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

Abstract. In isoprene-impacted environments, carbonyl nitrates are produced from NO₃-initiated isoprene oxidation, which constitutes a potentially important NO_x 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.