Abstract
Abstract. Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene-rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by the Master Chemical Mechanism (MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals an OH source that is not accounted for in MCM, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, 0.77 OH radicals (1σ error: ± 0.31) need to be additionally reformed from each reaction of OH with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant if the OH source is attributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OO formed in the MACR + OH reaction as suggested in the literature (Crounse et al., 2012). This fast isomerization reaction would be a competitor to the reaction of this RO2 species with a minimum of 150 pptv NO. The isomerization reaction would be the dominant reaction pathway for this specific RO2 radical in forested regions, where NO mixing ratios are typically much smaller.
Highlights
Because peroxyacetic nitric anhydride (PAN) is formed from acetaldehyde in the sunlit chamber, the PAN mixing ratio starts increasing before MACR is injected
Measured OH concentrations during MACR oxidation experiments in the atmosphere simulation chamber SAPHIR are underestimated by chemical models by approximately 50 % (NO mixing ratios approximately 90 pptv, HO2 concentrations approximately 5 × 5 108 cm−3 and T = 301 K)
Potential mechanisms behind the missing OH source can be the production of OH 10 from unimolecular isomerization and decomposition reactions of RO2 radicals (Peeters et al, 2009; Crounse et al, 2012; Asatryan et al, 2010; da Silva, 2012) and the decomposition of double activated RO2 (Asatryan et al, 2010; da Silva, 2012) both giving the same products
Summary
25 Experiments were conducted in the atmosphere simulation chamber SAPHIR in Jülich, Germany. The chamber allows to investigate the photochemical oxidation of organic compounds at atmospheric conditions with respect to temperature, pressure, radiation and concentrations of trace gases and radicals. The major primary source for OH in the chamber is photolysis of nitrous acid (HONO), which is released from the Teflon film depending on temperature, relative humidity, and strength of radiation (Rohrer et al, 2005). RO2 radicals of approximately (1–2) × 108 cm−3 were immediately formed in the humidified clean air when the chamber roof was opened. They were partly formed by photolytic processes. Sources of trace gases and radicals can be well-parameterized 10 from reference experiments, but except for the production of HONO they played only a minor role for the experiments after MACR had been injected
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