Abstract:

Objective: Under simulated forest fire preheating conditions, we studied the volatilization characteristics of volatile oils from fresh needles of Pinus yunnanensis. The aim of this study is to explore the mechanism by which volatile oils influence forest fire behaviors. Method: The volatile oil in P. yunnanensis needles was extracted by steam distillation, and the volatile oil content was determined by measuring the volume and mass. Different forest fire preheating conditions were simulated using an automatic headspace sampler, which was coupled with gas chromatography–mass spectrometry (GC–MS) to identify the composition of volatilized gases. The gas composition and content of volatile oils under the various preheating conditions were determined and compared for investigating the volatilization characteristics of the volatile oil. Result: 1) The volume content of volatile oils in fresh P. yunnanensis needles ranged from 10.61 to 15.43 mL?kg^?1, with a mean of 13.13 mL?kg^?1 and standard deviation of 2.42 mL?kg^?1. The mass content ranged between 7.86 and 9.74 g?kg^?1, with a mean of 8.52 g?kg^?1 and standard deviation of 1.05 g?kg^?1. 2) Under different preheating conditions with different temperatures and durations, a total of 48 volatile oil components were isolated and identified, most of which were terpenes, including monoterpenes, sesquiterpenes, and other oxygen-containing terpene derivatives, with trace amounts of acids, alcohols, and esters. Under the condition of preheating at 150 ℃ for 15 min, there were 27 terpenes in the volatile oil, accounting for 89.16% of the total content, including 8 monoterpenes, accounting for 54.70%, 14 sesquiterpenes, accounting for 31.86%, and 5 other oxygen-containing terpene derivatives, accounting for 2.60%. 3) The gases of volatile oils were increasingly released from P. yunnanensis needles with an increase in the preheating temperature and duration. The volatile oil slowly released at 50 ℃ and 75 ℃, with a considerably low yield of gases released. The yield of volatile oil released at 100 ℃ was notably greater than that at 50 ℃ and 75 ℃, and the volatile oil gases continued to release, without reaching an equilibrium state. The yield of volatile oil released at 125 ℃ increased substantially compared with that under the aforementioned three temperatures. After preheating for 30 min at 125 ℃, the headspace equilibrium state was reached with no further increase in the yield of volatile oil gases. When preheated at 150 ℃ for 15 min, the release of volatile oil was slowed down and gradually reached the equilibrium state. The yield of volatile oil gases released at 175 ℃ was the highest and did not reach the equilibrium state after 15 min of preheating. 4) Among the volatile oil components released by heating of P. yunnanensis needles, α-pinene was the compound with the highest content, and its relative contents ranged from 23.45% to 79.66% under the simulated preheating conditions. The highest relative content of α-pinene was obtained at the preheating temperature of 50 ℃. 5) Whereas the relative release of α-pinene under preheating at 175 ℃ for 15 min was 100%, the relative release of α-pinene under preheating at 50 ℃, 75 ℃, and 100 ℃ for 60 min was 14.2%, 25.1%, and 63.6%, respectively. The relative release of α-pinene under preheating at 125 ℃ for 30 min was as much as 90%, while the relative release of α-pinene reached 90% after only 15 min under preheating at 150 ℃. Conclusion: The higher the temperature, the easier it is for the volatile oil to rapidly release from P. yunnanensis needles. The volatile oil components in P. yunnanensis needles are released only when the preheating temperature meets a certain threshold, particularly for sesquiterpenes, but the release temperature is considerably lower than the boiling point of the component. The α-pinene, the primary volatile oil gas involved in combustion in fires, can be released at a low preheating temperature (50 ℃). Increase in the fire temperature and extension of the preheating duration can lead to a prominent increase in the release of α-pinene. Thus, the release and accumulation patterns of α-pinene in real fire environments are important topics for future research on extreme fire behaviors and high-intensity fires.

Key words: Pinus yunnanensis, volatile oil, volatile organic compounds, steam distillation, simulated preheating, headspace sampling, GC-MS