An Unknown Source of An Unknown Source of Atmospheric OHAtmospheric OH

Group Project - Spring ‘08Group Project - Spring ‘08

Atmospheric Chemistry, Dr. Atmospheric Chemistry, Dr. WangWang

Patrick, Shannon, CharlesPatrick, Shannon, Charles

BackgroundBackground

OH is the primary oxidant in the OH is the primary oxidant in the daytime tropospheredaytime troposphere

Field observations suggest that there Field observations suggest that there is a missing source of OH not being is a missing source of OH not being accounted for in modelsaccounted for in models

Major Primary Source of Major Primary Source of Atmospheric OHAtmospheric OH

Photolysis of OPhotolysis of O33 in presence of water in presence of water vaporvapor

O3 + hν O2 + O(1D) J(O3)

O(1D) + H2O 2OH k2

O(1D) + M O(3P) + M k3

ROH = 2J(O3)[O3]/(1+ k3[M]/k2[H2O])

λ Dependent Lifetime (λ Dependent Lifetime (ττradrad))

Since 1997 NO2 has been investigated as the missing source.

Quenching of NOQuenching of NO22* *

M = N2, O2, or H2ONO2 * + M NO2

Total quenching rate = 7.1 x 108 sec-1 resulting in NO2* lifetime of 1.4 ns.

Quenching rates for N2,O2, and H2O: 2.7E -11, 3.0E -11,1.7E-10 cm3 molec-1 s-1

OH from NOOH from NO22**

ΔH = - 25 kcal/mol Thermodynamically feasible!

(calculation based on 65 kcal/mol of energy over that of ground state NO2 – resulting from absorption of 440 nm)

NO2 * + H2O OH + HONO

Rate of OH ProductionRate of OH ProductionNO2 + hν NO2 * J(NO2)

NO2* + M NO2 k4

NO2 * + H2O OH + HONO k5

ROH = J(NO2)[NO2]/(1+ k4[M]/k5[H2O])

The Search Began…The Search Began… 1997 - Crowley (430-450 nm):1997 - Crowley (430-450 nm):

NO2 + hν NO2 *

NO2* + hν NO2**

NO2** NO + O(1D)

O(1D) + H2O 2OH

However, in the troposphere the 2 photon process will be inefficient due to low photolysis intensities and rapid collisional quenching of NO2*.

A significant source of OH?A significant source of OH?

NO2 * + H2O OH + HONO k5

k5 = 1.2 x 10 -14 cm3 molec-1 s-1

ROH = J(NO2)[NO2]/(1+ k4[M]/k5[H2O])

= 3 x 103 cm-3 s-1 (upper limit)

ConclusionConclusion

A 2 photon process leads to OH via O(A 2 photon process leads to OH via O(11D)D) No OH production observed at 532 nmNo OH production observed at 532 nm IfIf the reactivity of NO the reactivity of NO22* is representative * is representative

of the entire range (410-600) of the NOof the entire range (410-600) of the NO22 absorption spectrum, an upper limit of absorption spectrum, an upper limit of 2% to formation of OH via rxn 4 in 2% to formation of OH via rxn 4 in troposphere when solar zenith angles troposphere when solar zenith angles are highare high

ExperimentalExperimental Design for Design for ControlledControlled HONO HONO

StudiesStudies

Attenuated Total Reflectance – Long Path Attenuated Total Reflectance – Long Path Fourier Transform Infrared Spectroscopy = Fourier Transform Infrared Spectroscopy =

ATR-FTIRATR-FTIR

BackgroundBackground The idea is to describe simultaneously both the The idea is to describe simultaneously both the

gas-phase and adsorbed reaction products in gas-phase and adsorbed reaction products in water films found both in laboratory systems and water films found both in laboratory systems and the tropospheric boundary layerthe tropospheric boundary layer

This experiment: coupling a long path Fourier This experiment: coupling a long path Fourier transform infrared spectroscopy for gas-phase transform infrared spectroscopy for gas-phase components with an attenuated total reflectance components with an attenuated total reflectance probe to follow thin film surface chemistryprobe to follow thin film surface chemistry

This is then applied to the heterogeneous This is then applied to the heterogeneous hydrolysis of NOhydrolysis of NO22 and H and H22OO

The camber gas is replaced and photolysis of the The camber gas is replaced and photolysis of the surface products proceeds, HONO and HNOsurface products proceeds, HONO and HNO33, is , is meassured at two different wavelengths.meassured at two different wavelengths.

Schematic Diagram (ATR-FTIR)Schematic Diagram (ATR-FTIR) Cylindrical borosilicate ShellCylindrical borosilicate Shell Stainless steel endplatesStainless steel endplates Quartz tube lamp housing Quartz tube lamp housing High surface to volume ratioHigh surface to volume ratio Halocarbon wax to minimize reactivityHalocarbon wax to minimize reactivity

Environmental RegulationEnvironmental Regulation Relative humidity and Relative humidity and

temperature are monitored temperature are monitored inside the reaction chamber inside the reaction chamber using a Caisala gauge (HMP-using a Caisala gauge (HMP-238)238)

Pressure in the range from Pressure in the range from 10^3 to 0.1 Torr are measured 10^3 to 0.1 Torr are measured using a Leybold diaphragm using a Leybold diaphragm gauge (Ceravac CTR-91) and gauge (Ceravac CTR-91) and from 750 to 10^-4 Torr are from 750 to 10^-4 Torr are measured using a Leybold measured using a Leybold Pirani Gauge (Thermovac TTR-Pirani Gauge (Thermovac TTR-216)216)

A VOC-Edwards A VOC-Edwards iQDP80/iQMB250 dry pump is iQDP80/iQMB250 dry pump is used to evacuate the cell while used to evacuate the cell while minimizing the backflow of minimizing the backflow of organics associated with oil organics associated with oil pumpspumps

SpectrometerSpectrometer ThermoNicolet Nexus 670 FTIR ThermoNicolet Nexus 670 FTIR

was used to record the infrared was used to record the infrared spectra of the gas phase spectra of the gas phase products.products.

The IR beam is directed from The IR beam is directed from the FTIR via a set of transfer the FTIR via a set of transfer optics into the reaction cell optics into the reaction cell through a ZnSe window.through a ZnSe window.

Multiple reflections achieved Multiple reflections achieved using White Cell Optics, created using White Cell Optics, created a path length of 47.5ma path length of 47.5m

The mirrors in the cell were gold The mirrors in the cell were gold coated and protected with a coated and protected with a thin layer of SiO.thin layer of SiO.

The exiting infrared beam was The exiting infrared beam was focused using a 30° off-axis focused using a 30° off-axis parabolic mirror onto a liquid parabolic mirror onto a liquid nitrogen cooled mercury nitrogen cooled mercury cadmium telluride (MCT) cadmium telluride (MCT) detector.detector.

Attenuated total reflectance ProbeAttenuated total reflectance Probe

The ATR proble is a hollow The ATR proble is a hollow wavequide probe (Axiom wavequide probe (Axiom Analytical ATR-FTIR Probe Analytical ATR-FTIR Probe DRP-100) inserted through a DRP-100) inserted through a vacuum-tight fitting into the vacuum-tight fitting into the reaction chamberreaction chamber

The ATR-FRIR accesssory The ATR-FRIR accesssory base is placed into the base is placed into the sample compartment of a sample compartment of a second FTIR spectrometer. second FTIR spectrometer. The IR beam in the sample The IR beam in the sample compartment is directed into compartment is directed into the infrared transmitting ATR the infrared transmitting ATR crystal and undergoes total crystal and undergoes total internal reflection bouncing internal reflection bouncing nine time within the crystalnine time within the crystal

Infrared Crystals / IrradiationInfrared Crystals / Irradiation Two infrared crystals were Two infrared crystals were

used: AMTIR (Ge33As12Se55) used: AMTIR (Ge33As12Se55) and silicon. AMTIR has a high and silicon. AMTIR has a high IR throughput and wide spectral IR throughput and wide spectral range (6000cm-1-700cm-1) but range (6000cm-1-700cm-1) but the silicon crystal does not the silicon crystal does not transmit below 1500cm-1 but is transmit below 1500cm-1 but is more representative of the more representative of the chamber construction. It is chamber construction. It is used to verify AMTIR.used to verify AMTIR.

Irradiation of the cell is Irradiation of the cell is produced by using low pressure produced by using low pressure mercury lamps. The G30T8 as mercury lamps. The G30T8 as the strongest line at 254nm and the strongest line at 254nm and the black lamp, the the black lamp, the F30T8/350BL has a broad range F30T8/350BL has a broad range from 300 to 400nm.from 300 to 400nm.

ChemicalsChemicals Nitric oxide (Matheson, Nitric oxide (Matheson,

99%) purified by liquid 99%) purified by liquid nitrogen trap to remove nitrogen trap to remove impurities such as NOimpurities such as NO22 and and HNOHNO33..

Nitrogen dioxide Nitrogen dioxide synthesized by reaction synthesized by reaction purified NO with excess purified NO with excess oxygen (Oxygen Service oxygen (Oxygen Service Company, 99.993%) for at Company, 99.993%) for at least 2 hours after which least 2 hours after which NONO22 is condensed at 195K is condensed at 195K and the oxygen is pumped and the oxygen is pumped off.off.

Nitrogen (Oxygen Service Nitrogen (Oxygen Service Company, 99.999%)Company, 99.999%)

Air Air Deuterated waterDeuterated water

NO2 Experimental ProceduresNO2 Experimental Procedures NN22 was chosen instead of air was chosen instead of air

to minimize NO oxidation.to minimize NO oxidation. The system was allowed to The system was allowed to

react up to 20 hours at 296 react up to 20 hours at 296 +/- 1K with spectra +/- 1K with spectra continuously recordcontinuously record

After reaction, cell contents After reaction, cell contents are evacuated for 2 hours are evacuated for 2 hours dropping cell pressure to 10dropping cell pressure to 10--

22-10-10-3-3 torr. After pumping torr. After pumping stops, gas-phase and stops, gas-phase and surface film spectra are surface film spectra are rerecorded.rerecorded.

1.1. Filled with a HFilled with a H22O/NO/N22 mixture mixture

2.2. RH from 37 to 63% RH from 37 to 63%

3.3. Pressure of 640 torr Pressure of 640 torr

4.4. Flow NFlow N22 gas through a bubbler gas through a bubbler containing water and mixing it with containing water and mixing it with dry nitrogen in a 5L mixing bulb.dry nitrogen in a 5L mixing bulb.

5.5. Equilibrate for 30 minutes Equilibrate for 30 minutes

6.6. Background spectra recorded Background spectra recorded

7.7. NONO22 flushed into the cell as a flushed into the cell as a mixture in nitrogen mixture in nitrogen

8.8. Filled to a pressure of 1atm with Filled to a pressure of 1atm with NN22, yielding NO, yielding NO22 mixing ratio of 87- mixing ratio of 87-250ppm250ppm

HONO Photolysis ProcedureHONO Photolysis Procedure For the photolysis For the photolysis

experiments, the chamber was experiments, the chamber was filled to 1 atm with Nfilled to 1 atm with N22, and , and then irradiated for 4 hours then irradiated for 4 hours with either the low pressure with either the low pressure mercury lamp or the black mercury lamp or the black lamp increasing the chamber lamp increasing the chamber temperature by a maximum of temperature by a maximum of 3K.3K.

Gas-phase NOGas-phase NO22, HONO, and NO , HONO, and NO were quantified by the net were quantified by the net absorbance of their peaks at absorbance of their peaks at 2917, 1263nm or 790nm, and 2917, 1263nm or 790nm, and 1875 cm1875 cm-1-1 respectfully. respectfully.

NO2 absorption band at NO2 absorption band at 1620cm1620cm-1-1 was dropped due to was dropped due to HH22O inferenceO inference

Detection limits for these Detection limits for these gases using these bands are gases using these bands are 2ppm for NO2ppm for NO22 and NO and and NO and 0.2ppm for HONO.0.2ppm for HONO.

Concentrations of NOConcentrations of NO22 and NO and NO were determined based on were determined based on calibrations using mixtures of calibrations using mixtures of known concentrations in Nknown concentrations in N22

HONO concentrations were HONO concentrations were calculated using the 1263 or calculated using the 1263 or 790cm790cm-1-1 band and applying an band and applying an effective absorption cross effective absorption cross section of (3.7+/-0.4)E-19 section of (3.7+/-0.4)E-19 cmcm22/molec or (2.8+/-0.6)E-19 /molec or (2.8+/-0.6)E-19 cmcm22/molec to measure total /molec to measure total HONO based on a trans/cis HONO based on a trans/cis ratio of 2.3ratio of 2.3

ReactionsReactions

In NO2 heterogeneous hydrolysis, equilibrium with HNO3 and the nitrate ion are likely:

Generation of NO could occur in this reaction which is energetically accessible at wavelengths below 501nm.

There exist reactions, thermodynamically, that could generate NO and HONO from photolysis of surface-adsorbed nitric acid and water complexes:

The reaction enthalpy will also be modified by the surface, suggesting that the lamps have sufficient energy to drive photochemisty.

Results of Lab ExperimentResults of Lab Experiment

a. irradiation 254nm in absence of O and presence of OH scavenger C6H12

b. exposure to N2 at RH 30% without irradiation

c. refer to dd. exposure to N2 at RH 50% with

irradiation at 254nme. irradiation at 300-400nm

•At 50% RH, which was used for most of the experiments reported here, there is the equivalent of 2 layers of water on the crystal surface, •similar to that measured on glass and quartz in earlier studies.(70)•appropriate to assume that the ATR spectra on both the silicon and AMTIR crystals are representative of the chemistry that is occurring on the cell walls.

Assess aqueous enhancement Assess aqueous enhancement production of HONO production of HONO

Introduction of Water VaporIntroduction of Water Vapor Runs were carried out in which humidified NRuns were carried out in which humidified N22

was added to the cell after pumping. was added to the cell after pumping. When water was added to the cell in the When water was added to the cell in the

present studies after reaction followed by present studies after reaction followed by pump down, an increase in gas-phase HONO pump down, an increase in gas-phase HONO was also observed (filled squares). was also observed (filled squares).

When the 300-400 nm lamp is turned on When the 300-400 nm lamp is turned on during such a run at 50% RH, no during such a run at 50% RH, no enhancement in the HONO production the enhancement in the HONO production the cell walls is observed.cell walls is observed.

Atmospheric Implications Atmospheric Implications (Problematic)(Problematic)

There is some evidence for HONO production from reactions on dust particles. For example, Stutz and co-workersreported enhanced HONO concentrations during a dust stormin Phoenix, AZ.

To contrast, the addition of dry N2 does not result in gas-phase HONO. Thus either HONO or a HONO precursor must be present on the surface and interact with water vapor to form and/or desorb HONO.

This process is very important for the interpretation of HONO concentrations in air and their dependence on RH.

Although we did not observe bands in the ATR spectra that were assignable to adsorbed HONO, if it were complexed to nitrate ions or other surface species, the spectra could be shifted significantly compared to that expected based on gas-phase data.

The Search Continues…The Search Continues…

Reinvestigating Crowley’s Reinvestigating Crowley’s Closed CaseClosed Case

A ClueA Clue

Discrepancies between modeled and Discrepancies between modeled and measured values of HOmeasured values of HOxx (OH & HO (OH & HO22) ) associated with high solar zenith associated with high solar zenith angle chemistryangle chemistry Upper troposphereUpper troposphere Polar regionsPolar regions

A Possible Pathway:A Possible Pathway:mechanism developmentmechanism development

Primary mechanism Proposed mechanism

1

2

3

13 2

1 3

2 2

12

O + ( 320 nm) O( D) + O

O( D) + M O( P) + M

(where M is N or O )

O( D) + H O 2OH

j

k

k

h

5

6

7

2 2

2 2

2 2 2

2 2

NO + ( 420 nm) NO

NO + M NO + M

(where M is N , O ,or H O)

NO + H O OH + HONO

j

k

k

h

HONO + ( 390 nm) OH + NOh

5 7 2 2OH

7 2 6

2 [NO ][H O]R

[H O] [M]

j k

k k

1 3 2 3

OH3 2 2

2 [H O][O ]R

[H O] [M]

j k

k k

QuenchingQuenchingkk6,N26,N2 = 2.7 x 10 = 2.7 x 10-11 -11 cmcm33 molecule molecule-1-1 s s-1-1

kk6,O26,O2 = 3.0 x 10 = 3.0 x 10-11 -11 cmcm33 molecule molecule-1-1 s s-1-1

kk6,H2O6,H2O = 1.7 x 10 = 1.7 x 10-10 -10 cmcm33 molecule molecule-1-1 s s-1-1

OH ProductionOH Production

kk7,H2O7,H2O = ??? = ???

5

6

7

2 2

2 2

2 2 2

2 2

NO + ( 420 nm) NO

NO + M NO + M

(where M is N , O ,or H O)

NO + H O OH + HONO

j

k

k

h

5 7 2 2OH

7 2 6

2 [NO ][H O]R

[H O] [M]

j k

k k

A Possible Pathway:A Possible Pathway:rate constantsrate constants

5

6

7

2 2

2 2

2 2 2

2 2

NO + ( 420 nm) NO

NO + M NO + M

(where M is N , O ,or H O)

NO + H O OH + HONO

j

k

k

h

A Possible Pathway:A Possible Pathway:in contrast with Crowley et al.in contrast with Crowley et al.

8

9

10

11

2 2

2 2

12 2

12

NO + ( 450 nm) NO

NO + NO

NO N +O( D)

O( D)+H O 2OH

j

j

k

k

h

h

Testing the Hypothesis:Testing the Hypothesis:experimental setupexperimental setup

Reaction chamber filled with a Reaction chamber filled with a mixture of Nmixture of N22, NO, NO22, and H, and H22O with O with exposure to laser and sensorsexposure to laser and sensors

Crowley et al. Crowley et al. (1997)(1997)

Li et al. Li et al.

(2008)(2008)

WavelengtWavelengthh

430 nm < 430 nm < < 450 < 450 nm, 532 nmnm, 532 nm

560 nm < 560 nm < < 640 < 640 nmnm

OH OH detectordetector

resonance resonance fluorescencefluorescence

laser induced laser induced fluorescencefluorescence

Testing the Hypothesis:Testing the Hypothesis:experimental resultsexperimental results

Production of OH was verified by LIFProduction of OH was verified by LIF Timing was confirmed to be consistent with Timing was confirmed to be consistent with

bimolecular reaction rather than bimolecular reaction rather than photodissociation of heterogeneous photodissociation of heterogeneous chemistry products such as HONO or HNOchemistry products such as HONO or HNO33

QuickTime™ and a decompressor

are needed to see this picture.

Testing the Hypothesis:Testing the Hypothesis:experimental resultsexperimental results

Correct Correct dependencdependence on e on protons in protons in mechanismmechanism

InsignificanInsignificant reaction t reaction of excited of excited NONO22 with with excited Hexcited H22OO

Testing the Hypothesis:Testing the Hypothesis:source of OHsource of OH

The The congruent congruent

spectra spectra constrain OH constrain OH

to having NOto having NO22 as its source as its source

in these in these experiments.experiments.

Experimental Results:Experimental Results:pseudo-first order kineticspseudo-first order kinetics

5 7 2 2OH

7 2 6

2 [NO ][H O]R

[H O] [M]

j k

k k

2

2

77 2

2

I7 7

[OH][NO ][H O]

Assuming that [H O] is

large enough to be

approximately constant,

[OH] [NO ]

dk

dt

k t

Implications:Implications:OH productionOH production

5 7 2 2OH

7 2 6

2 [NO ][H O]R

[H O] [M]

j k

k k

OH concentration based solely on production by the proposed mechanism.

• Box model• [H2O] = 10 torr at ground level

Implications:Implications:comparative OH productioncomparative OH production

Up to a factor of Up to a factor of 0.50.5 increaseincrease in OH production at high SZA in OH production at high SZA and in polluted conditions considering this mechanismand in polluted conditions considering this mechanism

2

3

5 7 2 2[NO ]OH 7 2 6[O ]

1 3 2 3OH

3 2 2

2 [NO ][H O]

R [H O] [M]2 [H O][O ]R

[H O] [M]

j k

k kj k

k k

Case Closed:Case Closed:could this be the primary missing link?could this be the primary missing link?

Despite Crowley’s early Despite Crowley’s early discounting discounting of this of this reaction pathway for the production of OH, reaction pathway for the production of OH, analyzing the reaction at longer wavelengths analyzing the reaction at longer wavelengths shows that shows that excited NOexcited NO22 is a legitimate is a legitimate contributor contributor to OH in certain situations.to OH in certain situations.

For periods of time and locations in which For periods of time and locations in which solar zenith angle is highsolar zenith angle is high, this reaction , this reaction should should supplement supplement the primary production the primary production pathway in atmospheric chemistry models.pathway in atmospheric chemistry models.

Investigations of Investigations of HNOHNO33 and HONO and HONO as as products of products of heterogeneous chemistry heterogeneous chemistry of NOof NO22 could be further explored via this new could be further explored via this new instrument.instrument.

1. Crowley et al. OH formation in the photoexcitation of NO1. Crowley et al. OH formation in the photoexcitation of NO22 beyond beyond

the dissociation threshold in the presence of water vapor. J Phys the dissociation threshold in the presence of water vapor. J Phys Chem A (1997) vol. 101 pp. 4178-4184 Chem A (1997) vol. 101 pp. 4178-4184

2. Li et al. Atmospheric hydroxyl radical production from electronically2. Li et al. Atmospheric hydroxyl radical production from electronicallyexcited NOexcited NO22 and H and H22O. Science (2008) vol. 319 pp. 1657-1660O. Science (2008) vol. 319 pp. 1657-1660

3. Ramazan et al. New experimental and theoretical approach to the3. Ramazan et al. New experimental and theoretical approach to theheterogeneous hydrolysis of NOheterogeneous hydrolysis of NO22: Key role of molecular nitric acid : Key role of molecular nitric acid

and its complexes. J Phys Chem A (2006) vol. 110 pp. 6886-6897and its complexes. J Phys Chem A (2006) vol. 110 pp. 6886-6897

References