Formation of carbonyls and hydroperoxyenals (HPALDs) from the OH radical reaction of isoprene for low-NOx conditions: influence of temperature and water vapour content

The formation of gas-phase products from the reaction of OH radicals with isoprene for low-NO_x conditions (NO_x] ≤ 10¹⁰ molecule cm^?3) has been studied in an atmospheric pressure flow tube (Institute for Tropospheric Research-Laminar Flow Tube, IfT-LFT) operating in the temperature range of 293–343 K and a relative humidity of < 0.5 % up to 50 %. The photolysis of H₂O₂ or ozone photolysis in the presence of water vapour served as the NO_x-free OH radical sources. For dry conditions at 293 K, the measured yields of methyl vinyl ketone (MVK), 0.07?±?0.02, and methacrolein (MACR), 0.12?±?0.04, were in reasonable agreement with literature data. Beside the C₄-carbonyls, further product signals have been attributed tentatively to glycolaldehyde, methylglyoxal, hydroxyacetone, 3-methylfuran, C₅-hydroperoxyenals (HPALDs) and C₅-hydroxy-hydroperoxides. A simplified, “classical” reaction mechanism without efficient HPALD production describes well the observed yield for MVK and MACR. Unexpected high MVK and MACR yields of up to 0.65 in total were measured under conditions of a relative humidity of 50 % using both OH radical sources and two different measurement techniques for organics (proton transfer reaction mass spectrometry and gas chromatography with flame ionization detector). The reaction mechanism applied is not able to describe the strong increase of MVK and MACR yields with increasing water vapour content. The signal attributed to the HPALDs showed a distinct rise of about one order of magnitude increasing the temperature from 293 K to 343 K. A rough estimate leads to a HPALD yield of 0.32 at 343 K with an uncertainty of a factor of two. The results of this study do not support a predominant formation of HPALDs under atmospheric conditions in low-NO_x areas. The surprisingly high MVK and MACR yields measured for a relative humidity of 50 % and the formation of glycolaldehyde, methylglyoxal and hydroxyacetone necessitate further research.