3.1.Temporal distribution characteristics of PM2.5 concentration
Measurements of PM2.5 in the atmosphere were taken synchronously at four sampling sites in Fuxin using the gravimetric method. The results are shown in Fig. 2. The average annual concentration of PM2.5 was 39.68 µg·m− 3 in Fuxin during the sampling period from December 2021 to November 2022. Among them, 176 days exceeded the safe concentration limit (10µg·m− 3) of PM2.5 prescribed by the World Health Organization, accounting for 97.8% of the total sampling period. About 80% of the days exceeded the daily average concentration limit of PM2.5(35µg·m− 3) set by the United States, and 16 days exceeded the daily average concentration standard of PM2.5(75µg·m− 3) set by China, accounting for 8.9% of the total sampling days. It shows that PM2.5 pollution in Fuxin has improved (Zhao, Cui, and Zhai 2015). The average mass concentration of PM2.5 was 52.93 µg·m− 3 in winter and 39.18 µg·m− 3 in spring. The average mass concentration of PM2.5 in spring was about 26% lower than that in winter because, the heating was stopped in Fuxin at the end of March. The average mass concentrations of PM2.5 in summer and autumn were 32.29 µg·m− 3 and 34.31 µg·m− 3, respectively. The results were the same as those in Lanzhou and Xi'an (Li et al. 2018;Liu et al. 2023). The highest concentration of PM2.5 in Fuxin was recorded on December 18(159µg·m− 3), with an average daily humidity of 96%, an air quality index (AQI) of 209 and a prevailing wind direction of 1.02m/s, high humidity and low wind speed provide meteorological conditions for high PM2.5 pollution. According to the special geographical location in Fuxin, the wind direction has a great influence on the concentration of PM2.5. During the sampling period, the lowest PM2.5 concentration in Fuxin was found on May 11, with no rainfall, relative humidity of 44%, and PM2.5 concentration decreased to 3µg·m− 3, which is mainly due to the effect of “Rain in the cloud” on PM2.5“Nuclear condensation” and “Scoured below the cloud” on PM2.5“collisional coagulation” (Li et al. 2014; Xuan, Xue, and Lei 2019; Han et al. 2017;Bui et al. 2023).
3.2.Influencing factors of PM2.5 concentration
As it can be clear from Fig. 3 that, the annual variation trend of PM2.5 concentration with NO2 and SO2 was stable and relatively synchronous, and the correlation coefficients of PM2.5 with NO2 and SO2 were 0.777 and 0.655, respectively, during the sampling period. The industrial structure of Fuxin is relatively single, and the population in the urban area is only 600,000. Coal-fired smoke emission from power plants and thermal power plants is the main source of PM2.5. NO2 and SO2 in the urban area of Fuxin, and the concentration of three pollutants is relatively high in winter and spring, and relatively low in summer and autumn There is a large concentration gradient with the same trend between the three pollutant concentrations in summer and autumn, which is mainly due to the lack of the important emission source of heating coal combustion. The emission of PM2.5 and SO2 is greatly reduced, and the emission of NO2 is due to the contribution of motor vehicle exhaust, as compared with the winter and spring season, its concentration did not decrease significantly. At the same time, gaseous precursors such as SO2 and NO2 can produce secondary pollutants such as sulfate and nitrate aerosol through homogeneous or heterogeneous (particle surface) reaction, which can increase the concentration of PM2.5 (3).
The variation of PM2.5 concentration in winter and spring in Fuxin was larger than that in summer and autumn, and the variation of O3 concentration in winter and spring was smaller than that in winter and spring. The correlation coefficients between PM2.5 concentration, temperature and O3 were − 0.166 and − 0.038, respectively. In summer and autumn, the surface temperature is higher than the atmospheric temperature, and the atmospheric convection is favorable for the diffusion and dilution of PM2.5. The results showed that evergreen leaves could effectively retain PM2.5 (Yang et al. 2018;Zeng et al. 2023). At the same time, the amount of the light radiation increases because of the lower concentration of PM2.5. Due to this, it can easily excite the photochemical chain reaction of NO2 and other tail gas of motor vehicle, and strengthens the concentration level of O3. Previous studies have shown that, the concentration of PM2.5 and O3 in Fuxin have a staggered peak relationship (Zhao et al. 2021).
The northern part of Fuxin is the Horqin Sandy Land, bordered by the Liaohe Plain to the east, Nuerhu Mountain to the west, South link to Bohai Bay. It is a transitional zone between the Inner Mongolian steppe and the Rocky Mountains of North China. It presents a semi-enclosed hilly basin landform. The wind roses during sampling are shown in Fig. 4, southwesterly winds are the dominant wind direction in the Fuxin, and contribute significantly to the rapid accumulation of PM2.5 in Fuxin (Zhao et al. 2020).
According to the wind roses of Fuxin during the sampling period, the prevailing winds in the Fuxin region in winter and spring are northerly and Westerly, and the particulate matter in the Horqin Sandy Land, the largest sandy land in the world in the northwest, contributes directly to the PM2.5 concentration in the Fuxin region. It is easy to construct the phenomenon of temperature inversion, not conducive to the diffusion of pollutants which can promote the rise of atmospheric PM2.5 concentration. Wind direction will directly affect the air humidity and temperature in Fuxin, and then affect the secondary conversion of gaseous precursors such as SO2, NO2 and also affect the hygroscopic growth of PM2.5 particles and the fluctuation of PM2.5 concentration level.
3.3.Concentration level and time distribution of heavy metals in PM2.5
The measured results of the mass concentrations of heavy metals in atmospheric PM2.5 in Fuxin City during the sampling period are shown in Fig. 5, and their average annual concentrations were from high to low such as Zn(0.2947µg·m− 3) > Pb(0.0664µg·m− 3) > As(0.0225µg·m− 3) > Ba(0.0205µg·m− 3) > Mn(0.0187µg·m− 3) > Cu(0.0140µg·m− 3) > Cr(0.0095µg·m3) > V(0.0067µg·m3) > Ni(0.0061µg·m3) > Sb(0.0024µg·m3) > Cd(0.0019µg·m− 3) > Co(0.0007µg·m− 3). And the average concentrations of Pb and As are 1.2 and 3.75 times of the GB3095-2012 secondary standard limits, respectively, which are 3 and 1.13 times of the EU air standard limits. Pb and As can enter into the human body which results in several diseases related with respiratory system. As compared to other cities in Fushun, Jinzhou, Panjin and Anshan (Li 2017; Gu et al. 2016; Li et al. 2019; Wang et al. 2017), the concentrations of Cr in PM2.5 in Fuxin are more than 1.69、2.31、2.25 and 1.13 times of those in Fushun、Jinzhou、Panjin and Anshan, respectively. The possible reason for the increase concentration of Cr is the existence of leather industry which is responsible for the economic growth pole in Fuxin. As, lots of chrome alum and dichromate are used in the leather industry which discharge many leather tanning materials. In Fuxin, this leather industry is located in Xinqiu District which is 6 km apart from the city (Xie, Hou, and Chen 2018).
The variation range of Zn, Pb, As and Mn with the seasonal concentration was larger, and the maximum average concentration appeared in spring and minimum in summer. With the exception of mica As the main source of Mn, both Zn and As are associated with industrial processes (Tian et al. 2010; Jiao et al. 2014), with the highest values occurring in spring because of the emissions from coal-fired heating and metal smelting industries. Pb may be closely related to the combustion of fossil fuels in automobiles (Wang et al. 2015), the dominant wind direction in spring is northwest, and the Mn may be contributed by the Horqin Sand transported by the northern Horqin Sand Land and southern Bohai Bay Air Passage.
3.4.Source apportionment of heavy metals in PM2.5
The enrichment factor method (EF) was proposed by Gordon in the 1970s to judge the impact of man-made pollution sources other than natural sources on atmospheric particulate matter. Loska K and others believe that although the method has some shortcomings, it has a standardized formula, so it can still be used as a simple and good method to estimate the enrichment level of elements. The formula is:
$$\:\text{EF=}\frac{({C}_{i}\text{/}{\text{C}}_{n}{)}_{\text{particulate\:matter}}}{{\left({C}_{i}\text{/}{\text{C}}_{n}\right)}_{soil}}$$
1
In the above formula: Ci is the concentration of the ith element; Cn is the concentration of the selected reference element; The numerator part of the formula represents the amount of elements in particulate matter and the denominator part represents the amount of elements in soil. The reference elements are all elements in particulate matter, and the content in soil is relatively rich. The frequently used reference elements are Al, Fe, Ti. Fe has relatively stable chemical properties and is commonly used reference elements. Therefore, Fe is selected as the reference element in this paper.
From Fig. 6, the concentration index of Cd and Zn in PM2.5 of Fuxin atmosphere is more than 100 during the sampling period, which indicates that the concentration index of Cu, As, Sb and Pb are all in between 10 and 100. The Enrichment Index of V, Cr, Ni lies in the range of 2 ~ 10, considered as moderate enrichment which means there is less man-made influence. However, the enrichment index of Mn, Co and Ba is less than 2 elements indicating the slight man-made influence.
The concentration index of heavy metals in PM2.5 during winter and spring is generally higher as compared to summer and autumn, and the concentration index of Cd in spring is the highest, reaching 920.88 in the spring, much higher than 100, which shows that the concentration of heavy metals in PM2.5 during winter and spring is seriously affected by human activities. Zhang Song, You Fang and others have shown that the main source of atmospheric Cd is coal burning (Zhang et al. 2020; You et al. 2019). As a coal resource city with a century’s mining history, Fuxin has five coal-fired thermal power plants, such as Fuxin power generation, Jinshan district coal gangue thermal power, Eagle cement, Jiechao coal gangue thermal power, Fuxin mining group coal gangue thermal power plants. The enrichment indexes of Zn, Pb and As are 254.98,173.40 and 195.14, respectively, which are very high in spring. The reason for the higher enrichment factor in spring are mainly attributed to coal burning and motor vehicle emissions. The number of vehicles in Fuxin continues to grow, currently reaching 300,000 and the annual increase in motor vehicle emissions may contribute to Zn, Pb and As. Fuxin is prone to the accumulation of air pollutants due to its poor dispersion conditions owing to its “North-facing south “dustpan topography.
3.5.Health risk assessment of heavy metal elements in PM2.5
Human health risk assessment model was proposed by Environmental Protection Agency (EPA) in 1983. The risk assessment is divided into four steps: hazard identification, dose response, exposure assessment and risk characterization (US 2002). The data collecting from the International Cancer Research Institute and the EPA comprehensive risk information show that the pollutants are classified into carcinogens and non-carcinogens.
12 heavy metals in PM2.5 in the Fuxin atmosphere during the sampling period are shown in Table 2. The heavy metal ions which show the noncarcinogenic risks from high to low are Mn, V, Co, Cr, As, Pb, Sb, Cd, Zn, Cu, Ni and Ba. The non-carcinogenic risk coefficient HQ of 12 heavy metal elements was 1.08 × 10− 6 ~ 1.85 × 10− 2, which was lower than the EPA limit 1 (EPA 1989). Non-carcinogenic risk of heavy metals in Fuxin was generally, low during the sampling period, and the risk was gradually reduced in males, females and children.
Table 2
Risk of Respiratory Exposure of Heavy Metals to PM2.5 in Fuxin City during Sampling Period
Heavy Metal | HQ Adult male | HQ Adult female | HQ children | SF | ILCR |
V | 1.33×10− 2 | 1.20×10− 2 | 1.01×10− 2 | | |
Cr | 4.55×10− 3 | 4.11×10− 3 | 3.46×10− 3 | 0.84 | 9.76×10− 8 |
Mn | 1.85×10− 2 | 1.67×10− 2 | 1.40×10− 2 | | |
Co | 1.81×10− 3 | 1.64×10− 3 | 1.38×10− 3 | 9.8 | 8.92×10− 8 |
Ni | 4.23×10− 6 | 3.82×10− 6 | 3.21×10− 6 | 0.84 | 6.26×10− 8 |
Cu | 4.83×10− 6 | 4.37×10− 6 | 3.68×10− 6 | | |
Zn | 1.36×10− 5 | 1.23×10− 5 | 1.03×10− 5 | | |
As | 1.04×10− 3 | 9.37×10− 4 | 7.88×10− 4 | 15.1 | 4.14×10− 6 |
Cd | 2.66×10− 5 | 2.41×10− 5 | 2.03×10− 5 | 6.3 | 1.48×10− 7 |
Sb | 8.41×10− 5 | 7.60×10− 5 | 6.40×10− 5 | | |
Pb | 2.62×10− 4 | 2.37×10− 4 | 2.00×10− 4 | | |
Ba | 1.42×10− 6 | 1.28×10− 6 | 1.08×10− 6 | | |
The lifetime cancer risk of five heavy metals in Fuxin PM2.5 lies in the range of 6.26 × 10− 8 to 4.14 × 10− 6. The heavy metals which show high to low cancer risk are As, Cd, Cr, Co, and Ni, among them As shows the higher cancer risk (4.14 × 10− 6) lies in the range of carcinogenic risk (10− 6 ~ 10− 4). The results showed that As in PM2.5 had carcinogenic risk, and the carcinogenic risk values of other heavy metal elements were all lower than the threshold of carcinogenic risk. As is a symbolic element of coal combustion, which may be related to the relatively single energy structure of coal in Fuxin. The Haizhou Mine, once the largest open pit mine in Asia, is located only 3 km south of the urban area of Fuxin, and there are currently more than 200 sites of spontaneous combustion of residual coal, at the same time, a large amount of coal gangue and fly ash are piled up around the open-pit mine. Under the wind disturbance, the concentration of PM2.5 and As are the identifying elements of the coal-fired source in the urban area are greatly contributed. The results of health risk assessment of heavy metal elements in PM2.5 in Nanjing and Xi'an respectively showed that the lifetime cancer risk of As exceeded the threshold range of cancer risk (Zhao 2018; Zhao 2016), there is a certain risk of carcinogenesis to the main population in the region. Excessive As can interfere with the normal metabolism of cells, affect the process of respiration and oxidation, make cells pathological changes, and eventually cause various diseases. Therefore, Fuxin should strengthen the total emission control of coal-burning to reduce PM2.5 emissions and heavy metals in coal-burning exposure to the health risks of the local population.