Detection of HONO using Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS)

Transcription

1 Detection of HONO using Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) A. A. Ruth Department of Physics, University College Cork, Cork, Ireland Hans-Peter Dorn (FZ Jülich, J IEK-8) Ravi Varma (UC Cork)

2 Outline (1) Measurement Principle (IBBCEAS) (2) Experimental Setup(s) ) & Design (3) Data Analysis (4) HONO Detection (at UCC) (5) Instrument for the FIONA Campaign in Valencia (2010)

3 (1) Measurement Principle Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBB-CEAS)

4 Conventional Absorption- spectroscopy Sample I in Loss L I d One Pass I I (1 L in )

5 I I in (1 L) Lambert-Beer absorption loss I I in exp( d) absorption losses very small I I 1d in absorption coefficient I in I 0 = transmitted intensity without sample I I 1d 0 absorption losses very small 1 I (λ) 0 1 d I

6 Absorption spectroscopy using optical resonators R losses L R = reflectivity I in HR Mirror d For gases! Advantages: Long optical path length high detection sensitivity ( >10-10 cm -1 ) = n (n=number density,=abs. cross-section) High temporal (s min) & spatial (cm m) resolution High spectral resolution possible (~GHz)

7 Absorption spectroscopy using optical resonators R losses L R I in HR Mirror d + I b I a + I c = I one pass I a I = I in (1 R ) (1 L) (1 R ) +... I in (1 R ) (1 L) R (1 L) R (1 L) (1 R ) I in (1 R ) 2 R 2n (1 L) 2n n passes I n +...

8 Absorption spectroscopy using optical resonators R losses L R I in HR Mirror geometrical series 2 2n 2n in(1 ) (1 ) (1 ) n0 I I R L R L d + I b I a + I c = I 2 (1 R) (1 L) converges for R<1, L<1: I Iin R (1 L)

9 I I in 2 (1 R) (1 L) R (1 L) Lambert-Beer Absorption Losses I, I 0 : transmitted intensity with, without sample I 0 2 I 0 2 ln 4 R ( R 1) ( R 1) 2 d 2R I I Absorption losses very small. Mirror reflectivity very high (R1) 1 d I I 0 1 1R Multi pass

10 (2) Experimental Setup

11 Incoherent broadband cavity-enhanced LED absorption spectroscopy (IBBCEAS) Measures the transmission of an optically stable cavity consisting of two highly reflecting dielectric mirrors. F P M 1 Cavity M 2 Lab setup (closed path): M Lens Iris Outlet P = pressure gauge M = mirrors F = filter (bandwidth) CCD = charged coupled device detector LED = light emitting diode Inlet CCD Lens

12 Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) Measures the transmission of an optically stable cavity consisting of two highly reflecting dielectric mirrors. F M 1 Cavity M 2 Lab setup (open path): M LED Lens Iris M = mirrors F = filter (bandwidth) CCD = charged coupled device detector d R I, I 0 Reflectivity Intensity CCD Lens

13 Instrument design for FIONA (Open Path) Receiver Transmitter CCD M open path M Iris LED Achr. Telescope Power Meter Fibre AR coated loss optic for calibration of reflectivity

14 Mirror calibration with low loss optic I 0 Transmission without optic: I 0 (λ) Transmission with optic: I 1 (λ) Intensity λ I 1 Calibrated low loss of optic: L(λ) (must be known accurately) Loss Calculate mirror reflectivity: R(λ) R I I0 I 1 1 ( ) 1 L( ) Reflectivity λ λ

15 Intercomparison SAPHIR Jülich J (2007) Receiver ( NO 3 / N 2 O 5 ) Transmitter IBBCEAS

16 Intercomparison SAPHIR Jülich J (2007) ( NO 3 / N 2 O 5 ) Receiver Transmitter AR coated loss optic M M

17 Target species NO 3

18 (3) Data Analysis

19 Incoherent broadband cavity-enhanced absorption spectroscopy 1 I d I d 0 ( ) i i( ) n 0 i( x) dx 1 1 R = extinction [cm -1 ] i = cross-section [cm 2 molecule -1 ] ( abs, Ray, Mie,... ) n i = number density [molecule cm -3 ] d = effective cavity length [cm] I 0 = transmitted intensity without sample (rel. units) I = transmitted intensity with sample (rel. units) R = R R mirror reflectivity 1 2

20 Fit function for HONO retrieval (range nm) ( ) n ( ) n ( ) a a a ' ' 2 HONO HONO NO2 NO Nonlinear least-square fit parameters varied: n i (i = HONO, NO 2 ),, a j (j = 1, 2), a 3 =0 Reference spectra i convoluted for 0.5 nm resolution: HONO: J. Stutz et al., J. Geophys. Res. 2000, 105, NO 2 : J.P. Burrows et al., JQSRT 1998, 60, FIONA: Linear singular value decomposition fit. n i (i = HONO, NO 2 ), a j (j = 1 3). 1 Separate minimization.

21 (1 d I 0 R) 110 % 2 % 5 % 5 8 % Systematic Errors Measurement errors: Fit errors: Choice of fit range: ~2 % Max uncertainty of various analysis approaches: ~10 % ( 1 R) d I ±10 % < total systematic error < ±20 %

22 (4) HONO detection in Cork IBBCEAS for chamber studies

23 Simulation chamber at the Centre for Research into Atmospheric Chemistry (CRAC) 3 4m HR Mirror Holder Teflon bag Light source HONO formation: (R1) NO 2 + NO + H 2 O 2 HONO (R2) 2 NO 2 + H 2 O (+ h) HONO + HNO 3

24 R [%] T [%] Intensity [a.u.] Cross-section [10-19 cm 2 ] Intensity [a.u.] Wavelength [nm] LED spectrum Filter transmission Mirror reflectivity (R) 4 m 3 chamber 1.4 dm 3 vessel Cavity transmission spectrum (I 0 ) Region used for analysis HONO absorption J. Stutz et al. NO 2 absorption J. P. Burrows et al.

25 HONO & NO 2 Absorption Spectrum [10-7 cm -1 ] min: Gherman et al., EST, 42, 2008, HONO Wavenumber [cm -1 ] NO 2 background Wavelength [nm] HONO: ~2.7 ppbv NO 2 : ~2.4 ppbv [10-9 cm -1 ]

26 (5) Instrument for the FIONA campaign at EUPHORE FIONA = Formal Intercomparison of Observations of Nitrous Acid

27 Estimated Detection Limits Based on: Integration time of 1 min 7.5 m cavity length R = Signal-to-noise ratio of 2:1 HONO: ~ 0.25 ppbv NO 2 : ~ 0.66 ppbv Estimated Precision (from fit) HONO: <10 pptv, NO 2 : <10 pptv

28 (1 d I 0 Estimated Systematic Errors Measurement errors: Fit errors: NO 2 R) 1 % 3 % 8 % HONO 5 % Choice of fit range: ~2 % (1 R) 2 2 HONO d 2 I Overall systematic error: ca. ±10 %

29 Acknowledgement Staff at Forschungszentrum JülichJ (ICG-2) 2): Irish Environmental Protection Agency (EPA): STRIVE programme: 2008-FS-EH-2-S5 Science Foundation Ireland (SFI): STTF programme: 06/RFP/ CHP055