FTUV and FTIRHow to calibrate sigma?How to calibrate sigma?

• Determine the absorption cross section (sigma)• Error sources in determining sigma• Limitations in Lambert-Beer’s Law

• Resolution Effects• Resolution Effects• Saturation Effects

• ApplicationsNov 20 2008Nov 20 2008CHEM 5161

Methods to measure the b ti ti ( i )absorption cross section (sigma)

• Pressure calibration• Kinetic calibration• Cross calibration with other method• Calculation from first principlesCalculation from first principles

Glyoxal (CHOCHO) background• Unstable molecule: Cuvet spectra not practicle• Wavelength calibration is not well known• Wavelength calibration is not well known • Only low resolution spectra are available

• instrument function unknowninstrument function unknown• limits sensitivity• limits selectivity (NO2, Xe-lamp, Fraunhofer lines)limits selectivity (NO2, Xe lamp, Fraunhofer lines)

• Nonlinearities have been reported (but not yet explained)• up to 50% scatter in cross section valuesp %• Intercalibration of DOAS, FTIR, TDL, CDR not yet feasible

⇒ High resolution sigma (UV/Vis and IR)⇒ Good wavelength calibration

Vis absorption cross section1E 181E-18

1E-19

on [c

m2 ]

1E-20

IUP HRross

sec

ti

1E-21

IUP convoluted MPI NCARPlum et al

C

380 400 420 440 460 480 500 5201E-22

Plum et al. Zhu et al.

380 400 420 440 460 480 500 520

Wavelength [nm]

Introduction GlyoxalOH

• C2H2O2 or CHOCHO• smallest α-dicarbonyl-type compound• Natural sources: fermentation (beer wine yogurt products)

O H

• Natural sources: fermentation (beer, wine, yogurt products), biomass burning, BVOC oxidation, (oceans ?)

• Anthropogenic sources: emissions from mobile sources, AVOC oxidation, (energy sector, industrial processes ?)

• In urban air: Airborne AVOC oxidation source >> direct emissions from mobile sources (ca. 70% aromatics, 20% ( ,alkenes, 10% acetylene, virtually no glyoxal from alkanes)

• Residence time in the atmosphere: <1.2 hoursM j h l i h t l i• Major gas-phase loss process is photolysis and OH-reaction (60% / 40%). Source for H2, CO, HCHO and HO2-radicals.

Experimental setupLi ht• Light sources:– XBO 150W Xe-arc lamp– Glowbar

• FTS optics (Table)• Detector (Table)

Experimental conditions• White-cell coupled to the Bruker FTS 120 HR

absorption path: L = 163 cmabsorption path: L 163 cmGlyoxal pressure: 0.005 – 13 mbar (MKS Baratrons)

• Low resolution measurements:short integration times (10 min)Linearity of Lambert-beers law was demonstratedIR and UV integral cross-sections were simultaneously determinedIR and UV integral cross sections were simultaneously determined

• High resolution measurements:long integration times (12 hours)IR spectra: unapodized spectral resolution of 0.009 cm-1

UV spectra: unapodized spectral resolution of 0.06 cm-1

high S/N levelhigh S/N level• Wavenumber accuracy checked from NO2 spectra, linked to I2 spectra

UV/Vis absorption cross section1E-18

1E-19

on [c

m2 ]

1E-20

ross

sec

tio

1E-21

Cr

1E 2238000 36000 34000 32000 30000 28000 26000 24000 22000 20000

1E-22

Wavenumber [cm-1]

Corrections• Lamp drift F=1.01-1.25

(error <1-10%)• Leakage F=1.04 (<1%)• Photolysis F=1.02 (0.2%)• Wall deposition F=1.01 (0.2%)

Characterization:• Purity (better 99%)• HCHO formation (<1%) • Reproducible Synthesis (0.2%)• Interpolation method (<2%)

Example: 13 µbar glyoxal measured to 1.5% precision!

• Polimerization (<0.5%)• Column density (<0.5%)

p⇒ error estimate: <5% UV / <3% IR⇒ not better than 2-5 10-22 cm2

Linearity of Absorptiony p

35

40

ts]

IR-Absorption ABS(IR) = -0.007 (0.014) + 67.7 (0.3) x X

ts]

100

120

UV-absorption (pure glyoxal)

25

30

n [a

rb. u

nit

n [a

rb. u

nit

80

100( g y )

ABS(UV) = 0.04 (0.04) + 291.2 (1.1) x X UV-absorption (1013 mbar of N2)

15

20

Abso

rptio

n

Abso

rptio

n

60

5

10

15

nteg

ral U

V-A

nteg

ral I

R-A

20

40

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

5 InIn

Glyoxal Pressure [mbar]

0

Glyoxal Pressure [mbar]

Modelling deviations from Lambert Beers LawLambert-Beers Law

UV-vis – 0.25 nm FWHM Infrared – 1cm-1 FWHM

U t i ti i th t f l l l i t fUncertainties in the spectroscopy of glyoxal explain part of the scatter among available literature yields

Glyoxal absorption cross sectionO

O H

H

Table 4: Comparison with available literature values.

Reference Spectral resolutiona

UV-region (350 nm)

Vis-region (440 nm)

Integral σ-ratio

fit-coef.c shiftc

[10-20 cm2] [10-20 cm2] [rel.units] [rel.units] [nm]

UV / vis spectral ranges this work ~0.001 105 1.00 1.00

0.01 98

0.1 0.38 / 0.41b 53

1.0 0.39 26

SUNY [22] 0.01 0.63

SUNY [25] 0.01 0.51

SUNY [26] 0.01 46 0.48 0.44 0.56

EUPHORE [30] 0.17 33 0.89 0.86 0.10

MPI [24] 0.25 0.36 30 0.94 0.87 0.39

NCAR [23] 0.6 0.36 17 0.93 0.69 -0.03

SAPRC [15] n.n. 0.01 25 0.83 0.64 1.08

IR spectral range NCAR [23] 1 cm-1 1.05

NIES [44] 1 cm-1 0.80

FORD [34] 0.25 cm-1 0.98

BUGH [45] 1 cm-1 0.74

a FWHM value in nm, unless noted otherwise. b Notation: σ350nm / σ351nm. For ref. [25] sigma at 351 nm needs be compared. c Least squares fit, after adjusting for resolution (see section 3.1).

Volkamer et al., J. Photochem. Photobiol. A: Chemistry 172 (2005) 35.NASA recommended UV-vis absorption cross section (JPL 06-02).

Sample applicationsSample applications

1. UV spectra: glyoxal photolysis 2 First direct detection in the atmosphere2. First direct detection in the atmosphere3. Global maps from satellite4. Mechanism development of hydrocarbonsp y5. Source apportionment of HCHO6. Ocean sources of hydrocarbons

Glyoxal photolysis300 350 400 450300 350 400 450

1x10-19

2x10-19

3x10-19

0.5

1.0

2x1014

4x1014

6x1014

antu

m Y

ield

[rel

. uni

ts],

rptio

n C

ross

Sec

tion

[cm

2 ]

Quantum-Yield

Actin

ic P

hoto

n Fl

ux[p

h cm

-2 s

-1 n

m-1]

sigma

Actinic Photon Flux

Field measurements of J(Glyoxal) using spectral-

00.0300 350 400 450 500

0

1.0x10-5

1.5x10-5

Qua

Abso

r

sigma

x Fl

ux x

QY

s-1 n

m-1]

370-430nmPhotolysis: 20%

290-365nm Photolysis: 80%

radiometry requires good knowledge of sigma.

300 350 400 4500.0

5.0x10-6

sigm

a x

[ph

Wavelength [nm]300 350 400 450

150380 430nm330 375290 325

1.2x10-4 J w/ sigma IUPJ w/ sigma MPI

0 0

5.0x10-6

1.0x10-5

1.5x10-5

50

100

150380-430nmPhotolysis: 20%Error: 18%

330-375nm Photolysis: 40%Error: 60%

290-325nm Photolysis: 40%Error: 22%

sigm

a x

Flux

x Q

Y

[ph

s-1 n

m-1]

40% of overall J-value

sigm

a va

riabi

lity

[%],

sigm

a [r

el. u

nits

]

6 0 10-5

8.0x10-5

1.0x10-4

J w/ sigma MPI

yoxa

l) [s

-1]

300 350 400 4500.0 0

2.0x10-6

3.0x10-6

4.0x10-6

50

100

40% (available literature data)

ma

varia

bilit

y [%

]lin

es th

is s

cale

sigm

a x

Flux

x Q

Y [p

h s-1

nm

-1]

6% w/ sigma IUP

12% w/ sigma MPI

2.0x10-5

4.0x10-5

6.0x10

difference: 8%uncertainty: 6% (3-sigma)

J(gl

300 350 400 4500.0

1.0x10-6

0

sigm al

l s

wavelength [nm]

0 20 40 60 80 1000.0

SZA [degrees]

Mechanism development of VOCCH3

CH3

OH

CH3

CHO

CH3CH

O2

OH

CH

< 10 %> 90 %

BenzaldehydeCH3

OH

OO

CH3

OH

OO.

CH3

OH

O

OCH3 O

. .

O

CH3

CH3

OH

CH3

OH

CH3

OH

-dicarbonyls,αi.e. Glyoxal

OOIsomers of

Cresol

Flow systems

Simulation chambers

Volkamer et al. J. Phys. Chem. A 2001, 105, 7865-7874“The uncertainty in sigma is limiting mechanistic conclusions.”

DOAS measurement of Glyoxal as an indicator for fast VOC chemistry in urban airy

Volkamer R, Molina LT, Molina MJ, Shirley T, Brune WH (2005)GRL, 32, L08806.

http://dx.doi.org/10 1029/2005GL02261610.1029/2005GL022616

SCIENCE, June 3 2005, VOL 308, 1379http://www.sciencemag.org/

First direct (spectroscopic) measurement of glyoxal in the atmosphereglyoxal in the atmosphere

DOAS advantages:• Overlapping absorptionOverlapping absorption

structures due to different species can be separated

• Species not anticipated can be measured

• Warning against unexpected absorbers (residual)I i t ti• Immune against continuous (broad band) extinction due to e.g. aerosol or molecules

• High sensitivity since many

Direct vehicle emissions are a minor glyoxal source

High sensitivity, since many trace gas lines (bands) are used.

Direct vehicle emissions are a minor glyoxal source compared to VOC photochemistry !

Volkamer et al. (2005) GRL, 32, L08806.

Emitted and photochemical HCHO

bi

VOC chemistry Glyoxal

?Ambient HCHO

Emissions CO

?

80 E

Photo % Emis % Backg %

12

14 B

CHOCHO20Easter HCHO measured

CHOCHO model

30

40

50

60

70

%6

8

10

12

HC

HO

ppb

v

CHOCHO CO Const

10

15

20

HC

HO

ppb

00:00 06:00 12:00 18:00 24:000

10

20

30

LST

00:00 06:00 12:00 18:00 24:000

2

4

LSTApr 19 Apr 19 Apr 20 Apr 20 Apr 210

5

H

Garcia et al. (2006) ACP

Anthropogenic vs. biogenic glyoxal sourcesGlobal source of glyoxal:• 45 – 83 Tg/yr• 17 % from C2H2

• 70% biogenic VOC• ocean sources ?

Global source of a dicarbonyls:Global source of a-dicarbonyls: 185 Tg/yr

A th i ll t i dAnthropogenically triggered aerosol loss could determine 75 –95% of the atmospheric fate of CHOCHO

MODIS Chlorophyll-a, 2005 (ocean color)

CHOCHO

Volkamer et al. 2007 Myriokefalitakis et al. 2008Myriokefalitakis et al. 2008 Fu et al. 2008

CU Ship MAX-DOAStarget gases: CHOCHO, IO (OIO, I2, BrO, HCHO, NO2, SO2, O4)

SCIAMACHY TM4 modelCHOCHO

Science questions:q• Is the Pacific Ocean a source for halogens?• Is Glyoxal (CHOCHO) over the oceans a satellite measurement artifact? CHOCHO directly confirmed !=> Missing ocean sources for hydrocarbons in models !