1. How does the chemical composition of dust aerosols vary with particle size and what impact does it have on heavy metal enrichment?
The chemical composition of dust aerosols can vary with particle size, with smaller particles tending to contain higher amounts of heavy metals. This variation is due to the complex processes of dust generation and aerosolization, which may have some chemical selectivity. Dust generation and aerosolization are influenced by factors such as wind erosion, soil movements, and sandblasting processes. The sandblasting efficiency of a soil particle is highly sensitive to its size, with smaller particles being more easily ejected. As a result, dust aerosol particles are unlikely to have exactly the same composition as their parent soils. Laboratory studies have shown that smaller dust aerosols tend to have higher enrichment of heavy metals such as Mn, Cd, Zn, and Pb. The enrichment factors for heavy metals in dust aerosols are much higher for smaller dust aerosols. This variation in chemical composition and heavy metal enrichment has implications for air quality modeling and cancer risk assessment, as using problematic dust profiles from the US EPA's SPECIATE database may impact the accuracy of air quality model calculations. Therefore, it is crucial to utilize proper dust profiles that accurately represent the chemical composition of dust aerosols, especially for heavy metal loadings in the atmosphere.
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2. What soil types were collected in the dust source regions?
Fourteen soil samples were collected from various locations in dust source regions and Shanghai, China. The soil types represented in the samples include silty loam (S1), sand (S2, S4, S7, S10, S11, S12), sandy loam (S3), loam (S5, S6), loam sand (S8, S13), and silty clay loam (S9, S14). These soil types were collected from different regions such as Yinshan Mountain in central inner Mongolia, Hexi Corridor and Alxa Plateau, Xinjiang Province, and Shanghai Yangpu District. The soil samples were collected from the top 10 cm of the natural soil profile and were used to produce fine and coarse dust aerosols (PM2.5 and PM10) using a GAMEL dust aerosol generator. The pH of the soil was also measured before aerosolization. Detailed information about the soil types and their locations can be found in Table S2 and Fig. S1.
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3. How does GAMEL simulate sandblasting process?
GAMEL, a laboratory dust generator, simulates the sandblasting process by agitating soil samples in a PTFE flask. The optimal generation parameter is set at a frequency of 500 cycles/min with an airflow rate of 8 liter/min. The particle-free air is passed through the flask, and the dust aerosols are collected on a 47 mm PVC film held in a metal frame filter holder. The dust-PM2.5 and dust-PM10 are obtained with or without an 8LPM cyclone, respectively. Size-fractionated particle sampling is carried out with a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI 110R) to obtain dust aerosols in different size ranges. The GAMEL generator can produce realistic dust aerosols, as shown in Table S3 and S4.
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4. How were dust aerosol samples digested?
Dust aerosol samples were weighed and placed in 25 ml digestion tubes with 6 ml 69% HNO3. The Microwave Digestion process involved an initial temperature of 100°C for 5 minutes, then ramped to 140°C for 5 minutes, and finally at 180°C for 60 minutes. The whole process was held for 120 minutes. After digestion, the solution was acid-fed at 120°C for 1.5 hours, then deionized water was added to maintain a 25 mL volume. The samples were diluted with 2% HNO3 four times for further analysis. Three blank PVC film samples were also digested using the same method for background control. The heavy metal content was determined using an inductively coupled plasma mass spectrometer (ICP-MS). Internal standard elements Bi and Rn were introduced into the nebulizer to compensate for instrument drift and matrix effects. After each sample analysis, 2% dilute nitric acid was used to clean the injection line for 1 minute. A scanning electron microscope (SEM) equipped with an energy-dispersive X-ray detector was used for particle morphology examination at a voltage of 10 kV. Statistical analysis was performed using SPSS Statistics, with correlation analysis conducted through Spearman's correlation and significant differences determined using an independent sample T-test.
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