Atmospheric Photo-Thermal Oxidation of Methacrolein as a New Source of Radicals in Atmospheric Chemistry

Abstract:

About 110-165 million tonnes of methacrolein (MACR) is formed in the atmosphere every year from biogenic sources. Despite its importance in the atmosphere, the photochemistry of MACR has not been well documented, with only four experimental studies published.

This work studied known and new photochemical reactions of MACR. The alpha-cleavage of MACR was studied through the formation of the 2-propenyl radical with the velocity map imaging technique (VMI). The threshold energy for the cleavage on the T₁ electronic state was found to be 394 kJ/mol (304 nm) at around 10 K. A fluorescence assay by gas expansion (FAGE) experiment monitored the production of hydroperoxyl (HO₂) radicals from the photolysis of MACR over a wavelength range of 295-355 nm. The FAGE experiment gave a threshold energy for the formation of hydrogen radicals on the S₀ electronic state as 374 kJ/mol (320 nm). The same experiment found wavelength independent production of hydroperoxyl from the photolysis of MACR between 320-342 nm. This hydroperoxyl production cannot be explained through the above radical pathways, due to the photon energies being too low for these pathways to be accessible. The hydroperoxyl formation was attributed to oxygen abstracting hydrogen from photo-excited MACR, known as atmospheric photo-thermal oxidation (APTO). APTO is an unpublished mechanism that has recently been observed in formaldehyde.

Fourier transform infrared (FTIR) spectroscopy was used to obtain quantum yields of the final photochemical products of MACR in the wavelength range 290-340 nm. Experiments found that the addition of molecular oxygen increased the total quantum yield of MACR by up to 15-fold at wavelengths longer than 310 nm. This increase was attributed to the APTO mechanism.