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Hydroxyl radicals (OH) are the most important reagent for theoxidation of trace gases in the atmosphere. OH concentrations measured duringrecent field campaigns in isoprene-rich environments were unexpectedly large.A number of studies showed that unimolecular reactions of organic peroxyradicals (RO2) formed in the initial reaction step of isoprene with OHplay an important role for the OH budget in the atmosphere at low mixingratios of nitrogen monoxide (NO) of less than 100 pptv. It has also beensuggested that similar reactions potentially play an important role forRO2 from other compounds. Here, we investigate the oxidation ofmethacrolein (MACR), one major oxidation product of isoprene, by OH inexperiments in the simulation chamber SAPHIR under controlled atmosphericconditions. The experiments show that measured OH concentrations areapproximately 50% larger than calculated by the Master Chemical Mechanism(MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). Theanalysis of the OH budget reveals an OH source that is not accounted for inMCM, which is correlated with the production rate of RO2 radicals fromMACR. In order to balance the measured OH destruction rate, 0.77 OH radicals(1σ error: ± 0.31) need to be additionally reformed from eachreaction of OH with MACR. The strong correlation of the missing OH sourcewith the production of RO2 radicals is consistent with the concept of OHformation from unimolecular isomerization and decomposition reactions ofRO2. The comparison of observations with model calculations gives a lowerlimit of 0.03 s−1 for the reaction rate constant if the OH source isattributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OOformed in the MACR + OH reaction as suggested in the literature(Crounse et al., 2012). This fast isomerization reaction would be a competitor tothe reaction of this RO2 species with a minimum of 150 pptv NO. Theisomerization reaction would be the dominant reaction pathway for thisspecific RO2 radical in forested regions, where NO mixing ratios aretypically much smaller

OH regeneration from methacrolein oxidation investigated in the atmosphere simulation chamber SAPHIR