摘要

本工作对模拟的太阳光和紫外光（UV）照射下的水溶性羟自由基（OH）诱导的三种适度可溶前体物的生物化学氧化进行了系统的研究，其中包括4-甲基丁香酚（DMP）、丁香酚（Eug）和2，4，6-三甲基苯酚（TrMP）。采用空气黑炭粒子气溶胶质谱仪（SP-AMS）对水性二次有机气溶胶（aqSOA）的主要化学成分和元素组成进行了监测。

在日光和紫外光条件下，AqSOA的质量产率分别在80-190%和0-200%之间变化。在光照+OH条件下，AQSOA的氧碳比（O/C）和碳氧化状态稳定增加，但在UV+OH条件下，碳氧比（OC/C）和碳氧化态均呈先上升后下降的趋势。形成了苹果酸、乙醇酸、甲酸和草酸等有机酸，它们的总量约占SOA质量的12%。紫外-可见光谱的变化暗示了光吸收有机物的形成。结合气相色谱-质谱联用（GC-MS）和固相质谱（SP-AMS）结果，提出了反应途径。

在阳光+OH条件下，积聚、功能化和碎裂过程都参与了aqSOA的演化，其中更多的贡献来自于通过羟基化和氧化反应进行的功能化。UV+OH氧化反应机理以功能化为主，其次是碎裂，主要由有机碳（TOC）含量下降，有机酸小分子产物生成来表明。我们的工作强调了SP-AMS和GC-MS的结合是实验室研究水相反应的有力方法。

Figures:

Figure 1. Overall temporalchanges of reaction solutions under sunlight + OH and UV + OH conditions. (a,b) concentration of precursor, (c, d) TOC concentration, (e, f) pH value, (g,h) total organic acid, (i, j) formic and oxalic acid.

Figure 2. (a–c) Organic massnormalized by sulfate (△Org/SO42−) and (d–f) aqSOAmass yield as a function of reaction time for the three precursors under bothsunlight + OH and UV + OH systems.

Figure 3. High resolution massspectra (HRMS) of aqSOA at reaction times of 5–7 h under sunlight + OHcondition for (a) DMP, (b) Eug, and (c) TRMP. Signals for m/z > 100fragments are enhanced by a factor of 10 for clarity. The peaks are color-codedaccording to four ion categories: CxHy+, CxHyO1+, CxHyO2+, and HyO1+. Theelemental ratios (O/C and H/C) and OM/OC of the aqSOA are also shown in thelegends. (For interpretation of the references to color in this figure legend,the reader is referred to the Web version of this article.)

Figure 4. Variations ofelemental ratios (H/C, O/C) and OSc against reaction time, and the “Triangleplot” of aqSOA (a–c) under sunlight + OH or (d–f) UV + OH irradiation.

Figure 5. Temporal profiles oflight absorbance at two different wavelengths (250 and 280 nm) for (a–b)sunlight + OH and (c–d) UV + OH system, respectively.

Table 1. Products identifiedvia GC-MS detection for DMP degradation under sunlight + OH reaction systems.

Figure 6. Temporal variationsof major fragment ions measured by the SP-AMS during aqueous OH oxidation of(a–b) DMP (c–d) Eug and (e–f) TRMP under sunlight and UV irradiation.

Figure 7. Proposed DMP aqueousOH oxidation mechanism when exposed to sunlight irradiation. The red textrepresents the compounds listed in Table 1 via GC-MS. (For interpretation ofthe references to color in this figure legend, the reader is referred to theWeb version of this article.)

Conclusion:

The aqueous-phasephotochemical oxidation process of three modestly-soluble phenolic compoundswas studied systematically. Overall, degradation rate of precursors in UV + OHsystem was significantly higher than that of sunlight + OH system. Exposure toUV + OH irradiation, absorbance at 250 nm increased for the first 1 h ofphotolysis then decreased gradually, while absorbance at 280 nm decreasedagainst reaction time, suggesting a coupling of precursor decay and formationof complex reaction products. Exposure to sunlight + OH irradiation, thetemporal trends of absorbance at 250 nm and 280 nm were different from UV + OHsystem. The aqSOA mass yield reached a maximum at 1 h and then graduallydecreased to zero for UV + OH system. In addition, both O/C and OSc values increasedfirst and then decreased, suggesting that the first-generation oxidationproducts were continuously fragmented at the later stage. Eug-aqSOA was highlyoxygenated with O/C ratios in the range of 0.40–1.02 under UV + OH system. Forsunlight+·OH system, both O/C value and OSc increased with reaction time.Aqueous-phase photochemistry of three compounds can produce considerableamounts of organic acid, up to ~12% of the aqSOA mass.

Products analysis via GC-MSshowed the differences in the products were due to the different functionalgroups of the precursors and therefore reaction pathways. Our results reveal acombination of oligomerization, functionalization, and fragmentation processesin the chemical evolution of aqSOA under sunlight + OH system. Functionalization,such as hydroxylation and oxygenation, contributes more than oligomerizationfor three precursors under sunlight + OH irradiation. In other words, theseresults also give us some implications that the oligomerization reaction wasnot the dominant factor leading to the high SOA mass yield. For UV + OH system,we can infer that functionalization dominates first and fragmentation dominateslater.

In summary, our researchhighlights the importance of aqueous-phase oxidation modestly-soluble compoundsto SOA formation. These reactions can be implemented into atmospheric chemistrymodels in the future. More detailed identifications of possible oligomers arealso needed to better understand its role in aqueous-phase oxidation.

Published online: 24 December 2019

Reference：

Aqueous-phase oxidation of three phenolic compounds by hydroxyl radical:Insight into secondary organic aerosol formation yields, mechanisms, productsand optical properties

ZhaolianYe, Atmospheric Environment, https://doi.org/10.1016/j.atmosenv.2019.117240