Index Geophysics

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

In isoprene-impacted environments, carbonyl nitrates are produced from NO₃-initiated isoprene oxidation, which constitutes a potentially important NO_<i>x</i> reservoir. To better understand the fate of isoprene carbonyl nitrates, we synthesized a model compound, <i>trans</i>-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.

Xiong, Fulizi

Borca, Carlos H.

Slipchenko, Lyudmila V.

Shepson, Paul B.

2018-09-10

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10.5194/acp-16-5595-2016

https://acp.copernicus.org/articles/16/5595/2016/

eISSN: 1680-7324

eng