Abstract
Abstract. Theoretical, laboratory, and chamber studies have shown fast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene, largely due to unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospherically relevant NO levels (between 0.15 and 2 ppbv – parts per billion by volume) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations, and the best agreement was observed when at least 50 % of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 ppbv. For these levels of NO, up to 50 % of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through the photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehydic hydrogen shift (rate constant ∼10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum of methyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearly independent of the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (<1 h), forming the OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the distribution of oxygenated products, which affect radical levels and organic aerosol downwind of unpolluted isoprene-dominated regions.
Highlights
The hydroxyl radical (OH) is the main daytime oxidant controlling the removal and transformation of gaseous pollutants in the atmosphere (Levy, 1974)
While this mechanism can lead to numerical agreement with the Berndt et al yields, the argumentation is not based on actual quantitative theoretical work on each reaction step and may be unable to discriminate between alternative mechanisms or yields in this subtle, complex chemistry; in this paragraph, we briefly examine a few aspects of the mechanism that warrant further investigation
Measurements of OH reactivity, OH, HO∗2 and RO∗2 radical concentrations, and other important trace gases were compared to results from different model calculations all based on a stateof-the-art chemical mechanistic model (MCMv3.3.1) (Jenkin et al, 2015)
Summary
The hydroxyl radical (OH) is the main daytime oxidant controlling the removal and transformation of gaseous pollutants in the atmosphere (Levy, 1974). The most relevant isomerization reaction, the 1,6 αhydroxy-hydrogen shift (1,6-H shift), occurs for the Z-δ-RO2 radicals with a fast reaction rate coefficient (measured at 3.6 and 0.4 s−1 at 298 K for OH addition on C4 and C1, respectively, by Teng et al (2017) These experimental values are used directly within the Caltech mechanism and are in good agreement with the calculated rates in LIM1 (Peeters et al, 2014) (within 40 %). In addition to the above products, both experimental (Berndt et al, 2019) and theoretical (Müller et al, 2019) studies suggest the formation of an hydroperoxy-epoxy-carbonyl compound (∼ 0.15) Both currently available explicit isoprene oxidation mechanisms, i.e. the Master Chemical Mechanism (MCMv3.3.1) (Jenkin et al, 2015) and the Caltech mechanism (Wennberg et al, 2018), use a yield of 0.5 and 0.4, respectively, for HPALD, with the Caltech mechanism distinguishing between β- (0.15) and δ-HPALD (0.25). The global impact of the optimized isoprene mechanism on the OH radical concentration is shown
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