Comment on Monitored Compliance with Ozone Standards 

CASAC Reconsideration of the 2008 Ozone National 

Ambient Air Quality Standards 

Teleconference 

February 18, 2011 

Summary Comment

CASAC should reconsider and recommend changing the 1 ppb compliance requirement of the 

2008 O3 Standard to 1 pphm to match the current measurement precision of ozone monitors. 

Introduction In the early 1980’s the automated Ultraviolet (UV) Photometric method for 

measuring tropospheric ozone was hailed by the ambient monitoring community as a major 

innovation because it did not require the use of consumable reagents. However, even before the 

first UV-based monitor received “equivalency” status in June, 1977 comparison of its 

performance to the existing ethylene chemiluminescence (CL) “reference method” indicated a 

positive bias in the UV technique1

. 

Researchers and ambient monitoring specialists have since cataloged a variety of interferences 

and anomalous behaviors associated with UV monitors. Interferences include particles2

, 

mercury3, and aromatic hydrocarbon derivatives4,5. Anomalous behaviors include oscillation of 

monitor output tied to instrument shelter temperature swings6 and baseline shifts associated with 

calibration using dry air7

. Interferences in the UV method are noteworthy because (1) they are 

frequently positive and (2) virtually all State and Local monitoring agency sites in the United 

States now employ UV monitors. The anomalous behavior of UV monitors is troubling due to 

the difficulty in detecting such behaviors within hourly averages. 

Effects of Humidity on Monitor Bias 

Evidence has accumulated that changes in the water content of sample or calibration air entering 

a UV monitor6,7 create short-term instability. These findings, in turn, led to methods identifying 

O3 scrubbers in UV monitors that were prone to anomalous behavior8 and possible alternative 

scrubber configurations9

. An alternate calibration scheme using humidified air was also 

proposed but not adopted by EPA. 

More recent research7 has identified mechanisms responsible for moisture-based instability in 

UV monitors, especially those equipped with manganese dioxide (MnO2) scrubbers. This 

research has provided mitigating solutions to Nafion moisture equilibration in UV O3 that have 

achieved “equivalent” status10. 
Given the historic problems associated with changing humidity and dry calibration, regulators 

should consider implementing this new technology. 

Monitor Calibration/Drift/Noise/Sensitivity 

The biases and errors inherent to current monitoring networks have been recognized by 

researchers11. 

Calibration of the U.S. ozone monitoring network involves transferring the authority of a limited 

number of “Standard Reference Photometers” through a chain of calibration devices operated by 

State and Local agencies to field-based monitors. At each stage of this transfer imprecision 

increases and is generally thought to accumulate to 2.25-5.0% of the value reported. 

Equipment manufacturers recognize that component aging, temperature/line voltage variations 

and changes in air flow controllers accumulate as “span drift” which ranges from 0.5%/day to 

0.5%/month. EPA also requires manufacturers to specify the sensitivity of their monitors, 

denoted as the “Lower Detection Limit” (LDL). For current monitors this level is ~ 1 ppb; 

however, the LDL is specified as being twice the “zero noise” and zero noise is measured in zero 

air the absence of ozone. 

At the level of the reconsidered primary ozone standard (0.060-0.070ppm) the uncertainty due to 

calibration alone would be 1.4 - 1.6 ppb. Short-term drift adds 0.8 - 2.5 ppb and results in an 

uncertainty of 2.2 - 4.1 ppb due to these effects alone. This estimate agrees well with an 

examination of systematic bias in ozone measurements made by EPA12. However, the 

calibration and precision-check performance results are performed with ozone in dry zero air and 

thus are far removed from compliance measurements occurring in wet, interference-laden 

ambient air. The resulting interferences and anomalous behaviors noted above could easily 

compound this imprecision. Current monitors cannot be assumed to reliably report 

concentrations to three decimal places as required by the 2008 O3 standards. 

Conclusions 

The vast majority of field studies which utilized collocated UV and CL monitors have shown 

bias in the UV method and most of those biases are positive. The MnO2 ozone “scrubber” in UV 

O3 instruments has been identified as the source of bias and imprecision and monitor 

performance is heavily impacted by moisture content of the sample air. Calibrations at ambient 

moisture levels would be helpful however a “wet” calibration method has not been adopted. 

Regulators should consider implementing Nafion humidity equilibration to mitigate this problem 

and dry calibration issues. Regulators should also consider developing a procedure similar to the 

Method Detection Level employed in the Photochemical Assessment Monitoring Stations 

(PAMS) program for O3 instrument LDL determinations. Until O3 monitoring network 

precision improves, CASAC should recommend relaxing the current 1 ppb resolution 

requirement of compliance monitoring to 1 pphm (10 ppb). 
References: 

1. T.M. Royal and B.A. Grimes, Special Tests of Dasibi Ozone Analyzer, EPA Contract No. 68-

02-2277, Research Triangle Institute Project No. 43U-1221-13, April, 1977. 

2. R. Payson, Effects of Wildfire Smoke on UV Ozone Instruments, Regional Applied Research 

Effort, EPA Region 8, Oct. 2007. 

3. Y. Li et al, UV-Absorption-Based Measurements of Ozone and Mercury: An Investigation on 

Their Mutual Interferences, Aerosol and Air Quality Research, Vol. 6, No. 4, pp. 418-429, 2006 

4. T.E. Kleindienst et al, Comparison of Chemiluminescence and Ultraviolet Ozone Monitor 

Responses in the Presence of Humidity and Photochemical Pollutants, J. Air Waste Management 

Assoc., V. 43, Feb. 1993. 

5. C.W.Spicer et al, A Re-Examination of Ambient Air Ozone Monitor Interferences, J. Air Waste 

Management Assoc., Vol., 60, Nov. 2010. 

6. A.R. Leston, W. Ollison, Estimated Accuracy of Ozone Design Values: Are They 

Compromised by Method Interferences?, Tropospheric Ozone: Nonattainment and Design Value 

Issues, Air & Waste Mgmt. Assoc., Pittsburgh, PA, p. 451 (October 27-30, 1992). 

7. K.L. Wilson, J. W. Birks, Mechanism and Elimination of a Water Vapor Interference in the 

Measurement of Ozone by UV Absorbance, ES&T, 2006, 40. 

8. J. Maddy, A Test That Identifies Ozone Monitors Prone to Anomalous Behavior While 

Sampling Hot and Humid Air, Proceedings of the 91st Annual Air Waste Management Assoc. 

Meeting and Exhibition, June,1998. 

9. J. Maddy, Evaluating a Heated Metal Scrubber’s Effectiveness in Preventing Ozone Monitors’ 

Anomalous Behavior During Hot and Humid Ambient Sampling, Proceedings of the 92nd Annual 

Air Waste Management Assoc. Meeting and Exhibition, June , 1999. 

10. LIST OF DESIGNATED REFERENCE AND EQUIVALENT METHODS, Issue Date: 

August 24, 2010, www.epa.gov/ttn/amtic/criteria.html . 

11. D. Parrish, F. Fehsenfeld, One Step Forward and . . . : Some Cautionary Notes on Urban 

Ozone Measurements, NARSTO News, Vol. 3, No. 1, Winter/Spring, 1999, 

http://www.cgenv.com/Narsto . 

12., Memo D. Mikel, EPA-OAPS, to Louise Camalier, EPA-OAQPS, Ambient Ozone 

Systematic Error, 7/7/06.