Analysis of the Diurnal Changes in the Water-Soluble Ion Concentration in Wuhan between 2016 and 2019

Abstract

This study uses online monitoring data from the Hubei Environmental Monitoring Center’s Atmospheric Compound Pollution Automatic Monitoring Station from 2016 to 2019 to analyze the diurnal changes in the concentration of water-soluble ions in particulate matter in Wuhan. During the study period, the concentrations of SO₂, NO₃⁻, and SO₄²⁻ changed significantly, while those of NH₄⁺, NH₃, and Ca²⁺ exhibited minimal differences. SO₂ and NO₃⁻ showed an annually increasing trend, while NH₄⁺ and SO₄²⁻ exhibited an annually decreasing trend. The ion concentration was generally higher in the winter and spring and lower in the summer and autumn. The concentration of water-soluble ions was generally higher during the day than at night. However, the “weekend effect” on the change in ion concentrations was substantial and higher during the day than at night. This effect was the strongest for NO₃⁻ and the weakest for NH₃. These changes in the weekend effect of water-soluble ions in particulate matter clearly revealed the impact of periodic human activities on atmospheric pollution. Taken together, the results of this novel study reveal the diurnal pollution characteristics and “weekend effect” of water-soluble ions with high concentrations in atmospheric aerosols in Wuhan over a four-year period, thus providing relevant insights for Wuhan’s atmospheric mitigation plan.

1. Introduction

In recent years, with the accelerating economic growth and urbanization in China, environmental issues, including air pollution, have become increasingly severe while the people’s demand for an improved quality of life and environment has greatly increased [1]. According to the “Bulletin of the State of China’s Ecological Environment” [2], from 2016 to 2019, 338 cities across the country were at, or above, the prefecture level (note that Laiwu City was merged into Jinan City in 2019; therefore, the total number of cities was 337). From 2016 to 2019, the proportions of cities that exceeded the ambient air quality standard were 75.1%, 70.7%, 64.2%, and 53.4%, respectively, and severe pollution occurred in all cities for 2464, 2311, 1899, and 452 days, respectively. Particles with a diameter of 2.5 µm and 10 µm (PM_2.5 and PM₁₀, respectively) have become the primary pollutants causing air pollution in China. Atmospheric particulate matter can shorten the life span and increase morbidity [3]. Furthermore, a causal relationship was found between atmospheric PM_2.5 exposure and cardiovascular morbidity and mortality [4]. PM_2.5 also has acute adverse effects on the lung function of young healthy adults [5]. Moreover, the water-soluble ions in PM can directly affect the acidity of atmospheric precipitation and have an impact on atmospheric visibility, air quality, and human health [6,7].

Water-soluble inorganic ions (WSIIs) are major components of PM_2.5 in which sulfate, nitrate, and ammonium, known as secondary inorganic aerosols, predominate [8]. To date, the contribution of WSIIs to air pollution has been intensively studied in many cities in order to reduce PM levels. Meng [9] reported a WSII concentration of 63.1 ± 35.3 μg/m³, accounting for approximately 45.3% of the total PM_2.5 mass in Handan city, Hebei province, China. Hu [10] found that the average concentration of WSIIs was 44 μg/m³, accounting for 38% of the total PM_2.5 mass in Beijing, China. Moreover, Zhang [11] reported that the concentration of secondary inorganic aerosols increased significantly during the haze period, implying a greater contribution of secondary inorganic aerosols to the severe PM_2.5 pollution. The relative abundance of ions determines the hygroscopicity of aerosols and may aggravate the impairment of visibility [12]. Furthermore, these ions can increase the toxicity potential of organic compounds, such as polycyclic aromatic hydrocarbons (PAHs) and n-alkanes, by increasing their solubility and acting as surfactants [13]. In fact, Qiu [14] studied the seasonal characteristics of water-soluble ions in PM_2.5 in Jiangbei New Area, Nanjing and found that the concentration of water-soluble ions followed the same fluctuating trend throughout the four seasons; however, higher concentrations were detected in the winter while lower concentrations occurred in the summer. Zhang [15] further revealed the characteristics and sources of the chemical components of PM_2.5 during the winter in Jiashan and found that the PM_2.5 levels were 2.6 times those in the cleaning period and the sulfur and nitrogen conversion rates during the pollution period were higher than those during the cleaning period. Thus, PM, which is a complex mixture of particles and liquid droplets in the air, is a key traffic-related air pollutant in urban environments [16].

Although air pollution control has never ceased, the poor air quality in Wuhan remains a relatively serious issue, with a particular increase in the number of mixed air pollution days [17]. Indeed, PM has become the primary air pollutant in Wuhan. Fine particles remain in the air for a long time and can produce toxic substances that pose a significant threat to the air quality and human health. Water-soluble ions represent the main components of PM, with their proportion generally exceeding 50% [18]. In recent years, with the joint efforts of citizens, the atmospheric environmental issues in Wuhan have improved; however, the development prospects remain unclear. Moreover, research on water-soluble ions has primarily focused on changes in spatiotemporal characteristics and source analysis. Therefore, the current study applied hourly time data to reveal the diurnal characteristics of water-soluble ions in Wuhan city from a novel “weekend effect” research perspective.

2. Materials and Methods

2.1. Overview of the Research Area

Wuhan is located at 29°58’–31°22’ N and 113°41’–115°05’ E in the eastern region of Hubei province and extends to the Yangtze River. Wuhan has a northern subtropical monsoon climate, with abundant rainfall and sufficient heat, with rain and heat occurring in the same season. According to the “Bulletin of the State of the Ecological Environment in Wuhan” [19], there were 237, 255, 249, and 245 days with good ambient air quality and 129, 110, 106, and 120 polluted days in Wuhan in 2016, 2017, 2018, and 2019, respectively. The primary pollutant was PM_2.5 for 87 (67.4%), 68 (61.8%), 53 (50.0%), and 38 (31.7%) days in 2016, 2017, 2018, and 2019, respectively, while PM₁₀ served as the primary pollutant for 4 (3.1%), 3 (2.7%), 10 (9.4%), and 2 (1.7%) days in 2016, 2017, 2018, and 2019, respectively.

2.2. Sampling Site and Data Selection

As shown in Figure 1, the sampling site for this study was established in the Hongshan District of Wuhan City, on the roof of the Hubei Environmental Monitoring Center Station (30.54° N, 114.37° E), approximately 20 m above the ground. The surrounding terrain is flat and densely populated, with high traffic flow; thus, it represents a typical urban residential area. The data used in this research were obtained from the monitoring site. Among the eight PM water-soluble ions and five gases that are monitored by the superstation, four water-soluble ions and two gases were selected from 2016 to 2019. The “2 + 2 + 2” combination method was employed, including two anions (NO₃⁻ and SO₄²⁻), two cations (Ca²⁺ and NH₄⁺), and two gases (SO₂ and NH₃).

2.3. Research Methods

In this study, the online gas composition and aerosol monitoring system of the Hubei Environmental Monitoring Center (MARGAL Metrohm) was used to monitor water-soluble ions in the PM in Wuhan. MARGAL is composed of three main parts: a sampling system, a control system, and an analysis system [20]. The principle of the instrument is as follows: Air is pumped into the sampling tube where the cyclone cutter can screen the particle sizes. Once the screened sample enters the sampling box, the soluble gas is quantitatively absorbed by, and passed through, the wet rotating diffusion tube (WRD), at which point it is captured by the steam jet aerosol collector (SJAC). The vapor injection creates a supersaturated environment that condenses and turns the growing particles into a liquid sample. The samples from the WRD and SJAC are quantitatively collected into a syringe pump in the analytical chamber within 1 h and analyzed by quantitative loop injection ion chromatography.

The time-by-time data monitored by MARGAL were aggregated in Microsoft Excel to analyze the four-year trend of the selected samples. The ion data from each season, defined meteorologically as spring (March to May), summer (June to August), autumn (September to November), and winter (December to January), were summarized for all four years at the same time points to reflect the diurnal variation in ion concentrations in different seasons.

In addition to the monthly data, the weekday (Monday to Friday) and weekend (Saturday and Sunday) data were assessed separately to facilitate analysis of the “weekend effect” on water-soluble ions. After the data processing was completed, Microsoft Excel was used to create line graphs of the temporal and seasonal changes in water-soluble ions, and scatter plots were produced to illustrate the trends of the “weekend effect”. Cleveland [21] delimited the size of the weekend effect as:

$$w={\mathrm{C}}_{\mathrm{weekend}}-{\mathrm{C}}_{\mathrm{weekday}}$$

(1)

$$\mathrm{Weekend}\mathrm{effect}\mathrm{index}=\left({\mathrm{C}}_{\mathrm{weekend}}-{\mathrm{C}}_{\mathrm{weekday}}\right)/{\mathrm{C}}_{\mathrm{weekday}}\times 100\%$$

(2)

where w is the magnitude of the weekend effect and C_weekend and C_weekday are the four-year average concentrations of ions on weekends and weekdays, respectively.

To further explain the cause of changes in ion concentrations, the correlation between ions was calculated via Spearman’s correlation analysis. Importantly, the correlation between water-soluble ions characterizes their existing forms and sources in the atmosphere.

3. Results and Discussion

3.1. Concentration of Water-Soluble Ions over Time

A total of 26,840 h of ion data were collected throughout the study period. The effective collection rates of SO₂, NH₃, NO₃⁻, SO₄²⁻, NH₄⁺, and Ca²⁺ were 85.88%, 86.01%, 83.72%, 86.12%, 86.32%, and 85.73%, respectively. The average concentration of the four water-soluble ions and two gases during the sampling period was 6.855 μg/m³, with SO₄²⁻, NO₃⁻, and NH₄⁺ (collectively referred to as SNA) being the most abundant. These results are consistent with those of Shi et al. [22]. During the sampling period, the ratio of the mass concentration of SNA to the total water-soluble ion content in Wuhan (excluding HCl, HNO₂, HNO₃, SO₂, and NH₃) was 88.37%. Compared with other cities, the level in Wuhan was higher than that in Ho Chi Minh City, Vietnam and lower than that in Osaka, Japan [23]. Fine particulate matter mostly consisting of secondary inorganic aerosols is formed from gaseous precursors either through gas phase chemistry and growth by coagulation or through an aqueous-phase chemical transformation with particles left suspended after evaporation of the liquid [24]. For example, ammonium bisulfate (NH₄HSO₄) and ammonium sulfate (NH₄)₂SO₄ are produced by the reaction of ammonia (NH₃) and sulfur dioxide (SO₂) in the gas phase. Most of these aerosols are of the scattering type and primarily derived from human activities [25].

To reflect the changes in water-soluble ion concentrations in Wuhan more intuitively, the monitoring data for each time point were summarized and analyzed. As shown in Figure 2, the most apparent changes in the ion concentrations from 2016 to 2019 were observed for SO₄²⁻ and NO₃⁻. The concentration of SO₄²⁻ decreased significantly, while that of NO₃⁻ increased. Indeed, atmospheric sulfur emissions to the atmosphere in the form of sulfur dioxide (SO₂) and sulfate (SO₄²⁻) are of growing concern. In many urban areas, SO₄²⁻ from anthropogenic sources makes up a significant fraction of PM_2.5 [26]. Moreover, SO₂ reportedly originates primarily from coal combustion emissions, while NOx largely originates from motor vehicle exhaust emissions [27]. As a core city in the Yangtze River Economic Belt, a strategic fulcrum for the development of central China, and an important industrial base in the country, Wuhan has a complete industrial system of iron and steel, automobiles, optoelectronics, chemicals, metallurgy, textiles, shipbuilding, manufacturing, and medicine. This industrial development is primarily by coal, thus accounting for the significantly increased SO₂ concentration in Wuhan during the study period. Furthermore, SO₂ is a precursor for the secondary conversion of SO₄²⁻; however, the concentration trends of these two ions were inconsistent, indicating that the secondary conversion of water-soluble ions in particles is an extremely complicated process. In contrast, the concentrations of NO₃⁻, NH₃, and Ca²⁺ did not significantly change over the four years, while that of NH₄⁺ exhibited an overall downward trend.

The time-by-time variation in the concentration of water-soluble ions, excluding NH₃, exhibited an initial decreasing trend followed by an increase. Figure 2 shows that the lowest value of the hourly changes in SO₂, NO₃⁻, and NH₄⁺ and the highest values for NH₃ occurred in July or August of each year. From the perspective of interannual changes, the concentration of the various water-soluble ions in 2017 was relatively low.

3.2. Seasonal Changes in the Concentration of Water-Soluble Ions

The changes in the PM water-soluble ion concentration were season-dependent (Figure 3a). That is, the ion concentrations were generally higher during the winter and spring seasons and lower during the summer and autumn seasons. Ren [28] analyzed the pollution characteristics and sources of water-soluble ions in Taiyuan City and found that the water-soluble ion contents in atmospheric PM_2.5 decreased in the order of winter > autumn > spring > summer. Li [29] studied the characteristics of WSIIs in atmospheric PM_2.5 during the later period of Chengdu’s “Air Pollution Prevention Action Plan” and found that the water-soluble ion contents decreased in the order of autumn > winter > spring > summer with the magnitude of the difference between day and night also varying seasonally, indicating that the seasonal variations in the water-soluble ion concentrations also vary geographically.

Among the two cations, the concentration of ammonium ions was higher in autumn and winter, while that of calcium ions was the highest in autumn. Moreover, the concentration of anions was higher in winter and spring. Moreover, both gases exhibited significant seasonal concentration variations. The cold winter increases the anthropogenic emissions, while the meteorological conditions restrict the diffusion of pollutants, causing pollutants to become enriched and the concentrations of water-soluble ions to increase in the PM [18]. For example, Zhang’s research shows that the increase in air humidity and the deterioration of air dispersion conditions in winter are the most important causes of air pollution in Beijing [30]. SNA is the main component of water-soluble ions; the total concentration of the three associated ions in the four seasons, in descending order, is as follows: winter, spring, autumn, and summer. Dunea’s study showed a negative correlation between precipitation events and particulate matter concentrations [31], indicating that, during the rainy summer in Wuhan, the relatively concentrated precipitation promoted the removal of WSIIs from PM_2.5.

Changes in the daytime ion concentrations are shown in

Table 1. Seasonal variation in diurnal concentrations of different ions (μg/m³).

Ion	Season	8:00	9:00	10:00	11:00	12:00	13:00	14:00	15:00	16:00	17:00	18:00	19:00
NH4+	Spring	9.29	9.42	9.39	9.22	8.95	8.72	8.39	8.09	7.86	7.59	7.20	7.01
	Summer	6.26	6.49	6.63	6.70	6.41	5.85	5.58	5.31	5.11	4.93	4.73	4.54
	Autumn	6.69	6.96	7.29	7.39	7.28	6.97	6.63	6.34	6.19	5.95	5.88	5.68
	Winter	7.98	8.18	8.72	9.30	9.19	9.25	8.84	8.76	8.34	8.34	8.04	8.05
Ca2+	Spring	0.77	0.78	0.81	0.78	0.71	0.70	0.72	0.69	0.69	0.69	0.70	0.68
	Summer	0.87	0.92	0.91	0.79	0.71	0.62	0.58	0.60	0.64	0.66	0.67	0.63
	Autumn	1.18	1.29	1.33	1.29	1.35	1.23	1.36	1.40	1.35	1.16	1.23	1.33
	Winter	0.66	0.69	0.74	0.69	0.67	0.67	0.68	0.66	0.65	0.66	0.71	0.71
NO3−	Spring	22.29	21.56	20.28	19.10	18.28	17.46	17.09	16.04	15.65	15.22	14.25	14.09
	Summer	5.98	5.68	5.39	5.07	4.42	3.91	3.49	3.24	3.03	3.12	3.03	3.13
	Autumn	11.47	11.39	11.16	10.76	10.45	10.09	9.59	8.76	8.80	8.74	8.73	8.86
	Winter	19.28	19.32	19.87	20.51	19.79	19.63	18.50	18.41	17.96	18.61	18.21	19.29
SO42−	Spring	10.08	10.00	10.17	9.96	9.96	9.90	9.66	9.53	9.52	9.29	9.18	9.04
	Summer	8.15	8.26	8.61	8.68	8.97	8.50	8.39	8.31	8.19	7.96	7.84	7.82
	Autumn	8.35	8.42	8.53	8.55	8.67	8.43	8.39	8.54	8.53	8.29	8.27	8.22
	Winter	9.52	9.72	10.24	10.76	10.07	10.43	9.96	10.03	9.87	10.00	9.50	9.64
SO2	Spring	3.17	4.40	5.46	6.50	6.82	6.66	6.01	5.56	4.98	4.30	3.63	2.88
	Summer	3.01	4.00	4.77	5.66	5.30	4.42	3.91	3.26	2.92	2.55	2.20	1.90
	Autumn	3.45	4.54	6.05	7.09	7.26	6.48	5.68	5.12	4.76	4.09	3.49	3.00
	Winter	2.81	3.43	4.78	6.84	8.06	8.70	8.56	8.11	6.76	6.27	5.37	4.20
NH3	Spring	11.23	11.57	11.51	11.29	10.62	10.42	9.90	9.85	9.57	9.41	9.07	9.01
	Summer	14.00	14.64	14.78	14.81	14.01	12.87	12.61	12.02	11.61	11.14	10.53	9.66
	Autumn	8.68	9.04	9.52	9.75	9.08	8.94	8.31	7.83	7.47	7.31	7.51	6.57
	Winter	4.33	4.54	4.85	5.05	4.81	4.65	4.24	4.18	4.04	3.86	3.64	3.67

. The annual NH₄⁺ concentration exhibited a downward trend, except for winter, during which time it increased with time. Specifically, the NH₄⁺ concentration was the highest in spring before 10:00 and in winter after 10:00. It is worth noting that the annual changes observed in the NO₃⁻ concentration were consistent with those for NH₄⁺. The SO₂ concentration exhibited the most significant changes during the day and followed a unimodal trend across different seasons. Among them, the earliest peak was observed at 11:00 (5.66 μg/m³) in summer, the latest was at 13:00 (8.70 μg/m³) in winter, and in spring and autumn it peaked at 12:00. At night, the changes in ion concentrations between different seasons were relatively mild (

Table 2. Seasonal variation in night-time concentrations of different ions (μg/m³).

Ion	Season	20:00	21:00	22:00	23:00	0:00	1:00	2:00	3:00	4:00	5:00	6:00	7:00
NH4+	Spring	7.05	7.29	7.59	7.83	8.06	8.22	8.32	8.34	8.46	8.64	8.71	8.87
	Summer	4.54	4.66	4.83	5.02	5.26	5.46	5.63	5.67	5.70	5.71	5.75	6.03
	Autumn	5.67	5.82	5.89	5.97	6.11	6.07	6.07	6.07	6.17	6.22	6.32	6.43
	Winter	7.98	7.99	7.88	7.67	7.41	7.65	7.62	7.67	7.69	7.94	7.80	7.96
Ca2+	Spring	0.68	0.71	0.68	0.70	0.68	0.68	0.65	0.65	0.66	0.65	0.60	0.64
	Summer	0.64	0.66	0.68	0.71	0.73	0.73	0.76	0.76	0.78	0.74	0.72	0.80
	Autumn	1.35	1.33	1.24	1.27	1.44	1.24	1.25	1.13	1.21	1.17	1.10	1.18
	Winter	0.70	0.68	0.69	0.68	0.68	0.73	0.70	0.67	0.63	0.64	0.60	0.61
NO3−	Spring	14.40	15.77	17.30	18.18	18.94	19.60	20.29	20.42	20.75	21.55	21.81	21.90
	Summer	3.35	3.57	3.92	4.29	4.50	4.82	5.21	5.41	5.75	5.91	6.02	6.11
	Autumn	8.86	9.46	9.68	9.72	9.88	9.94	9.89	10.05	10.26	10.62	10.89	11.12
	Winter	18.97	19.23	19.02	18.14	17.96	18.72	18.27	18.53	18.23	19.37	18.80	19.68
SO42−	Spring	9.16	9.13	9.26	9.28	9.50	9.54	9.38	9.31	9.32	9.35	9.42	9.55
	Summer	7.74	7.87	7.90	7.72	8.03	8.02	7.95	7.84	7.71	7.60	7.62	7.90
	Autumn	8.18	8.17	8.17	8.11	8.11	7.95	7.90	7.84	7.95	7.90	7.96	8.08
	Winter	9.55	9.67	9.36	9.22	8.79	8.93	8.95	9.12	9.19	9.21	9.05	9.23
SO2	Spring	2.54	2.28	2.12	2.10	2.15	2.13	2.10	2.08	1.96	1.98	2.07	2.42
	Summer	1.59	1.59	1.68	1.92	2.23	2.41	2.58	2.35	2.29	2.25	2.43	2.65
	Autumn	2.79	2.65	2.72	2.97	3.11	3.26	3.30	3.23	3.13	2.96	3.07	3.15
	Winter	3.86	3.64	3.43	3.15	2.96	3.08	3.11	3.14	3.09	3.23	3.08	3.06
NH3	Spring	9.02	9.15	9.30	9.53	9.64	9.82	9.94	9.93	10.18	10.33	10.30	10.62
	Summer	9.80	9.98	10.38	11.02	11.33	11.85	12.22	12.64	12.49	12.52	12.42	13.26
	Autumn	6.71	6.98	7.02	7.20	7.59	7.64	7.71	7.74	7.88	7.92	8.09	8.13
	Winter	3.78	3.87	3.75	3.95	3.91	4.18	4.16	4.26	4.32	4.32	4.34	4.30

). Specifically, NH₄⁺, NO₃⁻, and NH₃ exhibited an upward trend in different seasons. Among them, the concentration of NH₄⁺ and NO₃⁻ followed the same trend, with both peaking in winter before 23:00 and in spring after 23:00. Additionally, the overt diurnal variation observed in the sulfur dioxide concentration may be related to human activities and factory emissions during the day.

3.3. “Weekend Effect” on the Changes in Water-Soluble Ion Concentration

The “weekend effect” refers to the weekly cyclic effect of the water-soluble ion concentration in the particles, in which the water-soluble ion concentration of the particles is lower during the working day than on the weekend [32].

The weekend effect index of the four water-soluble ions and two gases in Wuhan was calculated. A positive weekend effect occurs when the weekend concentration is greater than the working day concentration, while a negative effect occurs when the weekend concentration is lower than that of the working day [33]. All ions, excluding NH₃, exhibited a positive weekend effect, with NO₃⁻ having the highest weekend effect index (

Table 3. Changes in the concentration of water-soluble ions on weekdays and weekends from 2016 to 2019.

	Ca2⁺ (μg/m3)	SO42− (μg/m3)	NH₃ (μg/m3)	NO3− (μg/m3)	SO₂ (μg/m3)	NH4+ (μg/m3)	Average (μg/m3)	Standard Deviation
Weekday	0.827	8.743	8.684	10.823	3.804	7.040	6.654	3.369
Weekend	0.880	9.115	8.352	12.195	3.823	7.725	7.015	3.682
Rate	−6.07%	−4.08%	3.97%	−11.25%	−0.48%	−8.87%	−5.15%	/
Weekend Effect Index	6.46%	4.24%	−3.82%	12.67%	4.86%	9.73%	5.43%	/

). Furthermore, the NO₃⁻ concentration rate of the four water-soluble ions in PM also exhibited the largest change (11.25%) between working and non-weekdays, followed by NH₄⁺ (8.87%) and Ca²⁺ (6.07%). Meanwhile, SO₂ exhibited the smallest change (0.48%). Therefore, without considering the impact of meteorological conditions, it could be concluded that the source of SO₂ pollution on weekdays and weekends was relatively stable, while that of NO₃⁻ varied greatly.

The average concentration of the four water-soluble ions and two gases exhibited a significant weekend effect (Figure 4). That is, the concentration on weekends during each period was higher than that on weekdays, and the daily variation pattern showed a simple unimodal trend. The peaks appeared at 11:00 on both weekdays and weekends, with concentrations of 8.031 and 7.726 μg/m³, respectively. The weekend and weekday concentrations troughed at 19:00 and 20:00, with values of 6.304 and 5.919 μg/m³, respectively. The concentration distribution of Ca²⁺ differed between weekdays and weekends. During the weekend, the concentration was higher during the day. At night, the concentration began to decrease after 20:00 on the weekend; however, it began to increase on weekdays. Moreover, the concentration of Ca²⁺ during the first half of the night differed to that on weekdays with the concentration on weekdays being significantly higher than that on weekends. The concentration change curve on the weekend shows a multi-peak change trend, with peaks at 08:00 (0.99 μg/m³), 11:00 (1.04 μg/m³), and 18:00 (0.88 μg/m³), and the minimum concentration of 0.623 μg/m³ appeared at 07:00. The concentration changes on weekdays were relatively small, and only a small peak concentration of 0.874 μg/m³ appeared at 9:00.

The weekend effect index of NO₃⁻ was the highest (12.67%); however, the concentration change trends on weekdays and weekends were similar. The temporal distribution of the NO₃⁻ concentration on weekdays and weekends had strong regularity, increasing during the morning and evening traffic peaks; however, the peak on weekdays appeared at 07:00, two hours earlier than that on the weekend, with a concentration of 12.22 μg/m³. The peak concentration on weekends was higher than that on weekdays and appeared at 09:00 with a value of 13.84 μg/m³, while the troughs of both appeared at 18:00 with concentrations of 9.46 and 10.34 μg/m³, respectively. As an important secondary aerosol particle, NO₃⁻ originates from the NOx emissions of motor vehicle exhausts through a complex chemical oxidation process in the liquid or gas phase [34]. An increase in the NO₃⁻ concentration on weekends indicates that the impact of motor vehicle exhausts on NOx gas was greater than that on weekdays.

SO₄²⁻ was impacted by the weekend effect, which was characterized by a substantial increase in the average concentration over the weekend compared with that on the weekdays. The diurnal variation in SO₄²⁻ during weekdays followed a regular single-peak curve, with a peak at 11:00 and a trough at 06:00, and concentrations of 9.384 and 8.412 μg/m³, respectively. Meanwhile, the diurnal variation in SO₄²⁻ during the weekend was an irregular multi-peak curve, with a clear peak at 11:00 and a concentration of 9.760 μg/m³. Additionally, small peaks of 9.636 and 9.268 μg/m³ occurred at 08:00 and 22:00, respectively. The troughs of SO₄²⁻ on weekends appeared at 00:00, with a value of 8.64 μg/m³. As shown in Figure 4, the periods with the largest concentration difference between weekends and weekdays appeared in the morning and later in the night. As an important secondary aerosol particle, SO₄²⁻ originates from the oxidation of SO₂ gas through the liquid or gas phase. From sunrise, with the increased temperature, human activities, and pollution source emission intensity, the concentration of SO₄²⁻ gradually increases, and the peak at 11:00 may be related to changes in atmospheric diffusion. Generally, the atmospheric turbulence was strong between 10:00 and 16:00, hence the atmosphere was unstable, and the vertical diffusion ability was strong; therefore, high-altitude SO₂ was transported to the surface, near the ground, leading to an increase in the concentration of SO4²⁻ [35]. The SO₂ weekend effect index was 4.86%. The concentration distributions on weekdays and weekends were similar, and the concentration changes were large during the day and small at night. The maximum average concentration on weekdays and weekends appeared at 12:00, with concentrations of 6.518 and 6.742 μg/m³, respectively. After 12:00, the SO₂ concentration began to decrease; however, the rate of decrease on working days was significantly higher than that on weekends. After 20:00, the rate of decrease on weekends increased. The concentration of SO₂ on weekdays during the first half of the night was significantly higher than that on weekends, and the nodes where the concentration began to increase also differed. That is, the concentration on weekends began to increase at 07:00; however, the concentration on weekdays increased 1 h later.

NH₄⁺ exhibited a clear negative weekend effect. Nevertheless, the diurnal changes in the NH₃ and NH₄⁺ concentrations during weekdays and weekends were similar, with elevated levels during the day and low levels at night. Previous domestic and international studies demonstrated that agricultural production activities, such as nitrogen fertilizer application [36] and livestock breeding [37], are the main source of NH₃ emissions; however, more recent research further demonstrates that, due to the increasing scale of human activities, “urbanization”, including motor vehicle exhaust emissions, human settlements, and sewage treatment releases, has gradually become the main source of NH₃ [38]. Due to the recent dramatic increase in motor vehicles in Wuhan, large amounts of nitrogen compounds have been emitted into the atmosphere through motor vehicle exhausts, which chemically react with NH₃ to form ammonium nitrate [39]. Moreover, in the current study, the distribution of NH₃ and NH₄⁺ concentrations was relatively regular, increasing during the morning and evening traffic peaks, which further confirms that mobile sources in Wuhan contribute to the increase in NH₃ and NH₄⁺ concentrations. Meanwhile, the opposing weekend effects of NH₃ and NH₄⁺ indicate that the formation of NH₄⁺ is not solely determined by NH₃ emissions. Indeed, the ammonia–ammonium gas/particle conversion will be affected by the complexity of physical transmission and will also be affected by the chemical conversion process [40].

3.4. Correlation Analysis between Water-Soluble Ions

The correlation between water-soluble ions can explain their possible sources, their existing form, and the way they combine with each other. The correlation analysis of the main WSIIs was carried out by SPSS25.0. The results are shown in Figure 5.

NH₄⁺ primarily exists in the form of sulfate, nitrate, and hydrochloride in the atmosphere, with significant correlations observed among the three ions NH₄⁺, SO₄²⁻, and NO₃⁻. The correlation coefficients of NH₄⁺ with SO₄²⁻ and NO₃⁻ were 0.76 and 0.79, respectively, and statistically significant (p < 0.001; two-sided). Additionally, the correlation coefficient with NO₃⁻ was higher, indicating that NH₄⁺ was mainly in the atmosphere in Wuhan and existed in the form of NH₄NO₃, followed by (NH₄)₂SO₄ and NH₄HSO₄. Moreover, SO₄²⁻ and NO₃⁻ exhibited a significant correlation, indicating that the gas–particle transformation process of SO₄²⁻ and NO₃⁻ is affected by the same environmental factors. The primary source of these ions may be the combustion of fossil fuels [41].

4. Conclusions

Analysis of the diurnal changes in the concentration of water-soluble ions in PM in Wuhan over a four-year period from 2016 to 2019 showed that SNA comprised the majority of water-soluble ions in the PM, accounting for 88.37% of the molar concentration. At the same time, the correlation analysis results show strong correlations between NH₄⁺, SO₄²⁻, and NO₃⁻. Moreover, NO₃⁻ accounted for the highest proportion of water-soluble ions, indicating that vehicle emissions from the burning of fossil fuels represent the main source of PM_2.5 in Wuhan. The SO₂ emission levels also exhibited an annually increasing trend. This is inseparable from the historical origin of Wuhan as a major industrial base in central China. However, in the current situation of atmospheric pollution, this cannot be underestimated, and while pursuing economic growth, it is necessary to strictly limit the polluting gas emissions of enterprises and quantify the total emissions of companies.

Seasonal variations in the ion concentration were evident. Among them, SNA had the highest concentration in winter, while the two gases had the highest concentration in summer. Additionally, the highest weekend effect index was observed for NO₃⁻ (12.67%), and the lowest was observed for NH₃ (−3.82%). Regardless of the weekday or weekend, all ion concentrations were higher during the day than at night. Taken together, these results indicate that the government should encourage green travel and residents should gradually increase their environmental awareness and choose public transportation. Finally, while this study provides a detailed description of the diurnal characteristics of the monitored ion concentration data, it does not provide an analysis of the deep-seated causes of the circadian variation characteristics, which will require further investigation.