The changes of meteorological conditions on the north bank of Taihu Lake during the observation period are shown in Table 1 and Figure 2. The surface pressure, wind speed, and temperature change little around the onset of Meiyu. During the observation period, the average wind speed maintains about 2.0 m s⁻¹, while the air temperature keeps in the range of 19.0°C–33.0°C with an average of 25.2°C. The frequency and amount of precipitation increase significantly after the Meiyu onset, with the precipitation increasing from 105.4 mm (from June 4 to June 18) to 404.5 mm (from June 19 to July 11). The precipitation increase leads to significant increases in surface water-vapor pressure, relative humidity, and dew-point temperature. Specifically, the average surface water-vapor pressure is 25.3 hPa before the Meiyu onset, which is significantly lower than that (29.8 hPa) during the Meiyu period. The average dew-point temperatures (relative humidity) before and after the onset of Meiyu are 21.2 and 23.9°C (82.9 and 90.7%), respectively. Comparatively, the relative humidity after the onset of Meiyu is closer to saturation than before. Moreover, the wind direction changes obviously and frequently during the Meiyu period on the north bank of Taihu Lake.

Table 2 shows the daily mean concentrations of PM_2.5, PM_10, and main gaseous pollutants before and after the onset of Meiyu. The mass concentrations of five main air pollutants after the onset of Meiyu significantly decrease due to the rainfall influence. Specifically, the mass concentrations of PM₁₀ and PM_2.5 are 65.36 and 50.20 μg m⁻³ before the onset of Meiyu, which respectively drop to 38.27 and 29.05 μg m⁻³ after the Meiyu onset. For gaseous pollutants, the mass concentrations of SO₂, NO_2, and O₃ decrease from 12.10, 51.77, and 77.83 μg m⁻³ to 7.91, 41.51, and 51.89 μg m⁻³, respectively.

Before the Meiyu onset, the north bank of Taihu Lake experienced two pollution processes. The first process lasted from 0800 Beijing Time (BJT) on June 6 to 2000 BJT on June 9, and the second process lasted from 2000 BJT on June 12 to 2000 BJT on June 18. The mass concentrations of PM_2.5 and PM₁₀ peaked at 112 and 172 μg m⁻³ in the first process, and peaked at 103 and 148 μg m⁻³ during the second process (Figures 2D,E). Although the pollution process during the Meiyu period is not as long as that before the Meiyu onset, the mass concentrations of air pollutants during this period also experienced short fluctuations, where the mass concentrations of PM_2.5 reached 106 and 120 μg m⁻³ at 2000 BJT on June 21 and 2300 BJT on June 23, respectively (Figures 2D,E). According to the hourly precipitation (Figure 2) in the Meiyu period, these two fluctuations occurred during the intermittent period of precipitation. While the mass concentrations of air pollutants were significantly lower during other periods, which maintained below 30 μg m⁻³ for PM_2.5. The precipitation increase and changeable wind direction could be the reasons for the decline of air pollutants.

Table 3 depicts the differences between the concentrations of water-soluble inorganic ions before and after the onset of Meiyu. The average concentration of total water-soluble inorganic ions (ρ(TWSII)) in PM₁₀ is (28.59 ± 7.27) μg m⁻³, while the average ρ(TWSII) in PM_1.1, PM_1.1–2.1 and PM_2.1–10 are (11.22 ± 3.90) μg m⁻³ (5.18 ± 2.77) μg m⁻³ and (12.19 ± 2.89) μg m⁻³, respectively. Before the onset of Meiyu, the average ρ(TWSII) in PM_1.1, PM_1.1–2.1 and PM_2.1–10 are 13.36 ± 4.84, 6.55 ± 3.46, and 12.3 ± 3.46 μg m⁻³, and they are 9.34 ± 3.72, 3.98 ± 2.19, and 12.10 ± 5.3 μg m⁻³ after the onset of Meiyu, with the ρ(TWSII) decreasing by 30, 39 and 2%, respectively. The values of ρ (TWSII) in fine and coarse particles before the Meiyu onset are higher than those during the Meiyu period.

As shown in Figure 3A, the total proportions of four major water-soluble inorganic ions (NH₄ ⁺, Ca²⁺, SO₄ ²⁻ and NO₃ ⁻) in PM_1.1 and PM_1.1–2.1 are 82 and 87%, respectively. The major water-soluble inorganic ions for PM_2.1–10 areCa²⁺, NH₄ ⁺, SO₄ ²⁻ and Na⁺, accounting for 72% of the total. The anion with the largest proportion in PM_1.1, PM_1.1–2.1, and PM_2.1–10 is SO₄ ²⁻. Besides, the cation with the largest proportion in PM_1.1 and PM_1.1–2.1 is NH₄ ⁺, but it is Ca²⁺ in PM_2.1–10. It indicates that NH₄ ⁺ mainly exists in fine particles, while Ca²⁺ mainly in coarse particles. Furthermore, the concentration of NH₄ ⁺ decreases with the increase of particle size before and after the onset of Meiyu (Figure 3B).

The concentrations of NH₄ ⁺, Ca²⁺, SO₄ ²⁻ and NO₃ ⁻ rank top four in submicron particles, fine particles, or coarse particles before the Meiyu onset (Table 3 and Figure 3). The average concentrations of ten kinds of ions in PM_1.1 are in decreasing order of NH₄ ⁺>SO₄ ²⁻>Ca²⁺>NO₃ ⁻>Na⁺>K⁺>Mg²⁺>Cl⁻>F⁻>NO₂ ⁻, while those in PM_1.1–2.1 follow the order of NH₄ ⁺>SO₄ ²⁻>NO₃ ⁻>Ca²⁺>Na⁺>K⁺>Mg²⁺>Cl⁻>F⁻>NO₂ ⁻. For PM_2.1–10, the average concentrations are in order of Ca²⁺>NH₄ ⁺>SO₄ ²⁻>Na⁺>NO₃ ⁻>Mg²⁺>K⁺> Cl⁻>F⁻>NO₂ ⁻.

The concentrations of NH₄ ⁺, Ca²⁺, SO₄ ²⁻and NO₃ ⁻ are still the highest four after the Meiyu onset, with slight changes in the concentrations of different ions. The concentration of Ca²⁺ increases after the Meiyu onset, which is related to the road construction around the observation site (Wang et al., 2021). The concentrations of NH₄ ⁺, SO₄ ²⁻ and NO₃ ⁻ decrease significantly, while the concentrations of Na⁺, K⁺, Mg²⁺, Cl⁻, F⁻ and NO₂ ⁻ change little. During the Meiyu period, the average concentrations of ten kinds of ions follow the order of NH₄ ⁺>Ca²⁺>SO₄ ²⁻>NO₃ ⁻>Na⁺>K⁺>Mg²⁺>Cl⁻>F⁻>NO₂ ⁻ in PM_1.1, the order of NH₄ ⁺>Ca²⁺>NO₃ ⁻>SO₄ ²⁻>Na⁺>K⁺>Mg²⁺>Cl⁻>F⁻>NO₂ ⁻ in PM_1.1–2.1, and the order of Ca²⁺>NH₄ ⁺>SO₄ ²⁻>Na⁺>NO₃ ⁻>Mg²⁺>K⁺>Cl⁻>F⁻>NO₂ ⁻ in PM_2.1–10.

The changes of the mass concentrations of water-soluble inorganic ions in PM_1.1, PM_1.1–2.1, and PM_2.1–10 with time during the Meiyu period on the north bank of Taihu Lake are depicted in Figure 4. The ρ(TWSII) in PM_1.1, PM_1.1–2.1, and PM_2.1–10 before the Meiyu onset are generally greater than those during the Meiyu period, but there are some differences in the concentration changes. The ρ(TWSII) in PM_1.1–2.1 changes most obviously before and after the Meiyu onset, followed by PM_1.1, while the ρ(TWSII) changes little in PM_2.1–10. This indicates the effect of frequent precipitation during the Meiyu period on the removal of PM_1.1, PM_1.1–2.1, and PM_2.1–10, especially for PM_1.1–2.1.

Figure 4 also shows that the changes of ρ(TWSII) in PM_1.1, PM_1.1–2.1, and PM_2.1–10 with time are generally consistent, but there are some differences in some periods. During June 18–20, 2016, the ρ(TWSII) in PM_1.1 decreased first and then increased, while the change of ρ(TWSII) in PM_2.1–10 was opposite, and the ρ(TWSII) in PM_1.1–2.1 presented a consistent increase. Besides, the concentrations varied little in fine particles but changes drastically for coarse particles. This changing trend may be related to the compositions and formation mechanisms of water-soluble inorganic ions of different sizes.

Corresponding to the pollution processes mentioned in Concentration variations of PM2.5, PM10, and gaseous pollutants, the ρ(TWSII) in PM_1.1 and PM_1.1–2.1 increased obviously from June 5 to June 7 before the Meiyu onset, and the ρ(TWSII) in PM_1.1 and PM_1.1–2.1 decreased from June 7 to June 8, while the ρ(TWSII) in PM_2.1–10 increased to a maximum value before the Meiyu onset. From June 14 to June 16, the ρ(TWSII) in PM_1.1 and PM_1.1–2.1 increased to their maximums of 21.34 and 12.05 μg m⁻³ during the observation period, respectively. Meanwhile, the ρ (TWSII) of PM_2.1–10 decreased from June 14 to June 15 and increased from June 15 to June 16.

During the first process of air pollution (from 0800 BJT on June 6 to 2000 BJT on June 9) before the Meiyu onset, the mass concentrations of NH₄ ⁺, SO₄ ²⁻ and NO₃ ⁻ in PM_1.1, PM_1.1–2.1 and PM_2.1–10 increased, especially the concentrations of NH₄ ⁺ in PM_1.1 and PM_1.1–2.1. During the second process (from 2000 BJT on June 12 to 2000 BJT on June 18), the concentrations of NH₄ ⁺, SO₄ ²⁻ and NO₃ ⁻ in PM_1.1 and PM_1.1–2.1 increased, while the concentrations of these ions in PM_2.1–10 had no change, which is different from those in the first process. What is the reason for this?

Moreover, the two short fluctuations at 2000 BJT on June 21 and 2300 BJT on June 23 are only reflected as the increases in the concentrations of NH₄ ⁺, SO₄ ²⁻ and NO₃ ⁻ in PM_1.1 and PM_1.1–2.1. The above four fluctuations may be related to the change of SO₂ concentration before and after the Meiyu onset. On the one hand, the increase of SO₄ ²⁻ concentration in PM_1.1–_2.1 during the Meiyu period is due to the gas-to-particle conversion of smaller particles; on the other hand, the SO₄ ²⁻ converts to larger-size particles.

The particle-size distribution of water-soluble ions can characterize the source and formation mechanism of ions (Zhang et al., 2018). Figure 5A indicates that the mass concentration spectrums of total anions, total cations, and total ions within different particle-size sections present bimodal distributions before and after the Meiyu onset. The mass concentrations of fine particles peak at the particle-size of 1.1–2.1 μm, while the mass concentrations of coarse particles respectively peak at particle-size of 5.8–9.0 and 9.0–10.0 μm before and after the Meiyu onset, indicating that the particle-size corresponding to peak concentrations in coarse particles moves toward the larger particle sizes after the Meiyu onset. The variation characteristics for mass concentrations of anions, cations, and total ions are similar during the observation period, with the ions mainly concentrating in the section of fine particles. The peak concentration of ions in fine particles before the Meiyu onset is higher than that during the Meiyu period. While in coarse particles, the peak concentration of anions before the Meiyu onset is higher. Meanwhile, the peak concentrations of total cations and total ions before the Meiyu onset are lower than those during the Meiyu period.

Figure 6 displays the variations of the particle-size distributions of total anions, total cations, and total ions in water-soluble inorganic ions overtime during the observation. Before June 19, 2016, the total anions, total cations, and total ions in fine particles grew rapidly and reached their peak concentrations. During June 8–15 before the Meiyu onset, the concentration of total anions with the particle sizes of 4.5– and 0.65–1.1 µm in coarse particles increased more rapidly than those in other particle sizes (Figure 6A). The particle sizes for the peak of mass concentrations of total ions in coarse particles moved toward larger particle sizes after the Meiyu onset. The period with an increasing concentration for total anions with the particle size of 9–10 µm in coarse particles accounted for 47% of the total Meiyu period, while the concentration of the total anions at the particle size of 5.8–9.0 µm decreased during the same period. Meanwhile, the environmental relative humidity was greater than 90% (Figure 2C), indicating an obvious effect of the hygroscopic growth of the total anions in coarse particles. As shown in Figure 6B, the concentrations of total cations and total ions in coarse particles during the Meiyu period were higher than those before the Meiyu onset, with the maximum concentrations of total cations and total ions in coarse particles being 0.81 (0.95) and 3.28 μg m⁻³ (10.56 μg m⁻³) before (after) the Meiyu onset, respectively. The maximum concentration during the Meiyu period was 3.2 times that before the Meiyu onset. The main reason for the sharp increase in the peak concentration during the Meiyu period was that the concentrations of the cations at 5.8–9.0 and 9.0–10.0 μm increased sharply on June 19, and the changes of total cation concentrations in coarse particles were roughly the same on both June 18 and June 19, indicating the same source for the total cations on June 18 and 19. Figure 2 shows that the winds on June 18 and 19 were both southerly, and the daily means of temperature, relative humidity, and surface wind speed were 27.6°C (28.4°C), 75.6% (77.9%), and 3.0 m s⁻¹ (2.6 m s⁻¹) on June 18 (June 19), indicating no significant changes in meteorological elements. While the local anthropogenic emissions near the observation site have a more obvious effect on the concentration of total cations than meteorological factors do. During the observation period, the particle size of total ions has a more consistent distribution with that of total cations than total anions. It indicates that the cations in water-soluble inorganic ions in atmospheric particulate matters on the north bank of Taihu Lake played an important role in the chemical composition of ions during the Meiyu period in 2016 (Wang et al., 2021).

The particle-size distribution characteristics of different ions and their sources are further analyzed, with the size distributions of each ion before and after the Meiyu onset shown in Figure 7, and the time series for different particle-size distributions shown in Figure 5B. The particle size of SO₄ ²⁻ shows a bimodal distribution before the Meiyu onset and a multi-modal distribution during the Meiyu period. Before the Meiyu onset, the mass concentration of SO₄ ²⁻ peaks at a particle size of 1.1–2.1 μm in fine particles, while at 5.8–9.0 μm in coarse particles. After the Meiyu onset, the mass concentration of SO₄ ²⁻ presents a bimodal distribution with its peaks at particle-size of 0.43–0.65 μm and 1.1–2.1 μm in fine particles, and particle-size of 4.7–5.8 and 9.0–10.0 μm in coarse particles. As indicated in the sequence variation (Figure 7), before the Meiyu onset, the mass concentration of SO₄ ²⁻ in fine particles peaks at particle-size of 0.43–2.1 μm and the proportions for the frequencies in three different particle-size sections are 36, 21, and 43%, respectively. During the Meiyu period, the mass concentration of SO₄ ²⁻ in fine particles peaks at particle-size of 0–2.1 μm, and the proportions for the frequencies in different particle sizes are 19, 25, 6, and 40%, respectively. It indicates that SO₄ ²⁻ in fine particles is mainly generated by the reactions in the cloud and the gas-phase reaction. While the two reactions during the Meiyu period are weaker than those before the Meiyu onset. The particle size for the peak concentration of SO₄ ²⁻ in coarse particles moves toward larger particle sizes during the Meiyu period. This may be related to higher environmental humidity during the Meiyu period, resulting in the hygroscopic growth of sulfates in coarse particles.

The mass concentration of NO₃ ⁻ at different particle-size sections shows a bimodal distribution before the Meiyu onset, and a unimodal distribution after the Meiyu onset. The mass concentration of NO₃ ⁻ in fine particles peaks at particle-size of 1.1–2.1 μm, while at 5.8–9.0 μm in coarse particles. Figure 5B shows that before the Meiyu onset, the mass concentration of NO₃ ⁻ in fine particles peaks at particle-size of 0.43–2.1 μm, with the proportions of frequencies for particle-size in four particle-size sections accounting for 13, 25, 6, and 56%, respectively. The effect of gas-phase conversion (liquid-phase reaction) during Meiyu is stronger (weaker) than that before Meiyu. Before the Meiyu onset, the mass concentration of NO₃ ⁻ in coarse particles peaks at particle-size of 2.1–3.3 and 5.8–10.0 μm, and the proportions of frequencies for particle-size within the three particle-size sections account for 36, 57, and 7%, respectively. The peak particle-size of NO₃ ⁻ distributes in all five particle-size sections of 2.1–10 μm after the Meiyu onset, with the proportions of frequencies accounting for 50, 6, 6, 13, and 25%, respectively. It can be concluded that the sources of NO₃ ⁻ in coarse particles after the Meiyu onset are more complex than before. The proportion for NO₃ ⁻ having particle-size within 5.8–9.0 μm decreases significantly, which may be attributed to the removal effect of increasing rainfall during the Meiyu period. The concentration of NH₄ ⁺ at different particle-size sections presents a bimodal distribution before and unimodal distribution during the Meiyu, with the particle-size of peak concentration distributing in 1.1–2.1 and 5.8–9.0 μm (before the Meiyu), and 9.0–10.0 μm (during the Meiyu). NH₄ ⁺ is mainly found in fine particles, with the mass fraction accounting for 79.2% (Figure 5B).