Comment on egusphere-2023-476

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> We report improved synthetic routes to the isomeric isoprene-derived &beta;-epoxydiols (&beta;-IEPOX) in high yield (57&ndash;69 %) from inexpensive, readily available starting compounds. The syntheses do not require protection/deprotection steps or time-consuming purification of intermediates and can readily be scaled up to yield the target IEPOX isomers in gram quantities. Emissions of isoprene (2-methyl-1,3-butadiene, C₅H₈), primarily from deciduous vegetation, constitute the largest source of nonmethane atmospheric hydrocarbons. In the gas phase under low-nitric oxide (NO) conditions, addition of atmospheric hydroxyl radical (OH) followed by rapid addition of O₂yields isoprene-derived hydroxyperoxyl radicals. The major sink (&gt; 90 %) for the peroxyl radicals is sequential reaction with hydroperoxyl radical (HO₂), OH and O₂, which is then followed by the elimination of OH to yield a ~2 : 1 mixture of (2-methyloxirane-<em>cis/trans</em>-2,3-dilyl)dimethanol (<em>cis/trans</em>-&beta;-IEPOX). The IEPOX isomers account for about 80 % of the closed-shell hydroxyperoxyl products, and are rapidly taken up into acidic aerosols to form secondary organic aerosol (SOA). IEPOX-derived SOA makes a significant mass contribution to fine particulate matter (PM_2.5), which is known to be a major factor in climate forcing as well as adversely affects respiratory and cardiovascular systems of exposed populations. Prediction of ambient PM_2.5 composition and distribution, both in regional- and global-scale atmospheric chemistry models, crucially depends on the accuracy of identification and quantitation of uptake product formation. Accessibility of authentic cis- and trans-&beta;-IEPOX in high purity and in large quantity for laboratory studies underpins progress in developing models as well as identification and quantitation of PM_2.5 components.