The molecular compositions of polar organic compounds (POCs) in particles emitted from various vessels and excavators were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and possible molecular structures of POCs were proposed. POCs were extracted with purified water and sorted by elemental composition into three groups: CHO, CHON, and S-containing compounds (CHONS and CHOS). The results show the following. (i) CHO (accounting for 49 % of total POCs in terms of peak response) was the most abundant group for all tested off-road engines, followed by CHON (33 %) and CHOS (35 %) for diesel and HFO (heavy-fuel-oil)-fueled off-road engines. (ii) The abundance and structure of the CHON group in water extracts were different in terms of engine type and load. The relative peak response of CHON was the highest for excavator emissions in working mode, compared to the idling and moving modes. Furthermore, dinitrophenol and methyl dinitrophenol were potentially the most abundant emission species for high-rated speed excavators, while nitronaphthol and methyl nitronaphthol were more important for low-rated speed vessels. (iii) The composition and structure of the S-containing compounds were directly influenced by fuel oil characteristics (sulfur content and aromatic ring), with more condensed aromatic rings in the S-containing compounds proposed in HFO-fueled vessel emissions. More abundant aliphatic chains were inferred in diesel equipment emissions. Overall, higher fractions of condensed hydrocarbons and aromatic rings in POCs emitted from vessels using HFO cause strong optical absorption capacity. Different structures in POCs could provide a direction for qualitative and quantitative analysis of organic compounds as tracers to distinguish these emissions from diesel or HFO-fueled off-road engines.
A rapid increase in the number of off-road engines (e.g., vessels and
excavators) has resulted in large quantities of pollutant emissions, which
have severe impacts on air quality, human health, and climate change (Righi
et al., 2011; Li et al., 2016; Liu et al., 2016; Wang et al., 2018; Zhang et
al., 2018). In China alone, the deadweight capacity of vessels increased
from 51 million tons in 2000 to 266 million tons in 2016 (NBS, 2017). It was
reported that emissions from fishing boats accounted for 18.3 % of total
fine particulate matter (PM
Emission standards for off-road engines are not fully implemented in China,
especially for vessel emissions. Currently, the stage 3 emission standard has
been implemented for off-road diesel engines since 2016, while the stage 1
emission standard for emission from vessels will be implemented in 2020
(SEPA and SAQSIQ, 2015a, 2016). Furthermore, the oil quality for
off-road mobile sources cannot be guaranteed. According to the standard of
GB/T17411-2012, the sulfur content in oil used for vessels could reach
1 %–3.5 %, which was 200–700 times higher than those for China IV diesel
(SEPA and SAQSIQ, 2015b). There is a continued need to apportion the
contributions of off-road engines to atmospheric PM
Organic matter (OM) is one of the most important components in PM
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is an advanced technique with a high mass resolution of 0.00001
and is commonly used to determine the organic matter composition at a
molecular level in crude oil (Jiang et al., 2019). FT-ICR MS is usually
coupled with soft ionization techniques, such as electrospray (EST) and
atmospheric pressure chemical ionization (APCI). They are used to analyze
polar species and non-polar organic matters, respectively (Smith et al.,
2009; Smit et al., 2015). It should be noted that FT-ICR MS, without
chromatographic separation, can only detect molecular formulas and molecular
identification based on elemental composition alone. This is challenging
because most complex molecules have several stable isomeric forms (Laskin et
al., 2009). However, some traditional mass spectrometry methods are equipped
with quadrupole, ion trap, or time of flight, which have limited resolving
power compared with the FT-ICR MS. Recently, some studies have successfully
characterized the elemental components of polar organic compounds present in
the atmosphere or emitted by different sources using FT-ICR MS (Wozniak et
al., 2008; Laskin et al., 2009; Smith et al. 2009; Yassine et al., 2014). A
few of them have been undertaken in China, especially for source emission
(Lin et al., 2012a; Jiang et al., 2016; Mo et al., 2018; Song et al., 2018).
Song et al. (2018) reported that the most abundant group of HULIS emitted
from biomass burning and coal combustion was CHO, followed by CHON for
biomass burning and CHOS for coal combustion. In contrast, Wang et al. (2018) observed that CHON was the dominant compound emitted from straw
residue burning. In addition, the possible chemical structure of HULIS could
be determined by FT-ICR MS. Tao et al. (2014) compared the molecular
compositions of organosulfates in aerosols sampled in Shanghai and Los
Angeles. They found that the organosulfates in Shanghai had a low degree of
oxidation and unsaturation, indicating the presence of long aliphatic carbon
chains. Smith et al. (2009) reported that organic aerosol emitted from
biomass burning showed a clear trend of increasing saturation with
increasing molecular weight and exhibited a significant
This study aimed to hypothesize on the chemical characterization of polar organic aerosol constituents at the molecular level emitted from typical off-road engines by FT-ICR MS. To this end, studies were conducted (1) to identify the molecular composition of POCs from excavators in three different operation modes; (2) to determine the molecular composition and possible structure of POCs from vessels using heavy fuel oil (HFO) and diesel, respectively; (3) to explore the key factors affecting the composition and structure of POCs from HFO and diesel-fueled off-road engines; (4) to pave the way for the discovery of potential tracers for off-road engine emissions.
Four ships using HFO and diesel and four excavators covering different
emission standards and powers were chosen as representative off-road engines
in China. Detailed information about the four ships and four excavators is
presented in Table 1. Before conducting field sampling, the original fuel
was obtained directly from the fuel tank and sent to the testing company for
quality evaluation (Table 2). For excavator emission sampling, three
operation modes (idling, moving, and working) were selected, and sampling
time was approximately half an hour for each mode. The flow rate of the
PM
Technical parameters of test off-road engines.
Results of the fuel quality analysis.
Due to the limitations of organic matter load in filters and
cost-prohibitive analysis, the filters sampled from off-road engines with
the same operation modes or fuel quality were combined together to
characterize the comprehensive molecular compositions of POCs for off-road
engines in different operation modes and fuel quality. As shown in Table
S1, five samples (1, 2, 3, 4, and 5) were selected to conduct FT-ICR MS
analysis, which represented vessels using heavy fuel oil, vessels using
diesel, excavators in idling, moving, and working modes, respectively.
Sample 1 was combined with 25 % of the filter area from the two HFO-fueled
vessels, namely YK and YF; sample 2 was combined with 25 % of filter area
from two diesel-fueled vessels, namely GB1 and TB4; samples 3, 4, and 5 were
combined with 50 % of the filter areas from four excavators in idling,
moving, and working modes, respectively, namely CAT320, CAT330B, CAT307, and
PC60. The portions of filters (Table S1 in the Supplement) were cut and combined for 40 min,
subjected to ultrasonic extraction with 40 mL ultrapure water, and then
filtered using a 0.22
Both samples extracted with water or organic solvents were processed by a
solid phase extraction (SPE) method to remove ions, which disturbed the
results of FT-ICR MS. The majority of inorganic ions (e.g., ammonium, sulfate,
and nitrate) and low-molecular-weight organic compounds such as
isoprene-derived organosulfates and sugars could be removed during SPE
treatment (Gao et al., 2006; Lin et al., 2012b; Surratt et al. 2007), which
were not discussed in this research. The details of the solid phase
extraction method were presented by Mo et al. (2018). Briefly, the pH value
of water extracts was adjusted to 2.0 using HCl and then passed through an
SPE cartridge (Oasis HLB, 30
The molecular characterization of POCs was undertaken using negative-ion electrospray ionization (ESI) FT-ICR MS (Bruker Daltonics GmbH, Bremen, Germany) with a 9.4 T refrigerated
actively shielded superconducting magnet. Extracted solutions were injected
at flow rate of 180
C
In general, the range of detected peaks for excavators and vessels had
molecular weights between 150 and 900 Da, but most of the intensive peaks
occurred in the molecular weight range of 200–400 Da. The mass spectra for
excavators in different operational modes and vessels using different oils varied from each other. The number of peaks for POCs were 4734, 3097,
4731, 4554, and 2818 in excavator emissions in the idling, working, and
moving modes and vessel emissions using HFO and diesel, respectively. The
average molecular weight of excavator emissions in the working mode and
vessels using HFO were the lowest (
For excavators, CHO was the most abundant group of POCs in all three operation modes, accounting for 41 %, 46 %, and 48 % of all the formulas in terms of peak responses for the idling, working, and moving modes, respectively. S-containing compounds (CHOS and CHONS) were most abundant in the idling mode, while the relative peak response of the CHON group was highest in the working mode (Fig. 1). For vessels, CHO was the most abundant species group of POCs for both the vessel using diesel and the ones using HFO, accounting for 50 %–60 % of total peak intensity. However, CHOS accounted for almost 30 % of total ion intensity for vessels using HFO, higher than other off-road diesel engines. Furthermore, the chemical properties of POCs for vessels using HFO showed a larger degree of oxidation and unsaturation than other samples (Table S2). These differences in the composition of POCs might be attributable to variations in engine load, fuel supply, and air supply in different operation modes, which are discussed later.
As discussed in the Supplement (Sect. S3), the chemical properties
of extractions derived from water or dichloromethane (DCM) or MeOH were significantly different
(Supplement Figs. S1 and S2). And through comparing the optical properties between
water and
Mass spectra of POCs in water extractions for off-road diesel
engine emissions. Panels
The number of peaks for CHO compounds were 1746, 1287, 1797, 1561, and 1318
for excavators in the idling, working, and moving modes and vessels using
HFO and diesel, respectively. Considering the number of detected peaks for
CHO compounds, the compositions of the CHO group emitted from off-road engines
were more complicated than those from ambient samples while being relatively
comparable to those from other sources of emission (e.g., biomass: 1514–2296;
coal combustion: 918) (Lin et al., 2012a; Jiang et al., 2016; Song et al.,
2018). The average molecular weight of detected ions for CHO compounds for
excavators idling, working, and moving and vessels using HFO and diesel
were
The Van Krevelen (VK) diagram (
The Van Krevelen (VK) diagrams of CHO compounds for off-road
engines. Panels
The peaks intensity percentage for the CHON group to total ions was the
second largest in POCs emitted from off-road diesel engines, except for
vessels using HFO (Fig. 1). The fraction of nitrogen oxide declined with
increasing length of the straight-chain alkyl (Hellier et al., 2017), which
was consistent with the relative response of the CHON group for diesel and
HFO-fueled engine emissions. It was always considered that CHON mainly
originated from biomass burning emission (18 %–41 %), while the
percentage of peak responses for the CHON group to the total assigned ions
measured from off-road diesel engines was comparable or slightly smaller
than those emitted from biomass burning (Laskin et al., 2009; Wang et al.,
2017; Song et al., 2018). As shown by the average ratios of
For further discussion of possible chemical structures, the CHON group was
divided into 23 subgroups, including OxN1 (
The aromaticity equivalent (Xn) has been proposed to evaluate the
aromaticity of organic material with heteroatoms (e.g., N, S). When the value
of Xn exceeds 2.5, aromatic structures are most likely to be present within
the compounds, while a value of Xn higher than 2.7, indicates the presence
of condensed aromatic compounds (e.g., benzene core structure with Xn
Molecular composition and possible structure of CHON for excavators in three modes and vessels using HFO and diesel.
The last group of POCs was S-containing organic compounds, including CHOS and CHONS. As shown in Fig. 1 and Table S2, the percentage of peak responses for S-containing species to total assigned peaks from vessels using HFO (35 %) was higher than those from other vehicles, with 1, 3, 2, and 3 times more than those for excavators idling, working, and moving and vessels using diesel, respectively. However, the CHONS group for excavators was significantly higher than those for vessel emissions in terms of relative ion intensity. The high fraction of peak responses for S-containing species from vessels using HFO might be attributed to the high sulfur content in HFO. The maximum sulfur content in HFO detected in this study was 2.46 %, which was significantly higher than those in diesel (Table 2). In addition, for excavators in the idling mode, the fraction of the relative response of S-containing compounds was 32.5 %, while for the working and moving modes, they were 11.6 % and 17.1 %, respectively.
To facilitate further discussion, three subgroups for CHONS
(
S-containing compounds for vessels were highly unsaturated with 8.03 for the
average DBE value which was higher than those for excavators (6.77; Table S2). Furthermore, the fraction of compounds with Xn
The distribution of subgroups of S-containing compounds for off-road engines.
On an average, 88.5 %
The ratios of
CHO compounds were the most abundant species across all sources (biomass, coal, on-road vehicles, and off-road vehicles) in terms of ion intensity, while the fractions of CHON and S-containing compounds were different from anthropogenic source emissions. Furthermore, the possible chemical structures of these compounds for diverse sources varied sharply.
For CHO compounds, the average DBE values from excavator and vessel emissions were
For CHON compounds, almost all sources were reported to emit nitrophenol compounds, while the substituted groups were slightly different due to different numbers of N and O atoms. The fraction of relative peak response of CHON compounds, an important light-absorbing substance, could reach half of the POCs from biomass burning emission. Methyl-nitrocatechols produced from the oxidation of cresol and N-bases composed of C, H, and N elements were regarded as the biomarkers for biomass burning (Laskin et al., 2009; Wang et al., 2017). However, on comparing the signal intensity of nitroaromatics in ambient aerosol and fresh biomass burning smoke, Wang et al. (2017) found evidence to the contrary. Signal intensity was stronger in ambient aerosols than that in fresh biomass burning smoke, which indicated the existence of other sources or aging processes. Recently, nitrophenol was also detected in tunnel samples indicating traffic sources. In this study, we inferred that dinitrophenol were abundant in off-road diesel vehicle emissions, while nitronaphthol with one or more methyl groups was dominant for HFO-fueled vessel emissions.
Except for biomass burning, S-containing compounds were still an important
group of organic matter for coal combustion, on-road vehicles, off-road
diesel vehicles, and HFO-fueled vessels and at background sites, accounting for
48 %, 17 %, 8.9 %, 33 %, and 32 %, respectively, of total detected
organic matters. Organosulfates and sulfonates were one of the most
important HULIS, which were reported as the prominent S-containing compounds
at background sites due to aging reactions of organics with
In recent years, air pollution from ship emissions, especially in coastal
areas, has drawn increasing worldwide attention. In previous studies,
vanadium (V) and nickel (Ni) were widely used as specific tracers HFO-fueled vessel emissions (Liu et al., 2017b). It was reported
that the V content in HFO gradually decreased from 39.5 ppm in 2013 to 12.7 ppm in 2018 in China in compliance with the requirements for all vessels
within the China domestic emission control areas (DECAs) (Zhang et al., 2019). In
addition, as V and Ni could also originate from industrial emissions, the
uncertainties in estimating ship emission contributions to atmospheric
PM
In this study, we found that the molecular compositions of POCs emitted by
HFO-fueled vessels were different from those of other source emissions (e.g.,
off-road diesel engines, biomass, and coal burning). The results of this
study demonstrated that soot materials or oxidized polycyclic aromatic
hydrocarbons and S-containing species especially those with high aromaticity
and
On the other hand, it should be noted that these organosulfates and sulfonates have been previously reported to be mainly formed from secondary photochemical reactions via the oxidation products of VOCs and acidified sulfate seed particles or sulfuric acid in the atmosphere (Riva et al., 2015; Tao et al., 2014). However, the high abundance of S-containing species found in HFO-fueled vessel smoke indicates that not only secondary organic aerosols but also primary HFO-fueled vessel smoke could be an important source of organosulfates (Fig. S5). Therefore, ignoring the contributions of HFO-fueled vessel emissions to organosulfates might lead to the overestimation of the contribution of secondary organic aerosols in the atmosphere.
Although some useful characterizations of POCs from off-road engine combustion emissions were proposed, some issues still need to be resolved in the future. These include the following: (1) determination of the molecular structure of the distinctive compounds mentioned in this study should be further explored; (2) potentially different molecular structures of organosulfates from HFO-fueled vessel emissions and SOA should also be distinguished.
Data used in this study can be provided by Cui Min (15212219177@163.com).
The supplement related to this article is available online at:
MC and CL contributed equally to this work. MC wrote the paper in close cooperation with CL and got helpful direction by YC, JuL, and JZ. FZ, JiL, and YM were responsible for sampling and chemical analysis. BJ, CY, and MZ were familiar with data processes of FT-ICR MS and mass absorption efficiency. ZX and GZ provided key contributions to article structure and logic.
The authors declare that they have no conflict of interest.
This study was supported by the Natural Scientific Foundation of China (nos. 91744203 and 41773120), Guangdong Provincial Science and Technology Planning Project of China (no. 2017B050504002), and State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry (no. SKLOG-201732).
This research has been supported by the Natural Scientific Foundation of China (grant nos. 91744203 and 41773120), the Guangdong Provincial Science and Technology Planning Project of China (grant no. 2017B050504002), and the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry (grant no. SKLOG-201732).
This paper was edited by James Allan and reviewed by two anonymous referees.