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文献阅读笔记：Zhang-2012-AE

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雾过程促进含三甲胺颗粒物的增加。

Information for the paper

Title: Enhanced trimethylamine-containing particles during fog events detected by single particle aerosol mass spectrometry in urban Guangzhou, China

Author: Zhang, Guohua

Year: 2012

Journal: Atmospheric Environment

URL: https://doi.org/10.1016/j.atmosenv.2012.03.038

Introduction

  1. Recent measurements by Silva et al. (2008) showed that amines could account up to 20% of organic matter mass in fine aerosol during some wintertime periods in Utah, USA.

  2. Angelino et al. (2001) reported that up to 80% of single particles contained amine signals during some episodes in Riverside, CA.

  3. Besides, TMA can also participate in the formation of secondary organic aerosol. Several studies have shown that gas-phase TMA could form non-salt organic aerosol products through reaction with oxidizing agents such as O3, OH, and NO3 radical (Murphy et al., 2007; Silva et al., 2008).

Experimental section

  1. Peak identification described in this paper corresponds to the most probable assignments for each specific mass-to-charge ratio (m/z), and more details can be found in the published work by Liu et al. (2003) and Murphy and Thomson (1997a, b).

Results and discussion

Detection of TMA-containing particles with SPAMS

  1. TMA-containing particles are queried with an absolute peak area at mass-to-charge ratio (m/z) +59 ([N(CH3)3]+) greater than 20. A total of 126,918 mass spectra for TMA-containing particles were obtained, which contributed approximately 18% on average to the total detected submicron particles by number.

  2. It is necessary to point out that ~77% of TMA-containing particles also contain signals for ammonium, implying their similar formation pathway.

  3. The presence of [H(NO3)2]- is an indication of large contribution from nitrate in particles.

  4. In this study, no evidence showed the presence of non-salt organic products of TMA in particles because no fragments of common secondary organic products for TMA such as trimethylamine-Noxide at m/z 76 [TMAOH]+, dimethylnitramine at m/z -90 [CH3N2O2]- and m/z -74 [CH3N2O]- were observed through the whole sampling period.

  5. Several fog events were obtained during the whole sampling period, three of which corresponded to the spikes of TMA-containing particles.However, no obvious spike of TMAcontaining particles was observed during other fog events. Back trajectory of the air mass originated from southeast or northeast during these fog events, which indicates that long range transport might be not the dominant factor that affected the difference between these events. Other factors (e.g., the emission strength of TMA from local sources) might be candidates. This remains to be verified in further research.

  6. The number of TMA-containing particles was significantly enhanced exclusively when fog formed, especially during Fog 1, Fog 2 and Fog 3.

  7. As a tracer for aqueous-phase fog processing, the number of particles containing HMS also sharply increased during these events, consistent with the earlier studies. It provides evidence that fog processing assists the formation of TMA-containing particles since TMA preferentially resides in the gas phase due to its volatility.

  8. Additionally, significant correlation (r=0.95, p<0.01) between relative intensity of TMA and relative humidity was observed in this study, and the most enhanced TMA in the particle phase was obtained during Fog 3 with the highest RH (94±4%), which suggests that aerosol water content might play an important role in mass transfer of TMA from gas to aerosol in urban Guangzhou during late spring.

  9. Theoretical calculation also supports the enhanced gas-to-particle partitioning of semi-volatile species such as TMA/ammonium nitrate under elevated relative humidity and lower temperature conditions.

Size distributions of TMA-containing particles during fog events

  1. The TMA-containing particles show dominant peaks in smaller sizes on clear days and shift to larger sizes during the fog events.
  2. The result indicates that fog processes enhanced the growth of particles, directly contributed from water uptake due to the increase of RH and also the enhanced gas-to-particle partitioning of semi-volatile species (e.g., TMA and nitrate), which is consistent with the observations of a foggy event in London using ATOFMS by Dall’Osto et al. (2009b).

Mixing state of TMA-containing particles with secondary

  1. As shown in Figure S1, considerable fraction of TMA-containing particles internally mixed with nitrate, sulfate and ammonium, approximate 66%, 76% and 77% on average in number fraction, respectively.

  2. The number fraction for sulfate in TMA-containing particles decreased from ~88% on clear days to ~70% during the fog events, although several studies demonstrate enhanced production of sulfate through fog/cloud processes (Minami and Ishizaka, 1996).

  3. It could be seen that the dots are distributed along TMA-nitrate and TMA-sulfate axes during the fog events, indicating that nitrate and sulfate are more strongly associated with TMA compared to ammonium, which is consistent with the previous observation by Pratt et al. (2009) during summer in Riverside, CA.

Possible formation mechanism of particulate TMA

  • Gas-to-particle phase partitioning via direct dissolution:
N(CH3)3(g) + H2O ↔︎ NH(CH3)3 · OH(aq)
NH(CH3)3 · OH(aq) ↔︎ NH(CH3)3+(aq) + OH-(aq)
  • neutralize aqueous acids after dissolution
NH(CH3)3+(aq) + NO3-(aq) ↔︎ NH(CH3)3NO3(s)
NH(CH3)3+(aq) + SO42-(aq) ↔︎ [NH(CH3)3]2SO4(s)
  • Heterogeneous uptake of acidic gas (e.g., NO, NO2, N2O5 and SO2) to the aqueous surface by TMA of alkaline nature
NH(CH3)3 · OH(aq) + NOx(g) → NH(CH3)3+(aq) + NO3-(aq) (+NO2-(aq))
NH(CH3)3 · OH(aq) + SO2(g) → NH(CH3)3+(aq) + HSO3-(aq)
  • gas-phase acid-base reactions to form inorganic salts
N(CH3)3(g) + HNO3(g) ↔︎ NH(CH3)3NO3(s)
N(CH3)3(g) + HSO4(g) ↔︎ [NH(CH3)3]2SO4(s)
  • heterogeneous reaction between gas-phase TMA and acid
N(CH3)3(g) + H+(aq) → NH(CH3)3+(aq)

总结与思考

  1. Fog-event 促进含TMA颗粒物数量增加似乎已成为共识，但本文有个细节：不是每一个 fog-event 都能促进含TMA颗粒物数量的增加。文中有三个 fog-events 促进，而另有四个 fog-events 并没有看到明显的含TMA颗粒物数量的增加。作者通过后向轨迹分析排除了传输的影响因素，认为其他因素（如本地污染源的排放强度等）导致这种差异，但并未深入验证。

  2. 含TMA颗粒物的化学组成复杂，TMA只是其中一种物质，从质量浓度来讲，还是一种微量物质。那么，TMA对含TMA颗粒物化学性质的影响究竟有多大，就值得深入探究了。从这个角度切入，或许对解释上一点所提问题有所帮助。

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