CCQM-P28: An international comparison of ground-level ozone reference standards

Transcription

1 CCQM-P28: An international comparison of ground-level ozone reference standards R.I. Wielgosz, J. Viallon, M. Esler and P. Moussay

2 National and international networks for atmospheric ozone measurements NIST-EPA Standard Reference Photometer (SRP) US ambient O 3 measurements under EPA regulations an increasing number of worldwide National Metrology Institutes maintain SRPs Joint BIPM-NIST programme to maintain the comparability of the worldwide network of ozone reference standards

3 Directive 2002/3/EC: Ozone in Ambient Air Parameter Threshold Information Threshold 1 hour average 180 µg/m 3 Alert Threshold 1 hour average 240 µg/m 3 Long-term objective for the protection of human health Maximum daily 8-hour mean within a calendar year 120 µg/m 3 Continuous fixed measurement Uncertainty of individual measurement 15%

4 CCQM-P28 Participating institutes + BIPM - GPT Europe + WMO - EMPA NERI NMi NILU SP FMI VNIIM Environment Canada NIST KRISS NIES NPL LNE UBA CHMI UBA IEM-DEP ISCIII IMGC METAS CSIR-NML + ERLAP (EC-JRC) Reference : BIPM-SRP27

5 CCQM-P28 time table Date Institute Country Instrument National Standard Protocol July 2003 NIST USA SRP 0 SRP 2 B Sept 2003 ISC III Spain TEI 49C SRP 22 B Sept 2003 EC-JRC ERLAP EC TEI 49CPS UMEG B Oct 2003 Environment Canada Canada SRP 16 A Nov 2003 METAS Switzerland SRP 18 A Dec 2003 KRISS Korea KRISS-O3-SRP A Feb 2004 LNE France TEI 49CPS SRP 24 B Feb 2004 VNIIM Russia Dasibi 1003AH A Mar 2004 FMI Finland TEI 49CPS A Mar 2004 WCC-EMPA WMO SRP 15 A Mar 2004 UBA Austria SRP26 A May 2004 SP Sweden Environment SA 42M A May 2004 NPL UK SRP 20 A June 2004 NDENW Hungary TEI 49C UMEG B June 2004 UBA Germany SRP 29 A July 2004 NIES Japan SRP 35 / GPT A/B Sept 2004 CHMI Czech Rep SRP 17 A Sept 2004 CSIR South Africa API A Nov 2004 NERI Denmark API-M401 UMEG B Dec 2004 NILU Norway ML9811 A Jan 2005 NMi-VSL Nederland UMEG A Jan 2005 IMGC Italy KRISS-O3-SRP A Feb 2005 BIPM TEI 49C GPT B

6 SRP facilities at the BIPM Triad of SRPs maintained at the BIPM Measurement range: detection limit to 1000 nmol/mol Instruments: SRP27, SRP28, SRP31, SRP32, SRP33, Reference instrument: SRP27

7 SRP measurement in detail I 2,0 I 0 CELL #2 : OZONE, transmittance D 2 f 2 I 2 = k 2 D 2 I 2,0 I 2 I 2,0 I 0 CELL #2 : ZERO AIR f 2 I 2 = k 2 I 2,0 I 2 I 1,0 I 0 I 1 I 1,0 I 0 I 1 CELL #1 : ZERO AIR f 1 I 1 = k 1 I 1,0 CELL #1 :OZONE, transmittance D 1 f 1 I 1 = k 1 D 1 I 1,0 The ratio D is defined as D= (f 1 /f 1 ) (f 2 /f 2 )= (k 1 D 1 I I 1,0 /k /k 1 I 1,0 )(k 2 D 2 I 2,0 /k 2 I I 2,0 )=D )=D 1 D 2 D = e -cαl 1 e -cαl2 = e -cα(l1+l2) =Transmittance of a cell of length L T =L 1 +L 2 =2L 1 Pstd T 1 R T C = ln( D) x = ln( D) 2αL P T 2αL N P std A

8 Uncertainty budget x = 1 2αL R N A T P ln( D) Component (y) Value Source Uncertainty u(y) Distribution Standard Uncertainty Combined standard uncertainty Relative contribution to u(x) Measurement Scale Rect cm Optical Path 2L cm Repeatability Normal 0.01 cm 0.01 cm 0.01 % Pressure gauge Rect kpa Pressure P 100 kpa Difference between kpa 0.03 % Rect kpa cells Temperature T 295 K Temperature probe Rect K K 0.03 % Ratio of Scalers resolution Rect intensities D Repeatability Triang % Absorption Cross section α cm 2 Literaure value Normal 0.75% 0.75% 0.75% /molecule

9 Uncertainty as a function of ozone mole fraction Combined standard uncertainty u(x) / (nmol/mol) a) with u(α)=0 2 u ( T) u ( P) u ( L) u ( x) = x T P L 2 2 u 2 ( D) ( D ln( D) ) 2 b) with u(α)=0.75% mole fraction x / (nmol/mol)

10 Generalised Least Square Fits Both measured mole fraction x SRP27 and x SRPn have uncertainties A Generalised Least Square fit is used to determine the fit parameters and their uncertainties 273 Guest mole fraction x SRPn (nmol/mol) GLS Fit : x SRPn =a 0 +a 1 x SRP27 a 0, u(a 0 ) =? a 1, u(a 1 ) =? Main mole fraction x SRP27 (nmol/mol) Guest mole fraction x SRPn (nmol/mol) S xˆ i, yˆ ) ( i = n Main mole fraction x SRP27 (nmol/mol) 2 2 = xˆ i xi yˆ i yi u ( xi ) u ( yi ) i ( i x i, y ) Sum to minimise :

11 Summary of SRP comparisons versus [BIPM] SRP27 Slopes SRP2 AT NIST SRP28 SRP0 SRP28 AT BIPM SRP0 SRP19 SRP Intercepets (nmol/mol) Uncertainty calculation : Generalised Least Squares fitting using u(x SRP27 ) and u(x SRPN ). Results given with k = 2.

12 SRP27 vs SRP28 during comparison (July 03 Jan 05) Pilot Study Results SRP28 evolution Wednesday 9 March average standard deviation Slopes uly03 ept03 ept03 Oct03 Nov03 Dec03 Mar04 Apr04 Intercepets (nmol/mol) May04 May04 Jun04 Jun04 Jul04 Sep04 Nov04 Jan average standard deviation 0.12

13 CCQM-P28 Data treatment Collaboration with Wolfram Bremser (BAM) on the Generalised Least- Square regression with B_Least Comparison parameters : x = p _ 0 + p _ 1 A x B Questions : How many points in the ozone mole fraction range (12 actually)? How to take correlation into account : var(x i,x j ), var(x i,y i )

14 Pressure measurements P < 40 Pa P 1 (before)? SRP pressure probe P 1 (after)? Cell 1 P < 60 Pa assumed in the uncertainty budget for a well maintained SRP. P < 40 Pa Cell 2 P 2 (before)? P 2 (after)?

15 Pressure differences between gas cells The pressure difference between the cells is 40 Pa, less than the maximum difference assumed in the uncertainty budget pressure difference / Pa air in cell 1 ozone in cell2 air in cell 2 ozone in cell 1 PUMP OFF PUMP ON PUMP OFF BAR-CHEM2 in point 2 BAR-CHEM3 in point 1 13:04:48 13:05:31 13:06:14 13:06:58 13:07:41 13:08:24 13:09:07 13:09:50 13:10:34

16 Temperature measurements in the SRP Source block heated Probe 1 Probe 2 Temperature gradients in the SRP gas cells can cause a significant measurement bias

17 Temperature Control Unit Temperature differences in SRP gas cells reduced < 0.1 C

18 Efficiency of the temperature control unit temperature / c c remaining gradient is lower than 0.1 c :48 11:16 11:45 12:14 12:43 13:12 13:40 14:09 experiment duration /h AMR probe 0 AMR probe 1 AMR probe 2 SRP 31 probe

19 Optical path length determination C ( D) ln = α L P P std T T std Optical path length is assumed equal to the geometric pathlength,but: Reflections on the cell walls Multiple reflections on optics Hg lamp Cell length : Lc = 90 cm Light Path length L > Lc Reflecting optics : windows, filter, detector. Part of light as a path length greater that 3L... L under-evaluated, and C over-evaluated.

20 Towards a laser-based SRP The UV radiation will be obtained by Second Harmonic Generation The laser-based SRP33 will be compared to the lamp-based SRP32 to assess the optical bias in SRPs. SRP32 SRP33 laser 2.5W at λ nm λ/2 nm SHG in the BBO crystal

21 Absorption cross-section section 120 Conventional value 1.5 % uncertainty 250K, Bass K, Daumont K, Molina K, Hearn 61 cross-section / cm 2.molecules λ / nm Laser wavelength

22 Value of the absorption cross-section section at 253.7nm Absorption cross-section measured by different groups since 1953 (same technique = spectroscopy) A conventional value has been adopted (NIST, ISO) : Hearn E E-17 (k=1) Molecular absorption cross section /cm E E E E E-17 Hearn [1961] Griggs [1968] DeMore and Raper [1964] Molina and Molina [1986] Mauersberger [1986] Mauersberger [1987] Mauersberger [1987] Yoshino et al. [1988] Yoshino et al. [1993] Malicet et al. [1989] NIST SRP value Daumont et al. [1992] 1.080E-17 Inn and Tanaka [1953] 1.060E-17

23 Gas Phase Titration (GPT) O 3 SRP NO + O 3 NO 2 + O 2 GPT NO-CHEMILUMIN. traceable to gravimetric NO standards GPT NO2-CHEMILUMIN. traceable to Dynamically prepared NO 2 standards NO primary facility comparison of NO gravimetric standards NO 2 primary facility dynamic generation of NO 2 standards

24 Gas Phase Titration molbloc 0-50 ml n /min Alternative Reaction Vessel: 30 m x 4.8 mm ID FEP tube waste waste [NO] 50 µmol.mol -1 in N 2 NO dilution system waste O 3 TEI 49C NO NO 2 TEI 42C molbloc 0-50 ml n /min molbloc L n /min [NO] 2 µmol.mol -1 [O 3 ] nmol.mol -1 Reaction Vessel: 2 x 500 ml glass sphere NO + O 3 NO 2 + O 2 P zero air molbloc L n /min O 3 Generator Env waste Pressure Controller wetted surfaces electropolished stainless steel Pump wetted surfaces glass or FEP waste

25 Gas Phase Titration: Flow Measurement reaction vessel valves molblocs MFCs pressure controller

26 Gas Phase Titration (NO CHEMILUMINESCENCE ) GPT Experiment Sequence cycle 1 cycle 2 cycle 3 cycle 4 cycle st NO calibration [NO], [O 3] / (nmol/mol) NO + O 3 titration O 3 measurement 2nd NO calibration [O 3] s.d. = 0.16 nmol/mol (0.03%) 400 [NO] s.d. = 0.52 nmol/mol (0.09%) time / minutes

27 Comparison of SRP and GPT based ozone measurements 1200 SRP27 [O 3 ] vs GPT [NO] 1000 SRP27 [O 3] / (nmol/nmol) GPT [NO] / (nmol/mol)

28 BIPM future activities CCQM-P28 workshop, April 2005 at BIPM the maintenance of ground-level ozone reference standards to act as an international reference facility; the provision of an on-going comparison and calibration service for national ground-level ozone reference standards start autumn the development of ozone reference standards and gas-phase titration based reference methods for the measurement of ozone; the development and implementation of high-accuracy methods for the comparison of nitrogen oxide gas standard provided by NMIs.