Nighttime chemistry of furanoids and terpenes: Temperature dependent kinetics with NO3 radicals and insights into the reaction mechanism

In this study, the gas phase reaction of NO₃ radical with three furanoids, furan (F), 2-methylfuran (2-MF) and 2,5-dimethylfuran (2,5-DMF) were investigated using a relative rate method in a temperature regulated atmospheric simulation chamber (THALAMOS). As part of this study, the temperature dependence of two monoterpenes, α-pinene (α-P) and 2-carene (2-C), that were used as reference molecules, is also reported. The kinetic measurements were performed in the range of 263–373 K, atmospheric pressure using zero air as bath gas. The reaction was followed using Selected ion flow tube mass spectrometry (SIFT-MS) to monitor in real time the mixing ratios of the investigated species. The corresponding Arrhenius expressions obtained were:

$k_{\alpha -P+NO3}\left(263-378K\right)=\left(1.32\pm 0.16\right)\times {10}^{-12}\times e^{\frac{462\pm 70}{T}}{\text{cm}}^3{\text{molecule}}^{‐1}{s}^{‐1}$.

$k_{2-C+NO3}\left(296–433K\right)=\left(8.77\pm 2.71\right)\times {10}^{-13}\times e^{\frac{904\pm 96}{T}}{\text{cm}}^3{\text{molecule}}^{‐1}{s}^{‐1}$.

$k_{F+NO3}\left(263-353K\right)=\left(7.55\pm 1.96\right)\times {10}^{-13}\times e^{\frac{\left(254\pm 79\right)}{T}}{\text{cm}}^3{\text{molecule}}^{‐1}{s}^{‐1}$.

$k_{2-MF+NO3}\left(263-373K\right)=\left(7.76\pm 2.62\right)\times {10}^{-13}\times e^{\frac{922\pm 262}{T}}{\text{cm}}^3{\text{molecule}}^{‐1}{s}^{‐1}$.

$k_{2,5-DMF+NO3}\left(298–353K\right)=\left(2.58\pm 0.77\right)\times {10}^{-13}\times e^{\frac{1692\pm 136}{T}}{\text{cm}}^3{\text{molecule}}^{‐1}{s}^{‐1}$.

Where the estimated uncertainties include the 1σ precision of the fit and the estimated uncertainty in the precision of the Arhenius fit of the reference compound. In the case of furanoids, the negative temperature-dependence of the rate coefficients shows that the reaction mechanism is complex, indicating that the reaction involves the formation of an adduct that could lead either to NO₃ addition to the double bond, as well as, in the case of 2-MF and 2,5-DMF, the H-abstraction from the methyl group attached to the ring. In addition to the kinetics measurements, the major products formed from the reaction of furan and 2,5-DMF with NO₃ were studied. The two identified products from the reaction of furan with NO₃, were 3H-furan-2-one (C₄H₄O₂) and 2-butenedial (C₄H₄O₂), formed by NO₃ addition to the double bond of the furan ring. 3-Hexene-2,5-dione (C₆H₈O₂) and 5-methylfurfural (C₆H₆O₂) are the two major products of 2,5-DMF reaction with NO₃, linked to the addition and abstraction pathways, respectively. Considering that 5-methylfurfural is the only primary product of the H elimination pathway of 2,5-DMF, the determination of its formation yields as a function of temperature, allowed us to extract the temperature dependence of the rate coefficients of the abstraction and addition pathways.

Overall, the NO₃ oxidation of the studied furanoids, is expected to be the dominant tropospheric loss process, in the studied temperature range.