## 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 cha nging the 1 ppb compliance requirement of the 2008 O3 Standard to 1 pphm to match the curr ent measurement precision of ozone monitors.

Introduction In the early 1980’s the automated Ultr aviolet (UV) Photometric method for measuring tropospheric ozone wa s hailed by the ambient monitoring community as a major innovation because it did not require the use of c onsumable reagents. However, even before the first UV-based monitor received “equivalenc y” status in June, 1977 comparison of its performance to the existing ethy lene chemiluminescence (CL) “r eference method” indicated a positive bias in the UV technique 1

.

Researchers and ambient monitoring specialists ha ve since cataloged a variety of interferences

and anomalous behaviors associated with UV monitors. Interferences include particles 2

, mercury 3

, and aromatic hydrocarbon derivatives 4,5 . Anomalous behaviors include oscillation of monitor output tied to instrume nt shelter temperature swings 6 and baseline shifts associated with calibration using dry air 7 . Interferences in the UV method ar e 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 be haviors 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 monitor 6,7 create short-term instability. These findi ngs, in turn, led to methods identifying

O3 scrubbers in UV monitors that were prone to anomalous behavior 8 and possible alternative scrubber configurations 9 . An alternate calibration scheme using humidified air was also proposed but not adopted by EPA.

More recent research 7 has identified mechanisms responsible for moisture-based instability in UV monitors, especially those equi pped with manganese dioxide (MnO 2 ) scrubbers. This research has provided mitigating solutions to Nafion moisture equilibration in UV O3 that have achieved “equivalent” status 10

.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

researchers 11

.

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 accumu late to 2.25-5.0% of the value reported.

Equipment manufacturers recognize that componen t aging, temperature/lin e 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 ma nufacturers to specify the se nsitivity of their monitors,

denoted as the “Lower Detecti on Limit” (LDL). For current m onitors 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 effect s alone. This estimate agrees well with an examination of systematic bias in ozone measurements made by EPA 12

. However, the calibration and precision-check performance results are performed with ozone in dry zero air and thus are far removed from compliance measurem ents 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 d ecimal places as required by the 2008 O3 standards.

Conclusions The vast majority of field studi es which utilized collocated UV and CL monitors have shown bias in the UV method and most of those biases are positive. The MnO 2 ozone “scrubber” in UV

O3 instruments has been identified as the source of bias and imprecision and monitor performance is heavily impacted by moisture conten t 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 Phot ochemical Assessment Monitoring Stations (PAMS) program for O3 instrument LDL dete rminations. 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 Da sibi 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 Wa ter Vapor Interference in the Measurement of Ozone by UV Absorbance , ES&T, 2006, 40.

8. J. Maddy, A Test That Identifies Ozone Monito rs Prone to Anomalous Behavior While Sampling Hot and Humid Air , Proceedings of the 91 st Annual Air Waste Management Assoc. Meeting and Exhibition, June,1998.

9. J. Maddy, Evaluating a Heated Metal Scrubber’s Eff ectiveness in Preventing Ozone Monitors’ Anomalous Behavior During Hot and Humid Ambient Sampling , Proceedings of the 92 nd Annual Air Waste Management Assoc. Meeting and Exhibition, June , 1999.

10. LIST OF DESIGNATED REFERE NCE 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 Loui se Camalier, EPA-OAQPS, Ambient Ozone Systematic Error, 7/7/06.