Monoterpenes and isoprene, emitted primarily by vegetation, represent a globally dominant source of secondary organic aerosol (SOA) precursors. Previous studies have shown that isoprene suppresses particle formation from the oxidation of monoterpenes in both isoprene-rich regions and laboratory chambers, but a mechanistic understanding of such isoprene effect remains limited. Here we combined flow reactor experiments and kinetic model simulations to understand how isoprene affects particle formation from the ozonolysis of α-pinene. We find that in the O₃/OH “two-oxidant” regime (without an OH scavenger), α-pinene particle formation is substantially suppressed by the presence of isoprene, mainly because of the termination of organic peroxy radicals (RO₂) arising from α-pinene by those from the OH oxidation of isoprene to form C₁₁–C₁₅ species rather than the less volatile C₁₆–C₂₀ dimers. However, in the “O₃-only” regime (with an OH scavenger), such isoprene effect is diminished because of the reduced oxidation of isoprene and inefficient formation of isoprene-derived RO₂, and the small reduction in C₁₆–C₂₀ species due to RO₂ terminations may be compensated by low-volatility species formed via the reactions of stabilized Criegee intermediates. These results suggest that the OH chemistry plays an important role in the isoprene suppression of particle formation from α-pinene ozonolysis. Model simulations for conditions typical of isoprene-rich forested regions show that the formation of α-pinene-derived C₁₉ and C₂₀ species that play a pivotal role in particle nucleation and early growth, is reduced by 14%–48% in the daytime (high OH) and 2%–9% in the nighttime (low OH) for isoprene/α-pinene concentration ratios of 3 to 15. Therefore, we would expect that the isoprene suppression effect on particle formation from monoterpenes is strongly dependent on, among other factors, the OH level in the atmosphere.

中文翻译：

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羟基自由基化学在异戊二烯抑制α-P烯臭氧分解颗粒形成中的重要性

主要由植被排放的单萜和异戊二烯代表了全球主要的次要有机气溶胶（SOA）前体来源。以前的研究表明，异戊二烯可抑制富含异戊二烯的区域和实验室室内单萜类化合物氧化产生的颗粒，但对这种异戊二烯效果的机理了解仍然有限。在这里，我们将流动反应器实验与动力学模型模拟相结合，以了解异戊二烯如何影响α-pine烯的臭氧分解过程中的颗粒形成。我们发现，在O ₃ / OH“双氧化剂”体系（无OH清除剂）中，异戊二烯的存在基本上抑制了α-pine烯颗粒的形成，这主要是由于有机过氧自由基（RO _2）的终止）由α-pine烯经异戊二烯的OH氧化形成C ₁₁ –C ₁₅物种而不是挥发性较小的C ₁₆ –C ₂₀二聚体而产生。但是，在“仅O ₃ ”状态（使用OH清除剂）中，由于异戊二烯的氧化减少和异戊二烯衍生的RO ₂形成效率低，以及C ₁₆ –C ₂₀的小幅减少，这种异戊二烯效应得以减弱。RO ₂导致的物种可以通过稳定的Criegee中间体反应形成的低挥发性物质来补偿末端。这些结果表明，OH化学在异戊二烯抑制α-pine烯臭氧分解中的颗粒形成中起重要作用。对富含异戊二烯的林区典型条件的模型模拟表明，在颗粒成核和早期生长中起关键作用的α-pine烯衍生的C ₁₉和C ₂₀物种的形成在白天减少了14％–48％ （高OH）和夜间（低OH）的2％–9％（异戊二烯/α-pine烯的浓度比为3至15）。因此，我们预计异戊二烯对单萜颗粒形成的抑制作用强烈依赖于，除其他因素外，大气中的OH含量。