Response to Comment on "Atmospheric Hydroxyl Radical Production from Electronically Excited NO₂ and H₂O"

^* To whom correspondence should be addressed. E-mail: asinha{at}ucsd.edu

The reaction of electronically excited nitrogen dioxide with water, NO₂*+ H₂O, can be an important source of atmospheric OH radicals if its reaction rate is sufficiently fast (1–3). Carr et al. (2) report a rate for this reaction that differs from the rates we reported (3) by more than an order of magnitude. In their study, Carr et al. report being unable to detect any OH from the reaction in the vicinity of 560 nm, whereas we reported detecting OH products over the region from 560 to 635 nm by electronically exciting NO₂ in the presence of water vapor (3). The reasons for the differences in the results of the two studies are not obvious, although it is well known that the reaction under consideration is a difficult one to study because of rapid quenching of the electronically excited NO₂ by water and the low product yields (1–3). The main difference in the approaches between the two studies apparently involves the laser power density used to excite the NO₂ reagent and the calibration reaction used to quantify OH yields.

In our experiment, we used a lens to introduce the excitation laser into the reaction cell and electronically excite NO₂ (3). This was necessitated by the low electronic absorption cross section of NO₂ over the red end of the spectral range covered in our study. By contrast, Carr et al. did not use a lens, and thus their measurements used an unfocused laser beam (2). With regard to the OH calibration reaction, we used OH from the vibrational overtone–induced unimolecular dissociation of CH₃OOH (5_OH) to calibrate OH yields, whereas Carr et al. used the photodissociation of acetone followed by the reaction of the resulting acetyl radical with O₂ for their calibrations. Carr et al. suggest that the difference in their results and those of our study are likely due to multiphoton effects contributing to the OH signal in our study arising from the greater laser power density created by the presence of the lens. Carr et al. cite the presence of an intercept appearing in the linear power dependence plot in (3) as indicative of multiphoton effects. However, the authors do not provide any particular mechanism to suggest how the OH radicals we observed are produced by the multiphoton excitation process, nor do they provide any data on the effect of increasing laser power density in their measurements on the OH yield. In (3), we suggested that the intercepts in our power dependence plots likely arose from our detection electronics. Indeed, we looked at the power dependence for OH formation from the unimolecular dissociation of CH₃OOH (5_OH) and HNO₃ (5_OH), which are both linear processes, using the same experimental setup as that used for the NO₂*+H₂O study and found small intercepts in the power dependence plots for these unimolecular processes as well (intercepts of 1 to 2.5 mJ, corresponding to 6 to 8% of the peak power range). Hence, the presence of an intercept in an otherwise linear power dependence plot does not in itself signify the occurrence of multiphoton effects.

Based on the OH spectrum recorded and the action spectrum presented in (3), we have little doubt that we observed OH from the excitation of NO₂ in the presence of water. Could the OH we detected arise from multiphoton effects? As reported in (3), we considered the possibility of several multiphoton mechanisms and found no compelling evidence for their occurrence. First, multiphoton dissociation of NO₂ can result in the production of electronically excited oxygen atoms such as O(¹D). However, it is well known that a common characteristic of O(¹D) atoms is that they react rapidly with H₂ to produce OH radicals (4). When we introduced H₂ into the experimental cell in place of H₂O, we did not observe any OH being produced (3). This lack of OH formation, which would be expected if O(¹D) were present, nullifies the production of these electronically excited atoms through multiphoton excitation of NO₂. A second multiphoton process that was also considered involved the possibility of producing translationally hot ground state O(³P) atoms from the multiphoton dissociation of NO₂. These oxygen atoms could in principle then react with water to produce OH radicals. This scenario was ruled out on the basis that the reaction of O(³P)+H₂O is endothermic by roughly 17 Kcal/mole (4). Measurements of Brouard and Vallance (5) show that O(³P) atoms generated from the photodissociation of NO₂ at 308 nm have an average translational energy of 0.3 eV (6.9 kcal/mole) with a full-width at half-max of 0.2 eV. As we also observed OH production for NO₂ excitation wavelengths between 610 and 630 nm, which corresponds to a two-photon excitation energy equivalent to exciting between 305 and 315 nm, the results of Brouard and Vallance then suggest that the average translational energy of the oxygen atoms potentially generated by two-photon dissociation of NO₂ at these wavelengths will not be sufficiently energetic to initiate the O(³P)+H₂O reaction. Hence, this multiphoton mechanism for OH production was also ruled out. Third, the possibility of populating long-lived bound excited quartet electronic excited states of NO₂ through multiphoton excitation, which then reacts with H₂O, has also been considered. According to the calculations of Bera et al. (6), NO₂ has several excited quartet states in the vicinity of 3.61 to 4.37 eV. Accessing these bound excited electronic states of NO₂ from the ground state requires the involvement of spin-forbidden transitions. These bound excited states could potentially be accessed through sequential two-photon excitation by the intermediate A ²B₂ electronic state of NO₂ initially populated through excitation in the visible range. However, spectral features of bound-to-bound transitions such as these should be reflected in their characteristic structured excitation spectrum. The action spectrum generated from the NO₂*+ H₂O reaction in (3), however, match that expected from the traditional X ²A₁ A ²B₂ excitation of NO₂ in the visible range. Also, in addition to being spin-forbidden with respect to their photochemical formation, the subsequent bimolecular reaction of these quartet NO₂ states with H₂O to form OH + HONO is expected to be spin-forbidden as well. Finally, we also considered the possibility that the primary source of OH signal in our experiments were due to the collision of electronically excited nitrogen dioxide, NO₂*, with vibrationally excited water through the following reaction sequence.

(1)

(2)

The pseudo-first order rate constant for OH production through this mechanism would be expected to go as the square of the NO₂ concentration. Our experiments using different starting NO₂ concentrations did not show this squared dependence for the pseudo-first order rates; hence, this mechanism was ruled out as the main source of the observed OH signal, although it could contribute to small background levels as noted in (3). Thus, based on the above reasoning, it is difficult to justify the suggestion that the OH signal observed in our study and the rate we measured arise from multiphoton effects.

References and Notes

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In Science Magazine

TECHNICAL COMMENTS
Comment on "Atmospheric Hydroxyl Radical Production from Electronically Excited NO₂ and H₂O"

Scott Carr, Dwayne E. Heard, and Mark A. Blitz (17 April 2009)
Science 324 (5925), 336b. [DOI: 10.1126/science.1166669]
 |  Abstract »  |  Full Text »  |  PDF »  |  Supporting Online Material »

REPORTS
Atmospheric Hydroxyl Radical Production from Electronically Excited NO₂ and H₂O

Shuping Li, Jamie Matthews, and Amitabha Sinha (21 March 2008)
Science 319 (5870), 1657. [DOI: 10.1126/science.1151443]
 |  Abstract »  |  Full Text »  |  PDF »  |  Supporting Online Material »

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