Biogenic organic precursors play an important role in atmospheric new particle formation (NPF). One of the major precursor species is α-pinene, which upon oxidation can form a suite of products covering a wide range of volatilities. Highly oxygenated organic molecules (HOMs) comprise a fraction of the oxidation products formed. While it is known that HOMs contribute to secondary organic aerosol (SOA) formation, including NPF, they have not been well studied in newly formed particles due to their very low mass concentrations. Here we present gas- and particle-phase chemical composition data from experimental studies of α-pinene oxidation, including in the presence of isoprene, at temperatures (−50 and −30 ^∘C) and relative humidities (20 % and 60 %) relevant in the upper free troposphere. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber. The particle chemical composition was analyzed by a thermal desorption differential mobility analyzer (TD-DMA) coupled to a nitrate chemical ionization–atmospheric pressure interface–time-of-flight (CI-APi-TOF) mass spectrometer. CI-APi-TOF was used for particle- and gas-phase measurements, applying the same ionization and detection scheme. Our measurements revealed the presence of C₈₋₁₀ monomers and C₁₈₋₂₀ dimers as the major compounds in the particles (diameter up to ∼ 100 nm). Particularly, for the system with isoprene added, C₅ (C₅H₁₀O₅₋₇) and C₁₅ compounds (C₁₅H₂₄O₅₋₁₀) were detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C₅ and C₁₅ compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene oxidation products enhance the growth of particles larger than 15 nm. Additionally, we report on the nucleation rates measured at 1.7 nm (J_1.7 nm) and compared with previous studies, we found lower J_1.7 nm values, very likely due to the higher α-pinene and ozone mixing ratios used in the present study.

中文翻译：

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α-蒎烯成核纳米粒子的化学成分及低温下异戊二烯和相对湿度的影响

生物有机前体在大气新粒子形成 (NPF) 中起着重要作用。主要的前体物质之一是α-蒎烯，它在氧化后可以形成一套涵盖各种挥发性的产品。高度氧化的有机分子 (HOM) 包含一小部分形成的氧化产物。虽然已知 HOM 有助于二次有机气溶胶 (SOA) 的形成，包括 NPF，但由于它们的质量浓度非常低，它们在新形成的颗粒中尚未得到很好的研究。在这里，我们提供了来自α-蒎烯氧化实验研究的气相和颗粒相化学成分数据，包括在异戊二烯存在的情况下，在温度 ( − 50 和− 30  ^∘C) 和与自由对流层高层相关的相对湿度（20% 和 60%）。测量在 CERN Cosmics Leaving Outdoor Droplets (CLOUD) 室进行。通过与硝酸盐化学电离 - 大气压力界面 - 飞行时间（CI-APi-TOF）质谱仪耦合的热解吸微分迁移率分析仪（TD-DMA）分析颗粒化学成分。CI-APi-TOF 用于粒子和气相测量，应用相同的电离和检测方案。我们的测量显示C的存在_8-10单体和C _18-20 二聚物中的颗粒作为主要化合物（直径可达 〜  100纳米）。特别是对于加入异戊二烯的体系，C ₅ (C ₅ H_{检测到 10} O _5-7 ) 和 C ₁₅化合物 (C ₁₅ H ₂₄ O _5-10 )。该观察结果与先前观察到的此类化合物在气相中的形成一致。然而，尽管 C ₅和 C ₁₅化合物不容易成核，但我们的测量表明它们仍然可以在自由对流层条件下促进粒子生长。对于此处报告的实验，异戊二烯氧化产物最有可能促进大于 15 nm 的颗粒的生长。此外，我们报告了在 1.7 nm ( J _{1.7 nm} )处测量的成核速率，并与之前的研究相比，我们发现较低的 J_{1.7 nm}值，很可能是由于本研究中使用的α-蒎烯和臭氧混合比较高。