2013-05-05

1

Guillaume Marcotte,a,b

Patrick Ayotte,a,b

Stéphanie Pronovost,a

Patrick Marchand,a

Carine Laffon, b

Philippe Parentb

a-Département de chimie, Université de Sherbrooke, Sherbrooke, CANADAb-Laboratoire de Chimie Physique - Matière et Rayonnement,

Université Pierre et Marie Curie (Paris 06) et CNRS (UMR 7614), FRANCE

Ionization and Enhanced

Photochemistry of HNO3

Adsorbed onto Ice

NOx photochemical fluxes at polar sunrise� Wolff et al. and Dominé and Shepson have

reported unusually high NOx photochemical fluxes upon polar sunrise after the dark winter.

� This has been explained by the photolysis of nitric acid (reservoir species of NOx) that has been deposited onto the polar ice cap.

200 250 300 350 400

0

2

4

6

8

10

εε εε (

M-1/c

m)

εε εε x

10

3 (

M-1/c

m)

wavelength (nm)

1

2

Irra

dia

nce (

W/m

2/n

m)

0

5

10

Sea level

Sunlight blackbody radiation

Dominé and Shepson, Science 297, 1506 (2002).

Wolff et al. Geophys. Res. Lett. 29, 5823 (2002)

n → π*π→π*

2013-05-05

2

Polar boundary layer chemistryTable 1. Measured (or calculated) concentrations of gaseous species at

Alert at the end of April 2000, compared with values predicted by

models that include gas-phase chemistry only.

Species Value measured over snow Prediction by gas-phase chemistry

HCHO 200 pptv 70 pptv

CH3CHO 80 pptv 40 pptv

NOx 25 pptv 1 pptv

HONO 20 pptv 1 pptv

OH 0.03 pptv* 0.003 pptv

HO2 3.7 ppt* 0.9 ppt

O3 As low as 0.07 ppb 30-34 ppbv

Hg0 0.02 pptv 0.17 pptv

Dominé and Shepson, Science 297, 1506 (2002).

Nitrate photolysis within/onto ice

H NO3

H NO3

H NO3 H+ NO3-

Ionic

DissociationSticking

X

H NO3 H+ NO3-

+

+

Differences between absorbed and adsorbed nitric acid/nitrates could be:

Symmetry breaking (dipole forbidden transition)

Shifts in Absorption spectrum (better overlap with solar irradiance)

Caging effect (ΦΦΦΦ=0.1% at 270K)

pKa of adsorbed HNO3 (surface segregation)

NO2 O-+

NO2 O-+

hν

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3

Thermodynamics of HNO3 dissociation

�High T favor recombination1,3,4

�Poor solvation (as on surfaces1,2,4,5 or clusters3) favor molecular nitric acid

HOW ABOUT HNO3 AT THE SURFACE OF ICE, OR OF THE QLL AT LOW TEMPERATURE?1.Wang, Bianco and Hynes, JPCA, 113 (2009) 12952.Ardura and Donaldson, PCCP, 11 (2009) 8573.Miller and Gerber, PCCP,10 (2008) 10914.Shamay, Buch, Parrinello and Richmond, JACS, 129 (2007) 129105.Bianco, Wang and Hynes, JPCA, 111 (2007) 11033

3 4 5-10

-8

-6

-4

-2

0

2

4

pK

a

1000/T (1/K)

HF

HNO3

HCl

HBr

HNO3 at the surface of nitric acid

SFG suggests nitric acid can be found in two different molecular forms at the surface of water as shown below:

1600 1640 1680 17200

10

20

30

40

40% HNO3 in water

16% HNO3 in water

inte

ns

ity

wavenumber (cm-1)

PES indicates HNO3 is mostly dissociated at the surface of liquid water, behaving as a somewhat weaker acid.

T≈ 295K

Soule et al., JPCA, 111 (2007) 3349. Lewis et al., JPCC, 115 (2011) 21183.

T≈ 277K

2013-05-05

4

Outline

�Introduction and motivation

�RAIRS and NEXAFS of thin molecular solid films

�Adsorption, absorption and dissolution of HNO3

�Enhanced photolysis of surface-bound nitrates

�Summary and conclusions

QMS

Beam source 1

(BS1)

FTIRPt(111)

T = 20 - 1300 K

Θrot

Beam source 2

(BS2)

Ion sputter gun

(CI)

Nd:YAG/MOPO

220-1800 nm

10Hz, 8ns

P~1x10-10 torr

Molecular beam studies of ice chemistry

2013-05-05

5

Pt(111)

nFilm(ω)+ikFilm(ω)

nPt(ω)+ikPt(ω)

Reflection-absorption infrared spectroscopy of thin molecular solid films

• Reflectance spectroscopy : refraction, absorption, and interference effects from multiple reflections.

• Classical optics model: Iteratively solving Fresnel equations for stratified films.

• Compare specimens with similar optical thickness.

Optical effects in RAIRS

Vibrational signature for HNO3 and NO3-

Dissociation of HNO3-

can be deduced from

characteristic features in

vibrational spectra as

shown by Ritzhaupt and

Devlin:

•HNO3: large feature at

1750 and 950 cm-1

•NO3-: Strong doublet

between 1300-1500 cm-1

Ritzhaupt and Devlin, J.P.C., 95 (1991) 90

800 1000 1200 1400 1600 1800 2000

10

20

NO3

-

HNO3

Wavenumber (cm-1)

0

20

40

Ab

so

rba

nc

e (

x1

0-3)

2013-05-05

6

http://www.theochem.kth.se/research/xspectra/index.htmlhttp://www.e20.ph.tum.de/index.php?id=nexafs&type=98

NEXAFS spectroscopy in a nutshell

Fixed energy X-rays

TunableX-rays

A

5ML HNO3@95ML H2OT = 80K

Synchroton TEY

100ML HNO3

T = 80K

Synchroton TEY

395 400 405 410 415 420 425 430 435

0.0

0.2

0.4

Inte

nsity (

arb

. un.)

525 530 535 540 545 550 555 560

0.0

0.2

0.4

Photon Energy (eV)

N1s

O1s

π* σ *ΗΟ−ΝΟ2

NEXAFS spectra for NO3- and HNO3

2013-05-05

7

NO3- D3h

LUMO

HOMO

IP

a2’

a1’

HNO3 Cse’’e’’

a2’

a2’’e’

a’

a’

a’

a’a’

a’’

a’’

a’’a’’

//

N 1s N 1s

Energy

a1’ a’

NEXAFSUV

//

NEXAFS spectra for NO3- and HNO3

NEXAFS

//

Outline

�Introduction and motivation

�RAIRS and NEXAFS of thin molecular solid films

�Adsorption, absorption and dissolution of HNO3

�Enhanced photolysis of surface-bound nitrates

�Summary and conclusions

2013-05-05

8

Cryogenic nitric acid solutions: RAIRS

� Interferometry shows that films have same optical thickness

� Fresnel analysis yields optical parameters (n and k)

� Spectral signature for dissociation are Zundel continuum and peaks between 1000 and 2000 cm-1.

FTIR

H2O HNO3

1000 2000 3000 4000 5000

-2

0

2

0 5

10 15

15 20

30 40

40 50

60 80

100

Wavenumbers (cm-1)

0

2

4

A

bso

rban

ce (

x1

0-2)

0

2

4

HNO3

mole

fractions

T = 45K

XML HNO3 @ (50-x)ML H2OT = 45K

800 1000 1200 1400 1600 18000

2

40 50

60 80

100

Marchand and Ayotte - Figure 2

0 5

10 15

Wavenumbers (cm-1)

15 20

30 40

Ab

sorb

an

ce (

x1

0-2)

T = 45K

� Interferometry shows that films have same optical thickness

� Fresnel analysis yields optical parameters (n and k)

� Spectral signature for dissociation are Zundel continuum and peaks between 1000 and 2000 cm-1.

H2O

FTIR

HNO3

XML HNO3 @ (50-x)ML H2OT = 45K

Cryogenic nitric acid solutions: RAIRS

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1000 2000 3000 4000 5000

0.0

0.5

1.0

0

1

2

Experimental spectrum

Fresnel fit

Ab

so

rban

ce (

x10

)

Wavenumber (cm-1)

Inte

ns

ity

k

n

Fresnel analysis

Model input parameters:

n∞: obtained from Foster et al.

h: obtained from TPD or IR interferometry

θθθθIR : obtained iteratively to fit baseline where k=0

Pt(111)

nFilm(ω)+ikFilm(ω)

nPt(ω)+ikPt(ω)

7000 6000 5000 4000 3000 2000 1000

-0.01

0.00

0.01

0.02

T = 45K

Ab

sorb

an

ce

Wavenumbers (cm-1)

nHNO3/10H

2O

n=0 n=5

n=1 n=6

n=2 n=7

n=3 n=8

n=4

1800 1600 1400 12000.00

0.01

0.02

HNO3 adsorption on ASW: RAIRS

� Interference effects could mask weak Zundel continuum

� Low temperature and slow transport kinetics provide surface specificity

� Fresnel analysis of stratified films is made possible using optical constants derived from homogeneous mixed binary films.

HNO3

XML HNO3/10ML H2OT = 45K

2013-05-05

10

Experimental result Fresnel Model

1000 2000 3000 4000

0.0

0.5

1.0

1.5

2.0

ASW

1ML HNO3

2ML HNO3

3ML HNO3

4ML HNO3

5ML HNO3

Ab

so

rban

cex

100

Wavenumber (cm-1)

T=45K

Ab

so

rba

nce

x1

00

W aven u m ber (cm-1

)

10 00 2 00 0 3 000 400 0

0 .0

0 .5

1 .0

1 .5

2 .0

A

bs

orb

an

ce

x1

00

W aven u m ber (cm-1

)100 0 20 00 30 00 4 00 0

0.0

0 .5

1 .0

1 .5

2 .0

Then

Difference spectra: Experimental vs model

45K 80K

1st monolayer 20% 20%

2nd monolayer 50% 30%

3rd monolayer 80% 50%

4th monolayer 100% 80%

5th monolayer 100% 100%

1200 1600 2000

0

1

Wavenumber (cm-1

)

0

1

0

1

2

0

1

2

A

bs

orb

an

ce

x10

3

fourth monolayer

third monolayer

second monolayer

first monolayer

- =T=45K

2013-05-05

11

XML HNO3/100ML H2OT = 80K

Synchroton TEY

HNO3 adsorption onto ice by NEXAFS

HNO3

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Exposure (L)

HNO3

NO-

3

Total exposure

Langmuir fit

θ/θ

sat

400 405 410 415 420

0

1

2

0.19 L

0.38 L

0.56 L

0.83 L

525 530 535 540 545 550 555 560 565

0

1

2

3

O K-edge

Tota

l e

lectr

on y

ield

(a

rb.

un.)

Photon Energy (eV)

N K-edgeT = 80 K

� At low coverage, even at low temperature, HNO3 is extensively dissociated at the ice surface. As the HNO3 coverage increases, molecular HNO3 appears at the surface of ice.

� Metastable molecular HNO3 dissociates onto ice upon heating above 120K.

HNO3 adsorption onto ice by NEXAFS

0.4ML HNO3/100ML H2OT = 22.5K→150K

Synchroton TEY

400 410 420

0

1

2

3

4

5

To

tal e

lectr

on

yie

ld (

arb

. u

n.)

Photon Energy (eV)

22.5 K

80 K

100 K

120 K

140 K

150 K

0.57 L HNO3

on ice

0 30 60 90 120 150

0

20

40

60

80

100 NO

-

3

HNO3

To

tal co

nte

nt

(%)

Temperature (K)

2013-05-05

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Outline

�Introduction and motivation

�RAIRS and NEXAFS of thin molecular solid films

�Adsorption, absorption and dissolution of HNO3

�Enhanced photolysis of surface-bound nitrates

�Summary and conclusions

• Photolysis rate vary by more than one order magnitude over the 2≤pH≤13 range.

•For strongly acidic solutions (pH≤4), HONO becomes the majority photolysis product.

Nitrates photolysis : aqueous solutions

225 250 275 300 325 350 375 400 425 450

0

5

10

15

20

25

30

35

40

45

50

55

60

NO-3

NO-2

HONO

Wavelength (nm)

Mo

lar

extinction c

oeff

icie

nt (M

-1cm

-1)

250 300 350 400 4500,00

0,02

0,04

0,06

0,08

0,10

Reaction time

0.0h

0.5h

1.0h

1.5h

2.0h

2.5h

3.0h

3.5h

4.0h

Ab

so

rba

nc

e

Wavelength (nm)

250 300 350 400 4500,00

0,02

0,04

0,06

0,08

0,10Reaction time

0.0h

0.5h

1.0h

1.5h

2.0h

2.5h

3.0h

3.5h

4.0h

Ab

so

rba

nc

e

Wavelength (nm)

NO3-

NO2-

NO3-

HONO

T = 298K T = 298K

2013-05-05

13

Nitrate photolysis rates in KNO3 aqueous solutions agree with calculations

based on NO3- photolysis physical constants and Xe lamp specifications :

•0th order rate constant: k0 = 0,035 ± 0,002 Mh-1

•1st order rate constant: k1 = 0,036 ± 0,009 h-1

•Calculated rate constant: k1 = 0,054 ± 0,005 h-1

Nitrates photolysis : aqueous solutions

0 1 2 3 40,000

0,001

0,008

0,010

C=0.009*(1-exp(-t/28))

C=3.15x10-4*t

Co

nc

en

tra

tio

n (

M)

Time (h)

C=0,01-3.5x10-4*t

C=0.01*exp(-t/28)

UV-visT = 298K NO3-

NO2-

0.45ML HNO3/20ML H2OT = 90K

FTIR

Xe lamp ~10W

HNO3 versus NO3- photolysis: RAIRS

8ML HNO3

T = 90K

Xe lamp ~10WFTIR

1600 1400 1200 1000

0.01

0.02

0.03

Absorb

ance

Wavenumbers (cm-1)

Irradiation time

0 min

15 min

30 min

60 min

120 min

315 min

1500 1400 1300 1200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Ab

sorb

an

ce (

x10

-3)

Wavenumbers (cm-1)

Irradiation time

0 min

210 min

2013-05-05

14

•Surface-bound nitrates display photolysis kinetics that

are up to 4-6x faster than those dissolved within the bulk.

Nitrates photolysis : surface vs bulk NO3-

0 50 100 150 200 250 3000

10

20

30

40

50

60

70

80

90

100

Su

rviv

al ra

te (

%)

Time (min)

Surface 70 K Mixture

Surface 100 K Mixture

Surface 120 K Mixture

HNO3

35 K 70 K 90 K 100 K 120 K

0,00

-0,05

-0,10

-0,15

-0,20

-0,25

-0,30

-0,35

-0,40

Initia

l nitra

te d

estr

uction r

ate

(%

/min

)

RAIRS NEXAFS(x6)

Surface

Mixture

Temperature

0.45ML HNO3/20ML H2OT = 90K

FTIR

Xe lamp ~10W

8ML HNO3

T = 90K

Xe lamp ~10WFTIR

Nitrates photolysis : surface vs bulk NO3-

•1st moment of the NO3- band reveal enhanced photolysis rate for nitrates

that display largest gap between their nominally degenerate asymmetric NO

stretching bands (i.e., symmetry most perturbed from gas-phase D3h)

1500 1400 1300 1200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Ab

sorb

an

ce

(x1

0-3)

Wavenumbers (cm-1)

Irradiation time

0 min

210 min

0 50 100 150 200 250 300 350 400

80

85

90

95

100

105

110

115

120

125

130

135

140

Ce

nte

r to

ce

nte

r g

ap

(w

ave

nu

mb

ers

)

Time (min)

Surface 70 K Mixture

Surface 100 K Mixture

Surface 120 K Mixture

2013-05-05

15

Symmetry breaking in NO3-: vibrational spectroscopy

395 400 405 410 415 420 425 430

0

1

2

3

4

5

6

7

Inte

nsity (

arb

. un.)

Photon Energy (eV)

Pure HNO3

30 min UV

60 min UV

90 min UV

120 min UV

150 min UV

180 min UV

210 min UV

240 min UV

270 min UV

Nitric acid N K edge NEXAFS vs photolisis time

40ML HNO3

T = 20K

Xe lamp ~10W

HNO3 and NO3- photolysis by NEXAFS

Synchrotron TEY

400 405 410 415 420 425 430

0

1

2

3

4

16% HNO3 @ ASW N K egde versus photolysis time

inte

nsity (

arb

.un

.)

Photon Energy (eV)

Irradiation time

0 min

30 min

60 min

90 min

120 min

150 min

180 min

210 min

240 min

15ML HNO3@100 ML H2OT = 90K

Xe lamp ~10W

Synchrotron TEY

2013-05-05

16

400 405 410 415 420 425 430

0.0

0.5

1.0

1.5

0.15L HNO3/ASW N K edge versus photolysis time

inte

nsity (

arb

.un.)

Energy (eV)

Irradiation time

0 min

30 min

60 min

90 min

120 min

150 min

180 min

210 min

240 min

270 min

Nitrates photolysis onto ice by NEXAFS

0.07ML HNO3/100ML H2OT = 20K

Synchroton TEY

Xe lamp ~10W

0 60 120 180 240 300

0

10

20

30

40

50

60

70

80

90

100

110

100L HNO3 30K 90K

0.2L HNO3/100L H

2O 30K 90K

5L HNO3@100L H

2O 30K 90K

Surv

ival ra

te (

%)

Time (min)

•Surface-bound nitrates created

by dosing HNO3 onto ASW

display photolysis kinetics that

are faster than those dissolved

within the bulk of ASW.

Summary and Conclusions� On ASW, nitric acid mixes, dissolves and dissociates in

interfacial layers yielding an amorphous NAT superficial coating: molecularly adsorbed HNO3 is metastable.

� At higher temperatures (T>100K), dissolution and uptake of nitric acid by ASW are observed to be faster and more extensive.

� Photolysis rates for nitric acid/nitrates adsorbed onto ASW are significantly faster than that for those dissolved within ASW.

� Nitrate anions that display greater distortions from ideal gas phase D3h symmetry also appear to be photolyzedfaster.

2013-05-05

17

Acknowledgements

Sylvain Plessis and François Porzio for developing Fresnel model.

Discussions with Céline Toubin (Lille) and Casey Hynes (ÉNS/UCBoulder).

The TEMPO (Soleil, Paris) and BACH (Elettra, Italy) beamline staff.

0.00

0.05

0.10

0.15

0.20

0.25

Oscill

ato

r str

en

gh

t

Oscill

ato

r str

eng

ht

0.00

0.05

0.10

0.15

0.20

0.25

200

300

200

300

Tra

nsitio

n e

nerg

y (

nm

)

Tra

nsitio

n e

nerg

y (

nm

)

35 30 25 20 15 10 5

1300

1400

1500

Fre

qu

en

cy (

cm

-1)

Out-of-plane deformation (o)

C3v

D3h

C2v

0.00 0.03 0.06 0.09

1300

1400

1500

Fre

qu

en

cy (

cm

*1)

N-O stretch (10-10

m)

Symmetry breaking in NO3-: degeneracy and selection rules

2013-05-05

18

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0

50

100

150

200

Bianco

Bianco

Bianco

Bianco

Gap

(cm

-1)

∆RN-O

(10-10

m)

surface 70 K

mix 70 K

Hudson CaNO2

+

(aq)

Waterland 3 H2O

Waterland 2 H2O

Sullivan NAT

Fernandez NAT

Bianco

Hudson Ca2+

Symmetry breaking in NO3-: degeneracy and selection rules