Seasonal Variations and Correlation Analysis of Water-Soluble Inorganic Ions in PM 2 . 5 in Wuhan , 2013

Daily PM2.5 and water-soluble inorganic ions (NH4, SO42 ́, NO3 ́, Cl ́, Ca2+, Na+, K+, Mg2+) were collected at the Hongshan Air Monitoring Station at the China University of Geosciences (Wuhan) (30 ̋311N, 114 ̋231E), Wuhan, from 1 January to 30 December 2013. A total of 52 effective PM2.5 samples were collected using medium flow membrane filter samplers, and the anionic and cationic ions were determined by ion chromatography and ICP, respectively. The results showed that the average mass concentration of the eight ions was 40.96 μg/m3, which accounted for 62% of the entire mass concentration. In addition, the order of the ion concentrations was SO42 ́ > NO3 ́ > NH4 > Cl ́ >K+ > Ca2+ > Na+ > Mg2+. The secondary inorganic species SO42 ́, NO3 ́ and NH4 were the major components of water-soluble ions in PM2.5, with a concentration of 92% of the total ions of PM2.5, and the total concentrations of the three ions in the four seasons in descending order as follows: winter, spring, autumn, and summer. NH4 had a significant correlation with SO42 ́ and NO3 ́, and the highest correlation coefficients were 0.943 and 0.923 (in winter), while the minimum coefficients were 0.683 and 0.610 (in summer). The main particles were (NH4)2SO4 and NH4NO3 in PM2.5. The charge of the water-soluble ions was nearly balanced in PM2.5, and the pertinence coefficients of water-soluble anions and cations were more than 0.9. The highest pertinence coefficients were in the spring (0.9887), and the minimum was in summer (0.9459). That is, there were more complicated ions in PM2.5 in the summer. The mean value of NO3 ́/SO42 ́ was 0.64, indicating that stationary sources of PM2.5 had a greater contribution in Wuhan.


Introduction
With the rapid development of modern industrialization and urbanization and the sustainable growth of energy consumption and the number of motor vehicles, air contamination has gradually become the core constraint of sustainable urban progress and eco-civilization construction in recent decades.As a vital indicator of current domestic ambient air quality, Particulate Matter (PM) with aerodynamic diameters less than 2.5 µm (PM 2.5 ) has received extensive attention from society and academia.PM 2.5 not only reduces atmospheric visibility [1,2] but also severely damages organisms in the environment and public health [3,4].Numerous studies have revealed that the sources, material compositions and formation mechanisms of atmospheric PM 2.5 are very complicated [5,6], and PM 2.5 mainly contains black carbon [7], elemental carbon [8], crust elements [9,10], water-soluble ions [11,12], microelements [13,14], etc.Among these species, water-soluble ions could account for more than 80% of PM 2.5 's constituents [15] and are an important factor in the increase in PM 2.5 concentrations.
Atmosphere 2016, 7, 49 2 of 12 Nonetheless, PM 2.5 's constituents are different with the diversities of regional geographic conditions, meteorological conditions [16] and energy structures [17], and the constituents in the same district even show different varieties because of different economic development levels during different periods.The differences in PM 2.5 are primarily observed on the sources, composition structure, and concentration levels.
Wuhan is one of the most rapidly developing cities in China.Along with the increase in the speed of the urbanization process, the population is rising sharply, traffic pressure is constantly increasing, and problems from PM 2.5 pollution are also increasing gradually.Wuhan, as well as substantial areas of China, is experiencing chronic air pollution [18].Currently, there are some preliminary studies on the composition characteristics and concentration levels of water-soluble ions in PM 2. 5 in Wuhan [19][20][21][22].However, these studies lack long-term and continuous monitoring data and a comparison of seasonal differences.Based on this background, this study monitored PM 2.5 's water-soluble ions in Wuhan continuously throughout an entire year from 1 January to 30 December 2013, and then analyzed the concentration levels and correlations of water-soluble ions and the seasonal variation in the main ions in order to provide a theoretical foundation for the control and treatment of PM 2.5 pollution in Wuhan.

Overview of the Study Area
Wuhan is located in the middle and lower reaches of the Yangtze River, east of the Jianghan Plain, and its geographical location is between 113 ˝41 1 E and 115 ˝05 1 E (longitude) and between 29 ˝58 1 N and 31 ˝22 1 N (latitude).The climate is a subtropical humid monsoon climate, with abundant rainfall, sufficient sunshine, and four distinct seasons; in the summer, the temperature is high and precipitation is concentrated, while in the winter, the weather is moist and slightly cold.The average temperature reaches the lowest point of 3.0 ˝C in January and a peak of 29.3 ˝C in July.The summer period is as long as 135 days, and the spring and autumn periods both contain approximately 60 days.Wet and dry seasons are readily apparent, the rainfall is relatively adequate in the early summer, and the annual precipitation is 1205 mm.According to the ground monitoring datum in Wuhan, the winter has a prevailing north-northeast wind (NNE), while the summer has a prevailing south-southwest wind (SSW), and the rest of the seasons have a dominant southwest wind.The annual average wind speed is 1.1-1.2m/s, and light wind and calm wind are frequent.Air pollutants in northeastern provinces and cities easily drift to Wuhan with the airflow direction because of the controlled northeast monsoon in the winter, which could intensify Wuhan's air pollution.Therefore, Wuhan's air pollution is more serious in the winter than in other seasons.The wind rose diagram in Wuhan in 2013 is shown in Figure 1.
Atmosphere 2016, 7, 49 2 of 12 more than 80% of PM2.5's constituents [15] and are an important factor in the increase in PM2.5 concentrations.Nonetheless, PM2.5's constituents are different with the diversities of regional geographic conditions, meteorological conditions [16] and energy structures [17], and the constituents in the same district even show different varieties because of different economic development levels during different periods.The differences in PM2.5 are primarily observed on the sources, composition structure, and concentration levels.
Wuhan is one of the most rapidly developing cities in China.Along with the increase in the speed of the urbanization process, the population is rising sharply, traffic pressure is constantly increasing, and problems from PM2.5 pollution are also increasing gradually.Wuhan, as well as substantial areas of China, is experiencing chronic air pollution [18].Currently, there are some preliminary studies on the composition characteristics and concentration levels of water-soluble ions in PM2.5 in Wuhan [19][20][21][22].However, these studies lack long-term and continuous monitoring data and a comparison of seasonal differences.Based on this background, this study monitored PM2.5's water-soluble ions in Wuhan continuously throughout an entire year from 1 January to 30 December 2013, and then analyzed the concentration levels and correlations of water-soluble ions and the seasonal variation in the main ions in order to provide a theoretical foundation for the control and treatment of PM2.5 pollution in Wuhan.

Overview of the Study Area
Wuhan is located in the middle and lower reaches of the Yangtze River, east of the Jianghan Plain, and its geographical location is between 113°41′E and 115°05′E (longitude) and between 29°58′N and 31°22′N (latitude).The climate is a subtropical humid monsoon climate, with abundant rainfall, sufficient sunshine, and four distinct seasons; in the summer, the temperature is high and precipitation is concentrated, while in the winter, the weather is moist and slightly cold.The average temperature reaches the lowest point of 3.0 °C in January and a peak of 29.3 °C in July.The summer period is as long as 135 days, and the spring and autumn periods both contain approximately 60 days.Wet and dry seasons are readily apparent, the rainfall is relatively adequate in the early summer, and the annual precipitation is 1205 mm.According to the ground monitoring datum in Wuhan, the winter has a prevailing north-northeast wind (NNE), while the summer has a prevailing south-southwest wind (SSW), and the rest of the seasons have a dominant southwest wind.The annual average wind speed is 1.1-1.2m/s, and light wind and calm wind are frequent.Air pollutants in northeastern provinces and cities easily drift to Wuhan with the airflow direction because of the controlled northeast monsoon in the winter, which could intensify Wuhan's air pollution.Therefore, Wuhan's air pollution is more serious in the winter than in other seasons.The wind rose diagram in Wuhan in 2013 is shown in Figure 1.The sampling site is on the roof of the Institute of Atmospheric Environment at the China University of Geosciences, Hongshan District of Wuhan (14 ˝23 1 E, 30 ˝31 1 N), at an elevation of approximately 8 m above the ground (Figure 2).From 1 January to 30 December 2014, we collected PM 2.5 samples continuously and acquired 52 valid samples with Wuhan Tianhong Company's sampling apparatus (Type TH-150F).The sampling filter used a quartz fiber filter membrane (QFF, Φ90 mm, Whatman Company, Leicestershire, UK).The sampling time started at 10 a.m. on each Wednesday and was maintained for 24 h to the next day.
Atmosphere 2016, 7, 49 3 of 12 The sampling site is on the roof of the Institute of Atmospheric Environment at the China University of Geosciences, Hongshan District of Wuhan (14°23′E, 30°31′N), at an elevation of approximately 8 m above the ground (Figure 2).From 1 January to 30 December 2014, we collected PM2.5 samples continuously and acquired 52 valid samples with Wuhan Tianhong Company's sampling apparatus (Type TH-150F).The sampling filter used a quartz fiber filter membrane (QFF, Φ90 mm, Whatman Company, Leicestershire, UK).The sampling time started at 10 a.m. on each Wednesday and was maintained for 24 h to the next day.

Sample Analysis Method
The PM2.5 samples were weighed, and a quarter of the samples were cut up and placed into 50 mL polypropylene centrifugal tubes, to which was added 30 mL of ultrapure water.The samples were extracted at a constant temperature with an ultrasonic wave for 30 min and then stewed and filtered through a 0.45-µm-diameter micro-porous membrane.Furthermore, an inductively coupled plasma optical atomic emission spectrometer (Type ICAP6300, Thermo Fisher Scientific Inc, MA, U.S.) and an ion chromatograph (Type ICS-1100) were used to measure the concentrations of cations (K + , Ca 2+ , Na + , Mg 2+ , NH4 + ) and anions (Cl − , SO4 2− , NO3 − ).Stringent quality checks were executed during the sample analysis processes.

Concentration Level Analysis of PM2.5's Water-Soluble Ions
During the monitoring period, the total mass concentration value of the eight water-soluble ions of PM2.5 was 40.96 µ g/m 3 , which accounted for 62% of the entire mass concentration.The sequence of the concentrations of water-soluble ions in order from high to low was SO4 2− > NO3 − > NH 4+ > Cl − > K + > Ca 2+ > Na + > Mg 2+ , and the three secondary ions SO4 2− , NH4 + and NO3 − were the main water-soluble ions, which were separately converted from gas precursors SO2, NOx and NH3 and accounted for 92% of the total measured water-soluble ions.
The concentration level of SO4 2− was the highest of the eight water-soluble ions and was lower than the values for the northern cities Beijing and Tianjin and greater than the values for the southern cities Shanghai, Guangzhou and Hong Kong (Table 1, [15,[23][24][25][26][27]), mainly due to the

Sample Analysis Method
The PM 2.5 samples were weighed, and a quarter of the samples were cut up and placed into 50 mL polypropylene centrifugal tubes, to which was added 30 mL of ultrapure water.The samples were extracted at a constant temperature with an ultrasonic wave for 30 min and then stewed and filtered through a 0.45-µm-diameter micro-porous membrane.Furthermore, an inductively coupled plasma optical atomic emission spectrometer (Type ICAP6300, Thermo Fisher Scientific Inc, MA, USA) and an ion chromatograph (Type ICS-1100) were used to measure the concentrations of cations (K + , Ca 2+ , Na + , Mg 2+ , NH 4 + ) and anions (Cl ´, SO 4 2´, NO 3 ´).Stringent quality checks were executed during the sample analysis processes.

Concentration Level Analysis of PM 2.5 's Water-Soluble Ions
During the monitoring period, the total mass concentration value of the eight water-soluble ions of PM 2.5 was 40.96 µg/m 3 , which accounted for 62% of the entire mass concentration.The sequence of the concentrations of water-soluble ions in order from high to low was SO 4 2´> NO 3 ´> NH 4+ > Cl ´> K + > Ca 2+ > Na + > Mg 2+ , and the three secondary ions SO 4 2´, NH 4 + and NO 3 ´were the main water-soluble ions, which were separately converted from gas precursors SO 2 , NO x and NH 3 and accounted for 92% of the total measured water-soluble ions.The concentration level of SO 4 2´w as the highest of the eight water-soluble ions and was lower than the values for the northern cities Beijing and Tianjin and greater than the values for the southern cities Shanghai, Guangzhou and Hong Kong (Table 1, [15,[23][24][25][26][27]), mainly due to the emissions of industrial pollution sources and coal sources in Wuhan.The concentration levels of NO 3 ´and NH 4 + ions were basically identical to the concentration of SO 4 2´.The high concentration of NO 3 ´was based on the number of motor vehicles rising constantly in Wuhan in recent years.For example, take the NOx emissions (Table 2), we can find that the industrial NOx emission (stationary source) was the main source of NOx.Among them, NOx emission from thermal power industry was the primary source of pollution and accounts for 35.06% in the total NOx emission, followed by vehicle exhaust emissions accounts for 34%, suggesting that NOx emissions have a tendency to increase gradually.In addition, as seen from the seasonal distribution of NO 3 ´, the concentration level in the winter and autumn was significantly higher than that in the spring and summer because the high temperatures in the spring and summer accelerated the volatilization loss of nitrate.
The annual average concentration of NH

Seasonal Variation Characteristics of Water-Soluble Ions
The mass concentration variation of water-soluble ions in PM 2.5 presented distinctly seasonal distribution features.The sequence of the mass concentration levels in the four seasons was winter > spring > autumn > summer.The seasonal distribution of the cumulative concentration of eight water-soluble ions is shown in Figure 3.The concentration sum of the three main secondary ions (SO 4 2´, NO 3 ´, NH 4 + ) in the four seasons accounted for 79%, 46%, 67% and 85% of the total soluble-water ions, respectively, and was highest in the winter.The average mass concentration of the eight ions was 40.96 µg/m 3 , which composed 63% of the total mass concentration of the water-soluble ions.
winter > spring > autumn > summer.The seasonal distribution of the cumulative concentration of eight water-soluble ions is shown in Figure 3.The concentration sum of the three main secondary ions (SO4 2− , NO3 − , NH4 + ) in the four seasons accounted for 79%, 46%, 67% and 85% of the total soluble-water ions, respectively, and was highest in the winter.The average mass concentration of the eight ions was 40.96 μg/m 3 , which composed 63% of the total mass concentration of the water-soluble ions.As shown in Figure 4, the proportion of concentration contribution of the three main ions was SO4 2− (31.64%) > NO3 − (26.27%) > NH4 + (19.27%) in winter, and the same order in spring and autumn, but was SO4 2− (23.11%) > NH4 + (12.15%) > NO3 − (7.38%) in summer, implying concentration value of NH4 + was ascending comparing with the value of NO3 − .High temperature in summer is advantageous for the decomposition of solid material NH4NO3 and forming into gaseous materials NH3 and HNO3.After two-step chemical reactions (step one: NH3 + H2O = NH3•H 2O; step two: NH3•H 2O = NH4 + + OH − ) in the atmosphere, NH3 transforms into NH4 + compounds, causing the concentration level of NH4 + to rise.Atmosphere 2016, 7, 49 5 of 12

Seasonal Variation Characteristics of Water-Soluble Ions
The mass concentration variation of water-soluble ions in PM2.5 presented distinctly seasonal distribution features.The sequence of the mass concentration levels in the four seasons was winter > spring > autumn > summer.The seasonal distribution of the cumulative concentration of eight water-soluble ions is shown in Figure 3.The concentration sum of the three main secondary ions (SO4 2− , NO3 − , NH4 + ) in the four seasons accounted for 79%, 46%, 67% and 85% of the total soluble-water ions, respectively, and was highest in the winter.The average mass concentration of the eight ions was 40.96 μg/m 3 , which composed 63% of the total mass concentration of the water-soluble ions.
As shown in Figure 4, the proportion of concentration contribution of the three main ions was SO4    Similar to the seasonal variation tendency of all water-soluble ions, the concentration of SO 4 2´w as greatest in the winter, followed by the autumn, and was the lowest in the summer.The concentration value in the winter was 2.5 times that of the summer.One reason for the above situation is that citizens generally burn coal to keep warm in the winter.In addition, little rain and a dry climate in the winter cause SO 4 2´t o remain in the atmosphere for a long time, so its concentration is elevated.On the contrary, high temperatures and rainy weather in the summer are not conducive to the formation of SO 4 2´.
The concentration levels of Ca 2+ and Mg 2+ experienced similar seasonal varying trends, such that the values decreased as follows: winter > autumn > spring > summer.The concentrations of Ca 2+  and Mg 2+ in the winter were 1.9 times and 4.3 times those of the summer, respectively.As typical ions of flowing dust [29], the concentrations of Ca 2+ and Mg 2+ are immensely influenced by seasons and anthropic actions.On one hand, the winter climate with dry weather and little rain reduces wet subsidence of Ca 2+ and Mg 2+ ; on the other hand, with accelerating urbanization processes in recent years in Wuhan, a large number of surfaces from construction operation are emerging every year, thus increasing dust sources and resulting in the rise in the concentrations of Ca 2+ and Mg 2+ ions.Conversely, high temperatures and rainy weather in the summer provide beneficial conditions for the settlement of Ca 2+ and Mg 2+ compounds, which causes the concentrations of Ca 2+ and Mg 2+ ions to drop.

Concentration Equivalent Ratio Analysis of NO 3
´/SO 4 2Ć oncentration equivalent normality is defined as the number of equivalents per liter of solution, where the definition of an equivalent depends on the reaction taking place in the solution.For an acid-base reaction, the equivalent is the mass of the acid or base that can furnish or accept exactly 1 mole of protons (H+ ions).The mass concentration equivalent ratio of NO 3 ´and SO 4 2´c ould be used as relative significant index to measure the relative contribution of mobile source (vehicles) and fixed sources (coal) for nitrogen pollution and sulfur pollution in the atmosphere [24].Arimoto et al.
(1996) attributed the high ratio of NO 3 ´/SO 4 2´t o mobile sources, which had a greater contribution to the concentrations of regional atmospheric pollutants [30].The sulfur contents in gasoline and diesel in China were 0.12% and 0.2%, respectively.The NO x /SO x ratios from comburent of gasoline and diesel fuel were approximately 13:1 and 8:1, respectively.Coal's sulfur content is 1%; the ratio of NO X /SO X from coal's combustion is approximately 1:2.Therefore, NO X and SO X can act as tracers of mobile sources and fixed sources separately.When the concentration equivalent ratio of NO 3 ´/SO 4 2´e xceeds 1, it means that pollution sources of the observation point are dominated by mobile sources, while fixed sources play major roles when the ratio is below 1 [30].The equivalent ratios of NO 3 ´/SO 4 2´i n Wuhan were 0.73, 0.32, 0.70 and 0.83 in the spring, summer, autumn and winter, respectively.The annual average equivalent ratio of NO 3 ´/SO 4 2´i n Wuhan was 0.64, which is higher than the value of 0.73 in Changbai Mountain and the value of 0.46 in Nanjing, lower than the value of 0.83 in Shanghai, and essentially consistent with the value of 0.64 in Beijing [31].The results revealed that the main pollution source in Wuhan was a fixed pollution source, which was consistent with the research of Zhang et al. [22].

Charge Balance Analysis of Water-Soluble Ions
Previous studies showed that the charge balance of water-soluble ions in PM 2.5 could be used to analyze the importance of the contribution of water-soluble ions to the mass concentration of PM 2.5 [14,32,33].According to the analysis of data from the experiments, the charge balance figures of PM 2.5 's anions and cations in the four seasons in 2013 are drawn in Figure 5.The slope value of the linear fitting lines reached 0.9319 (R 2 = 0.9887), 0.9279 (R 2 = 0.9459) and 1.0158 (R 2 = 0.9844) in spring, summer and autumn, respectively.All values were nearly 1, while the slope value in winter only reached 0.8888 (R 2 = 0.9688), and had a relatively large gap with 1.These results revealed that the main ionic compositions in PM2.5 in spring, summer and autumn were SO4 2− , NO3 − , Cl − , Na + , K + , NH4 + , Mg 2+ and Ca 2+ , the eight ions that the experiments analyzed.By contrast, cationic charge numbers were slightly low in winter, revealing that there were some other cationic ions not detected except those had been measured in this study (Na + , K + , NH4 + , Mg 2+ and Ca 2+ ), such as H + [34], organic cations or heavy metal ions (Zn 2+ , Cu 2+ , etc.), which reflected that the ion components of PM2.5 in winter were much more complicated than that in spring, summer and autumn, and resulted from the more serious air pollution problems in winter compared with other seasons.Morever, existing research have shown that the mass concentrations of PM were higher in winter than other seasons, hence it carried a certain probability that PM2.5 contained organic cations [7] or heavy metal ions (Zn 2+ , Cu 2+ , etc.) in winter [35].This is not only a significant feature of the PM2.5 in winter, but also one of the reasons that the days of heavy pollution weather in winter were more than the days in the other three seasons.

Correlation and Seasonal Difference Analysis of Water-Soluble Ions
The existing forms of water-soluble ions in PM2.5 are diverse in different air pollution extents or different seasons, which have certain effects on atmospheric visibility, the PH of particulate matter, the viability of chemical reactions, etc.The correlation analysis method is usually used to study the existing forms of water-soluble ions [36].As the correlation coefficient between water-soluble ions increases, the correlation between water-soluble ions increases.
The Pearson correlation coefficients of the water-soluble ions of PM2.5 in all four seasons are shown in Table 3 to Table 6 below.High correlations existed between NH4 + and SO4 2− , NH4 + and NO3 − , Mg 2+ and SO4 2− , Ca 2+ and SO4 2− , K + and Cl − , Na + and Cl − , which were consistent overall in one season.Nevertheless, seasonal differences lie in water-soluble ions.The correlation levels between NH4 + and SO4 2− , NH4 + and NO3 − were significantly higher than the level in the summer, slightly exceeding the value in the autumn, while distinctly lower than the degree in the winter.The The slope value of the linear fitting lines reached 0.9319 (R 2 = 0.9887), 0.9279 (R 2 = 0.9459) and 1.0158 (R 2 = 0.9844) in spring, summer and autumn, respectively.All values were nearly 1, while the slope value in winter only reached 0.8888 (R 2 = 0.9688), and had a relatively large gap with 1.These results revealed that the main ionic compositions in PM 2.5 in spring, summer and autumn were SO 4 2´, NO 3 ´, Cl ´, Na + , K + , NH 4 + , Mg 2+ and Ca 2+ , the eight ions that the experiments analyzed.By contrast, cationic charge numbers were slightly low in winter, revealing that there were some other cationic ions not detected except those had been measured in this study (Na + , K + , NH 4 + , Mg 2+ and Ca 2+ ), such as H + [34], organic cations or heavy metal ions (Zn 2+ , Cu 2+ , etc.), which reflected that the ion components of PM 2.5 in winter were much more complicated than that in spring, summer and autumn, and resulted from the more serious air pollution problems in winter compared with other seasons.Morever, existing research have shown that the mass concentrations of PM were higher in winter than other seasons, hence it carried a certain probability that PM 2.5 contained organic cations [7] or heavy metal ions (Zn 2+ , Cu 2+ , etc.) in winter [35].This is not only a significant feature of the PM 2.5 in winter, but also one of the reasons that the days of heavy pollution weather in winter were more than the days in the other three seasons.

Correlation and Seasonal Difference Analysis of Water-Soluble Ions
The existing forms of water-soluble ions in PM 2.5 are diverse in different air pollution extents or different seasons, which have certain effects on atmospheric visibility, the PH of particulate matter, the viability of chemical reactions, etc.The correlation analysis method is usually used to study the existing forms of water-soluble ions [36].As the correlation coefficient between water-soluble ions increases, the correlation between water-soluble ions increases.
The Pearson correlation coefficients of the water-soluble ions of PM 2.5 in all four seasons are shown in Tables 3-6  Nevertheless, seasonal differences lie in water-soluble ions.The correlation levels between NH 4 + and SO 4 2´, NH 4 + and NO 3 ´were significantly higher than the level in the summer, slightly exceeding the value in the autumn, while distinctly lower than the degree in the winter.The correlations between Mg 2+ and SO 4 2´w ere higher in the spring, summer and autumn, but not in the winter, according to the sequence that the correlation coefficient spring > summer > autumn > winter.The correlation between Mg 2+ and Cl ´was higher than the level between Mg 2+ and SO 4 2´.The correlation of Ca 2+   and SO 4 2´f ollowed the order of spring > autumn > summer > winter, and the correlation between Ca 2+ and NO 3 ´was higher than that between Ca 2+ and SO 4 2´.The correlation between K + and Cl followed the order autumn > winter > spring, and the correlation level of K + and SO 4 2´w as obvious than the level of K + and Cl ´.NH 4 + , SO 4 2´a nd NO 3 ´in the weak acid environment is reversible reaction, and reaction process is as follows: H + `3NH + 4 `2SO 24 Ø pNH 4 q 3 HpSO 4 q 2 (2) NH 4 + as a kind of weak acid ion, is an incomplete reaction in aqueous solution, which existing in free form has not been involved in the charge balance in the solution.In the acidic environment, we can ignore the effects of free NH 4 + on the balance, the results as shown in Figure 6. Figure 6 presents the positive and negative charge balances of NH correlations between Mg 2+ and SO4 2− were higher in the spring, summer and autumn, but not in the winter, according to the sequence that the correlation coefficient spring > summer > autumn > winter.The correlation between Mg 2+ and Cl − was higher than the level between Mg 2+ and SO4 2− .The correlation of Ca 2+ and SO4 2− followed the order of spring > autumn > summer > winter, and the correlation between Ca 2+ and NO3 − was higher than that between Ca 2+ and SO4 2− .The correlation between K + and Cl − followed the order autumn > winter > spring, and the correlation level of K + and SO4 2− was obvious than the level of K + and Cl − .NH4 + , SO4 2− and NO3 − in the weak acid environment is reversible reaction, and reaction process is as follows: NH4 + as a kind of weak acid ion, is an incomplete reaction in aqueous solution, which existing in free form has not been involved in the charge balance in the solution.In the acidic environment, we can ignore the effects of free NH4 + on the balance, the results as shown in Figure 6. Figure 6 presents the positive and negative charge balances of NH4 + , SO4 2− and NO3 − in all four seasons.As is shown in these figures, the slope values (k) of the fitting line between the charge equivalent of NH4 + and the charge equivalent of SO4 2− +NO3 − were all less, but very close to, 1; meanwhile, the goodness of fit values (R 2 ) approximated 1.As a consequence, NH4 + in PM2.5 in Wuhan was neutralized by SO4 2− and NO3 − in all four seasons in 2013, which then existed with the forms of (NH4)2SO4, (NH4)3H(SO4)2and NH4NO3 in PM2.5.Synthetically, diverse forms of inorganic water-soluble ions in PM2.5 not only have some similar states or common characteristics but also exists some variation in four different seasons in Wuhan.The similarity or consistency was revealed at the aspect that the main compositions of PM2.5 were basically identical in four seasons, with their cations consisted of NH4 + , Mg 2+ , Ca 2+ , K + and Na + .In addition, there were several kinds of same particles in the four seasons, including (NH4)2SO4, NH4NO3 and CaSO4.The variation or diversity was reflected by the types of main particles compositions of Synthetically, diverse forms of inorganic water-soluble ions in PM 2.5 not only have some similar states or common characteristics but also exists some variation in four different seasons in Wuhan.The similarity or consistency was revealed at the aspect that the main compositions of PM 2.5 were basically identical in four seasons, with their cations consisted of NH 4 + , Mg 2+ , Ca 2+ , K + and Na + .In addition, Atmosphere 2016, 7, 49 9 of 12 there were several kinds of same particles in the four seasons, including (NH 4 ) 2 SO 4 , NH 4 NO 3 and CaSO 4 .The variation or diversity was reflected by the types of main particles compositions of PM 2.5 in four seasons.Among them, Na + ion mainly composited to form NaCl in spring (correlation coefficient between Na + and Cl ´reached 0.458 in Table 3), while forming NaNO 3 in summer, autumn and winter (correlation coefficients between Na + and NO 3 ´reached 0.423, 0.331 and 0.706 in Tables 4-6 respectively); K + composed to be K 2 SO 4 in summer (correlation coefficient between K + and SO 4 2ŕ eached 0.631 in Table 4), and then KCl in spring, autumn and winter (correlation coefficients between K + and Cl ´reached 0.537, 0.632 and 0.612 in Tables 3, 5 and 6, respectively); K + also formed KNO 3 only in autumn (correlation coefficient between K + and NO 3 ´reached 0.586 in Table 5); Mg 2+ composited MgCl 2 in winter (correlation coefficient between Mg 2+ and Cl ´reached 0.331 in Table 6) while MgSO 4 in spring, summer and autumn (correlation coefficients between Mg 2+ and SO 4 2´r eached 0.590, 0.469 and 0.441 in Tables 3-5 respectively); furthermore, Ca(NO 3 ) 2 also came into being in winter as a compound of Ca 2+ , with correlation coefficient between Ca 2+ and NO 3 ´reached 0.418 in Table 6, unlike other seasons that CaSO 4 was the main existing form.nitrogen oxides and particulate matter in addition to gradual elimination of Yellow Label cars and old cars by strict traffic law enforcement, in order to reduce the mobile sources of exhaust pollution.

Figure 2 .
Figure 2. The location map of sampling site.

Figure 2 .
Figure 2. The location map of sampling site.

Figure 3 .
Figure 3. Seasonal variation of water-soluble ions in Wuhan during the observation period.

Figure 4 .
Figure 4. Seasonal variation of eight inorganic ions accounts for the total mass concentration of PM2.5 in Wuhan during observation period.

Figure 3 .
Figure 3. Seasonal variation of water-soluble ions in Wuhan during the observation period.

Figure 3 .
Figure 3. Seasonal variation of water-soluble ions in Wuhan during the observation period.

Figure 4 .
Figure 4. Seasonal variation of eight inorganic ions accounts for the total mass concentration of PM2.5 in Wuhan during observation period.

Figure 4 .
Figure 4. Seasonal variation of eight inorganic ions accounts for the total mass concentration of PM 2.5 in Wuhan during observation period.

Figure 5 .
Figure 5.The charge balance of anion and cation water-soluble ions in Wuhan in four seasons.

Figure 5 .
Figure 5.The charge balance of anion and cation water-soluble ions in Wuhan in four seasons.

Figure 6 .
Figure 6.Positive and negative charge balances of NH4 + , SO4 2− and NO3 − in all four seasons in Wuhan.

Figure 6 .
Figure 6.Positive and negative charge balances of NH 4 + , SO 4 2´a nd NO 3 ´in all four seasons in Wuhan.

Table 1 .
4 + in the study was second only to that of Beijing and was relatively high in the winter and low in the summer.NH 4 + , converted from NH 3 , is an important ion that reacts with SO 4 2´a nd NO 3 ´in the aerosol phase to form secondary particles.NH 3 mainly comes from agricultural production, industrial emissions, vehicle exhaust emissions and other sources.Attributed to the sharp rise of motor vehicles in Wuhan, a large number of nitrogen compounds are emitted into atmosphere by vehicle exhaust and produce ammonium nitrate through a chemical reaction with NH 3 .Meanwhile, urban population growth (increasing the consumption of energy) and industrial economic expansion (such as thermal power industry, iron and steel industry and cement industry) are also important factors leading to an increase in ammonia emissions.Mass concentration of particulate matter (PM 2.5 ) and the water-soluble ions at different sites (µg/m ´3).
+and Cl ´, Na + and Cl ´, which were consistent overall in one season.
4 + , SO 4 2´a nd NO 3 ´in all four seasons.As is shown in these figures, the slope values (k) of the fitting line between the charge equivalent of NH 4 + and the charge equivalent of SO 4 2´+ NO 3 ´were all less, but very close to, 1; meanwhile, the goodness of fit values (R 2 ) approximated 1.As a consequence, NH 4 + in PM 2.5 in Wuhan was neutralized by SO 4 2´a nd NO 3 ´in all four seasons in 2013, which then existed with the forms of (NH 4 ) 2 SO 4 , (NH 4 ) 3 H(SO 4 ) 2 and NH 4 NO 3 in PM 2.5 .

Table 3 .
Pearson correlation of the water-soluble ions in PM 2.5 in the spring.

Table 4 .
Pearson correlation of the water-soluble ions in PM 2.5 in the summer.*: Significant at a level of 0.01 (2-tailed); *: Significant at a level of 0.05 (2-tailed); the bold data: descripted in content. *

Table 5 .
Pearson correlation of the water-soluble ions in PM 2.5 in the autumn.