Nighttime to daytime transition of the oxidation products of isoprene by NO3 radicals

Biogenic volatile organic compounds (BVOC) dominate the overall VOC emissions. Isoprene is the most common BVOC emitted from vegetation, accounting up to 50% of the total BVOC emissions. Despite being emitted in daytime it can accumulate in the stratified nocturnal layer. Thus, the oxidation of isoprene by nitrate radicals (NO₃) may be of high importance. A series of experiments were conducted in the atmospheric simulation chamber SAPHIR in Jülich, Germany, in order to investigate the gas and particle phase products of the oxidation of isoprene by NO₃, under a variety of conditions (e.g. high RO₂, high HO₂, nighttime to daytime transition, with and without seed aerosol) using a wide range of instrumentation. However, herein the focus is on the results of gas-phase product characterisation using high resolution time of flight chemical ionization mass spectrometers (HR-ToF-CIMS) using iodide or bromide as the primary reagent ion. The use of two HR-ToF-CIMS with different primary reagents provides possibilities to scrutinise the time profiles of isomers of selected products.

We will discuss qualitatively and quantitatively how the distribution of oxidation products change under different conditions, with a focus on the nighttime daytime transition of the major products and the role of subsequent OH oxidation on the products initially formed by NO₃ oxidation. Generally, the dominant gas phase products include compounds like nitrooxy hydroperoxide (INP) & dihydroxy nitrate (IDHN) (C₅H₉NO₅), carbonyl nitrate (ICN) (C₅H₇NO₅), hydroxy nitrate (IHN) (C₅H₉NO₄), hydroxy hydroperoxy nitrate (IHPN) (C₅H₉NO₆), as well as a C₄ compound (C₄H₇NO₅) among others.