Photooxidation of volatile organic compounds (VOCs) produces condensable oxidized organics (COOs) to yield secondary organic aerosol (SOA), but the fundamental chemical mechanism for gas-to-particle conversion remains uncertain. Here we elucidate the production of COOs and their roles in SOA and brown carbon (BrC) formation from m-xylene oxidation by simultaneously monitoring the evolution of gas-phase products and aerosol properties in an environmental chamber. Four COO types with the distinct functionalities of dicarbonyls, carboxylic acids, polyhydroxy aromatics/quinones, and nitrophenols are identified from early-generation oxidation, with the yields of 25 %, 37 %, 5 %, and 3 %, respectively. SOA formation occurs via several heterogeneous processes, including interfacial interaction, ionic dissociation/acid–base reaction, and oligomerization, with the yields of (20 ± 4) % and (32 ± 7) % at 10 % and 70 % relative humidity (RH), respectively. Chemical speciation shows the dominant presence of oligomers, nitrogen-containing organics, and carboxylates at high RH and carboxylates at low RH. The identified BrC includes N-heterocycles/N-heterochains and nitrophenols, as evident from reduced single scattering albedo. The measured uptake coefficient (γ) for COOs is dependent on the functionality, ranging from 3.7 × 10⁻⁴ to 1.3 × 10⁻². A functionality-based kinetic framework is developed to predict SOA production from the observed concentrations and uptake coefficients for COOs, which reproduces the measurement from m-xylene oxidation. Our results reveal that photochemical oxidation of m-xylene represents a major source for SOA and BrC formation under urban environments, because of its large abundance, high reactivity with OH, and high yields for COOs.

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基于功能的间二甲苯光氧化形成二次有机气溶胶

挥发性有机化合物 (VOC) 的光氧化会产生可冷凝的氧化有机物 (COO) 以产生二次有机气溶胶 (SOA)，但气体到颗粒转化的基本化学机制仍不确定。在这里，我们阐明了 COO 的产生及其在 SOA 和棕色碳 (BrC) 形成中的作用。- 通过同时监测环境室中气相产物和气溶胶特性的演变来氧化二甲苯。从早期氧化中鉴定出四种具有不同功能的二羰基、羧酸、多羟基芳烃/醌和硝基苯酚的 COO 类型，产率分别为 25%、37%、5% 和 3%。SOA 的形成通过多种非均相过程发生，包括界面相互作用、离子解离/酸碱反应和低聚，产率为 (20  ±  4) % 和 (32  ± 7) 分别在 10 % 和 70 % 相对湿度 (RH) 时的百分比。化学形态分析表明低聚物、含氮有机物和高 RH 下的羧酸盐和低 RH 下的羧酸盐占主导地位。鉴定的 BrC 包括 N-杂环/N-杂链和硝基苯酚，这从减少的单散射反照率可以看出。测得的 COO 吸收系数 ( γ ) 取决于功能性，范围从 3.7  ×  10 ^-4到 1.3  ×  10 ^-2。开发了一个基于功能的动力学框架，以根据观察到的 COO 浓度和吸收系数预测 SOA 的产生，它再现了m的测量值-二甲苯氧化。我们的研究结果表明，间二甲苯的光化学氧化是城市环境下 SOA 和 BrC 形成的主要来源，因为其丰度高、与 OH 的高反应性以及 COO 的高产率。