Seasonal variations of water-soluble ions in PM 10 at a WMO/GAW station in the Yangtze River Delta, China

. In order to understand the seasonal levels, formation mechanism and atmospheric chemical behaviours of water-soluble ions of PM 10 in the Yangtze River Delta (YRD) region, aerosol samples were collected from January 2nd to December 28th, 2017 at a WMO/GAW regional background station in Lin’an. The concentrations of PM mass and nine water-soluble inorganic ions were obtained. The annual average concentration of PM 10 was 59.9±33.9 μg m −3 , lower than those reported in previous studies, indicating air quality of YRD region was improved. Nine water-soluble inorganic ions was accounted for 30.2-45.1% of the total PM mass, while ammonium (NH 4+ ), sulfate (SO 42+ ), as well as nitrate (NO 3- ) were the major ions which contributed 86.3% to total ions. The NO 3- concentration was lowest in summer but highest in winter, suggesting it was likely influenced by thermodynamics. The levels of SO 42- in spring and winter were related to photochemical reaction and regional transportation. Except for the SNA, Ca 2+ was highest in four seasons likely due to sand storm and road fugitive dust. The annual mean ratio of [NO 3- ]/[SO 42- ] was nearly to 1, indicating mobile and stationary sources were equally important in Lin’an. The mean nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) were 0.22±0.13 and 0.41±0.13, respectively, suggesting secondary formation was significant in the atmosphere at the background station of YRD region.


Introduction
Since the strong link between air pollution and adverse health effects has been revealed, the elevated atmospheric particulate matters especially for inhalable particles (aerodynamic diameter less than 10 μm, PM 10 ) have become a public concern due to their abilities to induce chronic respiratory inflammation and acute cardiovascular disease, as well as the obvious impacts on haze formation and climate change [1,2]. In past few decades, the economically developed regions in China, such as the Jing-Jin-Ji area and the Yangtze River Delta (YRD) region, etc., have suffered from serious air quality which are mainly caused by PM emissions [3][4][5]. The chemical composition plays very important roles in atmospheric behaviors of PM like visibility degradation and secondary aerosol formation. For instance, carbonaceous species and water-soluble ions are considered as the main factors leading visibility reducing. Meanwhile, the three major ions (ammonium (NH4 + ), sulfate (SO 4 2+ ), as well as nitrate (NO 3 -) have been identified as the tracers for secondary inorganic aerosols in previous studies of source apportionment [6]. Huang et al. (2012) has demonstrated that secondary inorganic aerosols (SNA) was one of the three sources related with fog haze in the YRD region during field observations, rest of which were biomass burning and long-range transport [7].
In this study, annual PM10 samples were collected at a WMO/GAW regional background station in the YRD region, and the concentration of water-soluble ions were analyzed to understand their seasonal variations, formation mechanism and atmospheric chemical behaviors. The main aim of our work is to find out the existence forms of SNA and their relationships to indicate the characteristics of emission sources, which would provide a scientific basis for policy making and atmospheric environmental studies.

Sampling site
The samples were collected at a WMO/GAW background station in Yangtze River Delta, located in Lin'an, Hangzhou, China (119.73°E, 30.3°N). The station was in the top of a hill and surrounded by some farmland. A small town (about 200 inhabitants) and a little factory with production of charcoal from bamboo were seated in the north of Lin'an station (about 210 km southwest of the megacity Shanghai and 50 km west of the Hangzhou, the capital city of Zhejiang Province). The average annual temperature and humidity in Lin'an was 17.8℃ and 70.3%, which enjoys sub-tropical monsoon climate with abundant rainfall.
In this study, a total of 98 PM 10 samples were collected on quartz filter (Whatman, 47 mm, USA) in 2017 using a Minivol TM aerosol sampler (Airmetrics, USA) with flow rate of 5L min -1 . Two samples were collected in every week and the sampling period of all samples was 24h from 9:00 to 9:00 in next day, while one field blank filter was collected in every month. Before sampling, all quartz filters was prebaked for 6h at 600℃ in muffle furnace to remove the organics. The filters were balanced in a controlled relative humidity (30-40%) and temperature (23-26℃) environment before and after sampling for 48h prior to weighing on an analytical microbalance (Mettler-MX5, Mettler Toledo, Switzerland) with a 10 μg limit of detection. All filters were reserved in the refrigerator for -18℃ until analyzed.

Water-soluble ions analysis and quality control
Nine water-soluble inorganic ions (Cations: F -, Cl -, SO 4 2-, NO 3 -; Anions: Na + , NH 4 + , K + , Mg 2+ , Ca 2+ ) were analyzed in this study. A quarter of filter was put in a vial with 10 mL ultrapure water (18.2MΩ)，sonicated for 30 min in an ice-water bath and filtered with a 0.22 μm Teflon filter to remove the insoluble materials. Then extract was analyzed by ion chromatography (ICS-5000+, Dionex, USA). Cations determination was performed by the Dionex IonPac CS12A column using 20 mmol L -1 Methyl sulfonic acid as the eluent at a 1mL min -1 flow rate and detected by a CSRS 300 suppressor system. Anions determination was performed by Dionex IonPac AS11HC column using 30 m mol L -1 NaOH as the eluent at a 1mL min -1 flow rate and detected by a AERS suppressor system. The method limit of detection (LOD) was shown in Table1.

The level of PM10
The daily concentration of PM 10 and measurement water-soluble ions were shown in Fig. 1(a). The annual mean mass concentariton of PM 10 [9]. These results showed the air quality of YRD region was enhanced in recent years.
In this study, four seasons were indentified, e.g., spring (March 1st to May 31th), Summer (July 1st to August 31th), autumn (September 1st to November 30th) and winter (December 1st to February 28th). During observation, the hihgest PM10 mass concentration was observed in winter, with an annual average value of 72.9±41.4 μg m −3 , followed by 61.8±27.1 μg m −3 in spring, 58.16±35.67 μg m −3 in autumn and 42.17±23.12 μg m −3 in summer, respectively. So, Lin'an had the most serious particle pollution in winter and slightest particle pollution in summer, which was in accordance with Beijing, Shanghai and Guangzhou reported in previous researches [5,10,11]. Compared with other seasons, the lowest concentration in summer may be due to the abundant shower (about one third days) and the actions of rainwash can remove some particles suspended to the atmosphere [12].

Seasonal variations
As shown in Fig. 1(b) and (c), the levels of chemical compositions in PM 10 were various in four seasons, which was associted with different meteorological conditions (temperature, relattive humidity, wind speed and direction, etc.), the intensity of emission sources, as well as pollution transportation [3,15,16]. The total measurement ions contributed for 30.24-45.08% to PM 10 in four seasons. On the seasonal basis, the levels of F -, Cl -, SO 4 2-, NO 3 -and NH 4 + were highest in winter. The Na + , Mg 2+ ,and Ca 2+ peaked in autumn, while K + peaked in spring.
Compared with other three seasons, the mass concentration of NO 3 -and NH 4 + were lowest in summer, mainly due to easily decomposition of NH 4 NO 3 and formation of gaseous NH 3 and HNO 3 with the highest temperatue (27.72±3.76℃). The high level of SO 4 2-appeared in winter and spring, with mean value of 11.81±7.06 μg m −3 and 8.15±3.52 μg m −3 , respectively ( Fig. 1(c)). This phenomenon was also observed in previous study in this station. The high level of SO 4 2-was becasue of the photochemical reaction with both high concentration of ozone and sulfur dioxide in spring and was influenced by emissoions of regional transportation in winter [3]. Except for the SNA ions, The level of Ca 2+ was highest in every seasons, likely from sand storm and road fugitive dust owe to the roadway in the southwest (about 50m) of the station [17]. Besides, the highest concentration of Clpeaked in winter and was 1.5-4.3 times of other three seasons, mainly due to coal combustion and heating emissions [3]. For the K + , it showed weak seasonal variation at Lin'an station likely because of the continuous emissions from the charcoal factory (pyrolysis the bamboo) during sampling period.

[NO3 -]/[SO4 2-]
Generally, the ratio of [NO 3 -]/[SO 4 2-] can be used as a tracer to characterize the relative importance of mobile and stationary sources in the atmosphere [7]. The high value of [NO 3 -]/[SO 4 2-] showed the level of primary pollutants from the mobile sources was over the fixed sourecs. In our study, the annual mean ratio of

NOR and SOR
The secondary conversion was the major pattern for the formation of SNA and the levels of NO 3 -and SO 4 2-were strong related to atmospheric oxidation [4]. The nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) were the indicators for the secondary formations. The NOR and SOR were calculated as following, NOR=n-NO 3 -/(n-NO 3 -+n-NO 2 ) and SOR= n-SO 4 2-/(n-SO 4 2-+n-SO 2 ). The anunal mean NOR and SOR were 0.22±0.13 and 0.41±0.13, respectively. As shown in Fig. 3, the average value of NOR was lowest in summer, which was mainly relevant to the formation mechanism of NO 3 -that was mostly exsited with gaseous NO 3 -and not easily concentrated in particles with high temperature. Sachio et al. (1990) reported the value of SOR over 0.1 indicating sulfate was the production of SO 2 oxidized [21]. The seasonal mean values of SOR all exceeded 0.10, implying secondary formations was prominent in Lin'an ( Fig. 3).

Conclusions
The concentration of PM 10 mass and nine water-soluble inorganic ions were performed in Lin'an to reveal their chemical characteristics and seasonal variations. The annual average PM 10 concentration was 59.9±33.9 μg m −3 , lower than those reported in previous studies, indicating air quality improvement in YRD region. Nine water-soluble inorganic ions was accounted for 30.2-45.1% of the total PM 10 mass. The SNA were the major ions which contributed 86.3% to total ions. The great difference among ions in four seasons was the concentration of NO 3 -which was lowest in summer while highest in winter, suggesting it was likely influenced by thermodynamics. The greater SO 4 2-was related to photochemical reaction and emissoions of regional transportation. Except for the SNA, Ca 2+ concentrtion was higher than other ions in every seasons due to sand storm and road fugitive dust. The annual mean ratio of [NO 3 -]/[SO 4 2-] was 0.99±0.65, indicating mobile and stationary sources were equally important in Lin'an. The mean value of NOR and SOR were 0.22±0.13 and 0.41±0.13, suggesting great NO 2 and SO 2 were oxidized to form [NO 3 -] and [SO 4 2-], respectively, in the atmosphere at the background station of YRD region.