Articles | Volume 16, issue 9
Research article
04 May 2016
Research article |  | 04 May 2016

Photochemical degradation of isoprene-derived 4,1-nitrooxy enal

Fulizi Xiong, Carlos H. Borca, Lyudmila V. Slipchenko, and Paul B. Shepson

Abstract. In isoprene-impacted environments, carbonyl nitrates are produced from NO3-initiated isoprene oxidation, which constitutes a potentially important NOx reservoir. To better understand the fate of isoprene carbonyl nitrates, we synthesized a model compound, trans-2-methyl-4-nitrooxy-2-buten-1-al (4,1-isoprene carbonyl nitrate, or 4,1-isoprene nitrooxy enal), and investigated its photochemical degradation process. The measured OH and O3 oxidation rate constants (298 K) for this nitrooxy enal are 4.1(±0.7)  ×  10−11 cm3 molecules−1 s−1 and 4.4(±0.3)  ×  10−18 cm3 molecules−1 s−1, respectively. Its UV absorption spectrum was determined, and the result is consistent with TDDFT calculations. Based on its UV absorption cross section and photolysis frequency in a reaction chamber, we estimate that the ambient photolysis frequency for this compound is 3.1(±0.8)  ×  10−4 s−1 for a solar zenith angle of 45°. The fast photolysis rate and high reactivity toward OH lead to a lifetime of less than 1 h for the isoprene nitrooxy enal, with photolysis being a dominant daytime sink. The nitrate products derived from the OH oxidation and the photolysis of the nitrooxy enal were identified with an iodide-based chemical ionization mass spectrometer. For the OH oxidation reaction, we quantified the yields of two nitrate products, methyl vinyl ketone nitrate and ethanal nitrate, which together contributed to 36(±5) % of the first-generation products.

Short summary
Here we report on a detailed study of the photochemistry and fate of a nitrooxy enal that is produced from the reaction of NO3 with isoprene. We synthesized the 4,1-nitrooxy enal, purified it, and measured the O3 and OH reaction rate constants, and determined the atmospheric photodissociation rate constant for specified radiation conditions. The determined fast photolysis rate and high reactivity toward OH lead to a lifetime of less than 1 hour, with photolysis being a dominant daytime sink.
Final-revised paper