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The negligible E2 yield from hydroxyl-initiated oxidation could result from additional NOx chemistry that terminates the peroxy radicals before they can combine.

Peroxy radicals in the figure are indicated by a green label, E1 by a red label and E2 by a blue label.

The homogeneous recombination of two peroxy radicals via elimination of O2 produces the covalently bound dimer C20H30O14 chosen as the E2 surrogate.

The effective formation rate of the C=O-functionalized peroxy radical is calculated to be less than about 10−3 s−1, which is comparable to its wall deposition rate.

The timescale with respect to the subsequent autoxidation reaction, on the other hand, is expected to be of the order of seconds, by analogy with that for branched-chain peroxy radicals73.