, tables summarizing each agency's ambient air monitoring programs are provided. The tables list what parameters are being measured at each site.


For further information contact: Ranjit Bhullar, Manager Quality Assurance Section

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