Pressure-Dependent I-Atom Yield in the Reaction of CH2I with O2Shows a Remarkable Apparent Third-Body Efficiency for O2

The formation of I atom and Criegee intermediate (CH 2 OO) in the reaction of CH 2 I with O 2 has potential relevance for aerosol and organic acid production in the marine boundary layer. We report measurements of the absolute yield of I atom as a function of pressure for N 2, He, and O 2 buffer at 298 K. Although the overall rate coefficient is pressure-independent, the I-atom yield, correlated with CH 2 OO, decreases with total pressure, presumably because of increased stabilization of CH 2 IOO. The extrapolated yield of the I + Criegee channel under tropospheric conditions is small but nonzero, ∼0.04. The zero-pressure limiting I-atom yield is unity, within experimental error, implying negligible branching to IO + CH 2 O. The apparent collision efficiency of O 2 in stabilizing CH 2 IOO is a remarkable factor of 13 larger than that of N 2, which suggests unusually strong interaction or possible reaction between the chemically activated CH 2 IOO # and O 2 .