Impurity contribution to ultraviolet absorption of saturated fatty acids

TL;DR

This study reveals that the ultraviolet absorption of saturated fatty acids is primarily due to impurities, not the acids themselves, significantly impacting understanding of their photochemical behavior in atmospheric processes.

Contribution

It demonstrates that impurities cause the observed UV absorption in saturated fatty acids, clarifying previous uncertainties and refining photolysis rate estimates.

Findings

Impurities are responsible for UV absorption in saturated fatty acids.

Absorption cross sections are much smaller than those of atmospheric carbonyls.

Photolysis rates of fatty acids are significantly lower than previously estimated.

Abstract

Saturated fatty acids are abundant organic compounds in oceans and sea sprays. Their photochemical reactions induced by solar radiation have recently been discovered as an abiotic source of volatile organic compounds, which serve as precursors of secondary organic aerosols. However, photoabsorption of wavelengths longer than 250 nm in liquid saturated fatty acids remains unexplained, despite being first reported in 1931. Here we demonstrate that the previously reported absorption of wavelengths longer than 250 nm by liquid nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at most) intrinsically contained in nonanoic acid reagents. Absorption cross sections of nonanoic acid newly obtained here indicate that the upper limit of its photolysis rate is three-to-five orders of magnitude smaller than those for atmospherically relevant carbonyl compounds.