The OH radical plays an important role in controlling the oxidative capacity of the atmosphere. Attack by OH radicals, for example, is the first step in the removal of many pollutants in the troposphere. As a result, accurate modeling of the oxidizing or cleansing capacity of the atmosphere requires knowledge of all significant sources and sinks of these radicals. Over the past year we have been investigating various potential new sources of atmospheric OH radicals. Through our efforts we have discovered that the bimolecular reaction of electronically excited nitrogen dioxide, generated by absorption of visible light, with water molecules can lead to significant OH radical production that, apparently, are currently unaccounted for in tropospheric models.

            The primary production mechanism of tropospheric OH radicals is through the photolysis of ozone, which generates electronically excited O(¹D) atoms at wavelengths λ≤ 320 nm. The subsequent reaction of these O(¹D) atoms with atmospheric water molecules generates the OH radicals. An important constraint associated with the above mechanism is that it requires UV light. There are situations, however, such as those corresponding to high solar zenith angles (which is the angle the sun makes relative to the earth's surface normal), where the sun light traverses an extended optical path through the atmosphere resulting in the incoming solar flux being greatly depleted of its UV component due to scattering and absorption. For these scenarios alternate OH radical production mechanism involving photochemistries at longer wavelengths can become important. The reaction of electronically excited nitrogen dioxide with water can be a significant source of OH under these conditions.

            To better appreciate the OH generating potential of this new mechanism, we note that NO₂ has a broad absorption spectrum in the visible and near UV which starts around 250 nm and extends down to about 650 nm with a maximum around 410 nm. Absorption of solar radiation by NO₂ at wavelengths shorter than ~420 nm leads to rapid photodissociation and the formation of O(³P) atoms in the atmosphere.  While excitation of NO₂ in the blue end of its absorption band leads to photodissociation, excitation at longer wavelengths (λ>420 nm) leads to the formation of electronically excited NO₂ having fluorescence lifetimes of tens of microseconds. The long lifetime of these states arise from the mixing of the A²B₂ and B²B₂ excited electronic states with the X²A₁ ground electronic state. Upon photoexcitation to these long lived electronic states, most of the excited NO₂ will be quenched by collisions with bath gases such as N₂ and O₂. However, because water is a tropospheric trace gas in relatively large abundance, some of the photo-excited NO₂ molecules can also collide with water. It is well known that water quenches electronically excited NO₂ efficiently; however there has been no previous report of any reaction between NO₂* and H₂O.  If a significant fraction of the electronically excited NO₂ reacts with water, then the following mechanism for OH formation becomes possible:

 NO₂ + hν (λ<420 nm) → NO₂*                                                                      (4)

 NO₂* + M → NO₂ + M                     (M≡ N₂ or O₂ or H₂O)                         (5)

 NO₂* + H₂O → OH + HONO                                                                       (6)

We note that apart from OH radicals formed directly through reaction-6, photolysis of the HONO reaction product can also give rise to additional OH:

HONO + hν (λ<390 nm) → OH + NO                                                                       (7)

Applying steady state approximation to NO₂* in the above reaction sequence, one finds that the rate of OH formation through this mechanism is given by: 

R_OH = 2 j₄ [NO₂] [H₂O] / (k₆[H₂O] +k₅[M])                                                     (8)

In the above expression, j₄ is the photoexcitation rate constant for NO₂, k₅ is the total collisional quenching rate constant of NO₂* by “air”, and k₆ is the rate constant for the NO₂*+ H₂O reaction. A study of Eqs.8 suggests that if k₆ is sufficiently large and if the HONO molecule formed in reaction-6 is also readily photolyzed, then the yield of OH from the NO₂* + H₂O reaction can be a significant fraction of that arising from the more traditional O(¹D)+ H₂O reaction.  In order to test the viability of this new mechanism we have carried out laboratory studies consisting of rate measurement for the NO₂*+H₂O reaction in conjunction with laser induced fluorescence (LIF) detection of the OH products. Our initial results suggest that the rate of reation-6 is ~1.7x 10^-13 cm³/molecule sec, making it sufficiently fast to make the NO₂*+ H₂O reaction a significant source of tropospheric OH radicals.  Finally it is also worth mentioning that in addition to providing us the opportunity to pursue scientific research, support of this work by ACS-PRF has allowed the post-doc and graduate student working on the project to gain valuable scientific training.