An Unknown Source of Atmospheric OH Group Project - Spring ‘08 Atmospheric Chemistry, Dr. Wang Patrick, Shannon, Charles

Background OH is the primary oxidant in the daytime troposphere OH is the primary oxidant in the daytime troposphere Field observations suggest that there is a missing source of OH not being accounted for in models Field observations suggest that there is a missing source of OH not being accounted for in models

Major Primary Source of Atmospheric OH Photolysis of O 3 in presence of water vapor Photolysis of O 3 in presence of water vapor O 3 + hν  O 2 + O( 1 D)J (O3) O( 1 D) + H 2 O  2OHk2 O( 1 D) + M  O( 3 P) + Mk3 R OH = 2J (O3) [O 3 ]/(1+ k 3 [M]/k 2 [H 2 O])

λ Dependent Lifetime (τ rad ) Since 1997 NO 2 has been investigated as the missing source.

Quenching of NO 2 * M = N 2, O 2, or H 2 O NO 2 * + M  NO 2 Total quenching rate = 7.1 x 10 8 sec -1 resulting in NO 2 * lifetime of 1.4 ns. Quenching rates for N 2,O 2, and H 2 O: 2.7E -11, 3.0E -11, 1.7E-10 cm 3 molec -1 s -1

OH from NO 2 * ΔH = - 25 kcal/mol Thermodynamically feasible! (calculation based on 65 kcal/mol of energy over that of ground state NO 2 – resulting from absorption of 440 nm) NO 2 * + H 2 O  OH + HONO

Rate of OH Production NO 2 + hν  NO 2 *J (NO2) NO 2 * + M  NO 2 k 4 NO 2 * + H 2 O  OH + HONOk 5 R OH = J (NO2) [NO 2 ]/(1+ k 4 [M]/k 5 [H 2 O])

The Search Began… Crowley ( nm): Crowley ( nm): NO 2 + hν  NO 2 * NO 2 * + hν  NO 2 ** NO 2 **  NO + O( 1 D) O( 1 D) + H 2 O  2OH However, in the troposphere the 2 photon process will be inefficient due to low photolysis intensities and rapid collisional quenching of NO 2 *.

A significant source of OH? NO 2 * + H 2 O  OH + HONOk5 k5 = 1.2 x cm 3 molec -1 s -1 R OH = J (NO2) [NO 2 ]/(1+ k 4 [M]/k 5 [H 2 O]) = 3 x 10 3 cm -3 s -1 (upper limit)

Conclusion A 2 photon process leads to OH via O( 1 D) A 2 photon process leads to OH via O( 1 D) No OH production observed at 532 nm No OH production observed at 532 nm If the reactivity of NO 2 * is representative of the entire range ( ) of the NO 2 absorption spectrum, an upper limit of 2% to formation of OH via rxn 4 in troposphere when solar zenith angles are high If the reactivity of NO 2 * is representative of the entire range ( ) of the NO 2 absorption spectrum, an upper limit of 2% to formation of OH via rxn 4 in troposphere when solar zenith angles are high

Experimental Design for Controlled HONO Studies Attenuated Total Reflectance – Long Path Fourier Transform Infrared Spectroscopy = ATR-FTIR

Background The idea is to describe simultaneously both the gas- phase and adsorbed reaction products in water films found both in laboratory systems and the tropospheric boundary layer The idea is to describe simultaneously both the gas- phase and adsorbed reaction products in water films found both in laboratory systems and the tropospheric boundary layer This experiment: coupling a long path Fourier transform infrared spectroscopy for gas-phase components with an attenuated total reflectance probe to follow thin film surface chemistry This experiment: coupling a long path Fourier transform infrared spectroscopy for gas-phase components with an attenuated total reflectance probe to follow thin film surface chemistry This is then applied to the heterogeneous hydrolysis of NO 2 and H 2 O This is then applied to the heterogeneous hydrolysis of NO 2 and H 2 O The camber gas is replaced and photolysis of the surface products proceeds, HONO and HNO 3, is meassured at two different wavelengths. The camber gas is replaced and photolysis of the surface products proceeds, HONO and HNO 3, is meassured at two different wavelengths.

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

Environmental Regulation Relative humidity and temperature are monitored inside the reaction chamber using a Caisala gauge (HMP-238) Relative humidity and temperature are monitored inside the reaction chamber using a Caisala gauge (HMP-238) Pressure in the range from 10^3 to 0.1 Torr are measured using a Leybold diaphragm gauge (Ceravac CTR-91) and from 750 to 10^-4 Torr are measured using a Leybold Pirani Gauge (Thermovac TTR-216) Pressure in the range from 10^3 to 0.1 Torr are measured using a Leybold diaphragm gauge (Ceravac CTR-91) and from 750 to 10^-4 Torr are measured using a Leybold Pirani Gauge (Thermovac TTR-216) A VOC-Edwards iQDP80/iQMB250 dry pump is used to evacuate the cell while minimizing the backflow of organics associated with oil pumps A VOC-Edwards iQDP80/iQMB250 dry pump is used to evacuate the cell while minimizing the backflow of organics associated with oil pumps

Spectrometer ThermoNicolet Nexus 670 FTIR was used to record the infrared spectra of the gas phase products. ThermoNicolet Nexus 670 FTIR was used to record the infrared spectra of the gas phase products. The IR beam is directed from the FTIR via a set of transfer optics into the reaction cell through a ZnSe window. The IR beam is directed from the FTIR via a set of transfer optics into the reaction cell through a ZnSe window. Multiple reflections achieved using White Cell Optics, created a path length of 47.5m Multiple reflections achieved using White Cell Optics, created a path length of 47.5m The mirrors in the cell were gold coated and protected with a thin layer of SiO. The mirrors in the cell were gold coated and protected with a thin layer of SiO. The exiting infrared beam was focused using a 30° off-axis parabolic mirror onto a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. The exiting infrared beam was focused using a 30° off-axis parabolic mirror onto a liquid nitrogen cooled mercury cadmium telluride (MCT) detector.

Attenuated total reflectance Probe The ATR proble is a hollow wavequide probe (Axiom Analytical ATR-FTIR Probe DRP- 100) inserted through a vacuum- tight fitting into the reaction chamber The ATR proble is a hollow wavequide probe (Axiom Analytical ATR-FTIR Probe DRP- 100) inserted through a vacuum- tight fitting into the reaction chamber The ATR-FRIR accesssory base is placed into the sample compartment of a second FTIR spectrometer. The IR beam in the sample compartment is directed into the infrared transmitting ATR crystal and undergoes total internal reflection bouncing nine time within the crystal The ATR-FRIR accesssory base is placed into the sample compartment of a second FTIR spectrometer. The IR beam in the sample compartment is directed into the infrared transmitting ATR crystal and undergoes total internal reflection bouncing nine time within the crystal

Infrared Crystals / Irradiation Two infrared crystals were used: AMTIR (Ge33As12Se55) and silicon. AMTIR has a high IR throughput and wide spectral range (6000cm-1-700cm-1) but the silicon crystal does not transmit below 1500cm-1 but is more representative of the chamber construction. It is used to verify AMTIR. Two infrared crystals were used: AMTIR (Ge33As12Se55) and silicon. AMTIR has a high IR throughput and wide spectral range (6000cm-1-700cm-1) but the silicon crystal does not transmit below 1500cm-1 but is more representative of the chamber construction. It is used to verify AMTIR. Irradiation of the cell is produced by using low pressure mercury lamps. The G30T8 as the strongest line at 254nm and the black lamp, the F30T8/350BL has a broad range from 300 to 400nm. Irradiation of the cell is produced by using low pressure mercury lamps. The G30T8 as the strongest line at 254nm and the black lamp, the F30T8/350BL has a broad range from 300 to 400nm.

Chemicals Nitric oxide (Matheson, 99%) purified by liquid nitrogen trap to remove impurities such as NO 2 and HNO 3. Nitric oxide (Matheson, 99%) purified by liquid nitrogen trap to remove impurities such as NO 2 and HNO 3. Nitrogen dioxide synthesized by reaction purified NO with excess oxygen (Oxygen Service Company, %) for at least 2 hours after which NO 2 is condensed at 195K and the oxygen is pumped off. Nitrogen dioxide synthesized by reaction purified NO with excess oxygen (Oxygen Service Company, %) for at least 2 hours after which NO 2 is condensed at 195K and the oxygen is pumped off. Nitrogen (Oxygen Service Company, %) Nitrogen (Oxygen Service Company, %) Air Air Deuterated water Deuterated water

NO2 Experimental Procedures N 2 was chosen instead of air to minimize NO oxidation. N 2 was chosen instead of air to minimize NO oxidation. The system was allowed to react up to 20 hours at 296 +/- 1K with spectra continuously record The system was allowed to react up to 20 hours at 296 +/- 1K with spectra continuously record After reaction, cell contents are evacuated for 2 hours dropping cell pressure to torr. After pumping stops, gas-phase and surface film spectra are rerecorded. After reaction, cell contents are evacuated for 2 hours dropping cell pressure to torr. After pumping stops, gas-phase and surface film spectra are rerecorded. 1. Filled with a H 2 O/N 2 mixture 2. RH from 37 to 63% 3. Pressure of 640 torr 4. Flow N 2 gas through a bubbler containing water and mixing it with dry nitrogen in a 5L mixing bulb. 5. Equilibrate for 30 minutes 6. Background spectra recorded 7. NO 2 flushed into the cell as a mixture in nitrogen 8. Filled to a pressure of 1atm with N 2, yielding NO 2 mixing ratio of ppm

HONO Photolysis Procedure For the photolysis experiments, the chamber was filled to 1 atm with N 2, and then irradiated for 4 hours with either the low pressure mercury lamp or the black lamp increasing the chamber temperature by a maximum of 3K. For the photolysis experiments, the chamber was filled to 1 atm with N 2, and then irradiated for 4 hours with either the low pressure mercury lamp or the black lamp increasing the chamber temperature by a maximum of 3K. Gas-phase NO 2, HONO, and NO were quantified by the net absorbance of their peaks at 2917, 1263nm or 790nm, and 1875 cm -1 respectfully. Gas-phase NO 2, HONO, and NO were quantified by the net absorbance of their peaks at 2917, 1263nm or 790nm, and 1875 cm -1 respectfully. NO2 absorption band at 1620cm -1 was dropped due to H 2 O inference NO2 absorption band at 1620cm -1 was dropped due to H 2 O inference Detection limits for these gases using these bands are 2ppm for NO 2 and NO and 0.2ppm for HONO. Detection limits for these gases using these bands are 2ppm for NO 2 and NO and 0.2ppm for HONO. Concentrations of NO 2 and NO were determined based on calibrations using mixtures of known concentrations in N 2 Concentrations of NO 2 and NO were determined based on calibrations using mixtures of known concentrations in N 2 HONO concentrations were calculated using the 1263 or 790cm -1 band and applying an effective absorption cross section of (3.7+/-0.4)E-19 cm 2 /molec or (2.8+/-0.6)E-19 cm 2 /molec to measure total HONO based on a trans/cis ratio of 2.3 HONO concentrations were calculated using the 1263 or 790cm -1 band and applying an effective absorption cross section of (3.7+/-0.4)E-19 cm 2 /molec or (2.8+/-0.6)E-19 cm 2 /molec to measure total HONO based on a trans/cis ratio of 2.3

Reactions In NO 2 heterogeneous hydrolysis, equilibrium with HNO 3 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 Experiment a.irradiation 254nm in absence of O and presence of OH scavenger C 6 H 12 b.exposure to N 2 at RH 30% without irradiation c.refer to d d.exposure to N 2 at RH 50% with irradiation at 254nm e.irradiation at nm 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 production of HONO Introduction of Water Vapor Introduction of Water Vapor Runs were carried out in which humidified N 2 was added to the cell after pumping. Runs were carried out in which humidified N 2 was added to the cell after pumping. When water was added to the cell in the present studies after reaction followed by pump down, an increase in gas-phase HONO was also observed (filled squares). When water was added to the cell in the present studies after reaction followed by pump down, an increase in gas-phase HONO was also observed (filled squares). When the nm lamp is turned on during such a run at 50% RH, no enhancement in the HONO production the cell walls is observed. When the nm lamp is turned on during such a run at 50% RH, no enhancement in the HONO production the cell walls is observed.

Atmospheric Implications (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 N 2 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… Reinvestigating Crowley’s Closed Case

A Clue Discrepancies between modeled and measured values of HO x (OH & HO 2 ) associated with high solar zenith angle chemistry Discrepancies between modeled and measured values of HO x (OH & HO 2 ) associated with high solar zenith angle chemistry Upper troposphere Upper troposphere Polar regions Polar regions

A Possible Pathway: mechanism development Primary mechanismProposed mechanism

Quenching k 6,N2 = 2.7 x cm 3 molecule -1 s -1 k 6,O2 = 3.0 x cm 3 molecule -1 s -1 k 6,H2O = 1.7 x cm 3 molecule -1 s -1 OH Production k 7,H2O = ??? A Possible Pathway: rate constants

A Possible Pathway: in contrast with Crowley et al.

Testing the Hypothesis: experimental setup Reaction chamber filled with a mixture of N 2, NO 2, and H 2 O with exposure to laser and sensors Crowley et al. (1997) Li et al. (2008) Wavelength 430 nm < < 450 nm, 532 nm 560 nm < < 640 nm OH detector resonance fluorescence laser induced fluorescence

Testing the Hypothesis: experimental results Production of OH was verified by LIF Production of OH was verified by LIF Timing was confirmed to be consistent with bimolecular reaction rather than photodissociation of heterogeneous chemistry products such as HONO or HNO 3 Timing was confirmed to be consistent with bimolecular reaction rather than photodissociation of heterogeneous chemistry products such as HONO or HNO 3

Testing the Hypothesis: experimental results Correct dependence on protons in mechanism Correct dependence on protons in mechanism Insignificant reaction of excited NO 2 with excited H 2 O Insignificant reaction of excited NO 2 with excited H 2 O

Testing the Hypothesis: source of OH The congruent spectra constrain OH to having NO 2 as its source in these experiments.

Experimental Results: pseudo-first order kinetics

Implications: OH production OH concentration based solely on production by the proposed mechanism. Box model [H 2 O] = 10 torr at ground level

Implications: comparative OH production Up to a factor of 0.5 increase in OH production at high SZA and in polluted conditions considering this mechanism

Case Closed: could this be the primary missing link? Despite Crowley’s early discounting of this reaction pathway for the production of OH, analyzing the reaction at longer wavelengths shows that excited NO 2 is a legitimate contributor to OH in certain situations. Despite Crowley’s early discounting of this reaction pathway for the production of OH, analyzing the reaction at longer wavelengths shows that excited NO 2 is a legitimate contributor to OH in certain situations. For periods of time and locations in which solar zenith angle is high, this reaction should supplement the primary production pathway in atmospheric chemistry models. For periods of time and locations in which solar zenith angle is high, this reaction should supplement the primary production pathway in atmospheric chemistry models. Investigations of HNO 3 and HONO as products of heterogeneous chemistry of NO 2 could be further explored via this new instrument. Investigations of HNO 3 and HONO as products of heterogeneous chemistry of NO 2 could be further explored via this new instrument.

1. Crowley et al. OH formation in the photoexcitation of NO 2 beyond the dissociation threshold in the presence of water vapor. J Phys Chem A (1997) vol. 101 pp Li et al. Atmospheric hydroxyl radical production from electronically excited NO 2 and H 2 O. Science (2008) vol. 319 pp Ramazan et al. New experimental and theoretical approach to the heterogeneous hydrolysis of NO 2 : Key role of molecular nitric acid and its complexes. J Phys Chem A (2006) vol. 110 pp References