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文献阅读笔记：Denkenberger-2007-EST

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大气环境中的低聚物

Information for the paper

Title: Real-Time, Single-Particle Measurements of Oligomers in Aged Ambient Aerosol Particles

Author: Denkenberger, Kerri A.

Year: 2007

Journal: Environmental Science & Technology

URL: https://doi.org/10.1021/es070329l

Introduction

  1. While the formation and presence of polymers in aerosols during Los Angeles (LA) smog events was initially predicted by Haagen-Smit, oligomeric species have only recently been identified as an important component of the secondary organic aerosol (SOA) mass through laboratory studies.
  2. In the eastern LA basin, SOA has been found to comprise up to 80% of the total organic aerosol mass, and laboratory studies have suggested that possibly up to 50% of the SOA mass may be composed of oligomers, suggesting the potential importance of these species in ambient aerosols.
  3. Several laboratory-based mass spectrometry studies have reported the formation of SOA oligomers, characterized by a high mass envelope of peaks with a mass difference pattern of 12, 14, or 16 Da.

Experimental Procedures

  1. While ATOFMS measurements may be scaled for quantitative particle number and mass concentrations, scaling was not conducted for the results presented herein, which focus on mass spectral characteristics rather than overall particle concentrations.
  2. By comparing the existence and intensity of ion peaks in individual single-particle mass spectra, ART-2a classifies particles into separate clusters based on their mass spectral fingerprints.
  3. OC, amine-rich, V-rich, and EC-OC particle classes were found to contain oligomeric species. The amine-rich particle class was dominated by m/z 86 ((C2H5)2N=CH2+); particle phase amines are formed through photooxidation and gas particle partitioning. V-rich particles, attributed to ship and automobile emissions, were characterized by high intensity ions at m/z 51 (V+) and 67 (VO+) with less intense OC and amine ion peaks. EC-OC particles were dominated by m/z 12 (C+), 24 (C2+), and 36 (C3+) with less intense OC and amine ion peaks.
  4. Biomass particles are dominated by m/z 39 (K+) with less intense carbonaceous positive ions, such as m/z 27 (C2H3+/CHN+), 36 (C3+), and 37 (C3H+); the negative ions were dominated by m/z -62 (NO3-), -97 (HSO4-), and -125 (H(NO3)2-).
  5. The mass spectra of the calcium-EC particles are similar to the EC particles with the exception of an intense m/z 40 (Ca+) peak than suppresses the positive carbonaceous ions.
  6. The ammonium-rich positive mass spectra are dominated by m/z 18 (NH4+) and 30 (NO+)with less intense OC and amine ions; the negative ions are characterized by intense nitrate markers: m/z -62 (NO3-), -125 (H(NO3)2-), and -188 ((H2(NO3)3-).
  7. The metals particle type is characterized by particles containing intense ion peaks from metals such as lead (m/z 206-208 (Pb+)) or zinc (m/z 64, 66, 68 (Zn+)).
  8. To examine the loss of semivolatile species with heating, the summer unheated oligomer-containing average mass spectra was compared to the 200°C heated oligomer containing average mass spectra for the summer measurements. In particular, the intensities of ammonium (m/z 18 (NH4+)), amines (m/z 58 (C2H5NH=CH2+), 86 ((C2H5)2N=CH2+), 102 ((C2H5)3NH+), 118 (C2H5)3NOH+)), nitrate (m/z 30 (NO+/CH2=NH2+), -46 (NO2-), -62 (NO3-), -125 (H(NO3)2-)), and unidentified OC markers (m/z 116, 117) were observed to decreased with heating.

Results and Discussion

Mass Spectral Signatures of Oligomer-Containing Particles

  1. Signatures of oligomeric species were detected in submicrometer single-particle negative ion mass spectra obtained using A-ATOFMS during the SOAR field campaign in Riverside, CA in August 2005.
  2. As shown in Figure 1, these particles are characterized by high mass negative ions in the 200-400 mass/charge range with repetitive spacings of 12, 14, and 16, characteristic of oligomeric species; a corresponding oligomeric pattern was not observed in the positive ion mass spectra.
  3. The detection of the oligomers as negative ions suggests that the species are deprotonated oxidized organics (M-O)-; the mass/charge difference pattern can be attributed to the loss of carbon atoms, -CH2 groups, and oxygen atoms.
  4. In general, humicacids are considered refractory organic molecules that can only be thermally vaporized from -400-575 °C; therefore, the use of laser desorption ionization that results in extremely high temperatures, as in this study, or electrospray ionization, as in the previously mentioned study, are the most effective methods for detecting these high mass refractory organic compounds.
  5. However, not surprisingly, the negative oligomeric ion peaks observed by Gross et al, while exhibiting similar mass spacings, do not directly match the m/z values observed in this study, suggesting that different, or additional, oligomeric precursors are involved.
  6. The OC particles containing oligomers were dominated by carbonaceous marker ions at m/z 27 (C2H3+/CHN+), 37 (C3H+), and 43 (CH3CO+/CHNO+). The intensity of m/z 43 tracks ozone concentrations and serves as a marker for atmospheric aging and SOA.
  7. Furthermore, for all particle classes, the unheated oligomer containing particles contain a greater average peak area for m/z 43 as compared to the corresponding particles in the same classes that do not contain oligomers, showing that the particles containing oligomers are more heavily oxidized/aged.
  8. Furthermore, the oligomer-containing massspectra have a peak at m/z -195, indicating the presence of the sulfuric acid cluster H2SO4HSO4-, an indicator of particle acidity.
  9. For the OC and amine-rich particle types, the unheated oligomer-containing spectra also have a larger m/z -80 (SO3-) relative peak area as compared to those of the same particle type that do not contain oligomers. The presence of the intense peak at m/z -80 could indicate that organosulfates were present in the oligomeric particles measured in Riverside. Other ambient high mass OC studies have assigned m/z -80 (SO3-) to the fragmentation of organosulfates. Furthermore, recent SOA chamber studies suggest that the reaction of carbonyls with acidic aerosol sulfate in the atmosphere can result in organosulfate formation.

Comparison with SOAR II

  1. Although the summer and fall oligomeric species have similar mass spectral patterns and are present in similar particle classes, the mass range of the intense oligomeric ions only extends out to approximately m/z-311 in the fall as compared tom/z-365 in the summer, indicating that less particle aging and oligomerization has occurred during the fall.
  2. It is also important to note that while the summer and fall oligomers share similar mass spectral signatures, the exact identification of the molecular species of the oligomers is not possible due to mass resolution that is too low.
  3. On the basis of an in-depth SOAR comparison, particles were found to be less aged in the fall as compared to the summer, most likely due to the reduced ozone concentrations and solar radiation in the fall. Importantly, the peak area of m/z 43 (CH3CO+/CHNO+), an SOA marker, was observed to be lower in the oligomer-containing particles in the fall as compared to the summer.
  4. Thus, decreased photochemical activity and secondary OC precursor concentrations are also expected to partly account for the decreased degree of oligomerization in the fall.

Aerosol Size Dependence of Oligomer-Containing Mass Spectral Signatures

  1. The major cations and anions detected in Riverside are ammonium, nitrate, and sulfate. The relative acidity ratio is defined as the sum of the absolute average peak areas of nitrate (m/z -62 (NO3-)) and sulfate (m/z -97 (HSO4-)) divided by the ammonium ion signal intensity (m/z 18 (NH4+)).
  2. As shown in Figure 2c, the relative acidity ratio peaks in the summer for the 140- 160 nm unheated oligomer-containing particles, the size range that corresponds to the maximum oligomer peak intensities. This indicates that during the summer there is not as much ammonium in the smaller particles to balance the acidity of the sulfate and nitrate species.
  3. Since the sulfate peak area increases with decreasing particle size in the summer, this suggests that sulfate formation due to gas-particle partitioning occurs in the smaller submicrometer particles.
  4. Furthermore, for the unheated, oligomer-containing particles, the nitrate and ammonium peak areas are correlated and increase with respect to particle size, suggesting the presence of ammonium nitrate, which dilutes the oligomeric precursors and inhibits oligomer formation, particularly at larger submicrometer particle sizes.

Temperature Dependence of Oligomer-Containing Mass Spectral Signatures

  1. In the summer, the highest TD temperatures corresponds to the highest relative acidity ratio, suggesting an overall increase in the particle acidity due to the volatilization of ammonium nitrate. The higher acidity then serves to catalyze oligomer formation.
  2. In contrast, in the fall, the oligomeric mass difference pattern decreased with increasing temperature. Correspondingly, no increase in the relative acidity ratio was observed at the highest temperature.
  3. Therefore, the increased concentration of ammonium nitrate and decreased particle acidity in the fall accounts for the deviation in behavior of the fall oligomer-containing particles as compared to the patterns observed for the summer oligomer-containing particles.
  4. Therefore, evolution of the oligomeric species with heating in the summer is likely due to two factors: (i) the additional heat from the TD accelerates the kinetics of the formation of the oligomeric species and (ii) the particle matrix changes as the semivolatile species are removed with heating, leading to a more concentrated, acidic seed that catalyzes oligomer formation.
  5. Overall, secondary organic precursors, photochemical activity, and particle acidity appear to collectively be playing a role in oligomer formation and the degree of oligomerization.

总结与启发

  1. 作者使用单颗粒气溶胶质谱仪研究了大气中的低聚物。低聚物的谱图特征：负离子谱图中，质荷比分布在-200到-400之间，重复性间隔12、14和16个单位。
  2. 总体来说，低聚物聚合化的最高程度与颗粒物酸性的最大化有关。低聚物聚合化的最高程度取决于以下条件：（i）季节：在夏季检测到更多的低聚物，此时气溶胶的光化学老化程度更高，并且含有更多的硫酸盐；(ii) 颗粒物尺寸/酸度：夏季在较小的积累模式颗粒（140-200 nm）中检测到最大的颗粒物酸度和最高程度的低聚物； (iii) 温度：夏季在最高的热蒸馏器温度（175-200℃）下发生额外的低聚物。
  3. 夏季和秋季在颗粒大小和加热方面的低聚物行为的差异可能是由于铵的浓度增加，导致秋季的颗粒酸度降低，以及次级有机气溶胶在秋季较低的丰度，而这些次级有机气溶胶是形成低聚物的前体物。
  4. 本文首次使用了峰面积比来表示颗粒物酸度的方法，即以硝酸离子（m/z -62）和硫酸离子（m/z -97）的绝对峰面积之和与铵离子（m/z 18）的峰面积的比值。这个比值后来被广泛用于单颗粒质谱的相关研究中，虽然有不少文章在使用此方法时提到只是个粗略的估计，但是却忽略了仪器灵敏度对这个比值影响。在先前阅读的文献中有提到，单颗粒质谱仪对铵离子（m/z 18）的灵敏度很低，所以这个比值本身就会导致计算出的结果指向“颗粒物偏酸性”的结论。以我的观点来看，这个比值还是可以用的，但不能直接用，而是应该用来作比较。本文中夏季和秋季的对比就可以抵消仪器偏差。后来的很多研究虽然借鉴了这个比值的方法，但却有点“生搬硬套”，忽略了这个比值的真正价值和意义在于对比分析，而不是直接使用。不过本文在使用这个比值的时候还是有瑕疵的，比如分析含低聚物颗粒物的酸度时，可以计算一个不含低聚物颗粒物或所有颗粒物的酸度值，然后用于对比，这样更有说服力。
  5. 除了颗粒物酸度的计算外，作者对数据的处理手法上颇有些“巧妙”。比如，夏季的完整采样时间是7月30日到8月15日，作者发现 “a strong diurnal pattern with a similar range of average values was observed for ozone, PM2.5 mass concentrations, and ambient meteorological conditions, including wind direction, wind speed, ambient temperature, and relative humidity”，然后就采取了选取特定日的措施：“August 12, 2005 was found to be a representative day”。但是，在分析秋季数据时，却又使用了完整的采样数据“November 2-13, 2005”。
  6. 不得不说的是，作者的写作水平着实不错。可能是我看的文献还少，但作者的用词用句，在我看过的文献中还是相当有水平的，值得学习借鉴的单词和句子的写法非常多。

好词好句

  1. Unique high mass negative ions in the -200 to -400 mass/charge range with repetitive spacings of 12, 14, and 16 units, representative of oligomeric species, have been detected in single ambient submicrometer aerosol particles using real-time single-particle mass spectrometry during the Study of Organic Aerosols field campaign conducted in Riverside, CA (SOAR) in August and November 2005.
  2. Differences in oligomer behavior with respect to particle size and heating can be attributed to seasonal differences in photochemical oxidation, the relative amount of ammonium, and particle acidity.
  3. The ATOFMS SOAR oligomeric mass spectral pattern is nearly identical to that observed by triple-quadrupole MS for Riverside water-soluble organosulfate (containingm/z -97, HSO4-) molecules.
  4. Peak identifications within this paper correspond to the most probable ions for a given m/z ratio.
  5. OC and amine signatures are present in all of the oligomer-containing particles.
  6. Of the total unheated particles (789 007 particles) examined by ATOFMS during the fall, 33% (256 785 particles) contained oligomeric species.
  7. During the summer ATOFMS measurements, the ambient temperature ranged from 18 to 31 °C with a mean of 23 °C as compared to 8 to 26 °C with a mean of 16 °C during the fall measurements.
  8. Similarly, ozone concentrations reached as high as 111.2 ppb with a mean of 38.4 ppb for August 12 as compared to an upper limit of 71.8 ppb with a mean of 19.0 ppb for November 2-13.
  9. These results have potential implications for aerosol techniques that rely on thermal analysis, such as gas chromatography-mass spectrometry (GC-MS), which could be accelerating the formation of oligomeric species during the analysis step.
  10. The presence of secondary OC, amines, nitrate, and sulfate in the mass spectra of the oligomer-containing particles is indicative of the aged nature of these particles.

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