Wet deposition of polycyclic aromatic hydrocarbons in a remote area of Central South China from 2014 to 2017

In recent years, there has been a notable increase in the consumption of fossil energy, leading to a significant rise in environmental pollution, particularly in China due to its rapid development. This has resulted in the frequent occurrence of large‐scale fog and haze weather, highlighting the urgent need for environmental protection measures. To gain insights into the atmospheric conditions in China, an analysis was conducted on the wet deposition of polycyclic aromatic hydrocarbons (PAHs) in a remote region of Central South China from 2014 to 2017. The study revealed that the average concentrations and peak values of Ʃ16PAHs in 2014 and 2015 were considerably higher than those observed in 2016 and 2017. Furthermore, it was found that five‐ring PAH species were the predominant components during 2014 and 2015, indicating a shift in the main sources of PAHs. The peaks of Ʃ16PAHs were predominantly detected in samples collected during light rain in the winter, specifically on days without heavy rainfall. This can be attributed to the absence of heavy rain, which would otherwise reduce the concentration of air pollutants. Consequently, contaminants accumulated in the air are easily enriched in rainwater. The concentrations of Ʃ15Alkyl‐PAHs also exhibited a significant correlation with the number of rainfall days. Notably, a much higher annual average concentration of Ʃ15Alkyl‐PAHs was observed in 2017, which experienced fewer rainfall days. Coal combustion, petroleum sources, and vehicular emissions accounted for 58%, 12%, and 30% of the PAHs in the air, respectively. Despite improvements in air quality in China since 2016, it is crucial to address the elevated concentrations of PAHs in the atmosphere, particularly under adverse meteorological conditions characterized by reduced rainfall.

Air pollution in China in recent years has been one of the most serious events in the world.In January 2013, haze enveloped 30 provinces four times.In Beijing, there were only 5 days without haze.Fewer than 1% of cities in China meet the World Health Organization's recommended air quality standards, and seven of the world's 10 most polluted cities were in China.Subsequently, relevant regulations and restrictions for pollutant emissions have been implemented to improve the atmospheric environmental quality.Therefore, the number of haze days has been significantly reduced, and air quality has been significantly improved in the past several years.Feng et al. (2018) found that PAHs were the main pollutants in haze.Hence, wet deposition of PAHs from longterm monitoring can intrinsically reflect the variation in atmospheric contamination status.
Previous studies on PAHs in wet deposition were mainly carried out in metropolises or during severe pollution periods (Chen & Zhu, 2010;Finardi et al., 2017;Gaga et al., 2009;Guo et al., 2014;Liu, 2017;Yan et al., 2012;Ye et al., 2010), and those results showed significant influences from human activities.Study considering the long-term monitoring of wet deposition PAHs in remote rural areas was sparse, but they were of interest due to the lack of industries and sparse populations therein.
In this study, a remote rural area of Zhangjiajie City, Hunan Province-a world-famous scenic city in Central South China that has plenty of rainfall-was selected as the study site.Data were considered from 2014 to 2017 when regular levels of intense haze occurred in China.The wet deposition of PAHs, including PAHs and alkylated PAHs, was considered as indicators of long-term variations in atmospheric pollution in China.Alkylated PAHs were also considered because some alkylated PAHs are more toxic than PAHs and can be converted into more toxic nitro-PAHs under certain conditions (Walgraeve et al., 2010;Wei et al., 2015).

| Sampling sites and sample collection
The ability to remove atmospheric PAHs by dry/wet deposition is mostly dependent on regional climate conditions.In southern China, the influence of wet deposition is especially important due to abundant rainfall (Guo et al., 2014).Dry deposition predominates in northern China due to the lack of precipitation (Wang et al., 2016;Zhang et al., 2007).
The samples were collected in a remote village (111 10 0 42.1 00 E, 29 14 0 36.36 00N) with substantial forest coverage: Zhangjiajie City in Hunan Province, Central South China (Figure 1).The annual average temperature is $16.8 C, and the annual rainfall is $1400 mm.From the overall topography of China, it is in the transition zone between plain and mountain.According to the air quality monitoring results of 367 cities nationwide in 2015 (http://www.pm25.com), the air quality in this region was relatively good due to its being far away from highly polluted areas of the North China Plain.The wet deposition pollutants in the remote village can reflect the general Chinese air pollution situation.
Rainwater and snow were collected with a stainlesssteel basin, 1 m in diameter, during rain or snow.After the rain or snow ends, the collected rain or snow was put into sample bottles and then frozen for storage.Before sampling, the basin was rinsed with pure water and then with rainwater three times.To avoid interference from rainwater splashing off the ground, the basin was put on a dais, which was 2 m above the ground.The collected samples were stored at À18 C in PFA bottles when the rain or snow stopped.

| Sample preparation and gas chromatography-mass spectrometry analysis
The rainwater samples were extracted by dichloromethane and n-hexane twice, respectively.The extracts were combined and rotary-evaporated to approximately 2 mL and then cleaned and fractionated on a multilayer silica-alumina composite column, which consisted of 5% deactivated silica gel, 5% deactivated aluminum oxide, and anhydrous sodium sulfate from the bottom to the top.The column was pre-cleaned with n-hexane, and then the extracts were eluted with dichloromethane/ hexane (3:7, v/v).The eluents were collected and concentrated to near dryness under a gentle N 2 stream and then reconstituted in 0.2 mL isooctane.
The analysis of PAHs was performed on an Agilent Technologies 6890 gas chromatographer coupled with an Agilent Technologies 5975 mass spectrometer, and a DB-5MS (30 m, i.d.0.25 mm, film thickness 0.25 μm) capillary column, using selective ion monitoring mode.The operating conditions of the gas chromatographer-mass spectrometer were as described by Duan et al. (2015).
Sixteen PAHs were analyzed in terms of individual and total concentrations.The 16 PAHs measured were naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Acp), fluorene (Flu), phenanthrene (Phe), anthracene Quality assurance and quality control procedures were outlined in our previous study (Duan et al., 2015).The standard reference materials recoveries were 82.1%-112.8% for spiked blanks.Results reported in this work were not corrected for recoveries.The relative standard deviation for parallel samples (n = 6) was less than 15%.
The occurrence of PAHs in environmental matrices was usually at the parts per trillion (ng/L) level (Rianawati & Balasubramanian, 2009).The concentrations in this study were significantly lower than those in other areas, especially in urban areas (Table 1).The cumulative rainfall in 2014 was only 475.4 mm, and the cumulative rainfall in the other 3 years all exceeded 1000 mm (Figure 2).The rainfall in 2014 was significantly lower than that in other years, but the content of PAHs was not significantly higher than that in other years.
The annual average concentrations of Ʃ 16 PAHs as well as the peak values in 2014 and 2015 were significantly higher than in 2016 and 2017.Generally, the variation tendency was consistent with that of PM2.5 monitored in the nearest city (Zhangjiajie).The average annual concentrations of PM2.5 decreased from 2014 to 2017 (59.32, 48.65, 43.46, and 38.43 μg/m 3 in 2014, 2015,  2016, and 2017, respectively).The air pollution days decreased significantly during the 4 years (101, 63, 54, and 33 days in 2014, 2015, 2016, and 2017, respectively).Because a series of pollution control policies and measures have been strictly implemented in China, such as energy structure adjustment, industrial structure adjustment, and mobile source emission control, SO 2 , NOx, and dust emissions were, respectively, reduced by 55.7%, 39.4%, and 54.3% over the 4 years considered in mainland China, based on data from the National Bureau of Statistics of the People's Republic of China (www.stats.gov.cn) .This can explain why the PAH pollution was gradually alleviated from 2014 to 2017.Noticeable peaks in Ʃ 16 PAHs were observed on February 24, 2014, December 12, 2015, and January 12, 2017, with concentrations of 0.91, 1.16, and 0.23 ng/L, respectively.The peaks were generally found in the winter with low temperatures.Similar results were also obtained by other studies.Gaga et al. (2009) collected 62 samples of wet precipitation at an urban site in Turkey between December 2000 and June 2002.They attributed the higher 14-PAH concentrations in winter to coal combustion for space heating, the lower solar flux, the lower mixing height, and higher traffic intensity in winter (based on the local migration of residents to coastal areas for summer vacation) (Gaga et al., 2009).Based on a study of six locations in Rhode Island for 3 years, the highest 16-PAH deposition rates were observed in winter due to the use of fossil fuels and wood combustion for heating (Schifman & Boving, 2015).Finardi et al. (2017) also found biomass burning for heating purposes contributed to high concentrations of PAHs in the winter between November 2011 and July 2012 in Rome.The studies attributed the higher PAH levels in winter to the increased coal/gas combustion for domestic heating (Golomb et al., 1997;Leister & Baker, 1994;Park et al., 2001), decreased photochemical activity (Birgül et al., 2011), and lower mixing height ( Škrdlíkov a et al., 2011;Park et al., 2001).As there was no extra house heating in South China in the winter, the high levels of PAHs in the remote regions in winter may be transported by the East Asian winter monsoon from North China (Pei et al., 2018).
All peaks of Ʃ 16 PAHs were observed on days with light rain (February 24, 2014, December 12, 2015, and January 12, 2017), and there was no heavy rain in the month preceding those days.It was also demonstrated that precipitation-driven loss of PAHs was lower in the warm period than in the cold one (Siudek, 2022a).According to previous studies (Correll et al., 1999;Han et al., 2006;Hwang & Foster, 2006;Lim et al., 2007), intense precipitation events can swiftly remove atmospheric and surface particulate matter, including dust, and transport them into rivers.Concurrently, the pollutants adsorbed onto these particles were also carried away.Consequently, heavy rain can effectively eliminate PAHs from the Earth's surface by preventing their volatilization from the soil into the air.In this particular investigation, light rainfall events occurred during the winters of 2014, 2015, and 2017, resulting in noticeable peaks of PAHs.However, in the winter of 2016, only moderate rain was observed, and no significant peak in the total concentration of 16 PAHs (Ʃ16PAHs) was detected.This lack of a peak may be attributed to the absence of heavy rain over an extended period, which would have otherwise reduced the concentration of air pollutants, allowing them to accumulate and become enriched in the rainwater.This phenomenon has not received sufficient attention thus far.Future studies on wet deposition pollutants should focus on investigating the relationship between pollutant concentration and rainfall intensity.
The concentrations of Ʃ 15 Alkyl-PAHs were relatively higher than the concentrations of Ʃ 16 PAHs.The concentrations of Ʃ 15 Alkyl-PAHs in 2016 were lower than in 2014 and 2015.However, there was a relatively higher value of Ʃ 15 Alkyl-PAHs in 2017.The concentration peaks of Ʃ 15 Alkyl-PAHs were observed on November 24, 2014, July 16, 2015, and June 10, 2017 at 1.02, 0.81, and 2.39 ng/L, respectively; there was at least one heavy rain event in the month preceding those days, which also had relatively higher average temperatures (11, 27, and 24.5 C).No significant relationship exists between the concentration peaks of Ʃ 15 Alkyl-PAHs and the concentrations of PM2.5.The PAHs with alkyl chains were representative of petroleum sources (Latimer & Zheng, 2003;Sporstol et al., 1983).Therefore, the difference between the concentrations of Ʃ 15 Alkyl-PAHs and Ʃ 16 PAHs may mainly be due to the differences in the contributions from different sources.

| PAHs species composition characteristics
PAHs in this study were dominated by three-ring and four-ring species (Figure 3).According to previous studies, the distribution of PAHs in rainwater was predominantly in the dissolved phase (Park et al., 2001), and a high deposition value of low molecular weight PAH species was detected (Birgül et al., 2011;Wang et al., 2015;Yan et al., 2012 because low molecular weight PAH species have higher volatility and lower hydrophobicity compared with high molecular weight PAH species.Low molecular weight species relatively easily spread into the atmosphere in the form of vapor, whereas high molecular weight species tend to be stored in the soil and/or deposited in marine sediments (Li & Duan, 2015).
On peak days of Ʃ 16 PAHs (i.e., February 24, 2014, December 12, 2015, and January 12, 2017), the concentrations of four-ring PAH species were higher than those of the three-ring PAH species.However, the concentrations of the three-ring PAH species were greater than those of the fourring PAH species for the remainder of the time.On June 10, 2017, two-ring PAH was the most abundant species.Five-ring PAH species were one of the major components during 2014 and 2015, which are periods with more serious air pollution days than 2016 and 2017 (Figure 3).The decrease of Ʃ 16 PAHs in the last 2 years of the study may be consistent with lower pollution levels across the country.

| Possible influence factors
In 2017, there was a high annual average concentration of Ʃ 15 Alkyl-PAHs with significantly fewer rainfall days than the other years, especially during the rainy months of May-August (Table S1).
Ʃ 15 Alkyl-PAHs were widely detected from the emissions of coal burning (Hai et al., 2013), vehicles (Lian et al., 2008), and bituminous pavement (Possebon et al., 2018).The volatilized Ʃ 15 Alkyl-PAHs, from the bituminous pavement or other petroleum-related sources, accumulated in the air due to the lack of sufficient rainfall to clear the air.This assumption was confirmed by the distribution of the peaks of Ʃ 15 Alkyl-PAHs, which occurred after at least one heavy rain event in the previous month and on days with a higher-than-average air temperature.These heavy rain events within the previous month allow more alkyl-PAHs in the asphalt pavement to dissolve, and high air temperatures can also accelerate the volatilization of alkylated PAHs (Mahler et al., 2014).Alkylated PAHs also have acute toxicity to human health, so the utilization of bitumen and other processes that contain alkylated PAHs should be paid more attention (Witter, 2019).
According to the obtained results, the concentration peaks of Ʃ 16 PAHs appear in the winter, and the variation in Ʃ 16 PAHs showed a significant relationship with PM2.5.However, the concentrations of Ʃ 15 Alkyl-PAHs showed a significant relationship with rainfall days, and the peaks appeared in seasons with relatively higher average air temperatures.This was attributed to different sources and physicochemical properties.Alkyl-PAHs content was very high in petroleum products, and high temperature was more conducive to its volatilization.There were also two relatively higher values of Ʃ 16 PAHs on the days with relatively higher average air temperatures (0.53 and 0.296 ng/L on September 13, 2014 and June 10, 2017, respectively).The higher value of Ʃ 16 PAHs on September 13, 2014, may mainly be due to the serious air pollution in 2014, the year that had the most polluted days during the study period.Three days before the sampling day, an air pollution event had happened with a higher concentration of PM2.5.Compared with the pollution status of 2014 to 2016, the overall atmospheric pollution levels in 2017 had actually fallen.The reason for the higher concentrations of Ʃ 16 PAHs appearing on June 10, 2017, may be the same as the reason for the peak Ʃ 15 Alkyl-PAHs appearing in 2017: fewer rainy days and a higher temperature on the date considered.This conclusion was confirmed by the PAH species distribution during the period of 2014-2017 (Figure 3).The concentrations of two-ring PAH species were significantly higher on June 10, 2017; this was different from the PAH species distribution on the other sampling dates.Due to higher temperatures and fewer rainy days, more PAH molecules volatilized to the air and were unable to be removed by rainwater in a short period of time.Moreover, the low molecular weight species were easier to volatilize compared with the high molecular weight species.Therefore, the concentrations of two-ring PAH species were the highest.
The most heavily polluted areas in China were mainly in the eastern plains (including the Songliao Plain, North China Plain, and Middle-Lower Yangtze Plain) and the western plateau (mainly Sinkiang) (Chen et al., 2018;Kebin et al., 2011).The air pollution in the eastern plain seriously threatens nearly half of the Chinese population, and heavy air pollution continues to occur from October to March (Figure 4).
The main reasons for the formation of large-scale Chinese haze were suggested to be as follows.The first was the increase in pollutant emissions in the winter from coal and biomass burning for residential heating (You & Xu, 2010).The second was the chemical formation and transformation of PM2.5.The active gaseous substances (including volatile organic compounds, sulfur dioxide, nitrogen oxides, and ammonia) that were discharged into the atmosphere can be transformed into secondary organic/inorganic aerosols by chemical and physical atmospheric processes.These emissions contribute significantly to the haze (Mor an et al., 2016).Agricultural ammonia emissions contribute up to 30% of the PM2.5 in China (Zhao et al., 2017).In addition, the topography of the Qinghai-Tibet Plateau, El Niño, and other factors affect the occurrence of haze events by changing the climatic backgrounds, such as the East Asian Monsoon and mid-high-level westerly circulation (An et al., 2019).
Although the study site was located away from the highly populated areas of China (Figure 4), it could be affected by air masses from different parts of China.Therefore, long-term atmospheric pollution monitoring in the study area can demonstrate the general air pollution situation in China.

| Source identification
Partial least-squares regression (PLSR) was performed herein to assign the sources of the PAHs.The R 2 of the model reached 0.9865 when three variables were extracted, indicating that wet deposition PAHs in this region were primarily from these three sources (Table 2).
PC1 was related to all PAH species, and the loadings of alkylated PAH species were significantly greater than those of the other species.As PC1 represents typical characteristics of coal combustion emissions, the alkylated PAHs in the atmosphere were mainly from the direct combustion of lump coal, which has low combustion efficiency; in addition, this usage of coal is dominant in China.Although the use of cleaner natural gas has increased in recent years, the relatively cheap coal-based energy structure in China may last for many years to come due to significant coal reserves (1000 billion tons) and limited natural gas reserves (Liu et al., 2008;Zhong & Yang, 2000).As shown in Table S2, there were high scores for sampling days of February 6, 2014, February 17, 2014, February 24, 2014, September 13, 2014, and December 12, 2015on PC1. Therefore, on February 24, 2014, December 12, 2015, and September 13, 2014, the higher levels of Ʃ 16 PAHs were likely due to air pollution caused by coal combustion.From the distribution of discrete points that have high positive loadings of PC1, it can be determined that the discrete points occurred mainly in winter.Although no coal was used for heating in the winter in Hunan Province, the PAHs may be brought by wind from northern China.Decreased photochemical activity and lower mixing heights were favorable for the preservation of PAHs in the air (Siudek, 2022b).
There were high positive loadings for alkylated PAH species on PC2, whereas other PAHs had negative loadings.The alkylated PAHs were mainly from petroleum sources, and the PAHs were primarily from combustion sources.The high temperature in the summer allows the alkylated PAHs to easily release from the asphalt pavement or other petroleum-related sources.This conclusion can explain the distribution patterns of alkylated PAHs, especially the high values in summer when there is less rainfall.
PC3 was significantly correlated with PAHs.The results indicate that PAHs originate from automobile exhaust.Coal combustion and vehicle exhaust are usually regarded as the two largest contributors of PAHs in the atmosphere; thus, vehicle exhausts have a close relationship with the haze weather in China due to the increased number of vehicles (Taghvaee et al., 2018).The result of the source apportionment is in accordance with the variation tendency of the Ʃ 16 PAHs' concentration that showed a relationship with the concentration of PM2.5.
Based on the PLSR results, coal combustion, petroleum sources, and vehicular emissions accounted for 58%, 12%, and 30% of the ΣPAHs, respectively.

| CONCLUSION
In recent years, atmospheric pollution in China has received significant attention.From 2014 to 2017, atmospheric wet deposition samples were collected from a remote rural area in southern China, and the PAHs content was analyzed.The results showed that the average concentration and peak values of Σ 16 PAHs in samples collected during 2014 and 2015 were significantly higher than in those collected during 2016 and 2017.This is consistent with the overall improvement trend of China's atmospheric environment quality.The peak values of Σ 16 PAHs were typically detected in samples collected during the winter season with light rain, and there was no heavy rain in the preceding period.Due to the lack of heavy rain to reduce the concentration of air pollutants, the accumulated pollutants in the air are easily enriched in the rainwater during light rain.The concentration of Σ 15 Alkyl-PAHs also showed a significant relationship with the number of rainy days.The annual average concentration of Σ 15 Alkyl-PAHs in 2017 was much higher, and that year had fewer rainy days.Coal combustion, petroleum sources, and vehicle emissions accounted for 58%, 12%, and 30% of the PAHs in the air, respectively.

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I G U R E 1 Description of the sampling site.The terrain profile of the sampling area (red dot).Contours show the average annual PM2.5 distribution in 2015.

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I G U R E 2 Variations in PAH concentrations and weather indices from 2014 to 2017.The monthly rainfall data are from the local meteorological department.The temperature and PM2.5 data from the nearest city are from the China Meteorological Administration (http://www.cma.gov.cn) and the Ministry of Ecology and Environment of the People's Republic of China (http://www.mee.gov.cn),respectively.

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I G U R E 3 Concentrations of the PAH species during the period of 2014-2017.

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I G U R E 4 Monthly average PM2.5 content in China's atmosphere in 2015 (μg/m 3 ; red dots indicate the sampling location).The PM2.5 data were from https://www.aqistudy.cn/historydata/.The gray background indicates land.The color-filled boundaries in the maps are not national boundaries.They are the boundaries of the data range.
Wet deposition PAH levels obtained from other studies.
T A B L E 1 T A B L E 2 PLSR regression calculated Â loads.