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An Unknown Source of Atmospheric OH

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

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An Unknown Source of Atmospheric OH

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

  2. Background • 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

  3. Major Primary Source of Atmospheric OH • Photolysis of O3 in presence of water vapor 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])

  4. λ Dependent Lifetime (τrad) Since 1997 NO2 has been investigated as the missing source.

  5. Quenching of NO2* NO2 * + M NO2 M = N2, O2, or H2O 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

  6. OH from NO2* NO2 * + H2O OH + HONO 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)

  7. Rate of OH Production NO2 + hν NO2 * J(NO2) NO2* + M  NO2 k4 NO2 * + H2O OH + HONO k5 ROH = J(NO2)[NO2]/(1+ k4[M]/k5[H2O])

  8. The Search Began… • 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*.

  9. 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)

  10. Conclusion • A 2 photon process leads to OH via O(1D) • No OH production observed at 532 nm • If the reactivity of NO2* is representative of the entire range (410-600) of the NO2 absorption spectrum, an upper limit of 2% to formation of OH via rxn 4 in troposphere when solar zenith angles are high

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

  12. 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 • 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 NO2 and H2O • The camber gasisreplaced and photolysis of the surface productsproceeds, HONO and HNO3, ismeassuredattwodifferentwavelengths.

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

  14. Environmental Regulation • 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 LeyboldPirani 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

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

  16. 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-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

  17. 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. • 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.

  18. Chemicals • Nitric oxide (Matheson, 99%) purified by liquid nitrogen trap to remove impurities such as NO2 and HNO3. • Nitrogen dioxide synthesized by reaction purified NO with excess oxygen (Oxygen Service Company, 99.993%) for at least 2 hours after which NO2 is condensed at 195K and the oxygen is pumped off. • Nitrogen (Oxygen Service Company, 99.999%) • Air • Deuterated water

  19. NO2 Experimental Procedures • Filled with a H2O/N2 mixture • RH from 37 to 63% • Pressure of 640 torr • Flow N2 gas through a bubbler containing water and mixing it with dry nitrogen in a 5L mixing bulb. • Equilibrate for 30 minutes • Background spectra recorded • NO2 flushed into the cell as a mixture in nitrogen • Filled to a pressure of 1atm with N2, yielding NO2 mixing ratio of 87-250ppm • N2 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 • After reaction, cell contents are evacuated for 2 hours dropping cell pressure to 10-2-10-3torr. After pumping stops, gas-phase and surface film spectra are rerecorded.

  20. HONO Photolysis Procedure • Detection limits for these gases using these bands are 2ppm for NO2 and NO and 0.2ppm for HONO. • Concentrations of NO2 and NO were determined based on calibrations using mixtures of known concentrations in N2 • 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 cm2/molec or (2.8+/-0.6)E-19 cm2/molec to measure total HONO based on a trans/cis ratio of 2.3 • For the photolysis experiments, the chamber was filled to 1 atm with N2, 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 NO2, 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 H2O inference

  21. Reactions There exist reactions, thermodynamically, that could generate NO and HONO from photolysis of surface-adsorbed nitric acid and water complexes: In NO2 heterogeneous hydrolysis, equilibrium with HNO3 and the nitrate ion are likely: The reaction enthalpy will also be modified by the surface, suggesting that the lamps have sufficient energy to drive photochemisty. Generation of NO could occur in this reaction which is energetically accessible at wavelengths below 501nm.

  22. Results of Lab Experiment • 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. • irradiation 254nm in absence of O and presence of OH scavenger C6H12 • exposure to N2 at RH 30% without irradiation • refer to d • exposure to N2 at RH 50% with irradiation at 254nm • irradiation at 300-400nm

  23. Assess aqueous enhancement production of HONO • Introduction of Water Vapor • Runs were carried out in which humidified N2 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 the 300-400 nm lamp is turned on during such a run at 50% RH, no enhancement in the HONO production the cell walls is observed.

  24. 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 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.

  25. The Search Continues… Reinvestigating Crowley’s Closed Case

  26. A Clue Discrepancies between modeled and measured values of HOx (OH & HO2) associated with high solar zenith angle chemistry Upper troposphere Polar regions

  27. A Possible Pathway:mechanism development Primary mechanism Proposed mechanism

  28. Quenching k6,N2 = 2.7 x 10-11 cm3 molecule-1 s-1 k6,O2 = 3.0 x 10-11 cm3 molecule-1 s-1 k6,H2O = 1.7 x 10-10 cm3 molecule-1 s-1 OH Production k7,H2O = ??? A Possible Pathway:rate constants

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

  30. Testing the Hypothesis:experimental setup Reaction chamber filled with a mixture of N2, NO2, and H2O with exposure to laser and sensors

  31. Testing the Hypothesis:experimental results 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 HNO3

  32. Testing the Hypothesis:experimental results Correct dependence on protons in mechanism Insignificant reaction of excited NO2 with excited H2O

  33. Testing the Hypothesis:source of OH The congruent spectra constrain OH to having NO2 as its source in these experiments.

  34. Experimental Results:pseudo-first order kinetics

  35. Implications:OH production • OH concentration based solely on production by the proposed mechanism. • Box model • [H2O]= 10 torr at ground level

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

  37. 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 NO2 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. Investigations of HNO3 and HONO as products of heterogeneous chemistry of NO2 could be further explored via this new instrument.

  38. 1. Crowley et al. OH formation in the photoexcitation of NO2 beyond the dissociation threshold in the presence of water vapor. J Phys Chem A (1997) vol. 101 pp. 4178-4184 2. Li et al. Atmospheric hydroxyl radical production from electronicallyexcited NO2 and H2O. Science (2008) vol. 319 pp. 1657-16603. Ramazan et al. New experimental and theoretical approach to theheterogeneous hydrolysis of NO2: Key role of molecular nitric acid and its complexes. J Phys Chem A (2006) vol. 110 pp. 6886-6897 References

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