Characteristics of Water-Soluble Inorganic Components and Acidity of PM 2 . 5 in a Coastal City of China

To investigate the characteristics of water-soluble inorganic ions (WSIIs) of PM2.5 and aerosol acidity in a coastal city, 352 samples were collected at four sites representing four functional zones (FJ: living town; XY: industrial area; TZ: scenery areas; HR: harbor) in Xiamen, China in 2015–2016. Mass concentrations of PM2.5, nine WSIIs, total acidity and in-situ acidity of aerosol/non-sea-salt aerosol were measured. Overall, the mean mass concentrations of PM2.5 in the study area in spring, summer, autumn and winter were 57.5 ± 22.3, 28.1 ± 12.6, 46.8 ± 18.3 and 62.4 ± 22.1 μg m, respectively. WSIIs accounted for 36%–56% of PM2.5 at four sites. Secondary ions (non-sea-salt SO4, NO3 and NH4) contributed more than 74% to total WSIIs. Neutralization degree distributions indicated that 79.5% of PM2.5 was acidic. Total acidity and in-situ acidity both showed obvious seasonal variations, exhibited the peak values of 193.20 and 130.17 nmol m at HR site in summer. Sea-salt contributed 2.58%–17.74% to acidity in four seasons. The normalized ammonium concentration ([NH4]/[SO4]) and normalized nitrate concentration ([NO3]/[SO4]) showed greater correlation coefficients after eliminating the ammonium-poor data points (greater than 0.66), especially at HR site. NH4HSO4 was the major form of WSIIs in PM2.5 and the formation of aqueous HNO3 could enhance aerosol acidity.


INTRODUCTION
Atmospheric aerosols play a vital role in regional air quality which affects human health (Pope and Dockery, 2006;Vedal et al., 2009;Zhou et al., 2010) and global climate change (Remer et al., 2008;Levy et al., 2009;Fan et al., 2012;Deng et al., 2016).These impacts are highly associated with chemical components of aerosol.Watersoluble inorganic ions (WSIIs) are one of the major components of aerosols, especially for PM 2.5 .WSIIs in PM 2.5 in China have been reported by many previous literature which investigated the concentrations, size distributions and sources of WSIIs (Zhang et al., 2011;Zhao et al., 2011a;Chen et al., 2014;Kong et al., 2014;Meng et al., 2016;Tan et al., 2016).Secondary inorganic ions including NH 4 + , NO 3 -and non-sea-salt SO 4 2-can not only make a huge contribution to atmospheric visibility and haze formation, but also determine aerosol acidity.Aerosol acidity affected the chemical properties of aerosol and reaction activities of gas-to-particle conversions of volatile and semi-volatile compounds by altering the uptake of gas precursors on particles' surface (Zhang et al., 2007;Zhou et al., 2012).Higher acidity of aerosols implied lower acidic buffering capacity of atmosphere, which probably caused the acid rain by acting as cloud condensation nuclear and regional precipitation (Ren et al., 2011;Ge et al., 2016).Moreover, the forms of secondary inorganic ions could be transferred by the acid-catalyzed heterogeneous reaction and the mechanisms were not quite clear (Ullerstam et al., 2002;Manktelow et al., 2010;He et al., 2012).Previous studies showed that the acidities are general high in some representative cities in China (Pathak et al., 2004a, b).
Numerous studies of air pollutants in coastal cities have focused on local sources and long-range transport, rather than the interaction between different functional zones (Anjum et al., 2015;Jagdish et al., 2015;Jiang et al., 2015).Besides, the factors of pollution in the coastal cities might differentiate from inland cities. Sea-salt ions are one of the indispensable components of aerosols in the coastal cities, even the non-sea-salt ions take up a larger proportion in aerosols (Jung et al., 2014).Furthermore, sea-land breeze and air-sea exchange in the coastal cities had remarkable effects on accumulation and dispersion of pollutants (Sandra et al., 2003).The studies related to aerosol acidity mostly concentrated on the specific locations like the big or mega cities and mountains, but lacking the sites which located in land and sea intersections.It should be pointed out that non-sea-salt aerosol acidities' occupations still remain unknown.
To survey the WSIIs and aerosol acidity in the coastal cities, a representative city in the southeastern China, Xiamen, which was well-developed and multi-functional, was chosen to analyze in this study.The species, concentrations, spatialtemporal distribution of WSIIs, aerosol acidity and nonsea-salt aerosol acidity were investigated in detail.As far as we know, this is the first attempt to analyze the effects of sea-salt aerosol acidity in the coastal city.

Sampling Site Description
Four sampling sites were selected at four different functional zones in a coastal city, Xiamen (Fig. 1); The HR site is located in the Hairun port terminal which might be potentially influenced by ship emissions.The FJ site is located in a mix-functional area of residential, commercial and educational areas.The XY site is located in Xinyang, which is one of the largest industrial areas in Xiamen.The TZ site is located in the south foot of Mt.Tianzhu where there is a municipal forest park.The altitude of Mt.Tianzhu is 933 m.The distances of four sites to the adjacent seas are 0.6 km, 1.5 km, 4.4 km and 8.1 km, which could represent the coastal city.
Fig. 2 shows the wind rose of Xiamen during the sampling periods.The whole area was influenced by the prevailing winds blown from N to ESE, especially from NE.The seasonal average temperature and relative humidity (RH) are summarized in Table 1.The temperatures at four sites were close because of the disturbance of turbulence in such a coastal area.The RH at four sites TZ, FJ, HR and XY (in descending order) were at a high level; The TZ site had the highest humidity and the RH of XY site in the industrial area was the lowest.As a multi-functional area, the local anthropogenic emissions were complicated and the air pollutants were various, even though the major pollutants probably came from the long range transport (Zhao et al., 2011b;Yin et al., 2014).

Field Sampling
Daily PM 2.5 sampling campaign was synchronically performed by employing mini-volume air samplers (minivol, TAS, USA) equipped with 47 mm quartz filter at a flow rate of 5 L min -1 at four sites.Each sample was continuously collected for 23 h from 9:00 am to 8:00 am the next day.All air samplers were set on the 20 meters high building rooftops.Totally, 96, 92, 96 and 68 samples were collected at the four sites in spring (Apr 21, 2015-May 16, 2015), summer (Jul 29, 2015-Aug 28, 2015), autumn (Nov 2, 2015-Nov 26, 2015) and winter (Dec 15, 2015-Jan 3, 2016), respectively.Blank filters were prepared during each season for analysis.All the filters were pre-combusted at 600°C for 5 h.In addition, a weather and environment monitoring station was adopted at each site to obtain meteorological parameters and concentrations of routine pollutants.
The standard solutions and blank samples were tested before the samples.All standard solutions of 6 concentrations, 0.5 ppm, 1 ppm, 2 ppm, 5 ppm, 10 ppm and 20 ppm were obtained from the National Information Center for Standard Substance, China.The results of correlation coefficient of standard solutions were greater than 0.999.Each sample was corrected by the blank sample of each season.

Acidity Calculation
Two vital parameters are usually used to represent the aerosol acidity: total acidity (H + total ) and in-situ acidity (H + air ) (Li et al., 2014).H + total was the total amount of the deliquesced aerosol acid which was measured by strong acids, including sulfuric and/or nitric acid.H + air , which influenced the real chemical and photochemical reactions in the air, is the actual acidity of atmospheric aerosol.In contrary to the deliquesced aerosol of the solution, the liquid water content (LWC) of atmospheric aerosol is too low to measure the H + air in direct methods.The online version of Aerosol Inorganic Model IV (AIM-IV) was used to calculate the in-situ acidity in this paper.This method was adapted for the circumstance of available temperature and relative humidity (Clegg et al., 1998).Considering the sea-salt aerosol, AIM-IV covers Na + and Cl -which were the representative ions of sea-salt.Extended AIM Aerosol Thermodynamics Model is a community model for calculating gas/liquid/solid partitioning in aerosol systems containing inorganic and organic components and water, and solute and solvent activities in aqueous solutions and liquid mixtures (http://www.aim.env.uea.ac.uk/aim/aim.php).

Neutralization Degree and Acidity
Daily concentrations of NH 4 + , Na + , Cl -, NO 3 -and SO 4 2were used to calculate neutralization degree (F) and total acidity because of the great contribution of these ions to atmospheric acidity (Zhou et al., 2009).The contents of alkali metal ions (K + , Ca 2+ , Mg 2+ ) were too low to affect the acidity in the aerosol solutions.
Neutralization degree is an indicator of aerosol acidity and it can be estimated from the ratio of the mole concentrations of (NH 4 Total acidity was estimated from the difference between the molar concentration of (2 × SO 4 2-+ NO 3 -+ Cl -) and (NH 4 + + Na + ) by the following equation: In this equation, F < 1 indicates that the aerosol was acidic.Most cations are totally neutralized by ammonium when F > 1.
The output results from the model contained in-situ acidity ([H + ] air ) and HSO 4 -.In-situ acidity is defined as the moles of free hydrogen ions in the aqueous phase of aerosols per unit volume of air.It is noted that the model cannot predict the fully neutralized samples, which means only the acid aerosols were considered.

Backward Trajectories
Fig. 3 shows the back trajectories for the air mass of the study area during the four sampling periods.The Hybrid Single Particle Lagrangian Integrated Trajectory model was used to determine backward trajectories for air mass (http://www.arl.noaa.gov/raedy/hysplit4.html).In spring, three pathways for air mass came from the west, southwest and north.The west and north air mass may bring the pollutants from other places.On the contrary, the southwest air mass came from the sea would not cause pollution.The major pathway in summer, which accounted for 51%, surrounded at the low level in the study area.The atmospheric circulation in the planet boundary layer led to the exchange between marine surface and land surface.The other two path ways came from the sea in the east and south-west.In autumn and winter, the study area had similar air mass sources as the major pathways which came from

Overview of Aerosol Particles
Table 2 summarizes the seasonal concentrations of fine particulate matters at four sites.Annual concentrations at four sites all exceed the China Ambient Air Quality Standard of annual average value of PM 2.5 (grade II, 35 µg m -3 ) (PRC MEP, 2012) and were even much higher than the National Ambient Air Quality Standard annual average of 15 µg m -3 (USEPA, 1997).The highest concentration of PM 2.5 was observed at XY site as predicted because of the industrial emissions.The industrial exhausted SO 2 was probably oxidized to H 2 SO 4 , followed by condensation or nucleation of H 2 SO 4 both onto the pre-existing particles and into new particles with neutralization by ammonia and other basic ambient substance (Wall et al., 1988).The PM 2.5 concentration at the TZ site was the lowest and slightly lower than those at FJ and HR sites, implying that differences of PM 2.5 concentrations were almost negligible.Seasonal concentrations of PM 2.5 decreased in the order of winter > spring > autumn > summer.Like many cities in China, winter was the highest incident season and summer was the lowest season in Xiamen.Because there was no coal heating in Xiamen city in winter, meteorological factors might be more apparent.In order to investigate the chemical components of PM 2.5 , WSIIs concentrations were analyzed and will be discussed in the following section.

Chemical Composition of WSIIs
The anion equivalents and cation equivalents were calculated by converting mass concentrations of anions and cations (µg m -3 ) into mole concentrations (µmol m -3 ) by the following equations, The correlation coefficient between AE and CE at four sites were 0.938.The balance of cations and anions verified the validity of WSIIs measurements.
Fig. 4 illustrates the seasonal concentrations of 9 determined WSIIs at four sites.NO 3 -, SO 4 2-of cations and NH 4 + of anions were the dominated ions in total WSIIs; Na + , Cl -, K + , Ca 2+ , F -and Mg 2+ followed in descend order subsequently.This distribution of WSIIs was similar to PM 2.5 .Average percentages of seasonal concentrations of WSIIs in PM 2.5 were 36-56% at four sites.The seasonal concentrations of WSIIs are similar to the previous work in Xiamen (Yin et al., 2014).
Na + and Cl -are two representative ions of sea salt.Concentration of Na + was supposed to be lower than the ions which may generate from sea salt partly, because Na + came solely from sea salt.Nevertheless, mean concentration of Na + was higher than those of K + , Ca 2+ and Cl -.Other ions may deplete as well as Cl -because of the higher concentration of Na + .K + in fine particles can serve as an indicator of biomass burning, soil derived and sea salt (Zhang et al., 2008).In this study, the concentration ratio of Na + /Cl -were 0.10-6.20 (2.33 for average) in spring, 0.04-5.39(2.02 for average) in summer, 0.26-5.71(2.33 for average) and 0.08-6.83(0.98 for average) in winter, which were much higher than that in seawater (0.557, Wang and Shooter, 2001), which indicated Cl -loss.It has been confirmed that sometimes seasalt originated aerosols showed a large chloride deficiency compared with the original seawater composition (Rossi, 2003).The concentrations of Na + and Cl -at four sites were close, which indicated that the contributions of sea-salt to particulate matters were also close, as well as the deficiency of Cl -.The Cl -depletion is generally attributed to a series of chemical reactions involving alkali halides and strong inorganic acids, such as HNO 3 and H 2 SO 4 in both gaseous and aqueous phases (ten Brink, 1998) and occurs predominantly in the coarse-mode particles (Zhao and Gao, 2008).
To further understand the proportions of secondary ions, the seasonal mean concentrations of secondary ions at four sites were analyzed (Fig. 5).The mass percentages of secondary ions in total WSIIs at four sites during four seasons were 74-90%, indicating that the poor air quality of the study area mainly came from secondary reactions in atmosphere.The most ions in PM 2.5 were SO 4 2-which mainly converted from SO 2 precursor.Sulfuric acid and sulfate converted from SO 2 precursor probably caused the acid precipitation and sulfurous haze, which could greatly affect the atmospheric environment.Nitrate was formed by NO x precursors that were mainly came from stationary and mobile sources emissions.The concentrations of ions at HR site were probably influenced by ship emission which directly exhausted NO x and SO 2 , the precursors of nitrate and sulfate.The content of NO 3 -was lower than that of SO 4 2-in this work and had an obvious seasonal distribution which was similar to total WSIIs.Ammonia vapor (NH 3 ) could form fine particulate ammonium through reactions with acidic particles and the conversion depends on the acidic species, temperature and humidity in the atmosphere (Liu et al., 2008;Zhang et al., 2011;Zhao et al., 2016).In this paper, the concentrations   of ammonium were not sufficient to neutralize the acidity of PM 2.5 .The effects of ammonium will be discussed in the following part.
In order to figure the conversions of sulfur to sulfate and nitrogen to nitrate, sulfur oxidation ratios (SOR) and nitrogen oxidation ratio (NOR) were calculated by the following equations (Kaneyasu, 1999): Concentrations of SO 2 and NO x were obtained from the ambient air quality monitoring stations at the four sites.
As illustrated in Table 3, it should be pointed out that the SOR at FJ site in winter were the lowest.Higher SO 4 2accompanied lower SOR indicated that the sources might mainly come from the long range transportation.NOR in summer was the lowest that probably caused by temperature and NOR at four sites were close.The volatile NH 4 NO 3 could be decomposed NH 3 and HNO 3 at high temperatures that might lead to the lower NOR (Suzuki, 2008).It should be pointed out that particulate ions might be formed in other places and locally SO 2 and NO x might inadequately relate to SO 4 2-and NO 3 -.The uncertainty existed in SOR and NOR.

Neutralization Degree and Aerosol Acidity
In this paper, acidic aerosol was defined as the sample with a neutralization degree less than 1.79.5% of total samples were acidic aerosols (Fig. 6).The range of neutralization degree of spring samples was the largest, which showed that aerosol acidities were distinct; in summer and autumn, all the valid samples were acidic; in winter, only a few samples were alkaline.The distribution of neutralization degree was similar to the concentration of WSIIs to some extent.Compared with Fig. 4, the proportions of ammonia in summer and autumn were less than that of spring and winter.Ammonia played a key role in neutralizing the acidic aerosols in atmosphere.In the study area, anthropogenic ammonium was mainly exhausted from vehicle and industrial source, partly from agricultural source and urban green space (Hu et al., 2014).
As is obviously depicted in Fig. 7, total acidity was basically consistently with in-situ acidity, [HSO 4 -] and any other undetermined [H + ], which could be revealed as follows: Total acidity and in-situ acidity were illustrated in Table 4.Total acidity and in-situ acidity were significantly influenced by seasonal factors.Higher temperature in summer probably accelerated ammonia to volatile and gas SO 2 was converted to aqueous H 2 SO 4 .Along with the reduced ammonia and the elevated H 2 SO 4 in aerosol, neutralization degree values decreased rapidly.Acidities at XY, TZ and HR sites had similar seasonal distributions that the mean total acidity and in-situ acidity were the highest in summer and lowest in winter, which probably caused by the seasonal variations of temperatures, RH and surface radiation.At FJ site, the total acidity was the lowest in winter; in spring, summer and autumn, the acidities values were close; the in-situ acidity at FJ site had an approximate distribution to the other three sites.The acidities at HR site in spring, summer and autumn were higher than other three sites, especially in summer.Influenced by the low temperature and RH in winter, the acidities at four sites were the lowest generally.Annual total acidities at four sites decreased in the order of HR, FJ, TZ and XY.Moreover, the in-situ acidity in summer dominated the aerosol acidity; in the contrary, [HSO 4 -] was in charge in spring, autumn and winter.The variation of in-situ acidity was similar to that of total acidity.
Local emissions may be another factors impacting acidity.HR site was probably more affected by sea-salt and ship emission, which generated primary and secondary pollutants.XY site was more affected by the local industrial emission which could also exhausted SO 2 and NO x .Acidity of FJ site was mainly caused by vehicles which exhausted NO x much more.The ammonium from mountain could also alter acidity of TZ site.

Non-Sea-Salt Ions and Acidity
Sea-salt was one of the major sources for WSIIs in coastal city.It was worth of understanding the contribution of sea-salt to acidity.Sodium was supposed to be the only source which came from sea-salt solely and the composition    of sea-salt originated particles was the same as that of sea water (Kennish, 1994).The concentrations of non-sea-salt potassium (nss-K + ), non-sea-salt calcium (nss-Ca 2+ ), nonsea-salt chloride (nss-Cl -) and non-sea-salt sulfate (nss-SO 4 2-) were calculated by the following equations (Wang and Shooter, 2001): The concentrations and proportions of non-sea-salt and sea-salt ions and corresponding acidity are illustrated in Table 5.Most of the K + , Ca 2+ and SO 4 2-were consistent with non-sea-salt.In this study, more than 50% Cl -came from sea-salt aerosols.Several studies found that vegetation fire and anthropogenic burning of fossil fuel and garbage emissions could also enrich Cl -in fine particles (Andreae et al, 1998;Wang et al, 2011).As a vital composition of acidity, nss-SO 4 2-accounted more than 90% to the whole SO 4 2-in four seasons.The contributions of nss-SO 4 2-to acidity had an apparent seasonal distribution corresponding to total acidity.The proportions of non-sea-salt total acidity and non-sea-salt in-situ acidity, which was supposed to be correlative to nss-SO 4 2-, were less than the proportions of nss-SO 4 2-in spring, autumn and winter.In contrast, the proportions of acidity in summer were the highest and higher than that of nss-SO 4 2-.The variation of nss-SO 4 2- was not linear correlative to that of total acidity or in-situ acidity.Proportions of sea-salt aerosol acidity to acidity were quantified as 2.58%-17.74%.

Formation Path of Non-/Sea-Salt Ions
The logarithmic relationship between neutralization degree and in-situ acidity (R 2 = 0.73) is illustrated in Fig. 8.The F values decreased rapidly from 1.0 and tended to be slow since 0.3 approximately.In the contrast, the aerosol acidity raised more quickly when the F value was less than 0.3, which implied that F value of 0.3 was a threshold for different in-situ acidity over the region.Data points with F < 0.3 represented that the aerosol was much more acidic than the data points with 0.3 < F < 1.Previous study indicated that the neutralization degree and in-situ acidity had a logarithmic relationship with thresholds between 0.6 and 1 (Fu et al., 2015).
Nitrate particles were formed from two pathways: NH 4 NO 3 and HNO 3 by the following formation mechanisms:  ammonium and aqueous HNO 3 came from the hydrolysis of N 2 O 5 (Ansari et al., 1998;Chang et al., 2011).NO 3 -, SO 4 2-and NH 4 + were used to explain the formation pathways of nitrate.Meanwhile, these three abundant secondary ions in particles suggested the reactions between ammonia and nitrate in different concentrations of sulfate by using the ratio of [NH 4 + ]/[SO 4 2- ] and [NO 3 -]/[SO 4 2-] (mole concentration ratio), which were also named with normalized ammonium concentration and normalized nitrate concentration.These two deformations in different loadings of [SO 4 2-] presented the relationship of these three ions (Huang et al., 2011;He et al., 2012). 2-] at four sites.Among the products of NH 3 through gas-phase or aqueous-phase reactions with acidic species, (NH 4 ) 2 SO 4 was preferentially formed and was the least volatile species; NH 4 NO 3 was formed subsequently and NH 4 Cl was the last (Zhang et al., 2008).Ammonium neutralized sulfuric acid preferentially in more acid system and then the additional ammonium stabilized the nitric acid (Ansari et al., 1998;Pathak et al., 2004a, b;Pathak et al., 2009;Huang et al., 2016).Generally, ammonium poor was defined as that the ratio of ammonium to sulfate was less than 2 ([NH 4 + ]/[SO 4 2-] < 2).Considering the volatility of compounds ([(NH 4 ) 2 SO 4 ] < [NH 4 NO 3 ]), the reaction order was set as which ammonium were neutralized by sulfate firstly and then the nitrate in order to suggest the neutralized ammonium and the components of PM 2.5 .The data points were divided into two groups depending on the F values: with and without the data points whose F values were less than 0.3.An interesting finding was that when F values were less than 0.3, the [NH 4 + ]/[SO 4 2-] values were less than 1.0 around, indicating that the major form of ammonium of PM 2.5 was NH 4 HSO 4 and the study area was ammonium poor.Poor ammonium in PM 2.5 mainly combined with sulfate as the form of NH 4 HSO 4 , which indicated that the sulfate was not neutralized completely by ammonium and ammonium was not enough to neutralize nitrate acid.Thus, nitrate of PM 2.5 probably formed as HNO 3 which enhanced aerosol acidity by the second pathway mentioned above.With the elimination of ammonium poor data points, the correlation coefficients and slopes of correlation equations were increased at four sites, especially at HR sites.The trends of data points of which F values were between 0.3 and 1 simply increased at four sites.The data points whose F values were less than 0.3 had descend trends at FJ and HR sites and in the contrast, no clear variations at XY and TZ sites.Ammonium poor data points with lower ratio of through atmospheric processes while external mixing might be supplied by liquid water content.Elevated water content could uptake more components and released more H + in liquid phase aerosol (Yao et al., 2007).

CONCLUSIONS
In this paper, the concentration of WSIIs of PM 2.5 , aerosol acidity and non-sea-salt aerosol acidity were explored at four sites in a coastal city during four different seasons.Overall, annual PM 2.5 concentrations at different sites, HR, FJ, XY and TZ were 47.0, 46.2, 56.7 and 44.5 µg m -3 .The mass concentrations of WSIIs in PM 2.5 were 36-56% at four sites and the secondary ions (sulfate, nitrate and ammonium) were more than 67% of total WSIIs.79.5% samples were defined as acid aerosol by whose neutralization degrees less than 1.The total acidity and in-situ acidity at four sites peaked in summer and bottomed in winter.Sea-salt acidity contributed less than 20% and sea-salt sulfate accounted for less than 10%.The particles at HR site on the shore were the most acidic due to the moist air possibly.F = 0.3 was a threshold to recognize the relationship between the ratios of ammonium to sulfate and nitrate to sulfate, which had similar correlations after wiping off the data points of F < 0.3 values.NH 4 HSO 4 was the major formation of sulfate and ammonium in PM 2.5 .No excess ammonium to neutralization the nitrate hence the nitrate was formed to HNO 3 , which caused aerosol acidity.

Fig. 2 .
Fig. 2. Wind rose of the study area during sampling periods.

Fig. 3 .
Fig. 3. Clusters of backward trajectories for the air mass in four seasons in the study area.

Fig. 4 .
Fig. 4. Concentrations of 9 WSIIs in the study area during sampling periods.

Fig. 6 .
Fig. 6.Neutralization degrees in four seasons at four sites.

Fig
Fig. 9.The relationship between [NH 4 + ]/[SO 4 2-] and [NO 3 -]/[SO 4 2-] at four sites.(blue lines are the fit lines of data points between 0.3 and 1.0; red lines are the fit lines of all points).

Table 1 .
Seasonal temperature and relative humidity (± Std.Dev.) at four sites.

Table 3 .
Seasonal SOR and NOR at four sites.

Table 4 .
Total acidity, in-situ acidity concentrations in four seasons.

Table 5 .
Non-sea-salt and sea salt concentrations, acidity and proportions in four seasons.