1. Melinda Beaver 1, Keith Kronmiller 2, Rachelle Duvall 1, Surender Kaushik 1, Timothy Morphy 3, Patrick King 3, and Russell Long 1 1 US EPA Office of Research and Development 2 Alion Science and Technology 3 Teledyne API Direct and Indirect Methods for the Measurement of Nitrogen Dioxide (NO 2 ) Office of Research and Development National Exposure Research Laboratory 20 November 2013

2. 1 Research Question and Motivation: How do direct, optical measurements of NO 2 compare with the thermal conversion (FRM) and photolytic conversion techniques? EPA’s ORD interest in accurate NO 2 measurements supports: EPA’s monitoring networks Reference and equivalent method determinations and evaluations Ground-based satellite validation work with NASA

3. 2 Current NO 2 Regulations Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for criteria pollutants: * The new monitoring locations for the Jan 2010 primary standard will be sited in near roadway locations to capture areas of maximum concentration. (http://epa.gov/ttn/amtic/nearroad.html)http://epa.gov/ttn/amtic/nearroad.html * Currently, EPA has no NO y standard, but measurements are required in the Ncore network. NO 2 Primary Standards levelaveraging timeyear implemented 53 ppbAnnual1971 100 ppb*1 hr2010

4. 3 How is NO 2 (currently) measured? Federal (Automated) Reference Method (40 CFR, Part 50, Appendix F): -Gas-phase chemiluminescence -Indirectly measure NO 2 by conversion to NO, then NO is detected by chemiluminescence (NO + O 3  NO 2 *, NO 2 * = excited state); -Advantage  in use since the 1970s (long term record) -Disadvantages  non-specific; indirect thermal converter NO + O 3 rxn chamber O 3 generator PMT NO channel NO x channel Ambient air Heated bed - chemiluminescence NO 2 = NO x - NO

5. 4 The interference in the FRM monitor over predicts by ~50% during the hours surrounding noon. Dunlea et al. (2007) Atmospheric Chemistry and Physics - also observed this in Mexico City. FRM (ppb) FRM has possible interferences

6. 5 NO spikes  The indirect determination requires a slowly changing NO x distribution. Otherwise, negative spikes of NO 2 are possible: Indirect methods have possible interferences

7. 6 Federal Equivalent Method (FEM); designated in June 2012 Replace the metal bed reducer with a photolysis cell to photolyze NO 2 to NO (NO 2 + hν  NO + O; 350-420nm). -Use high-power light sources to maximize conversion to NO. - Advantage  more specific to NO 2 - Disadvantages  non-unity conversion efficiency; still indirect Alternative Technique: Photolytic- chemiluminescence Photolysis cell λ~ 400 nm NO + O 3 rxn chamber O 3 generator PMT NO channel NO x channel Ambient air photolytic-chemiluminescence

8. 7 UV/Vis Spectroscopy of NO 2 Data from Sander et al. (2006)

9. Cavity ringdown spectroscopy - instrument manufactured by Los Gatos Research, Inc. - 10 s time resolution - Advantage  DIRECT measurement - Disadvantages  not-necessarily specific to NO 2, but to any molecule that absorbs light at 405 nm 8 Direct Optical Techniques

10. Cavity attenuated phase shift spectroscopy (CAPS) - instrument manufactured by Teledyne API - Advantage  DIRECT measurement - Disadvantage  not-necessarily specific to NO 2, but to any molecule that absorbs light at ~450 nm - 2 versions: fast response (1 s) and ambient (10 s) 9 Direct Optical Techniques

11. 10 Two Ambient Evaluations Visalia Municipal Airport NASA DISCOVER-AQ collaboration Jan – Feb 2013 inlets ~35m from I-99 (60,000 AADT); 17% heavy duty truck traffic Thermal conversion, Photolytic conversion, and Cavity ringdown analyzers compared EPA RTP, on-site ambient measurement site SOAS-RTP collaboration August 2013 background/regionally influenced site Thermal conversion, Photolytic conversion, and Cavity ringdown, and CAPS analyzers compared Visalia, CARTP, NC

12. 11 Sampling and Calibration Considerations Gas-phase titration of NO (excess) with O 3 If response is outside ±10% of the expected response, data are excluded from the analysis.

13. y = 0.89*x+3.07 R 2 = 0.78 Visalia, CA (winter) y = 0.95x+0.71 R 2 = 0.88 12 Visalia, CA

14. 13 RTP, NC (summer) y = 1.04x-0.79 R 2 = 0.99 y = 1.08x-0.47 R 2 = 0.96 y = 0.97x-0.20 R 2 = 0.97 RTP, NC

15. 14 Influence of NO concentrations? Visalia, CA RTP, NC

16. |deltaNO| = absolute value of the diff() between a point and the following point. Influence of NO concentrations? 15 Visalia, CARTP, NC

17. fitR2R2 n All pointsy=1.04X-0.80.995447 |ΔNO| < 5ppb/min y=1.04x-0.80.995447 |ΔNO| < 0.4ppb/min y=1.04x-0.80.995445 Influence of NO concentrations on the indirect methods? fitR2R2 n All pointsy=0.89x+3.10.785760 |ΔNO| < 5ppb/min y=0.91x+2.40.873789 |ΔNO| < 0.4ppb/min y=0.96x+1.70.961921 Visalia, CARTP, NC

18. 17 Influence of NO concentrations on the direct method? fitR2R2 n All pointsy=0.95x+0.710.885760 |ΔNO| < 5ppb/min y=0.97x-0.010.913789 |ΔNO| < 0.4ppb/min y=1.0x-0.780.951912 fitR2R2 n All pointsy=1.08x-0.470.965447 |ΔNO| < 5ppb/min y=1.08x-0.470.965447 |ΔNO| < 0.4ppb/min y=1.08x-0.470.965444 Visalia, CA RTP, NC

19. 18 Currently evaluating NO 2 FRM, FEM, and direct optical methods Ambient datasets are currently being analyzed for their performance under a variety of atmospheric conditions (downwind of an urban area – Baltimore; background site – RTP, NC; near roadway – Visalia, CA; industrial/urban – LaPorte, TX). Direct optical methods may be preferable in near-roadway locations Re-evaluate using the compressed cylinder for NO 2 calibration Collect ambient NO 2 and NO y data in Denver, CO 2014 in collaboration with NASA DISCOVER-AQ Laboratory testing for NO 2 methods to include extended noise and stability testing; along with interference tests with H 2 O and NO NO 2 Research Initiatives and Next Steps