ABSTRACT
The Chinese Loess Plateau (CLP) receives and potentially contributes to Asian dust storms that affect particulate matter (PM) concentrations, visibility, and climate. Loess on the CLP has experienced little weathering effect and is regarded as an ideal record to represent geochemical characteristics of Asian paleo dust. Samples were taken from 2-, 9-, and 15-m depths (representing deposition periods from ∼12,000 to ∼200,000 yr ago) in the Xi Feng loess profile on the CLP. The samples were resuspended and then sampled through total suspended particulates (TSP), PM10, PM2.5, and PM1 (PM with aerodynamic diameters < ∼30, 10, 2.5, and 1 μm, respectively) inlets onto filters for mass, elemental, ionic, and carbon analyses using a Desert Research Institute resuspension chamber. The elements Si, Ca, Al, Fe, K, Mg, water-soluble Ca (Ca2+), organic carbon, and carbonate carbon are the major constituents (>1%) in loess among the four PM fractions (i.e., TSP, PM10, PM2.5, and PM1). Much of Ca is water soluble and corresponds with measures of carbonate, indicating that most of the calcium is in the form of calcium carbonate rather than other calcium minerals. Most of the K is insoluble, indicating that loess can be separated from biomass burning contributions when K+ is measured. The loess has elemental abundances similar to those of the upper continental crust (UCC) for Mg, Fe, Ti, Mn, V, Cr, and Ni, but substantially different ratios for other elements such as Ca, Co, Cu, As, and Pb. These suggest that the use of UCC as a reference to represent pure or paleo Asian dust needs to be further evaluated. The aerosol samples from the source regions have similar ratios to loess for crustal elements, but substantially different ratios for species from anthropogenic sources (e.g., K, P, V, Cr, Cu, Zn, Ni, and Pb), indicating that the aerosol samples from the geological-source-dominated environment are not a “pure” soil product as compared with loess.
Precise and representative chemical properties of dust are necessary to quantitatively evaluate the impact of mineral dust on the environment and climate. Efforts to obtain chemical characteristics for Asian paleo dust are valuable for reducing uncertainty in evaluation of the role of Asian dust in climate and environmental change of geologic times as well as during the modern period.
INTRODUCTION
Arid and semi-arid regions in northern and northwestern China are among the largest dust sources in the world. Large amounts of aeolian dust become airborne and are transported toward the east to be deposited in East Asia and beyond.Citation1 Asian dust has been detected in deep-sea sediments from the remote Pacific,Citation2 the atmosphere of the western United States,Citation3 Greenland ice cores,Citation4 and Europe.Citation5 Geological records trace Asian dust to 8–27 million yr before present (B.P.).Citation6,Citation7 Asian dust alters radiative transfer by scattering and absorbing solar and/or thermal radiation.Citation8 It is also one of the main sources of water-soluble Fe and P for ocean plankton growth and is expected to accelerate the productivity of marine and terrestrial ecosystems,Citation9,Citation10 influence the carbon cycle,Citation11 and change greenhouse gas levels.Citation11
Representative chemical characteristics of dust are needed to better evaluate the impact of suspendable dust on the environment and climate.Citation12,Citation13 The Asia Pacific Regional Aerosol Characterization Experiment (ACE-Asia) and the Chinese Dust Storm Research Project acquired receptor measurements of Asian outflows during dust storms.Citation14–22 Surface soil from the source regions has been collected, air-dried, size-segregated, and analyzed to obtain size-specific chemical source profiles.Citation23–25 Fine particle PM2.5 (particulate matter [PM] ≤ 2.5 μm in aerodynamic diameter) characterization is most useful because PM2.5 has long residence times in the atmosphere.Citation26 Limited measurements exist regarding the characteristics of Asian paleo dust. Surface samples do not tell the whole story, however, because suspendable Asian dust may have differing composition with time.
The loess-paleosol (dusts formed and deposited during geological time that may differ in chemical and physical characteristics from present-day surface dusts) sequence on the Chinese Loess Plateau (CLP) is a product of Asian dust deposition,Citation27 recording the evolution of Asian paleo dust since the Pliocene (∼22 Ma).Citation6 As a continental aeolian deposit, loess has experienced little weathering effect and is regarded as an ideal record to represent geo-chemical characteristics of pure Asian paleo dust.Citation28 Geochemical studies have been conducted on the loess to distinguish the provenance of Asian paleo dust and decipher East Asian monsoon variability by using different isotopic (e.g., Sm, Nd abundances and elemental ratios [e.g., Zr/Rb, Rb/Sr]).Citation29–33 However, these previous studies of bulk material have not examined loess properties relevant to suspendable dust in the atmosphere.
This study investigated multiple trace elements, water-soluble ions, and carbon fractions of size-segregated loess (i.e., total suspended particulates [TSP], PM10, PM2.5, and PM1 [PM with aerodynamic diameters <∼30, 10, 2.5, and 1 μm, respectively]) through separation of bulk loess samples by suspension and sampling through size-selective inlets onto filters followed by chemical analyses.Citation34 Size-differentiated chemical abundances were compared to those from the upper continental crust (UCC) and dust-dominated aerosol in Asian dust source regions.
METHODOLOGY
The CLP is one of the most extensive areas of loess deposition in the world. It spans an area of ∼440,000 km2 predominantly in the provinces of Shanxi, Shaanxi, and Gasnu between 33° north, −40° north and 98° east −115° east ().Citation1 Xi Feng (35°45′ north, 107°49′ east), a city in Gansu Province, is located in the central CLP. The loess deposit throughout the region is approximately 130 m thick and contains more than 30 major loess units inter-bedded with paleosol.Citation35 Nine loess samples were collected from three loess strata (L1LL1, L1LL2, and L2) of the profile. Loess strata samples were obtained by digging a 20-m-deep well in the upper portion of the CLP, and individual samples were collected with a plastic shovel in three typical loess strata. Paleosol samples were not collected because they suffer from strong chemical weathering after deposition and would not represent the original dust characteristics. Age of loess deposit is determined by correlation between the magnetism profiles and the mapping spectral variability in global climate project (SPECMAP) time series.Citation36 These samples are summarized in with their corresponding sampling depth, geological strata, and age of the loess deposit.
Table 1. Description of loess samples collected from the Xi Feng loess profile at the CLP
Figure 1. Map of China showing distributions of the CLP as well as major deserts and sand-lands. The loess sampling location is indicated by a black dot.
Samples were air-dried at 25 °C room temperature for 1 week and sieved through Tyler 30-, 50-, 100-, 200-, and 400-mesh sieves to obtain approximately 5 g of particles with physical diameters less than 38 μm. Approximately 0.1 mg of sieved material was placed in a 250-mL side-arm vacuum flask sealed with a rubber stopper. Air puffs into the flask introduced dust into a resuspension chamber.Citation34 Clean, filtered laboratory air was drawn into the chamber by the sample flow of 10 L/min through each of six channels equipped with greased PM10, PM2.5, and PM1 impactor inlets. TSP was collected on two channels under the dust cap without specific size segregation. The parallel channels for each size fraction used 47-mm Teflon-membrane filters (Pall Sciences) with 2-mm pore size in channel 1 for mass and subsequent elemental analysis and 47-mm quartz-fiber filters (Whatman Corporation, QM/A) in channel 2 for water-soluble ions, organic carbon (OC), elemental carbon (EC), carbon fractions, and carbonate carbon (CC) analyses. Teflon-membrane filters were periodically weighed during the resuspension process to avoid overloading; optimum loading on Teflon-membrane filters for chemical speciation is 1–3 mg per 47-mm filter.
Teflon-membrane filter samples were equilibrated in a relative humidity (25–30%) and temperature (21.5 ± 0.5 °C) controlled environment before gravimetric analysis to minimize particle volatilization and aerosol liquid water interferences. Filters were weighed on an MT5 microbalance (Mettler-Toledo Ltd.) with a sensitivity of ±0.001 mg. Filters were exposed to a low-level radioactive source (500 pCi of polonium-210) before and during sample weighing to remove static charge. The differences of re-weights for unexposed and exposed filters were below ±0.010 and ±0.015 mg, respectively.
Thirty-nine elements (e.g., Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Mo, Pd, Ag, Cd, In, Sn, Sb, Ba, Au, Hg, Tl, Pb, and U) were quantified on the Teflon-membrane filters with an Epsilon 5 energy dispersive X-ray fluorescence instrument (ED-XRF; PANalytical).Citation37 The excitation consisted of a gadolinium (Gd) anode tube, with Ti, Fe, Ge, Zr, Mo, Ag, and barium fluoride (BaF2) secondary targets. Characteristic X-ray emissions were detected by a solid-state, liquid nitrogen (N2) cooled Ge detector. Filters were loaded (and unloaded) into the sample holders in a high-efficiency particulate air (HEPA) filter laminar flow hood. One of every 10 samples was reanalyzed, and two MicroMatter multi-element quality assurance (QA) standards (Al, Cl, Ca, Ti, Fe, Se, Y, Mo, Ag, Sn, Ba, an W) were analyzed each day to verify lack of instrument drift.Citation37
Fourteen rare earth elements (REEs; La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were analyzed by inductively coupled plasma–mass spectrometry (ICPMS, Thermo Elemental, X Series). The entire Teflon filter was sliced into eight pieces, placed in a digestion vessel, and wetted with 0.2 mL of ethanol to counteract its hydrophobic tendencies. Two milliliters of a 2:1 HNO3:H2O solution were added, followed by a 1:4 HCl:H2O solution and 0.1 mL of Hartree–Fock (HF). The HF was needed to dissolve strongly bound mineral oxides in most geological samples. The capped digestion vessel was placed in a hot block for 90 min, cooled, and brought to 50 mL total volume with distilled-deionized water (DDW). The capped vessels were stored overnight with the cap side down before analysis. The ICP-MS was equipped with a concentric nebulizer with a cooled (2–3 °C) spray chamber that minimized oxide formationCitation38,Citation39 and a collision cell chamber to reduce polyatomic interferences.Citation40–42 The ICP-MS was optimized and calibrated daily for the elements of interest and maintained less than 2% oxide formation and less than 2% double-charged ions. A calibration curve from 0.001 to 500 μg/L was plotted each day. External standards, reagent blanks, and filter blanks were analyzed each day. Replicates, spikes, and QA standards were run at a rate of 10%.
Half of the quartz-fiber filter was extracted in DDW and analyzed for water-soluble chloride (Cl−), nitrate (NO3 −), and sulfate (SO4 2−) by ion chromatographyCitation43; for water-soluble sodium (Na+), potassium (K+), and calcium (Ca2+) by atomic absorption spectrophotometry (AAS); and for water-soluble ammonia (NH4 +) by automated colorimetry (AC). A 0.5-cm2 punch from the remaining half filters was analyzed for eight carbon fractions following the Interagency Monitoring of Protected Visual Environments (IMPROVE) thermal/optical reflectance (TOR) protocol.Citation44–47 This produced four temperature-specific OC fractions (OC1, OC2, OC3, and OC4 at 120, 250, 450, and 550 °C, respectively, in a 100% helium [He] atmosphere), a pyrolyzed carbon fraction (OP; determined when reflected laser light attained its original intensity after oxygen [O2] was added to the analysis atmosphere), and three EC fractions (EC1, EC2, and EC3 at 550, 700, and 800 °C, respectively, in a 98% H3/2% O2 atmosphere). IMPROVE OC is defined as OC1 + OC2 + OC3 + OC4 + OP, and EC is defined as EC1 + EC2 + EC3 – OP. The CC abundance was determined by acidification of the sample with HCl before thermal/optical analysis with subsequent detection of evolved carbon dioxide (CO2).Citation48 Mass fractions (abundances) were calculated for each measured component after blank subtraction. Uncertainties for individual samples were determined by error propagation of precisions derived from replicate measurements and standard deviations of filter blanks.Citation49
RESULTS AND DISCUSSION
Chemical Characteristics for Different Size Fraction
compares chemical abundances for TSP, PM10, PM2.5, and PM1 size fractions for the three sampled strata; also listed are composite source profiles for CLP surface dust from Cao's earlier paper.Citation24 Six major crustal elements (Si, Ca, Al, Fe, K, and Mg) show abundances (>1% with low variability). Ti and Mn abundances are in the range of 0.1–1%. Because O2 was not measured, the IMPROVE formulaCitation50 was used to estimate mineral mass as the weighted sum of aluminum, silicon, calcium, iron, and titanium oxides ([Soil] = 1.94 × [Ti] + 2.49 × [Si] + 2.42 × [Fe] + 1.63 × [Ca] + 2.2 × [Al]). This reconstructed mass accounts for 80–90% of total measured mass. Si is the most abundant species, but it shows the greatest variation (12–24%) among size fractions: 20–24% in TSP, 12–13% in PM10, and 13–15% in PM2.5 and PM1. Al, Fe, K, and Mg are more abundant in the PM2.5 and PM1 size fractions than in the TSP fraction. PM2.5 Al and Fe abundances are 70 and 40% higher than corresponding levels in TSP, respectively. PM2.5 and PM1 Ti and Mn abundances are 30% higher than those in TSP. REE abundances are higher in PM2.5 and PM1 than in TSP. La abundance ranges from 18 to 22 parts per million by weight (ppmw) in TSP, which increases to 31–42 ppmw in PM1. PM Ca abundances are less variable, ranging from 10 to 15%. Many trace element abundances are close or lower than their variability and show no consistent relationship to the size fraction.
Table 2. Massfractions in each layer for TSP, PM 10, PM 2.5, and PM1 size fractions
Cations (K+, Ca2+, Na+, NH4 +) and anions (Cl−, NO3 −, and SO4 2-) account for 9–13% of PM mass. Ca2+ (8–13%) is the most abundant ion, constituting 90% of total water-soluble ions. The ratios of Ca2+/Ca range from 0.7 to 1, indicating that most Ca is water-soluble. This indicates that the Ca multiplier in the IMPROVE formula, which assumes Ca, is probably incorrect because calcium oxide (CaO) is largely insoluble. A large Ca2+ abundance is consistent with the content of modern surface dust from the Zhenbeitai (ZBT) station in northern CLP ().Citation15 Water-soluble K+ is the second most abundant ion (0.1–0.4%), but this is much lower than the Ca2+ abundance. K+ abundances increase as the size fraction decreases, constituting 0.10–0.17% of TSP and 0.26–0.37% in PM1. Total K is 5–25 times the K+ abundance for all samples, indicating that most K is not water-soluble. Because the K abundance in biomass burning emissions is nearly all K+,Citation51–53 Asian dust contributions can be separated from vegetative burning using these markers. The remaining ionic species abundances are low, with 0.06–0.15% for SO4 2-, 0.02–0.08% for Na+, 0.02–0.05% for NH4 +, and 0.03–0.14% for NO3. Cl− is enriched for the larger size fractions, with TSP abundances of 0.08–0.15% and PM1 abundances of 0.04–0.08%.
Total carbon (TC = OC + EC + CC) accounts for 4–7% of measured mass. Average abundances of CC in PM loess are 2–5%, constituting 50–80% of TC. shows good correlation (0.75 < r < 0.95) between CO3 2−-C and Ca2+ in all size fractions, with mass ratios of CO3 2−-C/Ca2+ (0.2–0.29) close to the CaCO3 stoichio-metric ratio [C(12)/Ca (40) = 0.3]. CaCO3, in one of its many mineral forms, is an important component in surface as well as buried paleosols, similar to modern dust over the CLP.Citation54
Figure 2. Relationship between carbonate carbon (CO3 2-C) and water-soluble calcium (Ca2+) for the TSP, PM10, PM2.5, and PM1 fractions of loess.
OC constitutes 1.2–3.4% of mass, with OC/TC ratios between 0.25 and 0.5. The main component of OC is high-temperature (450 °C) OC3, accounting for 30–50% of OC in all size fractions. The EC abundance is mostly below the minimum detection limits. There is little or no variation in the abundances of all carbon fractions among the four size fractions.
Chemical abundances of loess PM from different depths are similar, indicating that Asian dusts share similar chemical profiles at least in the last two glacial and interglacial cycles. This may be attributed to the relative stability of the source of Asian paleo dust.
Composite profiles for nine loess samples, calculated by averaging the chemical abundances of individual profiles in each size fraction, are applied to comparisons below.
Comparisons with UCC
Elemental abundances from sampled loess were compared with the composite reference values for the Earth's UCCCitation55 in UCC abundances represent the average elemental composition of the Earth's surface and were usually used as a reference for pure or paleo dust. Suspendable loess has several abundances that are similar to those of the UCC, as indicated by ratios close to unity in Most notable are Mg, Fe, Ti, Mn, V, Cr, and Ni, which have ratios of 1–2. These elemental abundances would not be useful for distinguishing CLP Asian dust contributions from other dust sources.
Figure 3. UCC55-normalized abundances for TSP, PM10, PM2.5, and PM1 of loess. Samples for each size fraction represent the average of nine source profiles.
Positive deviations from unity of 2–6 are evident for Ca, Co, Cu, As, and Pb, whereas negative deviations are evident for Na, Si, K, P, Br, Rb, Sr, and Zr. Average abundances of Mg, K, Rb, Sr, Zr, and Ba are all 50–90% of that in UCC; Si is 30–80% of that in UCC; and Na and P are 10–20% of those in UCC. The loess Al abundance is similar to that of UCC for the PM2.5 and PM1 sizes, but it is lower for the TSP and PM10 fractions. The Y abundance in loess is similar to that of UCC for TSP and PM10, but it is higher for PM2.5 and PM1. Light REE abundances (La, Ce, Pr, and Nd) in UCC are similar to those of loess PM10, whereas heavy REE abundances (Ho, Er, Tm, Yb, and Lu) in loess are similar to those of UCC for PM2.5 and PM1.The differences of elemental abundances between loess PM and UCC reflect the petrography in the source regions of Asian dust, which is of typical abundances in CaCO3 Citation56 and does not exhibit the same composition as the globalupper crust. The use of UCC as a reference to represent pure or paleo Asian dust needs to be further evaluated.
Comparisons with Dust-Dominated Ambient Measurements in Source Regions
compares elemental ratios from these samples with those measured in other Chinese source regions.Citation14–16 The ratios of crustal elements such as Mg, Si, Fe, Ti, Mn, and Sr to Al in ambient samples are similar to those of PM loess. Ca/Al ratios varied from 1.8 ± 0.25 to 2.3 ± 0.12 for the loess samples, but their variability is higher in the ambient samples. The ambient Ca/Al ratio from the Taklimakan Desert is approximately 40% higher than the average ratios in PM loess, and they are 50–80% lower for the other samples.
Table 3. Comparison of elemental ratios (uncertainties) for loess PM and dust-dominated aerosol in source regions
Major differences between loess and ambient sample ratios are evident for species that might originate from other sources (e.g., K, P, V, Cr, Cu, Zn, Ni, and Pb). The K to Al ratio is 40–200% higher in ambient air than in PM loess. shows the anthropogenic fraction of disturbed or contaminated elements for PM2.5 of ZBT dust aerosol reported by Arimoto et al.Citation9 More than 70% of Pb, Zn, Cr, V, Ni, Cu, and Co in ZBT aerosol were attributed to non-crustal or anthropogenic origins. Contributions from non-loess sources are most apparent for Pb (97%), followed by Zn (95%), Cr (94.7%), V (91%), Ni (77%), Cu (76%), and Co (70%). These results indicate that anthropogenic activities may have disturbed or contaminated the chemical components of aerosol in the source regions. The modern aerosol samples from the geological source-dominated environment are therefore not a “pure” soil product as compared with loess.
Figure 4. The percentages of crustal and non-crustal origin for elements Pb,Zn, Cr, V, Ni, Cu, and Co of PM2.5 aerosol samples acquired from the ZBT station.15 The fraction of non-crustal origin (f non-crustal origin, in percent) for element X was calculated as follows: f non-crustal origin = 100 × [1 - (Al sample × X loess)/(Al loess × X sample)].
![Figure 4. The percentages of crustal and non-crustal origin for elements Pb,Zn, Cr, V, Ni, Cu, and Co of PM2.5 aerosol samples acquired from the ZBT station.15 The fraction of non-crustal origin (f non-crustal origin, in percent) for element X was calculated as follows: f non-crustal origin = 100 × [1 - (Al sample × X loess)/(Al loess × X sample)].](/cms/asset/4fdd2049-f2ff-468b-a60d-25fcc57fb359/uawm_a_10412087_o_f0004g.gif)
CONCLUSIONS
The chemical composition of Asian paleo dust derived from the CLP is similar throughout the past 198,000 yr in geological time. The elements Si, Ca, Al, Fe, K, Mg, Ca2+, OC, and CC are the major constituents (>1%) in loess among the four PM fractions (i.e., TSP, PM10, PM2.5, andPM1). Much of the calcium is water-soluble and corresponds with measures of carbonate, indicating that most Ca is in the form of CaCO3 rather than other calcium minerals. Most of the K is insoluble, indicating that loess can be separated from biomass burning contributions when K+ is measured. The loess has elemental abundances similar to those of the UCC for Mg, Fe, Ti, Mn, V, Cr, and Ni, but substantially different ratios for other elements (e.g., Ca, Co, Cu, As, and Pb), suggesting that the use of UCC as a reference to represent pure or paleo Asian dust needs to be further evaluated. The aerosol samples from the source regions have similar ratios to loess for Mg, Si, Fe, Ti, Mn, and Sr to Al, but substantially different ratios for species that might originate from anthropogenic sources (e.g., K, P, V, Cr, Cu, Zn, Ni, and Pb), indicating that the aerosol samples from the geological source-dominated environment are not a “pure” soil product as compared with loess.
As a natural atmospheric aerosol, dust plays an essential role on the regional and global climate and environmental changes.Citation8–13 Attempts have been made to simulate global dust distributions in several models.Citation57 Size-differentiated chemical characteristics of loess based on aerodynamic diameters reproduce the chemical characteristics of Asian paleo dust, which are valuable for reducing uncertainty in the evaluation of the role of Asian dust in climate and environmental change of geological times as well as in the modern period.
ACKNOWLEDGMENTS
This work was supported by grants from the Chinese National Science Foundation (40872211, 40925009), the State Key Laboratory of Loess and Quaternary Geology, DRI, and the Nazir and Mary Ansari Foundation. The authors are grateful to Steve Kohl and Brenda Cristani of DRI's Environmental Analysis Facility for their assistance in sample analysis.
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