Direct evidence for a substantive reaction between the Criegee intermediate, CH 2 OO, and the water vapour dimer †

The C 1 Criegee intermediate, CH 2 OO, reaction with water vapour has been studied. The removal rate constant shows a quadratic dependence on [H 2 O], implying reaction with the water dimer, (H 2 O) 2 . The rate constant, k CH 2 OO+(H 2 O) 2 = (4.0 (cid:2) 1.2) (cid:3) 10 (cid:4) 12 cm 3 molecule (cid:4) 1 s (cid:4) 1 , is such that this is the major atmospheric sink for CH 2 OO. The experiments were carried out using our newly constructed multiplexing absorption kinetics spectrometer coupled to laser flash photolysis. Full details about the setup will be described in a forthcoming publication. The essential details are as follows: the a multi-passed times

Direct evidence for a substantive reaction between the Criegee intermediate, CH 2  This reaction is only just exothermic and is the near exclusive channel at low pressure. 3 As the total pressure is increased the mechanism for this reaction switches to an association process: where at atmospheric pressure the CH 2 OO yield is 0.18. 3 Therefore this reaction and analogues using larger organic di-iodides are convenient sources of Criegee intermediates over a wide range of pressures, and in the last few years there has been a plethora of studies 4-7 that have used this type of reaction to determine direct properties of the Criegee intermediate, including the C2 species CH 3 CHOO. 8 Many of these new direct kinetic measurements on Criegee intermediates have determined rate constants significantly higher than older, indirect studies and their importance in atmospheric chemistry has been re-evaluated, in particular its reaction with SO 2 in competition with unimolecular decomposition 9 and photolysis. 4 The experiments were carried out using our newly constructed multiplexing absorption kinetics spectrometer coupled to laser flash photolysis. Full details about the setup will be described in a forthcoming publication. The essential details are as follows: the output from a xenon lamp was multi-passed 14 times through the 1.5 metre reaction cell and configured such that this probe beam was overlapped for the majority of this distance with the 248 nm excimer laser that passed along the length of the reactor. This probe beam was then directed via a fibre optic into a spectrograph (Jobin Yvon CP140-103) where the wavelengths 250-850 nm were simultaneously measured using a CCD image sensor (Hamamatsu S7031, back-thinned FFT-CCD). All the wavelengths were recorded for 1 millisecond intervals for a total of 200 milliseconds and transferred to a PC via a PCI interface board. All these data were processed by the PC using a custom built LabView program before the next photolysis laser pulse; the excimer laser was fired between 1-0.2 Hz. At each wavelength (l), the 50 points before the excimer laser pulse were averaged and assigned to I 0 (l) (intensity of the probe light), and all these I 0 (l) were compared to all the wavelength time points after the excimer laser fired, I(l). The program calculated DI/I 0 for each wavelength versus time, the time-resolved differential absorption signal for each wavelength.
An example of a spectrum at early time after photolysis is shown in Fig. 1, where it can be seen that the spectrum between 300-400 nm is dominated by the C1 Criegee intermediate. At longer wavelengths absorption by the IO radical is also observable (CH 2 I 2 photolysis produces a small amount of CH 2 , which reacts with O 2 to produce O( 3 P) 18 which in turn reacts with the precursor to produce IO 19 ). The IO is removed from the system much more slowly than CH 2 OO. H 2 O vapour was added to the system by passing the main gas, N 2 (BOC, OFN), through a bubbler filled with deionized water, where the pressure in the bubbler was measured and could be varied over range 1000-2000 Torr. [O 2 ] (B2 Â 10 17 molecule cm À3 ) was high enough to ensure R1 was rapid and the total pressure was varied between 50-400 Torr, where N 2 was the main buffer gas. At each pressure the kinetics of the system were recorded without H 2 O vapour and then the N 2 flow was switched to the H 2 O bubbler, where the pressure can be adjusted. These experiments were carried out at 294 K.
The features of the spectrum in Fig. 1, especially between 350-420 nm, are consistent with the absorption literature spectrum of CH 2 OO. 20,21 However, the present experiment records the differential absorption spectrum, DI/I 0 , and it needs to be corrected for CH 2 I 2 photolysis and IO before it can quantitatively be used to compare to the literature. Also, the spectrum is Fig. 1 has been corrected for scattered photons (4850 nm) hitting the CCD camera, see ESI † for further details. Hence this work cannot be compared with absolute crosssections from any previous study at present. It is at 350 nm where the cross-section value reported by Ting et al. 22 is ca. a factor of three times lower than the values reported by Beames et al. 4 and Sheps. 21 In our previous study, using a completely different absorption setup on CH 2 I 2 photolysis in the presence of O 2 at atmospheric pressure, we mis-assigned the Criegee intermediate spectrum as the CH 2 IOO from reaction (R1b). 23 If we now re-assign this spectrum as CH 2 OO and divide the cross-sections by 0.18, which our recent measurements have determined as the yield of Criegee intermediate at atmospheric pressure, 3 the spectrum is 40% lower at 350 nm than the crosssection value of Ting et al. 22 If the reaction of CH 2 OO with water is slow, then selfreaction 6 should dominate CH 2 OO decay. The CH 2 OO kinetic traces were analysed using an expression for second-order loss and it was observed that they were always better described by first-order kinetics, even for the traces at the lowest total pressure, see Fig. 2 for example. At this stage it is not clear what is causing the unexpected first order kinetics; a possible explanation is unimolecular decay: 7,9 CH 2 OOproducts (R3) It should be emphasised that experiments were always carried out in the absence of water vapour and then in the presence of water vapour, and therefore the difference between the pseudofirst-order decays can be attributed to the presence of water. The reaction with H 2 O vapour is slow but it will be pseudo-first-order, and reaction of the Criegee intermediate with water vapour is only significant at the higher total pressures, where more water vapour can be added. Therefore it is reasonable to describe the Criegee intermediate loss as a first-order process: and k 3 is the first-order rate constant for CH 2 OO removal other than H 2 O and was typically B200 s À1 . It is noted that in the study by Sheps 21 using similar concentrations (B5 Â 10 11 molecule cm À3 ) the loss of CH 2 OO was also observed to be reasonably described by single exponential behaviour and k obs was comparable (180 s À1 ) to this study. Second-order CH 2 OO loss rate constants have been measured in studies 20,24 that have used much higher concentrations than used in the present study. cm À3 , respectively. The spectrum was recorded 1 milli-second after the photolysis laser. The sharp peaks in the spectrum above 400 nm are due to IO, while the spectrum between 300-460 nm is the C1 Criegee intermediate, CH 2 OO. In red and blue the literature spectra of IO and CH 2 I 2 (inverted to aid clarity) have been added to guide the eye.
The early-time spectrum in Fig. 1 shows that both CH 2 OO and IO are present and from Fig. 2 it can be seen that the CH 2 OO is removed much faster than IO under all conditions, especially at high water vapour concentrations. The data were analysed at five different wavelengths, 353, 350, 346, 344 and 341 nm, using the equation: where [B] 0 exp(Àk b t) takes into account the small but significant contribution to the absorption from IO, k b is the rate constant for IO loss, and C takes into account CH 2 I 2 photolysis, which is significant up to 400 nm. In this analysis all the data at the five wavelengths were fitted simultaneously using eqn (E2), where k obs was treated as a global parameter and all the other parameters were local. At each total pressure the k obs was determined in the presence, k 2 0 + k 3 , and absence of H 2 O, k 3 .
Therefore subtracting k obs with and without H 2 O gives k 2 0 . As can be seen in Fig. 2, the fits to the data were good and k obs was defined with errors always less than 10%. The validity of using eqn (E2) is that k obs and not k b is significantly changing as [H 2 O] is added to the system, and therefore k obs vs. [H 2 O] is a good measure of reaction (R2). In Fig. 3   In the ESI † we report analysis of the data where both k 2 and k 4 are considered, and it is concluded that k 4 is overestimated by no more than 20%.
The value reported by Berndt et al. was k 4 = 1.01 AE 0.03 Â 10 À11 cm 3 molecule À1 s À1 , which is about a factor of two larger than our present value. So while both studies are in broad agreement in that reaction (R4) is operating, there is a significant discrepancy in the magnitude of the rate constant. In the present work, the rate constant k 4 is extracted from the change in k obs on addition of water vapour, where k 2 0 /k 3 o 10 and hence leads to larger than usual error in the bimolecular rate constant, see Fig. 4, but not as high as a factor of two. [H 2 O] was determined from measuring the temperature and pressure of the bubbler and it was assumed that the entire H 2 O equilibrium vapour pressure was delivered to the reactor. This is normally a reliable method to estimate the concentration of species introduced via a bubbler; previous work using a bubbler to deliver amines to a kinetic experiment has shown good agreement between calculated concentrations and values measured directly in the cell via UV absorption. 27 However, it is acknowledged that there is a potential to overestimate the water vapour concentration. Alternatively, there may be another reason for this discrepancy. The experiments from Berndt et al. 16 used an atmospheric pressure time-of-flight mass spectrometer, where gas was sampled via a small aperture into the low pressure environment of the mass spectrometer. This gas expansion promotes cooling, which promotes dimer formation, and if dimer formation is promoted more rapidly than the reduction in pressure, then 13 were not time-resolved; the contents of the reactor flowed into a cavity spectrometer and therefore it is possible that other secondary chemistry was responsible for NO 3 production. In a forthcoming paper it will be demonstrated using the current flash photolysis/UV/Vis absorption setup that NO 3 is not significantly made by reaction of Criegee with NO 2 , and the small amount of observed NO 3 is consistent with the iodine chemistry, INO 2 + IONO 2 -NO 3 + NO 2 + I 2 . Therefore the lack of change in the NO 3 signal versus added H 2 O indicates a lack of reactivity in iodine chemistry and not Criegee intermediate chemistry.

Conclusions
The Criegee intermediate, CH 2 OO, has been observed to react in the presence of water vapour. This is the first direct measurement to show that this reaction is occurring and its kinetics implies that the reaction is predominantly with the water dimer, (H 2 O) 2 , where k 4 = (4.2 AE 1.2) Â 10 À12 cm 3 molecule À1 s À1 . This result is in support of the recent indirect measurements by Berndt et al. 16 and indicates that Criegee intermediate chemistry is essentially independent of the method of generation via either ozonolysis or iodo-alkyl radical + O 2 . The observed loss contrasts with other previous studies, but we believe the discrepancies can be explained by either the use of relatively low concentrations of water (limiting dimer formation) or via secondary chemistry in more indirect studies monitoring products. The direct observation of Criegee intermediates as used in this study will be less susceptible to such systematic errors. Fig. 4 Plot of the removal rate constant, k 2 0 , in the presence of (H 2 O) 2 . The plot is reasonably linear and yields a bimolecular rate constant, k 4 = (4.0 AE 1.2) Â 10 À12 cm 3 molecule À1 s À1 , 2s error. X-errors are estimated to be 22% and are propagated into the k 4 determination.
Using the representative range in (H 2 O) 2 concentrations (molecule cm À3 ) reported by Vereecken et al., 29 8.5 Â 10 13 (mega city) to 5.5 Â 10 14 (tropical forest), results in firstorder loss rate for C1 Criegee intermediate ranging from 357-2310 s À1 . This is significantly greater than first order loss rates with other trace gases. In the atmospheric implications from Vereecken et al. 29 reaction (R4) was included, using a theoretical estimate of the rate constant, and it was concluded that water vapour was the dominant removal process. This assessment provides a better representation of Criegee chemistry compared to modelling studies that have not included reaction (R4). 30 Given the importance of Criegee intermediates, further studies of the reaction with water dimer are required to confirm the fast kinetics reported in this work and to identify the products of the reaction.