COVID-19 爆发前后聊城城区冬季含有机胺颗粒物特征及来源。

Information for the paper

Title: Characteristics and sources of amine-containing particles in the urban atmosphere of Liaocheng, a seriously polluted city in North China during the COVID-19 outbreak

Author: Li, Zheng

Year: 2021

Journal: Environmental Pollution

URL: https://doi.org/10.1016/j.envpol.2021.117887

Introduction

Laboratory experiments and theoretical calculations have demonstrated that amines are more alkaline than ammonia, although the gaseous concentration of amine is one or two orders of magnitude lower than ammonia (Cheng et al., 2018; Ge et al., 2011a; Sorooshian et al., 2008).
Amines can react directly with organic and inorganic acids such as sulfuric acid, nitric acid, and acetic acid to form particulate salts because of the strong alkalinity and high water solubility, thereby playing an important role in enhancing the atmospheric nucleation (Zhang et al., 2012).
Furthermore, amines can react with oxidants such as O₃, OH, and NO₃ radicals to form semi-volatile and non-volatile chemical species, which can significantly contribute to the formation of secondary organic aerosol (SOA) (Lee and Wexler, 2013; Murphy et al., 2007).
The mass concentrations and temporal distributions of amines in Chinese megacities (e.g., Guangzhou, Nanjing and Shanghai) have been studied extensively based on the traditional filter sampling methods (Cheng et al., 2020; Liu et al., 2018; Shen et al., 2017).
The gas-to-particle partitioning of amine is influenced by many factors, such as aerosol liquid water content (ALWC), particulate acidity, and atmospheric oxidation capacity, as well as meteorological conditions including relative humidity (RH) and temperature (Chen et al., 2019; Cheng et al., 2018).
In addition, the strong particle acidity could promote the gas-to-particle partitioning of diethylamine (DEA) (Ge et al., 2011b).

Methods

The calculation of ALWC in PM_2.5 samples was calculated using the ISORROPIA-II model, which treated the Na⁺—NH₄⁺—K⁺—Ca²⁺—Mg²⁺—SO₄^2-—NO₃^-—Cl^- system.
The 72 h air mass back-trajectories (500 m above the ground, 1h resolution) were simulated using the Hybrid Single-particle Lagrangian Integrated Trajectory (HYSPLIT) model, based on the meteorological data downloaded from the National Oceanic and Atmospheric Administration (NOAA) Global Data Assimilation System (GDAS, spatial resolution 1^°×1^°).
The Potential Source Contribution Function (PSCF) method was applied to identify the potential source regions and the individual contributions of three mainly amine-containing particles by using the TrajStat plugins on MeteoInfoMap software (Wang et al., 2009).

Overview of the campaign during the COVID-19 pandemic

As an important tracer of primary combustion emissions (Zhang et al., 2021), CO presented a decreasing trend similar to that of PM_2.5 concentration throughout the LCD.
O₃ can be used as an indicator of atmospheric oxidation capacity, because it mainly originated from the photochemical reaction of NO_x and volatile organic compounds (VOCs)
The formation of O₃ is principally originated from the NO_x-saturated regime in the urban atmosphere of China due to the shortage of HOx radicals during winter (Meng et al., 2021).
Secondary inorganic aerosols (SIA, sum of concentrations of SO₄^2-, NO₃^-, and NH₄⁺) were the dominant species of PM_2.5, which showed a decreasing trend in pace with the change of PM_2.5 and CO concentrations throughout the campaign.
The mass ratio of NO₃^- to SO₄^2- can be used as an indicator of the relative importance of vehicle exhaust versus the stationary sources of nitrogen and sulfur in the aerosol.The lower ratio of NO₃^-/SO₄^2- suggested that the contribution of stationary sources was more important than the mobile sources.

Characteristics of amine-containing particles

In addition, Healy et al. (2015) reported that the higher concentration of amines in the rural site than that in the urban and industrial sites was primarily due to the enhanced anthropogenic emissions of amines from agriculture activity and animal husbandry.
The total fractions of ¹⁰¹TEA—, ¹⁰²TEA—, and ¹⁴³DPA—containing particles were negligible, which was less than 0.5% of all detected amine-containing particles throughout the observation period.
In addition, oxidized organic species such as ⁸⁹HC₂O₄^- and ⁴³C₂H₃O⁺ were also observed during the whole campaign, indicating that the particles had experienced substantial aging processes.
As shown in Fig. S2, the signal intensity of ⁴⁰Ca⁺ was observed to be stronger during the LCD and post-LCD than that during the pre-LCD. Moreover, the determination coefficient (R2) between amine-containing particles and Ca-containing particles was higher during the LCD (0.47, P < 0.01) and post-LCD (0.58, P < 0.01) than that during the pre-LCD (0.11, P > 0.05), which indicated the vitally significant impact of dust source on the amine-containing particles during the LCD and post-LCD. The enhanced impact of dust source on the amine-containing particles can be attributed the increased wind speed during the LCD and post-LCD (Table 1).
To evaluate the acidic environment of amine-containing particles, the relative aerosol acidity (Rra) was defined as the ratio of total peak areas of nitrate (⁶²NO₃^-) and sulfate (⁹⁷SO₄^2-) to the peak area of ammonium (¹⁸NH₄⁺) (Denkenberger et al., 2007). Our previous study has obtained a significant correlation (R2 = 0.62, P < 0.01) between the in situ particle pH (pHis) calculated from inorganic ions using the ISORROPIA-II model and relative acidity ratio estimated by the SPAMS, confirming the validity of using Rra for evaluating particle acidity (Meng et al., 2021).
The amine-containing particles during the pre-LCD and post-LCD showed unimodal distributions and peaked at ~0.48 and ~0.54 μm, respectively. However, the unscaled size distributions of amine-containing particles during the LCD exhibited a bimodal distribution with a larger peak at ~0.48 μm and a smaller peak at ~0.66 μm.
The broad size peak and distribution range of amine-containing particles during the LCD and post-LCD were possibly resulted from more complex sources and/or gaseous amine condensation or reaction with the coarser particles during long-distance transport.

Gas-to-particle partitioning of amine-containing particles

The partitioning of gaseous amines to the particle phase is mainly through both acid-base reactions and direct dissolution (Liu et al., 2018).
Under relatively lower RH conditions, reacting with the acidic species (e.g., HNO₃ (g) and H₂SO₄ (g)) may be the dominant way for the gaseous amines condensed into the particle phase (Ge et al., 2011a, b).
However, the partitioning of amines from the gaseous phase to the particle phase is mainly via direct dissolution under relatively high RH conditions (Liu et al., 2018).
Box plots of relative peak area (RPA) of DEA under different RH were shown in Fig. 2(a). When RH increased from the range of lower than 45 % to the range of 85 %–100 %, the median RPA of the DEA containing particles increased from 3.4 % to 3.8 %, from 3.2 % to 3.5 %, and from 3.5 % to 3.8 % during the pre-LCD, LCD, and post-LCD, respectively. Such a small rise (0.3–0.5 %) in RPA of DEA under different RH indicated a minor impact of RH on the gas-to-particle partitioning of amine-containing particles.
High water content could enhance the acid-base reaction by dissolving ammonium salts into their ionic forms and shifting the gas-to-particle equilibrium of amines to the particle phase because of the high aqueous solubility of amines (Huang et al., 2012; Rehbein et al., 2011). ALWC is controlled by both SIA concentrations and RH (Yi et al., 2021).
As shown in Fig.2(b–d), the number of amines and amine/total ratio did not exhibit similar trends with RH and ALWC, suggesting that the effects of RH and ALWC on amine-containing particles were negligible.
It is noteworthy that the median RPA of the DEA containing particles was the highest during the post-LCD, followed by during the pre-LCD and during the LCD when RH was lower than 65 % (Fig.2(a)), which was in accordance with the distribution of Rra in section 3.1. The strong acidic conditions can facilitate the partitioning of gaseous DEA into the aqueous phase by the formation of aminium salts through reacting with acids of HCl, HNO₃ and H₂SO₄ (Chen et al., 2019). These results implied that the aerosol acidity was a key factor influencing the gas-to-particle partitioning of amines when RH was lower than 65%. Moreover, the ratio of amine/total detected particles correlated well (R2 > 0.30, P < 0.01) with Rra during the pre-LCD, LCD and post-LCD, respectively, again suggesting that the enhancement of aerosol acidity could promote the partitioning of amines from the gaseous phase to the particle phase.
Lower temperature can facilitate gaseous amines to enter the particle phase via gas–particle partitioning (Huang et al., 2012). The fractions of amine-containing particles showed a negative correlation with temperature during the whole sampling period (R2 = 0.32, P < 0.01), indicating that the lower temperature can promote the distribution of amines to particle phase.
In addition, no significant correlations (R2 < 0.38, P < 0.01) were obtained between O₃ and amine-containing particles during three observation periods, indicating the effect of O₃ on the formation of amine-containing particles was minor.
As the oxidation product of TMA, trimethylamine oxide (TMAO) (m/z = 76) was identified in this study to further investigate the effect of O₃ on the amine-containing particles.
However, the temporal profiles of TMAO and TMAO/TMA ratio exhibited similar diurnal variations with RH and ALWC during the whole observation period (Fig. S5). These results demonstrated that the probable formation of TMAO was closely associated with the aqueous oxidization reaction at night, rather than the O₃-dominated photochemical oxidation, thus the number count and fraction of TMAO showed higher values in nighttime (139.9 ± 48.2; 14.0 ± 8.1 %) than in daytime (74.1 ± 51.5; 10.5 ± 9.9 %) in the whole observation period. It has been reported that the aqueous oxidation launched by NO₃ radicals in nighttime significantly contributed to the production of TMAO, but the contribution of photochemical oxidation of amines with O₃ was negligible (Lian et al., 2020b; Tang et al., 2013). Therefore, the increased concentration of O₃ during the LCD and post-LCD was not a vital influencing factor determining the enhanced TMAO.

Potential source contribution function

As shown in Fig.S6, the amine-containing particles were classified into six types, including amine-organic carbon (A-OC), amine-elemental carbon (A-EC), A-EC and OC combined particles (A-OCEC), A-OCEC aged, sea salt, and biomass burning (BB), which accounted for 95 % of all amine-containing particles.
The A-OC particles were the most abundant type, contributing to approximately 40.0% of the total amine particles in the whole sampling period.
There were no significant correlations between A-OC types with SO₂ and NO₂ (R² < 0.4, P < 0.01), respectively, indicating that the impact of fossil fuel combustion on A-OC particle type was negligible.
Previous study demonstrated that EC in amine-containing particles were mostly derived from fresh mobile emissions (Huang et al., 2012). The fraction of A-EC particle during the LCD (1.1 %) was about 2.5 times lower than that during the pre-LCD (3.8%) and post-LCD (3.7%), which may be attributed to the sharp reduction of on-road vehicle numbers because of the lockdown measurement.
As shown in Fig.S7, the fraction of BB particle type showed an increasing trend from 4.2% during the pre-LCD to 7.0% during the LCD to 12.1% during the post-LCD. Similarly, the gradual increase of the number of fire spots from the pre-LCD to the LCD was also observed according to the National Aeronautics and Space Administration (NASA) satellite observation (Fig.3), suggesting that the increased BB in Liaocheng and surrounding regions during the post-LCD significantly contributed to the formation of amine particles in the urban areas of NCP.
Previous study has confirmed that different types of Ca²⁺ can be proposed as the tracers to identify the sources of dust. CaSO₄ mainly comes from the local area, but CaCO₃ is largely from Chinese dust origin (Yuan et al., 2008). The correlation (R² = 0.62, P < 0.01) between peak area of calcium and sulfate during the LCD was higher than that during the pre-LCD (R² = 0.22, P < 0.01) and post-LCD (R² = 0.28, P < 0.01). In addition, the peak area of calcium was strongly correlated with that of carbonate both during the pre-LCD (R² = 0.72, P < 0.01) and post-LCD (R² = 0.68, P < 0.01), but such a robust correlation was not obtained during the LCD (R² = 0.31, P < 0.01). These results implied that the amine-containing particles were mainly from local and surrounding sources during the LCD, while those were mainly affected by long-range transport during the pre-LCD and post-LCD.

Impact of the fog events on amine-containing particles

The number of aminecontaining particles was significantly enhanced when fog events occurred (Fig.S8), which was also observed in previous studies.
The ratio of amine/total detected particles during fog events (0.27) was 1.4 times higher than that (0.19) during clear days, indicating that fog events could promotes the formation of amine-containing particles.
As a marker of aqueous-phase fog process, the counts of HMS particle also dramatically increased during fog events. It provided evidence that fog events could promote the aqueous formation of amine-containing particles.
Moreover, the fraction of amine-containing particles was closely correlated with RH (R² = 0.74, P < 0.01) and ALWC (R² = 0.70, P < 0.01) during Fog 1, but such strong correlations were not obtained during clear days (R² < 0.45, P < 0.01) and in the whole sampling periods (R² < 0.1, P > 0.05) as discussed in section 3.3. These results suggested that both RH and ALWC played a vital role in the partitioning of gaseous amines into the aerosol phase when fog process was formed.
Theoretical calculation have demonstrated that the lower temperature and higher RH conditions facilitated the gas-to-particle partitioning of amine (Hu et al., 2008).
Apparently, the higher water content during fog events greatly enhanced the acid-base reaction in amine-containing particles.

Summary and conclusions

The increased aerosol acidity and lower temperature conditions was favorable for the gas-to-particle partitioning of amine-containing particles, while the impacts of RH and ALWC were minor.
The fraction of A-EC particle (1.1%) during the LCD was about 2.5 times lower than that in the pre-LCD (3.8%) and post-LCD (3.7%), owing to the sharp reduction of onroad vehicle numbers in the LCD.
The fog events could promote the aqueous formation and growth of amine-containing particles, due primarily to the enhancement of RH and ALWC.

启发与思考

使用离子浓度数据基于 ISORROPIA-II 模型计算 PM_2.5 的气溶胶含水量（ALWC）。是否可以使用单颗粒气溶胶质谱仪相应离子的峰面积来粗略计算ALWC？
PM_2.5 中 NO₃^- 与 SO₄^2- 的比值可以用来表征移动源与固定源的贡献大小。
含有机胺颗粒物与含钙颗粒物的相关性分析。风速是影响含钙颗粒物数量的因素之一。钙离子可以作为粉尘源的示踪粒子。 CaSO₄主要来自本地污染源，CaCO₃则主要来自传输源。
使用硝酸(⁶²NO₃^-)与硫酸(⁹⁷SO₄^2-)的峰面积之和与铵峰面积的比值(¹⁸NH₄⁺)来粗略计算气溶胶酸性（relative aerosol acidity, Rra），以此表征颗粒物酸度。
在这篇研究中，相对湿度（RH）和气溶胶含水量（ALWC）对有机胺（主要为DEA）颗粒物的气-粒分配并没有太大影响，而在相对湿度低于65%的条件下，气溶胶酸性成为影响有机胺颗粒物的气-粒分配的一个重要因素，说明酸碱反应比直接溶解对有机胺颗粒物的气-粒分配更为重要。
夜间 NO₃ 自由基的液相氧化反应促进了TMAO的形成，而臭氧氧化的贡献则微乎其微。
理论计算证明低温、高湿的气象条件可促进有机胺的气-粒分配。

扩展阅读

Denkenberger-2007-EST: 质谱数据粗略计算颗粒物酸度
Yuan-2008-AE: Dust颗粒物
Hu-2008-AE: 有机胺颗粒物特性的理论计算

理乐志

文献阅读笔记：Li-2021-EP