Characteristics of Particulate Matter at Different Pollution Levels in Chengdu, Southwest of China

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Article Characteristics of Particulate Matter at Different Pollution Levels in Chengdu, Southwest of China

Yi Huang 1,2,*,†, Li Wang 1,†, Xin Cheng 2, Jinjin Wang 2 , Ting Li 2, Min He 2, Huibin Shi 2, Meng Zhang 2, Scott S. Hughes 3 and Shijun Ni 2

1 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, College of Ecology and Environment, Chengdu University of Technology, Chengdu 610059, China; [email protected] 2 Department of Geochemistry, Chengdu University of Technology, Chengdu 610059, China; [email protected] (X.C.); [email protected] (J.W.); [email protected] (T.L.); [email protected] (M.H.); [email protected] (H.S.); [email protected] (M.Z.); [email protected] (S.N.) 3 Department of Geosciences, Idaho State University, Pocatello, ID 83209, USA; [email protected] * Correspondence: [email protected] † These authors contributed equally to this work and should be considered dual-ﬁrst authors.

Abstract: Air pollution is becoming increasingly serious along with social and economic development in the southwest of China. The distribution characteristics of particle matter (PM) were studied in Chengdu from 2016 to 2017, and the changes of PM bearing water-soluble ions and heavy metals and the distribution of secondary ions were analyzed during the haze episode. The results showed that at

different pollution levels, heavy metals were more likely to be enriched in fine particles and may be used as a tracer of primary pollution sources. The water-soluble ions in PM2.5 were mainly Sulfate- 2− − + Citation: Huang, Y.; Wang, L.; Nitrate-Ammonium (SNA) accounting for 43.02%, 24.23%, 23.50%, respectively. SO4 , NO3 , NH4 2− Cheng, X.; Wang, J.; Li, T.; He, M.; Shi, in PM10 accounted for 34.56%, 27.43%, 19.18%, respectively. It was mainly SO4 in PM at Clean 3 3 + − H.; Zhang, M.; Hughes, S.S.; Ni, S. levels (PM2.5 = 0~75 µg/m , PM10 = 0~150 µg/m ), and mainly NH4 and NO3 at Light-Medium 3 3 3 Characteristics of Particulate Matter levels (PM2.5 = 75~150 µg/m , PM10 = 150~350 µg/m ). At Heavy levels (PM2.5 = 150~250 µg/m , 3 2− + − at Different Pollution Levels in PM10 = 350~420 µg/m ), it is mainly SO4 in PM2.5, and mainly NH4 and NO3 in PM10. The Chengdu, Southwest of China. contribution of mobile sources to the formation of haze in the study area was significant. SNA had Atmosphere 2021, 12, 990. https:// significant contributions to the PM during the haze episode, and more attention should be paid to doi.org/10.3390/atmos12080990 them in order to improve air quality.

Academic Editor: Alina Barbulescu Keywords: particulate matter; heavy metals; Sulfate-Nitrate-Ammonium; pollution levels; mobile sources Received: 27 June 2021 Accepted: 26 July 2021 Published: 31 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in PM10 refers to particles with an aerodynamic equivalent diameter less than or equal to published maps and institutional afﬁl- 10 µm in ambient air, and PM2.5 refers to particles with an aerodynamic equivalent diameter iations. less than or equal to 2.5 µm in ambient air. The composition of atmospheric particles is complex, including heavy metals, water-soluble ions, carbonaceous components, and so on from multiple sources [1,2]. In addition, with small particle sizes and large surface area, atmospheric particulates have adverse effects on the atmospheric environment and public health. In recent years, there have been many haze pollution incidents occurring in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. a number of locations across China, which has caused more and more public concerns and This article is an open access article attention paid to air pollution [3–5]. distributed under the terms and The Sichuan Basin is in southwestern China. The topography of hills and basins, conditions of the Creative Commons coupled with the climate conditions of high humidity and low wind speed, leads to Attribution (CC BY) license (https:// atmospheric pollution easily in this area [6,7]. It is the fourth highest haze area following creativecommons.org/licenses/by/ the Beijing-Tianjin-Hebei area, Yangtze River Delta, and Pearl River Delta. Its pollution 4.0/). characteristics are of high particle concentration and low visibility [8–10]. The special

Atmosphere 2021, 12, 990. https://doi.org/10.3390/atmos12080990 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 990 2 of 13

terrain and the humid climate of Chengdu are not conducive to the diffusion of particulate matter and are prone to secondary pollutant (SNA) conversion and generation [6,10,11]. In recent years, although the implementation of pollution prevention and control measures has improved the air environment in Chengdu, the region still has a problem with air pollution [12]. The research topics in this region mainly include the analysis of particulate pollution characteristics [13,14], the impact of meteorological conditions on particulate pollution [15], the lag effect of particulate pollution on related diseases [16], and source apportionment [17]. It is reported that the heavy metals in the atmospheric particulate matter (PM) in Chengdu are mainly arsenic (As), lead (Pb), copper (Cu), nickel (Ni), zinc (Zn), iron (Fe), and manganese (Mn) [18,19]. Among them, arsenic (As) mainly comes from industrial smelters. Pb, Cu, Ni, and Zn mainly come from the exhaust of motor vehicles and the wear of tires and brake pads whereas Fe and Mn are mainly from dust generated during vehicle driving [20,21]. Sulfate-Nitrate-Ammonium (SNA) are water-soluble ions that greatly contribute to PM concentration [22,23]. Air pollution in Chengdu has obvious seasonal distribution characteristics, which are closely related to the meteorological factors of the city [24]. It is reported that the mass concentration of SNA is the highest in winter and the lowest in summer [25]. The high temperature in summer and autumn is conducive to the conversion of sulfur dioxide (SO2) to sulfate 2− (SO4 ) while adverse to the stable existence of ammonium nitrate (NH4NO3). Although the low temperature in winter inhibits the conversion of gaseous precursors, it is beneﬁcial to the stable existence of NH4NO3 [26,27]. However, it is known from the previous studies that the concentration of particulate matter increases with the increase of pollution, but the mechanism of particle concentration and composition change is different under different pollution levels [28]. A recent study showed that the rapid increase of PM2.5 at light pollution level in Beijing was caused by regional transportation, while the rise from heavy to severe was mainly caused by an increase in the proportion of secondary inorganic components [29]. The air pollution in cities in southern China has been easily overlooked. Up to now, there is no detailed report on the various characteristics of atmospheric particulate matter at various pollution levels in Chengdu, southwest China according to our investigation. The purpose of this study is to ﬁnd out how heavy metals and water-soluble ions in PM in Chengdu, China during the haze periods are distributed and changed at different pollution levels. Therefore, we investigated PM2.5 and PM10 in Chengdu in southwest China. The changes in heavy metal elements and water-soluble ions corresponding to the pollution level and their contribution to particulate matter are discussed. The effects 2− − + of SO4 , NO3 , and NH4 on the particulate matter were emphatically explored. The secondary production of sulfate and nitrate will be shown to be important in high pollution level scenarios, and the same with the heavy metal analysis.

2. Materials and Methods 2.1. Study Site and Sample Collection Chengdu is located in the western part of the Sichuan basin, surrounded by the western part of the Longquan Mountains and the eastern part of the Qionglai Mountains. The sampling site was located in Shilidian, Chenghua District, Chengdu (104◦080 E, 30◦400 N), the capital of Sichuan Province in the western part of the Sichuan Basin. Chengdu is densely populated, about 1000 people/km2 [30,31], with the annual temperature 15.2~16.6 ◦C, the annual precipitation 873 mm~1265 mm, the annual sunshine 23–30%, the average annual wind speed 1.3 m/s, and the average annual relative humidity 80% [32]. Shilidian is surrounded by major cities in Sichuan, including Deyang and Mianyang (Figure1). Atmosphere 2021, 12, 990 3 of 13 Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 13

FigureFigure 1.1. Sichuan region ofof ChinaChina andand samplingsampling sitesite inin Chengdu.Chengdu.

FromFrom MarchMarch 20162016 toto JanuaryJanuary 2017,2017, PMPM2.52.5 andand PMPM1010 were collectedcollected atat thethe ChengduChengdu UniversityUniversity ofof TechnologyTechnology inin thethe urbanurban areaarea ofof ChengduChengdu withwith nono chemicalchemical enterprisesenterprises andand talltall buildings.buildings. The The sampling sampling period period was was 24 24 h h and and we we collected collected 72 72 PM PM2.5 2.5samplessamples and and 72 72PM PM10 samples10 samples at the at thesame same time, time, with with a total a total of 144 of 144samples. samples. The Thesampling sampling instrument instrument was wasa TH-150C a TH-150C medium medium flow atmospheric flow atmospheric sampler sampler (Wuhan (Wuhan Tianhong, Tianhong, Wuhan, Wuhan, China), China), with a withcalibrated a calibrated flow rate flow of rate 100 of L/min. 100 L/min. Two kind Twos kindsof filter of filtermembranes membranes made made of quartz of quartz and andTeflon, Teflon, respectively, respectively, (Whatman, (Whatman, Buckinghamsh Buckinghamshire,ire, UK) were UK) werechosen. chosen. The Teflon TheTeflon filters filtersare used are for used the for heavy the heavymetal metalanalysis analysis because because Teflon Teflon filters filtershave low have heavy low heavy metal metalback- backgroundground content, content, and the and quartz the quartz filters filters are us areed usedfor the for water-soluble the water-soluble ion analysis. ion analysis. After Afterthe samples the samples were were collected, collected, the thesampling sampling membranes membranes were were placed placed in in clearly markedmarked samplesample boxesboxes immediately.immediately. At the samesame time,time, thethe meteorologicalmeteorological datadata atat thethe ShilidianShilidian meteorologicalmeteorological monitoring monitoring station station were were recorded, recorded, including including temperature, temperature, air pressure, air pressure, wind speed,wind speed, relative relative humidity, humidity, etc. Samples etc. Samples were collected were collected under stable under weather stable conditions, weather condi- with weaktions, windwith atweak speeds wind less at thanspeeds 1.5 less m/s, than thus 1.5 the m/s, contribution thus the contribution from pollutants from transported pollutants longtransported distances long are distances likely small. are likely The small. samples The in samples this study in this mainly study represent mainly represent the local atmosphericthe local atmospheric conditions conditions in Chengdu. in Chengdu. 2.2. Mass Concentration Analysis 2.2. Mass Concentration Analysis Before sampling, the Teflon filter membrane (Whatman, Φ90 mm, Buckinghamshire, Before sampling, the Teflon filter membrane (Whatman, Ф90 mm, Buckinghamshire, UK) is equilibrated for at least 24 h at a temperature of 20 ± 5 ◦C and a relative humidity of UK) is equilibrated for at least 24 h at a temperature of 20 ± 5 °C and a relative humidity 50 ± 5%. Quartz filter membranes (Whatman, Φ90 mm) were wrapped in aluminum foil, of 50±5%. Quartz filter membranes (Whatman, Ф90 mm) were wrapped in aluminum baked in a muffle furnace (SX-8-13, Beijing) at 500 ◦C for 4 h to remove the background foil, baked in a muffle furnace (SX-8-13, Beijing) at 500 °C for 4 h to remove the back- organic matter, and then placed in the same environment as the Teflon filters for at least 24 h. ground organic matter, and then placed in the same environment as the Teflon filters for After the filter membrane, use a one-hundred thousandth balance (Sartorius, Göttingen, at least 24 h. After the filter membrane, use a one-hundred thousandth balance (Sartorius, Germany, CPA225D) was used to weigh each filter 3 times to ensure that the difference Göttingen, Germany, CPA225D) was used to weigh each filter 3 times to ensure that the between any two weighing values did not exceed 0.04 mg. After the filter membranes were difference between any two weighing values did not exceed 0.04 mg. After the filter weighed, they were all wrapped in aluminum foil, and put in a sealed bag, and stored membranes were weighed, they were all wrapped in aluminum foil, and put in a sealed at −4 ◦C until analyzed. A pretreated blank filter membrane was used as a background. bag, and stored at −4 °C until analyzed. A pretreated blank filter membrane was used as a Before sample collection, the cutting head of the sampler filter membrane grid, sealing gasket,background. and other Before places sample that collection, may be inthe contact cutting with head the of filterthe sampler membrane filter were membrane wiped twogrid, to sealing three timesgasket, with and high-gradeother places pure that absolute may be ethanolin contact to with prevent the impuritiesfilter membrane from were wiped two to three times with high-grade pure absolute ethanol to prevent impu- entering the filter membrane during the sampling process. Refer to “Ambient Air PM10 rities from entering the filter membrane during the sampling process. Refer to “Ambient and PM2.5 Measurement-Gravimetric Method” (HJ618-2011) for details on the method Air PM10 and PM2.5 Measurement-Gravimetric Method” (HJ618-2011) for details on the used to calculate the mass concentrations of PM2.5 and PM10. method used to calculate the mass concentrations of PM2.5 and PM10. 2.3. Heavy Metals Analysis 2.3. HeavyThe concentrations Metals Analysis of heavy metals of the samples were then analyzed. Before the experiment,The concentrations all Teflon vials of heavy were thoroughlymetals of the cleaned samples with were 20% then hot analyzed. nitric acid Before solution the (70experiment,◦C) anddeionized all Teflon watervials were to avoid thoroughly contamination. cleaned with Subsequently, 20% hot nitric 1/2 acid of a Teflonsolution filter (70 was°C) and dissolved deionized with water 1 mL to of avoid nitric contamination. acid (HNO3) andSubsequently, 1 mL hydrofluoric 1/2 of a Teflon acid (HF)filter inwas a dissolved with 1 mL of nitric acid (HNO3) and 1 mL hydrofluoric acid (HF) in a

Atmosphere 2021, 12, 990 4 of 13

closed-cap Teﬂon vial for 48 h at 180 ◦C. After that, the mixed solution was steamed to near ◦ dry, and then re-dissolved twice with 1 mL HNO3 (120 C). After the last re-dissolution, HNO3 (1 mL), Rh solution (1 mL of 1000 ng/mL), and 5 mL deionized water were added and kept in Teﬂon vials for 6 h (100 ◦C). At this point, the sample pre-treatment was completed. The concentrations of the heavy metals were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer Corp., Norwalk, USA). The reference material GSS-4 was used to ensure the analytical accuracy with recovery between 94.3% and 103.6%. In addition, for 10% of the samples analysis was repeated and reagent blanks were also used to check the quality of the analysis. A total of eight metal elements were measured, including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), vanadium (V), and zinc (Zn). Their detection limits are: As (0.30 ng/m3), Cd (0.01 ng/m3), Cr (0.10 ng/m3), Cu (0.04 ng/m3), Ni (0.04 ng/m3), Pb (0.03 ng/m3), V (0.08 ng/m3), and Zn (0.10 ng/m3).

2.4. Water-Soluble Ions Determination The main steps for the determination of water-soluble ions in the sample are as follows: putting 1/4 of the quartz filter into a 50 mL PET bottle with 20 mL of ultra-pure water and sonicated (25 ◦C, power 50%, Kunshan Ultrasound Instrument Co., Ltd., Kunshan, China, KQ-700DB) for 0.5 h. The bottle was transferred to a water bath shaker (Changzhou Putian Instrument Manufacturing Co., Ltd., Putian, China, SHA-CA) at room temperature and kept shaking for 30 min. The extract was then filtered through a 0.22 µm filter membrane. − − − 2− Anions of fluoride (F ), chloride (Cl ), nitrate (NO3 ), sulfate (SO4 ), and cations of + + + 2+ 2+ sodium (Na ), ammonium (NH4 ), potassium (K ), calcium (Ca ), and magnesium (Mg ) were determined by ion chromatography (Metrohm 792). The anion column used was a Metrosep A Supp 5—150/4.0; the cation column used in ion chromatography is Metrosep C4-150. The flow rate was 0.7 mL/min. The sampling time for each run was 20 min. The anion eluent was sodium carbonate/sodium bicarbonate, fully dissolve the two in ultrapure water, and dilute them in a 100 mL volumetric flask, as a stock solution. The stock solution diluted 100 times is used as the eluent for anion determination. The cation eluent was 7.25 mM HNO3 and 0.02 M methanesulfonic acid. The ultrapure water, reagent solutions, and samples used in the test were filtered through a 0.45 µm filter membrane. − 3 − 3 − 3 Their detection limits are F (0.010 µg/m ), Cl (0.012 µg/m ), NO3 (0.027 µg/m ), 2− 3 + 3 + 3 + 3 2+ SO4 (0.030 µg/m ) and Na (0.019 µg/m ), NH4 (0.020 µg/m ), K (0.025 µg/m ), Ca (0.037 µg/m3) and Mg2+ (0.020 µg/m3).

2.5. SOR and NOR Analysis The concentrations of sulfate, nitrate, and ammonium are related to the concentration of gaseous precursors: sulfur dioxide (SO2), nitrogen oxides (NOx), and ammonia (NH3), and their conversion rates to particles generated in the atmosphere. Here SOR (sulfur oxidation rate) and NOR (nitrogen oxidation rate) are used to describe the formation of secondary aerosol species. The measured values of SO2 and NO2 come from the Chengdu Shilidian permanent monitoring site. Based on Ma et al. [29], the calculation formulas of SOR and NOR are: 2− 2− SOR = nSO4 /(nSO4 + nSO2) − − NOR = nNO3 /(nNO3 + nNO2) where n is the molar concentration of the species. When SOR > 0.1, it indicates that there 2− is a process of SO2 oxidation to SO4 in the particles. When NOR > 0.001, it is said that − there is a process of oxidation of NO2 to NO3 in the particulate matter. The higher value of SOR or NOR, the higher the oxidation rate of the pollutant [33]. Atmosphere 2021, 12, 990 5 of 13

3. Results and Discussion 3.1. PM Mass Concentration Thirty samples, eight samples, twenty-one samples, and thirteen samples were analyzed in spring, summer, autumn, and winter, respectively. The concentrations of PM2.5 and PM10 showed obvious seasonal distribution characteristics. The changes of PM2.5 con- Atmosphere 2021, 12, x FOR PEER REVIEW 3 3 5 of 133 centration with seasons (spring to winter) were: 98.62 µg/m , 66.75 µg/m , 84.02 µg/m , 3 and 159.74 µg/m . PM10 concentration changes with seasons (spring to winter) were: 169.87 µg/m3, 107.22 µg/m3, 167.16 µg/m3, and 260.30 µg/m3. The concentrations of 3.PM Results2.5 and and PM Discussion10 were the highest in winter and the lowest in summer. An inversion 3.1.easily PM forms Mass inConcentration winter, which prevents particles from diffusing. While the movement of atmospheric molecules and the atmospheric oxidation capacity is enhanced because of the Thirty samples, eight samples, twenty-one samples, and thirteen samples were an- high temperature in summer, which is conducive to the diffusion of atmospheric particles. alyzed in spring, summer, autumn, and winter, respectively. The concentrations of PM2.5 According to the “Ambient Air Quality Index (AQI) Technical Regulations (Trial)” (Min- and PM10 showed obvious seasonal distribution characteristics. The changes of PM2.5 istry of Environmental Protection of China, 2012), the PM concentration is divided into four lev- concentration with seasons 3(spring to winter) were:3 98.62 μg/m3, 66.75 μg/m3, 84.023 els (Clean: PM2.5 = 0~75 µg/m , PM10 = 0~150 µg/m ; Light-Medium: PM2.5 = 75~150 µg/m , μg/m3, and 159.74 μg/m3 3. PM10 concentration changes 3with seasons (spring to3 winter) PM10 = 150~350 µg/m ; Heavy: PM2.5 = 150~250 µg/m , PM10 = 350~420 µg/m ; Severe: were: 169.87 μg/m3, 107.22 μg/m3, 167.163 μg/m3, and 260.30 μg/m3. The concentrations of PM2.5 > 250 µg/m , PM10 > 420 µg/m ). The particulate matter concentration exceeding PM2.5 and PM10 were the highest in 3winter and the lowest3 in summer. An inversion easi- the clean level (PM2.5 = 0~75 µg/m , PM10 = 0~150 µg/m ) is defined as a haze incident. lyHaze forms incidents in winter, during which the samplingprevents periodparticles mainly from occurreddiffusing. from While March the movement to May 2016 of and at- mosphericNovember 2016molecules to January and the 2017 atmospheric (Figure2), so oxidation PM in these capacity periods is enhanced was analyzed. because Figure of 3thea highshowed temperature that there werein summer, 32 samples which at the is cleanconduc levels,ive to 55 the samples diffusion at the of Light-Medium atmospheric levels,particles. and 12 samples at the severe levels. It is worth noting that the pollution level based on PM2.5 did reachAccording the Severe to the levels “Ambient on 3 January Air Quality 2017. Index (AQI) Technical Regulations (Trial)” (Ministry of Environmental Protection of China, 2012), the PM concentration is divided The concentrations of PM2.5 that increase with the change in pollution levels were intoon average four levels 48.80 (Clean:µg/m PM3, 109.842.5 = 0~75µg/m μg/m3,3 and, PM 186.2110 = 0~150µg/m μg/m3 for3; Light-Medium: Clean, Light-medium, PM2.5 = 3 3 3 75~150and Heavy μg/m levels,, PM10 respectively. = 150~350 μ Theg/m concentrations; Heavy: PM2.5 of = PM 150~25010 increase μg/m with, PM the10 = change 350~420 in pollutionμg/m3; Severe: levels PM were2.5 > 98.49250 μµg/mg/m3, 3PM, 226.5310 > 420µg/m μg/m3,3 and). The 383.21 particulateµg/m3 matterfor Clean, concentra- Light- tionmedium, exceeding and Heavy the clean levels, level respectively (PM2.5 = 0~75 (Figure μg/m3b).3, FromPM10 = the 0~150 perspective μg/m3) ofis thedefined increase as a hazein particle incident. concentration, Haze incident thes growth during ratethe ofsampling PM10 is period faster thanmainly the occurred growth rate from of March PM2.5, toindicating May 2016 that and coarse November particles 2016 (PM to2.5 January–10) have 2 a017 certain (Figure contribution 2), so PM to in the these growth periods of PM was10. analyzed.PM2.5/PM Figure10 from 3a Clean showed to Heavythat there pollution were 32 decreases samples at ﬁrst the and clean then levels, increases 55 samples slightly, at theindicating Light-Medium PM2.5 contributed levels, and the12 samples most to at PM the10 atsevere Clean levels. levels It andis worth the least noting to PMthat10 theat pollutionSevere levels. level based on PM2.5 did reach the Severe levels on 3 January 2017.

100 95 2.0 90 85 80 1.5 75 RH (%)

70 (m/s) Ws 1.0 65 60 0.5

) 450 300 3 400 ) 3 250 350 Clean Level 300 200

(μg/m 250 150 (μg/m 10 200 150 2.5 100 PM 100 150 μg/m3 50 3 PM 75 μg/m 50 0 0 35 97 30 96 25 20

95 T (℃) 15 P (KPa) P 94 10 93 5 0 9 /1 /8 3 9 /5 0 6 9 /7 8 5 2 7 /2 5 9 4 /2 2 /1 /7 3 0 9 /1 /8 3 9 /5 0 6 9 /7 8 5 2 7 /2 5 9 4 /2 2 /1 /7 3 /2 /2 4 4 /1 /2 5 /1 /1 /2 6 /1 /1 /2 /2 0 /1 /1 /2 2 /2 1 1 /2 /2 /2 4 4 /1 /2 5 /1 /1 /2 6 /1 /1 /2 /2 0 /1 /1 /2 2 /2 1 1 /2 3 3 4 4 5 5 5 6 9 9 9 1 1 1 1 1 2 1 3 3 4 4 5 5 5 6 9 9 9 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 Date M-D (2016-2017) Date M-D (2016-2017)

Figure 2. Time series of changes in PM mass concentration and related meteorologicalmeteorological conditions.conditions.

The concentrations of PM2.5 that increase with the change in pollution levels were on average 48.80 μg/m3, 109.84 μg/m3, and 186.21 μg/m3 for Clean, Light-medium, and Heavy levels, respectively. The concentrations of PM10 increase with the change in pollution levels were 98.49 μg/m3, 226.53 μg/m3, and 383.21 μg/m3 for Clean, Light-medium, and Heavy levels, respectively (Figure 3b). From the perspective of the increase in particle concentration, the growth rate of PM10 is faster than the growth rate of PM2.5, indicating that coarse particles (PM2.5–10) have a certain contribution to the growth of PM10. PM2.5/PM10 from Clean to Heavy pollution decreases first and then increases slightly, indicating PM2.5 contributed the most to PM10 at Clean levels and the least to PM10 at Severe levels.

Atmosphere 20212021,, 1212,, 990x FOR PEER REVIEW 6 of 13

40 PM10 PM2.5 PM2.5/PM10 0.495 35 PM 35 a 2.5 400 b PM10 383.21 30 )

3 0.490 10 25 300 μg/m (

20 /PM 20

18 0.485 2.5 226.53 264.98

15 14 200 PM 12

PM conc. 186.21

Number of samples of Number 10 0.480

100 98.49 109.84 5 3 1 0 48.80 0 0.475 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Polution Levels Pollution Level

Figure 3. The number of samples that PM2.5 and PM10 were at different pollution levels (a); the concentrations of PM2.5 and Figure 3. The number of samples that PM2.5 and PM10 were at different pollution levels (a); the concentrations of PM2.5 and PM10 at different pollution levels (b). PM10 at different pollution levels (b).

There isis aa correlationcorrelation between between the the concentration concentration of of PM PM and and related related climatic climatic conditions condi- (Tabletions (Table1). The 1). concentration The concentration of PM isof significantlyPM is significantly negatively negatively correlated correlated with wind with speed, wind speed,temperature, temperature, and ozone, and ozone, and significantly and significantl positivelyy positively correlated correlated with relative with relative humidity, hu- 2 2 2.5 10 midity,atmospheric atmospheric pressure, pressure, CO, NO CO,2, and NO SO, and2. The SO correlation. The correlation for PM for2.5 andPM PM and10 PMwith withtemperature, temperature, CO, CO, and and NO 2NOare2 are similar. similar. PM PM2.5 2.5has has a a stronger stronger correlation correlation withwith relative humidity, atmospheric pressure, ozone,ozone, andand SOSO22, whilewhile PMPM1010 has a stronger correlationcorrelation with wind wind speed. speed. This This shows shows that that the the influenc influencee of meteorological of meteorological conditions conditions on fine on par- fine particlesticles is greater. is greater.

Table 1. Correlation coefﬁcientcoefficient ofof PMPM withwith meteorologicalmeteorological parametersparameters andand gas-phasegas-phase species.species.

℃ −3 2 −3 3 −3 2 −3 Ws (m/s) RH (%) P (kPa) T ( ) CO (μg·m ) NOCO (μg·m ) NO2 O (μg·m O) 3 SO (μSOg·m2 ) Ws (m/s) RH (%) P (kPa) T (°C) −3 −3 −3 −3 PM2.5 −0.75 0.98 0.93 −0.99 0.99 (µg·m 0.98) (µg·m ) −0.99(µ g·m ) (0.96µg·m )

PMPM10 2.5 −0.98− 0.750.87 0.980.77 0.93−0.99 −0.970.99 0.99 0.98 0.98 −0.91 −0.990.93 0.96 PM10 −0.98 0.87 0.77 −0.99 0.97 0.98 −0.91 0.93 3.2. Heavy Metals Characteristics and the Potential Use 3.2. HeavyThe content Metals of Characteristics heavy metals and in thePM Potential at different Use pollution levels is shown in Figure 4. The contentThe content of heavy of heavy metals metals varies in greatly PM at at different different pollution levels of levels pollution is shown (average in Figure values4 . Theare shown content in of Tables heavy S1 metals and S2). varies At greatlyeach pollution at different level, levels the heavy of pollution metal (averagecontent in values PM10 was significantly higher than that of PM2.5. With the increase of pollution level, the total are shown in Tables S1 and S2). At each pollution level, the heavy metal content in PM10 amount of heavy metals in the particles gradually increased, but the degree of increase was significantly higher than that of PM2.5. With the increase of pollution level, the total graduallyamount of decreased. heavy metals In PM in2.5 the and particles PM10, the gradually order of increased, heavy metal but thecontent degree at each of increase pollution level was Zn > Cu> Pb > Cr > As > Ni > V > Cd. It is reported that Zn, Cu, Cr, Pb gradually decreased. In PM2.5 and PM10, the order of heavy metal content at each pollution mainlylevel was come Zn >from Cu> exhaust Pb > Cr emissions > As > Ni >of Vmotor > Cd. vehicles It is reported or the that wear Zn, of Cu, brake Cr, Pbpads mainly and tirescome [34–36], from exhaust and Pb, emissions As, Ni come of motor from vehicles coal and or petroleum the wear of combustion brake pads and[13,37]. tires Cd[34 is–36 re-], latedand Pb, to As,industrial Ni come processes from coal [13,38], and petroleum and V may combustion come from [13,37 mining]. Cd is or related soil fertilizer to industrial use processes[39]. Lead, [ 13zinc,,38 ],and and copper V may account come from for mininga relatively or soil high fertilizer proportion, use [39 which]. Lead, is related zinc, and to automobilecopper account exhaust. for a Urban relatively traffic high jams proportion, are becoming which more is related and more to automobile serious, leading exhaust. to frequentUrban traffic braking jams and are start-up becoming ofmore vehicles, and morewhich serious, aggravates leading the toemission frequent of braking heavy met- and alsstart-up in the ofexhaust vehicles, gas. which Beijing aggravates is the city thewith emission the largest of heavynumber metals of cars in in the China, exhaust and gas.car exhaustBeijing ishas the been city studied with the in largestBeijing numberas a factor of [40,41]. cars in Chengdu China, and is the car second-largest exhaust has been city instudied the country in Beijing for as car a factorownership, [40,41]. so Chengdu the contribution is the second-largest of car exhaust city into theChengdu’s country forat- mosphericcar ownership, particulate so the contributionmatter is also ofsignificant car exhaust [42]. to Chengdu’s atmospheric particulate matterThe is relative also significant percentage [42]. content of all heavy metals is almost constant at each pollution level. The content of heavy metals (HM) per particle at different pollution levels is shown in Figure S1 (Supplementary Information). It can be seen that the heavy metals per particle changes with the increase of particle concentration. The heavy metals content per particle in PM2.5 is always higher than that for PM10.

Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 13

At the Light-Medium level, the ratio of heavy metals in PM2.5 to heavy metals in PM10 is the largest, indicating that heavy metals are mainly concentrated in fine particles at this pollution level. At the Heavy levels, the content of heavy metals in PM10 and PM2.5 is the smallest, and the contribution of heavy metals in PM2.5 to that in PM10 is the smallest, indicating that heavy metals enriched in coarser particles may be discharged into the atmosphere at this pollution level. At the severe level, the heavy metal content increased sharply. For example, on 3rd January 2017, it was found that the wind speed was the lowest during the study period (0.5 m/s). The wind speed on the previous day (2 Janu- Atmosphere 2021, 12, 990ary) was relatively higher (0.9 m/s) and from the northwest. It is speculated that the 7 of 13 heavy metal content on January 3rd sharply increased due to the metal sources carried by the wind from the northwest of Chengdu.

1800 (b)100 (a) Zn )

3 1600 V 250 90

Pb ) 3 80

1400 Ni (%) 200

Cu 2.5 70 (ng/m 1200 Cr 2.5 Cd 60 1000 150 As 50 800 PM 2.5 40 Conc.(μg/m 600 100

2.5 30 400 50 20 PM

200 PM in Fraction

Species in PM in Species 10 0 0 0 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level 400 (d)100 (c)1600 Zn

) 90

3 V 350 1400 Pb ) 3 80

Ni (%) 1200 300 Cu 10 70 (ng/m Cr 10 1000 60 Cd 250 800 As 50 PM 10 200 40 600 Conc.(μg/m

150 10 30 400 20 PM

200 100 PM in Fraction

Species in PM in Species 10 0 50 0 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level

Figure 4. (a,b) are the content and percentage of heavy metals in PM2.5 at different pollution levels; Figure 4. (a,b) are the content and percentage of heavy metals in PM2.5 at different pollution levels; (c,d) are the content and (c,d) are the content and percentage of heavy metals in PM10 at different pollution levels. percentage of heavy metals in PM10 at different pollution levels.

3.3. Ions in PM The relative percentage content of all heavy metals is almost constant at each pollution 3.3.1. Ions Characteristicslevel. The content at Different of heavy Pollution metals (HM) Levels per particle at different pollution levels is shown The ionsin in Figure PM have S1 (Supplementary significant differences Information). at different It can be pollution seen that levels the heavy (Figure metals 5). per particle From Clean changesto the subsequent with the increase pollution of particlelevels, the concentration. ion content Thein the heavy particles metals increased content per particle in PM is always higher than that for PM . gradually (see Tables2.5 S3 and S4 for the average values).10 The order of ion content in PM2.5 At the Light-Medium level, the ratio of heavy metals in PM to heavy metals in PM was SO42− > NO3− > NH4+ > Cl− > K+ > Na+ > Ca2+ > F− > Mg2+. Among them,2.5 SO42−, NO3−, 10 is the largest, indicating that heavy metals are mainly concentrated in ﬁne particles at this and NH4+ accounted for 43.02%, 24.23% and 23.50% of the total ion content, respectively. pollution level. At the Heavy levels, the content of heavy metals in PM10 and PM2.5 is The order of ion content, in PM10 was SO42− > NO3− > NH4+ > Ca2+ > K+ > Cl− > Na+ > Mg2+ > the smallest, and the contribution of heavy metals in PM2.5 to that in PM10 is the smallest, F−, while SO42−, NO3−, NH4+ accounted for 34.56%, 27.43%, 19.18% of the total ion content, indicating that heavy metals enriched in coarser particles may be discharged into the respectively. The results showed that the secondary ions (SO42−, NO3−, NH4+) were the atmosphere at this pollution level. At the severe level, the heavy metal content increased main ions in Chengdu atmospheric particles. sharply. For example, on 3rd January 2017, it was found that the wind speed was the lowest However, unlike the heavy metal percentage distribution, the relative percentage of during the study period (0.5 m/s). The wind speed on the previous day (2 January) was each ion varies significantly at different pollution levels. In PM10, the percentage of Ca2+ relatively higher (0.9 m/s) and from the northwest. It is speculated that the heavy metal was significantly higher than that of PM2.5, indicating that Ca2+ is more likely to be en- content on January 3rd sharply increased due to the metal sources carried by the wind riched in coarse particles. Coarse particles often are dust, and CaCO3 is a major compo- from the northwest of Chengdu. nent of dust, which is the same as previous studies in Chengdu [13,27]. From Clean to the subsequent3.3. Ionspollution in PM levels, the relative percentages of NO3−, NH4+ gradually in- 3.3.1. Ions Characteristics at Different Pollution Levels The ions in PM have signiﬁcant differences at different pollution levels (Figure5). From Clean to the subsequent pollution levels, the ion content in the particles increased gradually (see Tables S3 and S4 for the average values). The order of ion content in 2− − + − + + 2+ − 2+ PM2.5 was SO4 > NO3 > NH4 > Cl > K > Na > Ca > F > Mg . Among them, 2− − + SO4 , NO3 , and NH4 accounted for 43.02%, 24.23% and 23.50% of the total ion content, 2− − + 2+ + respectively. The order of ion content, in PM10 was SO4 > NO3 > NH4 > Ca > K > − + 2+ − 2− − + Cl > Na > Mg > F , while SO4 , NO3 , NH4 accounted for 34.56%, 27.43%, 19.18% 2− of the total ion content, respectively. The results showed that the secondary ions (SO4 , − + NO3 , NH4 ) were the main ions in Chengdu atmospheric particles. Atmosphere 2021, 12, x FOR PEER REVIEW 8 of 13

creased, but the relative percentages of Ca2+, K+, Na+, and SO42− gradually decreased, and the relative percentages of Mg2+, Cl−, and F− were basically stable. This result demonstrated atmospheric polluting processes in Chengdu were mainly caused by particles with ions such as NO3− and NH4+ during the research period. It is reported that with the control of SO2 pollution in China, the sulfate content in PM has been significantly reduced [43,44]. At the same time, NO3− and SO42− will interact, and NOx will catalyze the conversion of SO2 to SO42− [45]. The oxidation of a large amount of SO2 will not only produce SO42− but also promote the formation of NO3− on water particles [46]. Therefore, SO2, as the precursor of sulfate, is oxidized, as NOx is converted to NO3−, and the conversion of SO2 should be slow and reduced. Figure S2 (Supplementary Information) shows the content of ions per particle at the different pollution levels. The content of ions in particles is obviously different at different pollution levels. From Clean to Heavy or Severe, the content of ions in PM2.5 and PM10 decreased gradually. The ion content per particle in PM2.5 is always greater than that for PM10 at each pollution level, but the ratio of ion content per particle in PM2.5 to Atmosphere 2021, 12, 990PM10 decreases gradually from Clean to Heavy. The results indicate that ions may 8 of 13 mainly enrich fine particles, but the proportion of ions in coarser particles gradually increases as the particle concentration increases.

(a) 300 (b) 100 Mg2+ ) 120 2+ 3 Ca 90

+ ) 105 K 250 3 + 80 NH4 (%) + 90 Na 2.5 70 (μg/m 2- 200 SO4 2.5 60 75 - NO3 - 50 60 Cl 150 F- 40

PM Conc.(μg/m 45 2.5 100

2.5 30 30 20

50 PM 15 PM in Fraction 10 Species in PM 0 0 0 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level (c)180 450 100 Mg2+ (d)

) 2+ 3 160 Ca 400 90 K+ ) 3 + 80

140 (%) NH4 350 + Na 2.5 70 (μg/m 120 SO 2- 300 10 4 60 - 100 NO3 Cl- 250 50 80 F- 200 40 PM Conc.(μg/m 60 10

10 30 150 40 20 PM

20 100 PM in Fraction 10 Species in PM 0 50 0 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level

Figure 5. (a,b) are the content and percentage of water-soluble ions in PM2.5 at different pollution Figure 5. (a,b) are the content and percentage of water-soluble ions in PM2.5 at different pollution levels, respectively; levels, respectively; (c,d) are the content and percentage of water-soluble ions in PM10 at different (c,d) are the content and percentage of water-soluble ions in PM at different pollution levels. pollution levels. 10 However, unlike the heavy metal percentage distribution, the relative percentage of 3.3.2. Characteristics of Sulfate-Nitrate-Ammonium (SNA) 2+ each ion varies significantly at different pollution levels. In PM10, the percentage of Ca 2− 2+ − + Figure 6was shows significantly the changes higher of parameters than that of related PM2.5, indicatingto SNA (SO that4 , Ca NO3is, moreNH4 ) likely at dif- to be enriched ferent pollutionin coarse levels. particles. From Clean Coarse to the particles subsequent often arepollution dust, andlevels, CaCO SOR3 isis aalways major component greater than ofNOR, dust, but which the degree is the samethe two as incr previouseases with studies the in pollution Chengdu levels [13,27 are]. differ- From Clean to the − + ent. NOR in subsequentPM2.5 increased pollution from levels,0.11 (Clean) the relative to 0.22 percentages (Severe), and ofNO SOR3 increased, NH4 gradually from increased, 2+ + + 2− 0.43 (Clean) butto 0.61 the (Severe). relative percentagesNOR in PM of10 increased Ca ,K , from Na , 0.16 and SO(Clean)4 gradually to 0.29 (Heavy), decreased, and the and the SOR relativeincreased percentages from 0.52 of(Clean) Mg2+ ,to Cl 0.60−, and (Heavy). F− were Early basically studies stable. have Thisshown result that, demonstrated when SOR isatmospheric greater than polluting0.1, there processesis a photochemical in Chengdu reaction were of mainly SO2 in caused the atmosphere by particles with ions − + [47]. This resultsuch indicates as NO3 thatand SO NH2 is4 moreduring susceptible the research to se period.condary conversion than NO2. The sulfate and nitrateIt is reported in this study that with were the largely control formed of SO2 throughpollution secondary in China, reactions. the sulfate content in PM − 2− has been significantly reduced [43,44]. At the same time, NO3 and SO4 will interact, 2− and NOx will catalyze the conversion of SO2 to SO4 [45]. The oxidation of a large amount 2− − of SO2 will not only produce SO4 but also promote the formation of NO3 on water

particles [46]. Therefore, SO2, as the precursor of sulfate, is oxidized, as NOx is converted − to NO3 , and the conversion of SO2 should be slow and reduced. Figure S2 (Supplementary Information) shows the content of ions per particle at the different pollution levels. The content of ions in particles is obviously different at different pollution levels. From Clean to Heavy or Severe, the content of ions in PM2.5 and PM10 decreased gradually. The ion content per particle in PM2.5 is always greater than that for PM10 at each pollution level, but the ratio of ion content per particle in PM2.5 to PM10 decreases gradually from Clean to Heavy. The results indicate that ions may mainly enrich ﬁne particles, but the proportion of ions in coarser particles gradually increases as the particle concentration increases.

3.3.2. Characteristics of Sulfate-Nitrate-Ammonium (SNA) 2− − + Figure6 shows the changes of parameters related to SNA (SO 4 , NO3 , NH4 ) at different pollution levels. From Clean to the subsequent pollution levels, SOR is always greater than NOR, but the degree the two increases with the pollution levels are different. NOR in PM2.5 increased from 0.11 (Clean) to 0.22 (Severe), and SOR increased from 0.43 Atmosphere 2021, 12, x FOR PEER REVIEW 9 of 13

The formation of SNA is closely related to meteorological conditions (relative humidity and temperature) [48–50]. When the relative humidity is low, the main reaction is a gas-phase reaction, and when the relative humidity is high, the main reaction is a heterogeneous reaction on particles [51,52]. According to Pandis and Seinfeld [53], the liquid-phase oxidation of SO2 may be an important way to generate SO42−, while NO3− is mainly generated by gas-phase oxidation of NOx. So, the effect of humidity on SO42− is more significant. With the increase in pollution levels, the relative humidity increased from 76% to 83%. As the air approached saturation, the particle concentration increased, and the temperature decreased (PM2.5: 21.7–11.8 °C; PM10: 19.8–17.8 °C). It can be seen that nitrate and sulfate in this study tended to form through heterogeneous reactions with the change of pollution level. Sulfate and nitrate are important hygroscopic ions, which can promote the hygroscopic growth of atmospheric particles and have a great impact on visibility and temperature [3,54,55]. The NO3−/SO42− ratio has large differences at different pollution levels, which gradually increase with the increase of pollution lev- Atmosphere 2021, 12, 990els, and the aerosol ions will be easier to absorb moisture [56]. NO3− represents mobile 9 of 13 source, and SO42− represents fixed source. The NO3−/SO42− ratio is often used to indicate whether particulate matter is dominated by mobile source or fixed source. The NO3−/SO42− ratio increased from 0.52 (Clean) to 0.95 (Severe) in PM2.5, and increased from (Clean) to 0.61 (Severe). NOR in PM increased from 0.16 (Clean) to 0.29 (Heavy), and the 0.57 (Clean) to 1.20 (Heavy) in PM10. The results10 show that the contribution of pollution caused by mobileSOR increasedsources to from the increase 0.52 (Clean) of PM to is 0.60 gradually (Heavy). increasing. Early studies In the have fine shownparti- that, when cles, it is a mainlySOR is fixed greater pollution than 0.1, source there at is different a photochemical pollution reactionlevels. While of SO in2 in the the coarse atmosphere [47]. particles, it Thisis a mainly result indicates fixed pollution that SO 2sourceis more at susceptible Clean and toLight-Medium secondary conversion levels, and than NO2. The mainly mobilesulfate sources and at nitrate the Heavy. in this study were largely formed through secondary reactions.

(a)300 300 1.0 22 (b) PM NOR PM RH T 86 2.5 0.6 2.5 - 2- 250 SOR NO3 /SO4 20 250 0.9 )

3 84 ) 0.5 200 3 18 200 0.8 2- 82 4 0.4 (μg/m 150 150 0.7 /SO -

16 (μg/m 3 2.5 T(°C)

80 RH(%) 2.5 0.3 100 14 100 0.6 NO NOR or SOR PM

78 PM 50 12 50 0.2 0.5 76 0 10 0 0.1 0.4 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level 21 (d) 0.7 PM10 RH T PM NOR 400 400 10 (c) 83 SOR NO -/SO 2- 1.2 3 4 0.6 ) 3

20 ) 82 3 300 300 0.5 2- 1.0 4 (μg/m /SO 81 19 0.4 - (μg/m 3 10

200 T(°C) 200 RH(%) 10 0.8 0.3 NO

PM 80 NOR or SOR 100 18 PM 100 0.2 79 0.6

0 17 0 0.1 Clean Light-Medium Heavy Severe Clean Light-Medium Heavy Severe Pollution Level Pollution Level

Figure 6. Changes of (a) RH, T, PM2.5; (b)SOR, NOR,− NO2−3−/SO42−, and PM2.5; (c) RH, T, PM10; (d) Figure 6. Changes of (a) RH, T, PM2.5;(b)SOR, NOR, NO3 /SO4 , and PM2.5;(c) RH, T, PM10;(d) SOR, NOR, SOR, NOR, NO3−/SO42− and PM10 at different pollution levels (RH: relative humidity; SOR/NOR: NO −/SO 2− and PM at different pollution levels (RH: relative humidity; SOR/NOR: sulfur/nitrogen oxidation rate; T: 3 4 sulfur/nitrogen10 oxidation rate; T: temperature). temperature).

Figure 7 showsThe the formation correlation of SNA between is closely SNA related and toPM meteorological at different pollution conditions levels. (relative humidity The relative andcontribution temperature) of SNA [48 to–50 the]. Whenincrease the of relative PM2.5 and humidity PM10 at is each low, pollution the main level reaction is a gas- 2− is different. phaseAt Clean reaction, levels, and the when contribution the relative of humiditySO4 to the is high, increase the main of PM reaction2.5 is 11.5%, is a heterogeneous which is muchreaction larger onthan particles the contribution [51,52]. According of other ions, to Pandisand the andrelative Seinfeld contribution [53], the of liquid-phase SNA to PM10 is very small. At Light-Medium levels, the contributions of2− SO42−, NH4+, − oxidation of SO2 may be an important way to generate SO4 , while NO3 is mainly NO3− to PM differ (PM2.5: 7.18%, 10.9%, 9.93%; PM10: 10.4%, 14.0%, 14.0%). Sulfates con- 2− generated by gas-phase oxidation of NOx. So, the effect of humidity on SO4 is more tributed moresignificant. to PM2.5 at With the Clean the increase levels because in pollution of the levels, trend the to relativeform ammonium humidity sul- increased from fate during the76% formation to 83%. As of theSNA air in approached the inhomogeneous saturation, phase, the particle which concentrationimpeded the for- increased, and ◦ ◦ mation of ammoniumthe temperature nitrate. decreased This result (PM is2.5 reflected: 21.7–11.8 in moreC; PM contribution10: 19.8–17.8 of C).nitrate It can to be seen that nitrate and sulfate in this study tended to form through heterogeneous reactions with the change of pollution level. Sulfate and nitrate are important hygroscopic ions, which can promote the hygroscopic growth of atmospheric particles and have a great impact − 2− on visibility and temperature [3,54,55]. The NO3 /SO4 ratio has large differences at different pollution levels, which gradually increase with the increase of pollution levels, and − the aerosol ions will be easier to absorb moisture [56]. NO3 represents mobile source, and 2− − 2− SO4 represents fixed source. The NO3 /SO4 ratio is often used to indicate whether − 2− particulate matter is dominated by mobile source or fixed source. The NO3 /SO4 ratio increased from 0.52 (Clean) to 0.95 (Severe) in PM2.5, and increased from 0.57 (Clean) to 1.20 (Heavy) in PM10. The results show that the contribution of pollution caused by mobile sources to the increase of PM is gradually increasing. In the fine particles, it is a mainly fixed pollution source at different pollution levels. While in the coarse particles, it is a mainly fixed pollution source at Clean and Light-Medium levels, and mainly mobile sources at the Heavy. Figure7 shows the correlation between SNA and PM at different pollution levels. The relative contribution of SNA to the increase of PM2.5 and PM10 at each pollution level 2− is different. At Clean levels, the contribution of SO4 to the increase of PM2.5 is 11.5%, Atmosphere 2021, 12, 990 10 of 13

which is much larger than the contribution of other ions, and the relative contribution of 2− + SNA to PM10 is very small. At Light-Medium levels, the contributions of SO4 , NH4 , − NO3 to PM differ (PM2.5: 7.18%, 10.9%, 9.93%; PM10: 10.4%, 14.0%, 14.0%). Sulfates Atmosphere 2021, 12, x FOR PEER REVIEW 10 of 13 contributed more to PM2.5 at the Clean levels because of the trend to form ammonium sulfate during the formation of SNA in the inhomogeneous phase, which impeded the formation of ammonium nitrate. This result is reflected in more contribution of nitrate PM at the Light-Mediumto PM at the levels, Light-Medium compared levels, to the compared Clean levels. to theAt CleanHeavy levels.levels, Atthe Heavycon- levels, the 2− + − tributions of contributionsSO42−, NH4+, ofNO SO3−4 to ,PM NH 4are, NO significantly3 to PM are different significantly (PM2.5 different: 24.0%, (PM 3.76%,2.5: 24.0%, 3.76%, 11.7%; PM10: 11.7%;24.0%, PM40.8%,10: 24.0%, 59.7%). 40.8%, This result 59.7%). shows This resultsulfate shows is more sulfate likely is to more be enriched likely to be enriched in fine particlesin fineat each particles level. atNitrate each level. and ammonium Nitrate and salts ammonium are easily salts concentrated are easily concentrated in fine in fine particles at Cleanparticles and atLight-medium Clean and Light-medium pollution levels, pollution while levels,they are while easily they concentrated are easily concentrated − + in coarse particlesin coarse at Heavy particles pollution at Heavy levels pollution (NO3− levels: 59.7%; (NO NH3 4:+: 59.7%; 40.8%). NH The4 :secondary 40.8%). The secondary conversion ofconversion SO2 is mainly of SO liquid-phase2 is mainly liquid-phasereaction, which reaction, is closely which related is closely to relative related to relative humidity. Thehumidity. relative Thehumidity relative of humiditythe Heavy of level the Heavy is the levellargest is the(83%), largest so the (83%), contribu- so the contribution of sulfate to particles is also the largest at this level. The secondary reaction of NO is tion of sulfate to particles is also the largest at this level. The secondary reaction of NO2 2 is a mainly gas-phasea mainly gas-phasereaction. The reaction. atmospheric The atmospheric temperature temperature is lower than is lower other than levels other levels at Heavy levels, which is not conducive to the secondary generation of NO . However, it at Heavy levels, which is not conducive to the secondary generation of NO2. However, 2it has been reported that it is conducive to the stable existence of NH4NO3. Therefore, the has been reported that it is conducive− to the stable existence of NH4NO3. Therefore, the contribution− of NO3 to PM is relatively large at Heavy levels [57]. contribution of NO3 to PM is relatively large at Heavy levels [57].

70 2- SO 2-=-0.00102*PM +20.72 (C) 70 SO4 =-0.115*PM2.5+20.86（C） C L H 4 10 2- 2- SO =0.104*PM +8.91 (L)

) 60

) 4 10 SO4 =0.0718*PM2.5+19.48（L） 3 3 60 2- 2- SO4 =0.240*PM10-50.32 (H) SO4 =0.240*PM2.5-7.59（H） 50 50 2 -5 2 C:SN =14 R =7*10 C:SN =18 R =0.21 2 2 L:SN =35 R =0.28 40 L:SN =20 R =0.022 40 2 2 (μg/m H:SN=3 R =0.61 (μg/m H:SN=12 R =0.11 2-

2- 30 4 4 30 20 SO SO 20 10 （a）（b） 0 10 0 30 60 90 120 150 180 210 240 270 50 100 150 200 250 300 350 400 40 + + NH =0.00322*PM +6.78 (C) 70 NH4 =-0.00724*PM10+7.16 (C) 35 4 2.5 + + )

) NH =0.140*PM -12.55 (L) NH4 =0.109*PM2.5+1.55 (L) 4 10 3

3 60 30 + NH +=0.408*PM -123 (H) NH4 =-0.0376*PM2.5+30.19 (H) 4 10 50 2 25 C:SN =18 R2=0.0001 C:SN =14 R =0.0056 2 L:SN =20 R2=0.32 40 L:SN =35 R =0.27 20 2

2 (μg/m (μg/m H:SN=12 R =0.022 H:SN=3 R =0.97

+ 30 + 4 4 15 20 10 NH NH 10 5 （c） 0 （d） 0 0 30 60 90 120 150 180 210 240 270 50 100 150 200 250 300 350 400 40 70 NO -=0.000927*PM +7.21 (C) - 3 2.5 NO3 =0.0114*PM10+10.66 (C) 35 - NO =0.0993*PM +2.95 (L) 60 - 3 2.5 NO3 =0.140*PM10-5.44 (L) ) - ) 3 30 NO =0.117*PM +1.63 (H) 3 - 3 2.5 50 NO3 =0.597*PM10-178 (H) 2 -5 25 C:SN =18 R =3*10 C:SN =14 R2=0.0069 2 40 L:SN =20 R =0.26 L:SN =35 R2=0.33 2 20 H:SN=12 R =0.077 2 (μg/m (μg/m H:SN=3 R =0.99 - - 30 3 15 3 10 20 NO NO 5 （e） 10 （f） 0 0 0 30 60 90 120 150 180 210 240 270 50 100 150 200 250 300 350 400 3 3 PM2.5(μg/m ) PM10（μg/m ）

Figure 7. Linear regression2− of SO42− with (a) PM2.5 and (b+) PM10, NH4+ with (c) PM2.5 and (d−) PM10, Figure 7. Linear regression of SO4 with (a) PM2.5 and (b) PM10, NH4 with (c) PM2.5 and (d) PM10, NO3 with (e) PM2.5 NO3− with (e) PM2.5 and (f) PM10 at different pollution levels (p < 0.05; C: Clean; L: Light-Medium; and (f) PM at different pollution levels (p < 0.05; C: Clean; L: Light-Medium; H: Heavy; SN means sample number). 10 H: Heavy; SN means sample number).

4. Conclusions The present study analyzed the distribution and changes of heavy metals and water-soluble ions in PM2.5 and PM10 during the haze periods from March 2016 to January 2017 in Chengdu, China at different pollution levels. It revealed the concentration of PM was closely related to meteorological conditions and the effect on fine particles is more significant. Heavy metals were more easily enriched in fine particles at different pollution levels, and the relative percentage content was basically stable. However, the relative percentage of water-soluble ions varied with the pollution level, and the relative per-

Atmosphere 2021, 12, 990 11 of 13

4. Conclusions The present study analyzed the distribution and changes of heavy metals and water- soluble ions in PM2.5 and PM10 during the haze periods from March 2016 to January 2017 in Chengdu, China at different pollution levels. It revealed the concentration of PM was closely related to meteorological conditions and the effect on fine particles is more significant. Heavy metals were more easily enriched in fine particles at different pollution levels, and the relative percentage content was basically stable. However, the relative percentage of water-soluble ions varied with the pollution level, and the relative percentage − + of NO3 and NH4 increased gradually. The water-soluble ions in the particles during 2− − + the study were mainly SO4 , NO3 and NH4 and mainly from secondary reactions. Furthermore, the contribution of SNA to the increase of PM was variable at different 2− + − pollution levels. It was mainly SO4 in PM at Clean levels, and mainly NH4 and NO3 2− + at Light-Medium levels. At Heavy levels, it is mainly SO4 in PM2.5, and mainly NH4 − and NO3 in PM10. Mobile sources are contributing more to the occurrence of haze in Chengdu, which should have more attention paid to it. The results of this research not only enrich the air pollution research in Chengdu, China, but also provide a reference for the urban air pollution research with the same background. The deficiency lies in the lack amount of PM10 samples under Heavy and Severe pollution levels. The next step will be to study the source analysis of PM quantitatively.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/atmos12080990/s1, Figure S1: Changes of heavy metals per particle at different pollution levels, Figure S2. Change in ions per particle at different pollution levels, Table S1: Average mass 3 concentrations of heavy metals in PM2.5 at different pollution levels (ng/m ), Table S2: Average 3 mass concentrations of heavy metals in PM10 at different pollution levels (ng/m ), Table S3: Average 3 mass concentration of ions in PM2.5 at different pollution levels (µg/m ), Table S4: Average mass 3 concentration of ions in PM10 at different pollution levels (µg/m ). Author Contributions: Y.H.: Conceptualization, Resources, Funding acquisition; L.W.: Data curation, Writing—Original draft preparation; X.C.: Methodology, Validation; J.W.: Methodology, Supervi- sion; T.L. and M.H.: Investigation; H.S. and M.Z.: Visualization; S.S.H.: Writing—Reviewing and Editing; S.N.: Project administration. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 41977289 and the Major Science and Technology Projects of Sichuan Province, grant number 21CXTD0015. Institutional Review Board Statement: Not applicable. Acknowledgments: This study was supported by the National Natural Scientific Foundation of China (41977289), and the Science and Technology Department of Sichuan Province (2021JDTD0013). Conflicts of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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