A Multidimensional Study of the Reaction CH2I + O2: Products and Atmospheric Implications

The CH2I + O-2 reaction has been studied using laser flash photolysis followed by absorption spectroscopy, laser-induced fluorescence spectroscopy and mass spectrometry. The rates of formation of IO and CH2O were found to be dependent upon the concentration of CH2I2 under pseudo-first-order conditions ([O-2]>=[CH2I2]), demonstrating that IO and CH2O are not formed directly from the title reaction, in contrast to recent investigations by Enami et al.([1, 2]) It is proposed that the reaction proceeds via the formation of the peroxy radical species CH2IO2, which undergoes self-reaction to form CH2IO, and which decomposes to CH2O + I, and that in laboratory systems IO is formed via the reaction I + CH2IO2. The absorption spectrum of a species assigned to CH2IO2 was observed in the range 310-400 nm with a maximum absorption at 327.2 nm of sigma >= 1.7 x 10(-18) cm(2) molecule(-1). A modelling study enabled the room temperature rate coefficients for the CH2IO2 + CH2IO2 self-reaction and the I + CH2IO2 reaction to be confined within the ranges (6-12) x 10(-11) cm(3) molecule(-1) s(-1), and (1-2) x 10(-11) cm(3) molecule(-1) s(-1), respectively. In the atmosphere, CH2IO2 will slowly react with other radicals to release iodine atoms, which can then form IO via reaction with ozone. Slow formation of IO means that lower concentrations are formed, which leads to a lower propensity to form particles as the precursor molecule OIO forms at a rate which is dependent on the square of the IO concentration.