3.1.Temporal changes of the pollution characteristics in four haze pollution episodes in northeast China during 2017–2019
Table 1 and Figure 2 show the PM2.5 concentration (µgm−3) and the concentrations of its chemical components during the sampling period at each sampling site. In EP3, only four sampling sites were analyzed, and they were not included in the calculation of total PM2.5. The PM2.5 concentration was 66.3–158.5µgm−3, with an average of 104.7µgm−3. This was 1.4 times China's national environmental standard (24 h average of 75µgm−3) and 4.2 times the World Health Organization (WHO) standard (~25µgm−3). The highest concentration was observed at HST, with the stations following the order of HST>BX>SP>SJ>YS>ZD>DLS>SY>GZC>BH>BL>DQ>SY>YJ. The PM2.5 concentrations were 158.5, 144.1, 129.8, 124.5, 122.9, 111.3, 95.8, 93.9, 91.5, 90.8, 84.7, 77.3, 74.3, and 66.3µgm−3.
The chemical component concentrations at each sampling site were in the range of 52.4–109µgm−3, accounting for 63.8–82.1% of the PM2.5, with an average of 72% (Table 2). Total carbon (TC) was the component with the highest concentration (31.2µgm−3), accounting for 29.8%of the PM2.5. This was followed by water-soluble ions, with a concentration of 29.3µgm−3, accounting for 28% of the PM2.5. The winter heating period in northeast China has a duration of five months, and the large amounts of coal-fired emissions resulted in the contribution of organic matter to PM being much higher than in other seasons (~30–40%, Sun et al., 2013). Organic carbon (OC) was the most important component of PM2.5, with a concentration of 26.3µgm−3, followed by NO3− and SO42−, with concentrations of 8.3 and 7.0µgm−3, respectively. Total carbon, NO3−, SO42−, and NH4+accounted for 29.8%, 7.9%, 6.7%, and 5.3%, respectively, and in Beijing-Tianjin-Hebei accounted for 28%,19%,12%, and 11%, respectively, in the same time period (Wang, 2021a). The proportion of TC in PM2.5 in northeast China was higher than in the Beijing-Tianjin-Hebei region, but the other components had a lower proportional content. The absolute concentration and proportional content of NO3− in northeast China has surpassed that of SO42−, and it has therefore become the most important secondary inorganic component of PM2.5. The rapid increase in the NO3−concentration during the pollution period has become one of the key factors in the explosive growth of PM2.5. The reason for this is the use of low-sulfur coal in the northeast. Low-sulfur coal reduces sulfur emissions and greatly reduces the overall concentration and proportional content of SO42− in PM2.5. In the winter the low T reduces the pH of PM2.5, which is conducive to NO3− production (Shi et al., 2019). The chemical component concentrations followed the order of NH4+>EC>Cl->K+>Na+>Ca2+>Mg2+>F−, with concentrations of 5.5, 4.9, 4.2, 1.7, 1.5, 0.8, 0.2, and 0.1µgm−3 respectively. SO42−, NO3−, and NH4+ (SNA) accounted for 19.9% of PM2.5 and 71% of total water-soluble ions. Organic matter and SNA were the main components of PM2.5. The main chemical components that contributed to haze pollution in northeast China were organic matter and inorganic water-soluble ions. These components were mainly derived from photochemical oxidation and the liquid-phase oxidation reactions of gaseous precursors.
Table 1
Average concentrations (µgm-3) and the proportional content (%) of the chemical components of PM2.5
|
PM2.5
|
F−
|
Cl−
|
NO3−
|
SO42−
|
Na±
|
NH4+
|
K±
|
Mg2+
|
Ca2+
|
OC
|
EC
|
TWSIs*
|
TWSIs/ PM2.5
|
TC
|
TC/ PM2.5
|
HST
|
158.5
|
0.4
|
6.7
|
9
|
9.6
|
1.5
|
7.3
|
2.3
|
0.1
|
0.4
|
50.1
|
8.4
|
37.2
|
23.5
|
58.5
|
36.9
|
SJ
|
124.5
|
0.1
|
4.7
|
9.5
|
7.7
|
1.6
|
6.5
|
2.2
|
0.1
|
0.5
|
34.3
|
6
|
32.9
|
26.4
|
40.3
|
32.4
|
BX
|
144.1
|
0.2
|
4.8
|
10.8
|
8.1
|
1.4
|
6.5
|
2.8
|
0.2
|
0.5
|
30.4
|
6.3
|
35.2
|
24.4
|
36.7
|
25.5
|
ZD
|
111.3
|
0.1
|
4.4
|
6.2
|
7.1
|
1.4
|
4.6
|
2
|
0.1
|
0.9
|
28.5
|
4
|
26.8
|
24.1
|
32.5
|
29.2
|
BL
|
84.7
|
0.1
|
1.7
|
4.1
|
4.5
|
1.7
|
2.8
|
1.2
|
0.1
|
0.4
|
28
|
7.9
|
16.6
|
19.6
|
35.9
|
42.4
|
DQ
|
77.3
|
0.1
|
2.5
|
6.3
|
5.4
|
2.3
|
3.4
|
1
|
0.1
|
1.3
|
16.4
|
2.8
|
22.4
|
29
|
19.2
|
24.8
|
YJ
|
66.3
|
0.1
|
2.6
|
5.3
|
5.8
|
2
|
3.7
|
1.5
|
0.1
|
1
|
15.6
|
2.5
|
22.1
|
33.3
|
18.1
|
27.3
|
BH
|
90.8
|
0.1
|
4.1
|
9.4
|
6.5
|
1.3
|
5.9
|
1.5
|
0.1
|
0.6
|
20.9
|
3.4
|
29.5
|
32.4
|
24.3
|
26.7
|
GZC
|
91.5
|
0.1
|
4.4
|
11.2
|
7.5
|
1.6
|
6.6
|
1.8
|
0.1
|
0.9
|
20.8
|
3.8
|
34.1
|
37.3
|
24.7
|
26.9
|
DLS
|
95.8
|
0.1
|
5
|
9.3
|
7
|
1.2
|
4.8
|
1.1
|
0.8
|
1.8
|
21.4
|
6.2
|
31.1
|
32.4
|
27.6
|
28.9
|
SY
|
74.3
|
0
|
3
|
4.2
|
4.5
|
0.9
|
3.2
|
1
|
0.1
|
0.4
|
16.8
|
4.2
|
17.3
|
23.3
|
21
|
28.3
|
YS
|
122.9
|
0.1
|
5
|
8
|
6.2
|
1
|
6.1
|
1.7
|
0.1
|
1.2
|
31.7
|
5.3
|
29.5
|
24
|
36.9
|
30.1
|
SP
|
129.8
|
0.1
|
6.4
|
11
|
7.7
|
1
|
7.2
|
2.4
|
0.1
|
0.2
|
35.4
|
4.8
|
36.1
|
27.8
|
40.2
|
31
|
SY
|
93.9
|
0.1
|
4
|
12.5
|
9.7
|
1.8
|
8.3
|
1.3
|
0.3
|
1.1
|
18.6
|
2.9
|
39.1
|
41.6
|
21.5
|
22.9
|
average
|
104.7
|
0.1
|
4.2
|
8.3
|
7
|
1.5
|
5.5
|
1.7
|
0.2
|
0.8
|
26.3
|
4.9
|
29.3
|
28
|
31.2
|
29.8
|
average/PM2.5
|
100
|
0.1
|
4.0
|
7.9
|
6.7
|
1.4
|
5.3
|
1.6
|
0.2
|
0.8
|
25.1
|
4.7
|
28.0
|
/
|
29.8
|
/
|
The common feature of the PM2.5 chemical component analysis was that the Harbin-centered area was more polluted than Changchun (Table 1, Figure 3). Harbin and its nearby areas had the highest OC and EC levels, the SO42− and NO3−concentrations were highest in Shenyang, and the K+ concentration was higher in rural areas than in urban and suburban areas.
Table 2
The concentrations of PM2.5 and its chemical components in three pollution episodes (µgm-3)
|
PM2.5
|
F−
|
Cl−
|
NO3−
|
SO42−
|
Na+
|
NH4+
|
K+
|
Mg2+
|
Ca2+
|
OC
|
EC
|
EP1
|
86.1
|
0.1
|
4.3
|
6.5
|
7.2
|
1.6
|
4.6
|
1.4
|
0.1
|
0.6
|
23.6
|
2.6
|
EP2
|
86.9
|
0.1
|
2.4
|
5.6
|
4.1
|
1.4
|
5.2
|
1.5
|
0.1
|
0.7
|
20.4
|
4.8
|
EP4
|
122.4
|
0.1
|
5.2
|
10.8
|
7.3
|
0.5
|
5.3
|
1.8
|
0.2
|
0.4
|
27.4
|
5.6
|
total
|
98.5
|
0.1
|
3.9
|
7.6
|
6.2
|
1.2
|
5
|
1.5
|
0.1
|
0.6
|
23.8
|
4.3
|
Comparing the three pollution episodes shown in Table 2, EP4 (in 2019) had the highest concentrations of PM2.5 and its chemical components, while there was little difference between EP2(in 2018) and EP1(in 2017).
3.2 The evolution of PM2.5 and its chemical composition and the causes of pollution during the fourth pollution episode
Figure 4 shows the temporal changes of PM10 and PM2.5, their chemical components and precursor emission gas concentrations, meteorological factors, and the height of the boundary layer during the four pollution episodes. The evolution of EP1–EP4 in terms of the PM2.5 concentration and its chemical composition, and the causes of pollution are analyzed below.
EP1 was formed by the combined effect of the accumulation of local coal emissions, secondary conversion under a low T and high RH, and regional transmission. Wind direction and wind speed were the primary meteorological factors that affected the pollutant concentration. The important indicators of the beginning and end of the pollution episodes was the change of wind direction and speed. Pollution started to accumulate on December 26, 2017, and when the wind direction changed to the southwest, PM2.5 concentrations increased in various locations. The PM2.5 concentration peaked on January 1, 2018, when the wind speed was very low (Figures 4a, b; Figure 5). The wind turned to the north on January 1, the wind speed increased, and the PM2.5 concentration decreased rapidly, with the pollution episode ending on January 9. Whenever the wind direction turned to the south and the wind speed decreased, the PM2.5 concentration gradually accumulated, but when the wind direction was northerly the wind speed increased and the concentration decreased rapidly (Figures 4 and 5). The trends of SO2 and NO2 were similar to those of PM2.5, indicating that emissions had a great impact on pollution. The SO2 and NO2 levels in EP1 were the highest of the entire three years. The boundary layer height (BLH) followed the opposite pattern to that of the pollution concentration (Figure 4x). A low BLH results in poor diffusion of pollutants and high pollutant concentrations, which plays an important role in the rise and fall of the pollutant concentration (Figure 4v). The effects of air T and RH were also significant, and pollution in EP1 increased as the T increased (Figure 4p). However, the T on January 2–8 was higher than on the most polluted day December 31–January 1. It was apparent that T did not play a decisive role in pollution episodes. When the RH was high, the pollution was heaviest (Figure 4w). A high RH was very important for pollution formation.
The OC/EC ratio is used to characterize carbon aerosol emissions and conversion characteristics. Different ratios represent different sources of pollution. A ratio between 1.0 and 4.2 typically represents exhaust emissions from diesel and gasoline vehicles (Schauer et al., 2002); and a ratio of 2.5–10.5, represents emissions from coal combustion (He et al., 2004); while ratios of 9–12.3 and 16.8–40, represent biomass combustion emissions (Cao et al, 2005;Zhang et al.,2007). The average OC/EC ratio of the four episodes was 6.6 (1.5–25.6) (Figure 4s), indicating that they were all affected by secondary aerosol. Compared with the other three EPs, the OC/EC ratio was highest in EP1. The rural station SP had the highest OC/EC ratio, with a value exceeding 15 on six days. This was mainly due to the burning of rural straw. At the same time, the K+, NH4+, and Cl− concentrations, which were indicative of combustion emissions were also high, which indicates that they not all emissions were from straw combustion (Cao et al., 2010). The OC/EC ratio at HST was between 5–12.5, indicating that the main source of its air pollution was emissions from coal combustion. The OC/EC ratios at the other stations were also largely in this range, indicating emissions from coal combustion.
The SO42−, NO3−, and NH4+ concentrations in EP1 were very high (6.5, 7.2, and 4.6µgm−3, respectively, Table 2), with only the concentrations in EP4 being higher. The SO42− and SO2 concentrations, which were representative of coal burning, were higher than those of NO3− and NO2, indicating that the emissions from coal combustion were the most important factor in EP1 (Figures 4p,q, and r). The ratio of secondary pollutants (SNA) to EC is used to evaluate the degree of secondary conversion. The higher the value, the more serious the secondary pollution (Figure 4, Yang et al., 2018; Lin et al., 2009). Elemental carbon is usually considered to be a tracer of a pollution source. Comparing the ratio with the primary emissions (Ca2+, Mg2+) and other components, if the primary emissions are high but the ratio is not high, the pollution is caused by the primary emissions, otherwise it is caused by secondary conversion or regional transmission. The SO42−/EC and NO3−/EC ratios were lowest in EP3 among the four episodes, of which Shenyang and Siping had the highest values (>5). The Ca2+ and Mg2+ concentrations were not high, and therefore there were obvious regional transmission characteristics. At the other sampling sites, the SO42−/EC and NO3−/EC ratios were very high, indicating that SO42− and NO3− were formed by secondary conversion. During the secondary conversion process, the T was low and RH was high. The SNA and OC concentrations increased and decreased with the rise and fall of RH (Figure 4). Ge et al. (2012) found that fog promoted the generation of secondary aerosols (SNA and oxidizing organic matter) in the atmosphere. A high RH is conducive to the absorption of the main chemical components in the moisture on the surface of PM2.5, and the pollutant concentration will then increase rapidly (see section 3.3 for details). Therefore, it was concluded that EP1 was a composite pollution process formed by local emissions from coal combustion, secondary conversion under a low T and high RH, and regional transmission.
EP2 was expected to be dominated by straw burning pollutants due to the large-scale straw burning that occurred during the sampling process. The chemical component with the highest concentration was OC(Figure 4g),with a peak concentration of 180µgm−3 at HST. The concentrations at the surrounding BL, BX, and SJ sites were also very high, indicating that straw burning made a significant contribution to OC. The most representative ion for straw burning is K+ and its concentration was very high, with only EP4 having a higher concentration (Figure 4m). The K+ concentrations at HST, SJ, SP, BX, BL, and ZD were 5–7 times higher than the typical atmospheric concentration in the main straw burning area. The different sampling sites that were affected by straw burning displayed different characteristics. At HST and BX, OC and K+ increased significantly at the same time, while the K+ concentration at SP increased more significantly than the OC concentration. There was a significant increase in OC at BL, whereas K+ did not increase significantly. These results may be related to the different local emission sources, with HST receiving a large contribution from straw combustion and motor vehicle emissions, SP being impacted by straw combustion, and BL being impacted by residential coal burning in the suburbs.
EP3 was a cross-regional transmission process. Figure 6a clearly shows the gradual reduction of PM2.5 and other pollutants as they were transported from southwest to northeast. The jump point of the PM2.5 concentration changed from SY→BH→SJ→HST (Figure 6b), which indicated that EP3 was clearly dominated by a secondary transmission process. The SO42−/EC and NO3−/EC ratios were the highest among the four pollution episodes, and the peak values of the four sampling sites were in the range of 6.7–18.9 (Figure 4p, q). This further confirms that the event was a regional transfer process.
EP4 was the pollution episode with the highest PM2.5 concentration and the heaviest pollution during the entire observation period. Its main features were a high PM2.5 and O3 concentration, and a high mid-winter T. The SO2 and NO2 concentrations were lower than those of EP1 and EP3. The period was characterized by low emissions, and therefore the high levels of pollution were mainly a consequence of the strong oxidation conditions and high winter T.
There were three pollution peaks in EP4 (Figure 4), with high concentrations of the main chemical components (OC and SNA). The first peak was on February 15. The PM10 concentration was highest at BX and YS, which were the representative rural sampling sites, and it was much higher than the PM2.5 concentration (Figure 4), indicating that there was a large proportion of coarse particles. However, the NO3−/EC and SO42−/EC ratios were not very high, indicating that primary pollutants were the major contributors to the pollutant load. The BX site had the highest SO42− and CO concentrations, as well as high concentrations of NO2. OC, EC, Cl-, NO3−, NH4+, and K+ and other coal-burning emission index ions (Wang et al., 2021a; Hu et al., 2015). The YS site also had high concentrations of chemical components. These sites were located in rural areas, with few motor vehicles, and the high concentrations were likely caused by high emissions of loose coal and biomass combustion.
The second peak occurred on February 21. The wind direction at each station was southerly. The PM2.5 concentration at BX, HST, and SJ was high (Figure 4b), the weather conditions were stable, and the wind speed was low to calm(i.e., less than 2m/s) (Figure 5). The T continued to increase and the RH was conducive (about 50%)to the conversion to secondary pollutants. The high concentration at HST was related to the high SO2, NO2, and CO emissions on the previous day. On February 20th in BX, the NO3− and SO42−concentrations were high, the OC concentration at SJ was high, and the EC concentration at BX and SJ was high. These results indicated the secondary generation of local coal emissions under suitable T and RH conditions in a small area. The pollution process occurred under the joint action of atmospheric diffusion (see section 3.3 for details).
The PM2.5 concentration in the third peak was the highest during the entire observation period. There were three main reasons for this.
① The photochemical generation of high levels of secondary organic carbon (SOC). The high levels of SOC (Figure 4t, estimated by the OC/EC minimum ratio method, SOC=OC – EC×(OC /EC)min. Li et al., 2018a)accounted for the highest proportion of OC (Figure 4u). The O3 concentration and T were also the highest during the entire observation period (Figure 4e and v). The high O3 concentration enhances the atmospheric oxidability, which coupled with the high T, was conducive to SOC formation. Li (2018a) and Hu et al. (2012) in Zibo and the Pearl River Delta both found that SOC and O3 increased in a similar manner.
②A large amount of secondary generation of SNA. The SNA concentration was only higher in EP3 due to regional transmission. However, the SO2 concentration was the lowest among the four pollution episodes, and the NO2 concentration was the same as in EP1. Due to the low emission levels, the high pollution load was created by high secondary pollutant concentrations. There were two main reasons for this. First, under a relatively high T (0–5°C) and RH (40–60%), which was not high enough to limit the reaction, the gas emitted by local coal and motor vehicles generated a high SNA concentration through homogeneous and heterogeneous phase reactions (see section 3.3 for details).
Similarly, SJ, HST, DQ, ZD, and SY had high PM2.5 concentrations, which may be related to their higher local emissions. The OC, EC, and Cl− concentrations at SJ and ZD were high on February 26–28, the OC and EC concentrations at DQ and HST were high on February 28, and the SNA concentration at SY had been continuously increasing since February 27. This indicates that the peaks were mainly the local secondary conversion of primary emissions. At this time, the SO42− concentration was high (Figure 4k, peak value 6.1–20.4µgm−3), but still lower than the NO3− concentration. The NO3−/ SO42−ratio was mostly >1, with the highest value reaching 2.5 (Figure 4r).
The reason why the NO3− concentration was higher than the SO42− concentration was related to the low SO2 emission concentration, which limited SO42− formation. It may also be related to the high O3 concentration and the strong gas oxidation was conducive to the gas-particle conversion of NO3−.The high O3 concentration had an important effect on NO3− and SO42− formation. Studies have found that under suitable light conditions, the photolysis of O3 will produce OHˑ and promote NO3− and SO42− formation (An et al. 2014) (see section 3.3.1 for details). Wang et al. (2018) found that the low T of urban high-rise buildings and the high O3 concentration in the residual layer at night can promote the gas-particle conversion of NO3− and the heterogeneous reaction of N2O5. In this study, NO3− formation was dependent on the combination of a high T in winter and a high O3 concentration. This involved the formation of low-T gas-particles at night, and the formation of high-T photochemical reactions during the day. It can be seen that the increase in the O3 concentration greatly increased the NO3− and SO42− concentration, and it is therefore very important to strengthen the control of O3 and NOx pollution.
③The superimposed effect of cross-regional transmission has caused the rapid growth of secondary pollutants. The SO42−/EC and NO3−/EC ratios in Figure 4q and r are very high, indicating that secondary formation occurs through the superimposition of regional transport and secondary reactions. The OC/EC ratio was not high (Figure4s, between 1.7 and 8.4, with an average of 4.9), indicating that it was mainly controlled by emissions from coal combustion and motor vehicle exhaust emissions. It can be seen from the graphs of the PM2.5 concentration and wind field in Figure 7 that the most polluted area was near Harbin (HST). At this time, the wind direction and wind speed of each station changed from northerly to southerly, and the wind speed decreased rapidly. This shows that the atmosphere was in a static and stable state, and the static and stable meteorological conditions were also conducive to the accumulation of secondary particulate matter. In addition, the wind field at the high-concentration center of the BL site had an obvious counterclockwise convergence, and the wind speed was higher than that in the surrounding areas, which accelerated the accumulation of pollutants at the site.
It can be seen that EP4 was a compound pollution process. The Cl− and OC concentrations were the highest among the four episodes, indicating that coal combustion made a substantial contribution, and the K+ and Mg2+ concentrations were also the highest among the four episodes. Biomass combustion and industrial emissions also made a large contribution. Except for the high Ca2+concentrationatthe BX and ZD sites on February 21st, the levels at the other stations were low. The main source of Ca2+ is usually sand and dust emissions (Xu et al., 2017b), indicating that the impact of sand and dust was small in EP4. Therefore, the pollution formation during this episode occurred through the homogeneous and heterogeneous reaction of pollutants emitted by local coal combustion, biomass combustion (K+), and industrial emissions (Mg2+) under a moderate T and strong gas oxidizing conditions (high O3). It was formed by a significant increase in the secondary pollutant concentrations, such as SOC and SNA, and then superimposed on a composite pollution process controlled by regional transmission.
3.3 The influence of meteorological factors (e.g., T and RH) on secondary pollutant (SO 4 2- and NO3-) formation in the haze pollution process
3.3.1 Nitrate formation
The rapid increase in secondary pollutant (SO42− and NO3−) concentrations is an important reason for haze formation, and meteorological factors also play an important role in secondary pollutant formation.
Nitrate formation in the northeastern urban agglomeration was investigated. The relationship between [NH4+]excess and NH4+/ SO42− (molar concentration ratio; Pathak et al., 2009) was used to determine the occurrence of NO3− under different pollution processes and different weather conditions, and homogeneous and heterogeneous chemical reactions. The [NH4+]excess= ((NH4+/ SO42−- 1.5 )×[SO42−]) was calculated during the entire observation period. Figure 8 shows its correlation with NO3−. During the pollution process, NO3− and [NH4+]excess had a strong linear relationship (R2=0.81), indicating that NO3−was mainly generated through heterogeneous reactions (Pathak et al., 2005)). The NO3− concentration increased with the increase in ([NH4+]excess (Figure 8a). Therefore, the NO3−formationin the northeastern urban agglomeration occurred mainly through the homogeneous gas-phase reaction of atmospheric ammonia (NH3) and nitric acid(HNO3).
The NO3− formation conditions and accumulation mechanisms differ under the influence of different emission and meteorological factors. In recent years, the NOx concentration was often higher than that of SO2. The low slope value of Figure 8a (0.62) indicates that the NH4+ concentration in northeast China was much higher than that of NO3−, which was conducive to NO3− formation. Because emissions from coal combustion make a large contribution to atmospheric NH4+ and Cl− (Xu et al., 2017b), the Cl− concentration was added to evaluate the relationship between [NH4+]excess and NO3− (Figure 8b). The correlation coefficients for the relationships between [NH4+]excess and NO3− +Cl− in all pollution processes were greater than 0.82, and for EP4 the value was 0.99. The slope was also very high, indicating that after NH4+ neutralized all of the NO3− and SO42− (forming ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4)), there may be a surplus, which would continue to react with Cl− and other anions, mainly in the form of ammonium chloride (NH4Cl). In EP1 and EP4, the slopes were 1.1 and 1.14, and the intercepts were 0.09 and 0.12, respectively, indicating that some of the NO3− +Cl− was not completely neutralized by NH4+. In Figure 4n, it can be seen that the Ca2+ concentration in EP1 was higher than in the other pollution episodes, and the contribution of sand dust was also greater. The remaining NO3− and Cl− may exist in the form of calcium nitrate (Ca(NO3)2) and calcium chloride (CaCl2). The lowest slope of EP2 was 0.68, indicating that after all the NO3− +Cl− was neutralized, there was still a large amount of NH4+. The main feature of EP2 was that straw burning produced a large amount of NH3, and therefore the NH4+ concentration was also high. The slope of EP3 was 1.04, indicating that NH4+was present in excess and Cl- emissions from coal combustion were not high. The K+ concentration in EP4 was high. The contribution of biomass combustion was also high, which may have enabled potassium to exist in the form of potassium nitrate (KNO3) and potassium chloride (KCl).
Nitrate in the EP1 process was formed under the conditions of a medium T and high RH. At this time, the warming period in winter was occurring (Figure 8d, 5v), and the RH was also high when NO3− peaked (Figure 8c, 5w). This shows that NO3−formation occurred when the T was mild in winter and the RH was high. A high T was conducive to the homogeneous gas-phase reaction of NO2 reacting with OHˑ to produce gaseous HNO3(Harris et al., 2013). A T not higher than 4°C is also conducive to NO3− formation. The reaction therefore proceeded in the positive direction (Sun et al., 2012). The subsequent reaction between gaseous HNO3 and NH3 needs to occur at a higher RH, because although NO3− is mainly formed through a homogeneous reaction in the gas-phase, a high RH is conducive to the accumulation of NO3− in the particles (Shi et al., 2014). Therefore, a high RH promotes NO3− formation.
The EP2 process was clearly impacted by straw burning emissions, with rapid NO3− formation in an NH3-rich environment. Straw burning in the vicinity of several cities, coupled with southerly winds, was conducive to the spread of pollution from south to north. In particular, the wind speeds at several sampling sites were high on December 15 (Figure 5, 4–5ms−1), which caused pollution to rapidly spread to the area. On the December16 the wind speed decreased rapidly, and the calm winds were conducive to the accumulation of pollutants. At this time, at SP, YS, and other locations, the RH was high (40–60%, Figures 4w and 8c), and the T was low (-15 to -11°C, Figure 4v), and therefore straw burning pollutants accumulate in wet weather conditions to produce haze. There are records of haze in various locations.
In the EP3 pollution process, the NO3− concentration was the highest among the four pollution episodes (Figure 4j, Figure 9a). It was formed by the secondary pollutants in the high RH and medium T environment during the transmission process, and the RH has a great influence on it. The RH was also the highest of the four periods at around 50–80% (Figures 9a, c and Figure 4w), and the T was centered at -10–0°C (Figure 4v).
The NO3−formation in the EP4 process occurred under a relatively high T and moderate RH in winter. Only EP3 had a higher NO3− concentration, with the NO3− concentration in the third peak period being especially high (Figures 4j, 9a). The RH was only between 40–60% (Figures 4w, and 8a, c). Although the RH was not high, the deliquescent RH of the particulate matter was still higher than the weathered RH, which caused it to continue to deliquesce without being weathered, and a high water content was maintained in the particulate matter. In conjunction with the high winter T and strong atmospheric oxidability, serious haze pollution could still occur. The pollution concentration sometimes exceeded the concentration recorded when the RH was higher and the T was lower (such as in EP1). It can be seen that the influence of RH on NO3− in northeast China was not as obvious as its influence on SO42−. Li et al. (2017), Sun et al. (2013), and Wang et al. (2014) also reached similar conclusions in a study of pollution in the Beijing-Tianjin-Hebei region. Wang et al. (2014) estimated that the influence of fog was responsible of 70% of the SO42− formation in winter in Beijing, while the impact on NO3−was less than 30%. Under the condition where a certain RH was maintained, a higher T was more conducive to the homogeneous reactions involved in NO3− formation.
In the EP4 process T conditions were very favorable, with the highest Ts recorded during all observations (between -4 and -2°C, Figure 8a, d). A higher T is more conducive to the occurrence of photochemical reactions. In addition to the above-mentioned low RH conditions that did not affect NO3− formation and the warmer T conditions in winter, the oxidative properties of the atmosphere were also important for HNO3 formation. During EP4, there were high O3 concentrations (Figure 4e), and nitrous acid (HONO) peaks also appeared at the same time as the first two peaks of NO3− (Figure 9b). The SO42−, NO3−, HONO, HNO3, and NH3 concentrations in Figure 10 were measured by the Marga instrument in the Shenyang Atmospheric Composition Station. At this time, NO3− and SO42− formation occurred through the gas-phase oxidation reactions of OH· with NO2 and SO2, respectively (Seinfeld and Pandis, 2006). The main sources of OH· were the photolysis of O3 and HONO (Ji et al., 2016). Although the scattering, reflection, and refraction of haze pollution in winter will weaken the amount of solar radiation reaching the ground and affect the generation of OH·, high HONO concentrations can produce large amounts of OH· under suitable light conditions, ensuring the uniform progress of the homogenous NO3− formation reaction. During the daytime, HONO photolysis produces OH·, OH· reacts with VOC and NOx in a multi-chain reaction, and this increases the O3 concentration. The O3 further promotes OH· production through photolysis, and OH· continues to oxidize NO2 and SO2, which promotes HNO3 and SO42− formation (An et al. 2014). The reaction rate is related to the amount of liquid water adsorbed on the particle-air interface. Strong correlations have been reported between aerosol and HONO concentrations, and the specific surface area of an aerosol and the HONO concentration (Finlayson-Pitts et al., 2003). Part of the reaction product is separated from HONO when the reaction surface returns to the atmosphere, while HNO3 stays on the reaction surface and re-participates in a series of atmospheric chemical reactions, increasing the NO3−concentration in the atmosphere. Therefore, the high atmospheric oxidability formed by the high O3 and HONO concentrations in the EP4 process promoted photochemical reactions and produced a large amount of NO3−.
Similarly, during EP4, a high NH3 concentration was observed, which reacted with HNO3 (Figure 9b) and promoted massive NO3− aerosol formation. Due to the use of low-sulfur coal and the widespread use of desulfurized rubber as pollution control measures, SO2 concentrations have decreased, but NH3 emissions have risen at the same time. The NO3− concentration increased significantly with the increased NH3 emissions (Figure 9a). The relatively warm T (-4 to -2°C) was conducive to an acceleration of the neutralization reaction of NH3 and HNO3, which promoted NO3− formation. Many studies have also shown that a sufficient source of NH3 will effectively promote NO3− and SO42− particulate formation, resulting in the rapid accumulation of particulate matter (2015; Guo et al., 2017). A high NH3 concentration promotes NO3− aerosol formation in a ubiquitous atmospheric chemical phenomenon that has also been observed in other cities in northern China (Sun et al., 2018). In addition, under a high RH and rich NH3 conditions, SO42− will also be generated in large quantities. The pH of the aerosol surface is close to 7, and NO2 oxidizes SO2 to generate SO42− (Wang et al., 2016a). However, Li et al. (2018b) later found that on the surface of aqueous sodium bisulfite (NaHSO3) (pH 3~6), NO2 cannot directly oxidize S(IV) to form SO42−, but instead generates NO3− and HONO. Based on this analysis, the possibility of HONO oxidation of S(IV) is a more important mechanism of NO3−formation. In this study, the NO3− formation in northeastern China was mainly due to the mild T, high RH, high atmospheric oxidability (O3 and HONO), suitable light conditions, and a high NH3 concentration. A homogeneous gas-phase reaction occurred between OH· and NO2, SO2, and NH3.
3.3.2 The relationship between the SO42- concentration and water vapor pressure
To characterize the relationship between ion concentrations and RH, the relationship between ambient water vapor pressure(e*) and the SO42− concentration was determined for different pollution periods, under different RH, T, and sulfur oxidation rate (SOR) conditions, and with different precursor gas concentrations (mainly SO2 and NO2). The ambient water vapor pressure was calculated using the formula e*=e×RH/100 (e refers to the saturated water vapor pressure, where e=6.112exp (17.67t/(t+243.5), t refers to the T (°C)). The SOR refers to the molar ratio of sulfur in SO42− to the total sulfur, and is calculated with the formula SOR= SO42−/( SO42−+SO2).Figure 10 shows the changes in the SO42− concentration with e*, with air temperature, different pollution periods, RH, and the precursor gases shown in different colors.
The trend of the fitted line in Figure 10a shows that SO42− increased with an increase in e*, but the increase was not very obvious, indicating that SO42− formation was complicated. Taking e*=1.5 and the SO42− concentration=10µgm−3 as boundaries, the figure could be divided into four areas. Figure 11 shows the statistical values of various pollutants, T, and RH in the four areas.
Area ①. This feature mainly occurred in EP4 and EP3 (Figure 10a, b). This area of the figure had the highest SO42− concentration (14.7µgm−3), the highest T and O3 (3.7°C and 71.6µgm−3,Figure 10a, Figure 11), coupled with high HONO and NH3 concentrations (Figure 9b). The degree of oxidation of NO2 and SO2 was enhanced, and the SOR (0.3, Figure 10d), SO42− (14.7µgm−3), and NO3− (22.3µgm−3) increased the most. Although the SO2 concentration was low, the NO2 concentration was high, and the prevailing conditions still supported SO42− formation. At the same time, the RH was not high (Figure 10d, Figure 11, 63.8%), which affected the aqueous phase reaction of SO42− to a certain extent. In area①, the correlation between SOR and O3was highest (0.473,Table 3), and the correlation with T (0.275) was higher than the correlation with RH (0.110),
In area①, the correlation between SOR and O3 was highest (0.473 ,Table 3), and the correlation with T (0.275) was higher than with RH (0.110), indicating that SO42−was mainly transformed by the SO2 gas-phase instead of aqueous oxidation. Therefore, the SO42− formation in area ①was mainly the homogeneous gas-phase reaction of the OH· oxidation of NO2 and SO2. The source of OH· was the photolysis of O3 and HONO, which plays a key role in atmospheric chemistry (Ji et al., 2016). The oxidation rate of OH· is usually several times or even hundreds of times higher than in other pathways (Tang et al., 2006). Many studies have shown that the gas-phase oxidation of SO2 by OH· and peroxyl radicals is important for SO42− formation (Xie et al., 2009). Therefore, the photochemical activity had a very important influence on the gas-phase conversion of SO2 to SO42−during the haze events. Previous studies have shown that the heterogeneous chemical reaction of SO42− is an important pathway for SO42− formation during heavy haze episodes in northern China. The reaction conditions were a low T, high RH, and weak light (Elser et al., 2016), and it was speculated that SO42− formation is the gas-phase reaction of OH· oxidation NO2 and SO2. The reaction conditions were warm winter Ts and high O3, HONO, and NH3 concentrations.
The SO42− concentration in area ② was slightly lower than that of area①, and these conditions mainly occurred in EP1. The conditions that favored SO42− formation were the highest SO2 and NO2 concentrations, and the highest RH (63.8%). The disadvantages for SO42− formation were the very low T (-16°C), weak light levels, low O3 concentration, and slightly lower SOR (Figure 11). When using the anion/cation equivalent ratio (AE/CE) to evaluate the aerosol acidity (Wang et al., 2016), it was found that the average AE/CE of total cations (NH4+, Na+, K+, Ca2+, Mg2+) and anions (SO42−, NO3−,Cl−) was 1.07±0.21. After the cations had completely neutralized the anions, there were still small amounts of cations remaining. This indicated that PM2.5 was approximately neutral or weakly alkaline. Cheng et al. (2016) also studied the causes of SO42− formation under the same conditions (high NO2, low O3, and high RH) during Beijing haze, and speculated that NO2 may be an important oxidant for fine aerosols under conditions with high RH and NH3 neutralization. A high pH value will draw more SO2 into the aerosol droplets, thereby increasing the rate of SO42− formation in the NO2 reaction pathway. It is more likely that NO2 will oxidize SO2 in the aqueous phase on fine aerosols with a high RH (>60–70%) and sufficient neutralization (pH~7) (Wang et al., 2016a). Therefore, it was speculated that the increase in SO42− production under high NO2 and SO2 concentrations, high RH, and low O3 levels in northeast China haze events may have a specific mechanism. Under neutral or weakly alkaline conditions, the high pH will pull more SO2 into the aerosol water, and the high RH will also make it easier for NO2 to react with SO2 droplets in an aqueous oxidation to increase the SO42− production rate. This is another important reaction pathway for the production of high SO42−concentrations in northeast China in addition to the gas-phase transformation under the influence of the photochemical activity in area①. However, due to the unique low-T environment and low SO2 emissions in northeast China in winter, the aqueous phase reaction of SO42− in area ② was actually substantially weakened, and the gas-phase SO42− formation in area ①, with its high atmospheric oxidability, was greater than in area ②.
Hong et al. (2019) described seven pollution episodes that occurred in Shenyang from January to April, 2014.The EP1–EP3 reported by Hong et al. (2019) were similar to the EP1 in this study(area ②). The emission and meteorological conditions were similar, with high SO2 and NO2 concentrations, high RH, low O3 concentrations, and weak light conditions.EP5 in Hong et al. (2019) was similar to EP4 in this study(area①), both of which had low SO2 and NO2 concentrations, low RH, high O3, HONO, and NH3 concentrations, and a high atmospheric oxidability. The SO42− formation was also very similar. The pollutant formation pathway in area ① might be high-photochemical oxidation or gas-phase oxidation, while area ② was an aqueous phase reaction process. Moreover, the SO42− concentration produced by gas-phase oxidation was higher than that produced by aqueous phase oxidation. However, gas-phase oxidation often occurred after March in late winter and early spring. Most of the time SO42− was produced by an aqueous phase reaction, but the highest concentration and the fastest formation occurred through the gas-phase reaction. Studies in Beijing-Tianjin-Hebei and other regions have shown that the aqueous reaction is the most important reaction pathway for the rapid growth of SO42− (Sun et al., 2013). Huang et al. (2015) also reported that the conversion of SO2 to SO42− during winter haze occurred mainly on the surface of aerosol droplets through catalytic oxidation in the presence of transition metals, rather than by gas-phase oxidation. Wang et al. (2016a) studied the heavy pollution formation process in Beijing-Tianjin-Hebei and the entire North China region in autumn and winter. They also found and confirmed that the aqueous oxidation of NO2 on atmospheric fine particles is an important SO42− formation mechanism during haze episodes in China. The gas-phase oxidation process was not important. However, in northeast China, due to the inhibitory effect of the low T in winter on the SO42− aqueous reaction, there is rapid SO42− and NO3− production, and the rapid gas-phase oxidation process plays an important role. This is the main difference between northeast China and other regions. However, the aqueous reaction is still the most common way of producing SO42− in northeast China.
The SO42− concentrations in areas③ and ④were similar. The largest advantage of area ③in terms of SO42− formation was the high RH, but there were low SO2 and NO2 concentrations and a low T, which inhibited the heterogeneous oxidation of NO2 and SO2 to a certain extent, resulting in a low SO42− concentration. The biggest advantage of area ④ in terms of SO42− formation was the high T in winter, but the low SO2 and NO2 concentrations and slightly lower O3 concentration than in area ③ resulted in the degree of homogeneous oxidation of NO2 and SO2 in the gas-phase being less than that of area ①. In addition, the low RH affected the aqueous phase oxidation process causing a low SOR. Therefore, the SO42− concentration was not high.
Table 3
The correlation between various pollutants, relative humidity (RH), and temperature (T) in areas① and ②.
|
NO3−
|
SO42−
|
T
|
RH
|
e*
|
SO2
|
NO2
|
SOR
|
NOR
|
O3
|
NO3−
|
1
|
.663**
|
-.056
|
.212
|
.071
|
.009
|
.144
|
.217
|
.533**
|
.115
|
SO42−
|
.822**
|
1
|
-.336*
|
.434**
|
-.130
|
.305*
|
.290
|
.063
|
.202
|
-.054
|
T
|
.348**
|
.110*
|
1
|
-.681**
|
.820**
|
-.020
|
-.010
|
-.248
|
-.044
|
-.081
|
RH
|
.029
|
.135**
|
-.524**
|
1
|
-.153
|
.261
|
.249
|
-.056
|
-.038
|
.011
|
e*
|
.463**
|
.231**
|
.828**
|
-.009
|
1
|
.163
|
.169
|
-.354*
|
-.088
|
-.116
|
SO2
|
.123**
|
.338**
|
-.207**
|
.184**
|
-.115**
|
1
|
.817**
|
-.793**
|
-.585**
|
-.657**
|
NO2
|
.344**
|
.426**
|
.050
|
.089*
|
.122**
|
.687**
|
1
|
-.780**
|
-.696**
|
-.657**
|
SOR
|
.490**
|
.428**
|
.275**
|
-.110*
|
.268**
|
-.519**
|
-.316**
|
1
|
.820**
|
.642**
|
NOR
|
.752**
|
.566**
|
.354**
|
-.064
|
.400**
|
-.198**
|
-.169**
|
.773**
|
1
|
.576**
|
O3
|
.216**
|
.044
|
.628**
|
-.412**
|
.505**
|
-.459**
|
-.288**
|
.473**
|
.390**
|
1
|
Note: **. The correlation was significant at the 0.01 level. *. The correlation was significant at the 0.05 level. The lower left side of the table represents area ①, and the upper right side represents area ②.
3.3.3 The influence of T and RH on the concentration of important chemical components
(1) Changes with T:
The NO3−concentration increased with increasing T (Figure 12). Higher Ts are conducive to NO3− formation in the gas-phase, but Ts below 4°C will not support volatilization and therefore will affect the gas particle distribution in the NH3+HNO3↔NH4NO3 reaction. The formation of SO42− was similar to that OC and followed a V-shaped pattern. Higher Ts in winter were beneficial to SO42− formation because with high Ts the solar radiation is strong, the rate of photochemical oxidation is rapid, and the OH· concentration increases, which in turn strengthens the oxidation of SO2. Under a high RH, the water content of aerosols is relatively high, and O3 is dissolved in aerosol water where it reacts with dissolved SO2 to promote the liquid-phase oxidation process (Cheng et al., 2016). Heating emissions increase as the T falls, and therefore large amount of SO2 are generated. The OC increases with an increase in T, with a higher T meaning that the photochemical reaction was strong, which was conducive to OC formation(Li et al.,2016,2019).
(2) Changes with RH:
The highest OC, NO3−, and SO42− concentrations were observed when RH was in the range of 80–90% (Figure 12). Under a high RH, many atmospheric heterogeneous reactions are accelerated, and the reaction products are conducive to water absorption and the deliquescence of particulate matter. They will therefore have a significant role in promoting haze formation. The coexistence of pollutant particles and fog forms a positive feedback mechanism, which will continuously promote the conversion of the gaseous pollutants discharged from the primary to secondary aerosol. This will result in their concentration continuing to increase. The SO42− concentration in this study increased with an increase in RH. This shows the importance of the heterogeneous reactions (liquid phase reactions) under a high RH in SO42− formation.
Unlike OC and SO42−, more than 70–60% of the NO3− was produced when the RH was 50–30%. The phenomenon in which the NO3− concentration produced by the EP4 process (low RH) was higher than that of produced during the EP1 process (high RH) was explained in section 3.3.1. Wang et al. (2014) also reported a high NO3− production under a low RH. This shows that the dependence of NO3− on RH was not as obvious as that of SO42−. Many studies have reported the phenomenon in which the change of the NO3− concentration at different RH levels in winter is much smaller than that of SO42− (Sun et al., 2013). The NO3− formation was more complicated than that of SO42−. Section 3.3.1 of this article shows that a high atmospheric oxidability, high NH3, high NO2, and high [NH4+]excess were all important in NO3 formation.
Compared with NO3− and SO42−, RH had little effect on the changes in the OC concentration. In this study, the RH during EP4 was not high, but the OC and SOC concentrations were the highest during the whole three years (2017–2019), which was related to the high O3 concentration, high atmospheric oxidability, and high T during this episode (see section 2.1 for details). Sun et al. (2013) reported that among the organic components, those emitted from coal combustion increased most obviously with an increase in RH. The influence of fog on SO42− formationin winter in Beijing can be high, accounting for 70% of the overall formation, while its influence on NO3− was less than 30%. Our results showed that RH had the largest impact on SO42−, followed by NO3−, and had the least impact on OC.