We evaluate the applicability of an optical transmission measurement method commonly used for the analysis of the Black Carbon (BC) content of aerosol sample, to determine the BC content of loess sediments. A number of different sample pretreatment procedures are developed and compared, leading to an optimum preparation process. The results include: 1) Subtraction of the optical attenuation values before and after heating of the sample filters (“ Δ ATN ”) varies linearly with the sample mass. The slope of the regression line provides the best determination of BC concentration. 2) When the sample mass is small, (NaPO 3) 6 pretreatment is best for BC measurement, and the BC concentration results are given by the slope of the regression between Δ ATN and sample mass, for a series of samples of varying mass. 3) HF pretreatment accompanied by centrifugation and rinsing may produce a negative bias on the result. 4) Replicate measurements of BC for loess samples showed a maximum deviation less than 5.6%, suggesting that measurements of the BC concentration of a sequence of loess samples could determine variations to this degree of significance. 5) The overall trends of BC concentration in loess section sequences were similar for all chemical pretreatments. The BC concentration result for replicate samples is comparable when pretreated by the same procedure.
BC particles are ubiquitously present in our environment: both suspended as aerosols in the atmosphere, and in liquid and solid media after deposition to soils, loess, and aquatic sediments (both lacustrine and marine) [
Various methods have been developed for the measurement of the BC content of samples [
The methods most frequently used for the determination of the BC content of soil and sediment samples are the chemical, thermal, and chemical-thermal oxidation (CTO) methods, based on differences in the rates of degradation or oxidation between the BC and non-BC components [
The removal of carbonates by chemical pretreatment is used in most oxidative methods. Removal of silicate by a mixture of HCl and HF was also used in the TOR method [
For aerosol samples, BC is quantitatively measured by optical methods, and this analytical technique is based upon a determination of the “Attenuation” i.e. “blackness”.
The Optical Transmission analytical method is rapid, non-contact, non-contaminating and non-destructive. Now both OT-21 and Aethalometer which is a real-time BC optical measurement instrument are widely used to measure BC content of aerosol samples in worldwide [
This optical method to measure BC content by using OT-21, was developed in 1993 [
It is commonly used for the analysis of the BC content of aerosol samples collected on quartz filters [
It is known that the periodic variability of the composition of loess-paleosol sequences reflects changes between dry and wet periods [
The purpose of this study was to investigate rapid, convenient and quantitative methods to measure the BC content of loess samples. This work establishes the optimum experimental procedures and improves a measurement method. We also evaluate carefully the effect of different pretreatment procedures on the results of the BC measurement of sediment samples.
The loess and paleosol samples used in this study were taken from a loess-paleosol section located at (34˚34′N, 109˚32′E) near Yangge town, Weinan, in Shanxi province in the southern Chinese Loess Plateau. This section has been studied extensively [
In this study, we employed the BC measurement method using Optical Transmission analysis described by Ahmed et al. (2009). We used the same Optical Transmissometer (Model OT-21, Magee Scientific, California USA) as is commonly used in atmospheric aerosol research [
The OT-21 has a movable tray with two locations for holding filters. The outer position is used for the sample, while the inner position holds the reference (blank filter). The instrument compares the intensity of 880-nm light transmitted through the sample, to the intensity transmitted through the reference blank filter. The “Attenuation” (ATN) is defined as:
where I and I0 are the intensities transmitted through the sample and blank filters respectively.
The Attenuation is related to the surface loading density of BC by the relation
where the BC density (in units of μg∙cm−2) is determined by dividing ATN by the “Specific Attenuation” coefficient σ. The usual value of σ recommended by the instrument manufacturer is 16.6 cm 2∙μg−1 for the analysis at 880 nm of samples collected on quartz fiber filters. The BC density is then converted to a BC concentration fraction by mass.
Specific methods-development steps were required in order to apply this technique to the analysis of sediment samples. These were: 1) to weigh a sequence of different masses for each sample; 2) to pre-treat the samples by seven different procedures for removal of carbonate, OC and silicate; 3) to collect the samples by filtration onto quartz-fiber filters; and 4) to measure the BC content. These procedures are described in detail in the following sections.
Previous studies show that the BC content of loess is generally low, in the range of 0.010% - 0.054% (Han et al., 2007), and 0.041% - 0.572% (Wang et al., 2005). Thus, the BC content of samples may be below detection limits for small sample masses. However, too large a sample mass may result in an inaccurate measurement of BC content if the sample filter is too dark to be measured by optical methods, or if grains of the sample drop off from the filter.
Before the samples were weighed, all soils were lightly ground with an agate mortar in order to remove adhesion aggregates and to mix the samples uniformly. Aliquots of 5 mg, 10 mg, 15 mg, 20 mg and 25 mg were weighed out for each sample (
In loess samples, there are three forms of carbon: BC (or elemental carbon); OC; and carbonate. Various pretreatment procedures were used to remove carbonate and OC from loess samples. Following the methods of other authors [
Seven pretreatment procedures were designed for use in this study. In all procedures except the HF pretreatment, approximately 3 ml of (NaPO3)6 was added as a dispersant (
(1) “(NaPO3)6 pretreatment”: Add 30% (NaPO3)6 as dispersant
This pretreatment was intended to disperse any cemented aggregates in the loess samples. Approximately 10 ml
Sample location | Sample number | loess-paleosol formation units |
---|---|---|
Weinan | WN05-I-A-32 | Paleosol of Holocene Optimal, S0, i.e. Marine isotopic stage-1(MIS-1) |
WN05-I-A-103 | Loess of the last glacial maximum (LGM), L1-1, i.e. Marine isotopic stage-2 (MIS-2) | |
WN05-II-A-8 | Weakly-developed paleosol of the last glacial period, L1-2, i.e. Marine isotopic stage-3 (MIS-3) | |
WN05-III-A-24 | Loess of the last glacial period, L1-5, i.e. Marine isotopic stage-4 (MIS-4) | |
WN05-IV-A-6 | Paleosol of the last interglacial period, S1, i.e. Marine isotopic stage-5 (MIS-5) |
Procedures | Sample mass (mg) | Pretreatmentsa | ||||
---|---|---|---|---|---|---|
1 | 5 | 10 | 15 | 20 | 25 | Only dispersant |
2 | 5 | 10 | 15 | 20 | 25 | 1) 2 N HCl; 2) dispersant |
3 | 5 | 10 | 15 | 20 | 25 | 1) 30% H2O2; 2)dispersant |
4 | 5 | 10 | 15 | 20 | 25 | 1) 2 N HCl; 2) 30% H2O2; 3) dispersant |
5 | 5 | 10 | 15 | 20 | 25 | 1) 30% H2O2; 2) 2 N HCl; 3) dispersant |
6 | 10 | 15 | 20 | 25 | 200b | 1) 2 N HCl; 2) 48% HF |
7 | 10 | 15 | 20 | 25 | 200 | 1) 2 N HCl; 2) 30% H2O2; 3) 48% HF |
aAcid treatment and H2O2 treatment in all procedures were similar and waited for 24 hrs to ensure that the reaction could run to completion. bIn HF-added pretreatments, 200 mg mass of samples follows the former work of Wang et al. (2005) [
of de-ionized water was put into the tubes with the weighed samples, and then ~3 ml of (NaPO3)6 were added. The tubes were shaken lightly to mix the sample uniformly, and then kept at room temperature for at least 24 hrs.
(2) “HCl pretreatment”: Add 2N HCl for carbonate removal
The samples were digested in 3 - 10 ml of 2 N hydrochloric acid (HCl) at room temperature for at least 24 hrs to remove carbonates.
(3) “H2O2 pretreatment”: Add 30% H2O2 for OC removal
The samples were digested in 3 - 5 ml of 30% H2O2 at 140˚C. After the bubble of OC reaction disappeared, the residues were laid at room temperature for at least 24 hrs to remove OC completely.
(4) “HCl + H2O2 pretreatment”: Add 2 N HCl and then 30% H2O2
The samples were first digested in 3 - 10 ml of 2 N HCl at room temperature for at least 24 hrs to remove carbonates. Then, OC removal was under the same condition as procedure (3).
(5) “H2O2 + HCl pretreatment”: Add 30% H2O2 and then 2 N HCl
OC removal was under the same condition as procedure (3). Then, 3 - 10 ml of 2 N HCl was added to remove carbonates under the same conditions.
(6) “HCl + HF pretreatment”: Add 2 N HCl and then 48% HF
The samples were first digested in 3 - 10 ml of 2 N HCl at room temperature for at least 24 hrs to remove carbonates. Next, 10 - 15 ml of 48% HF: 2 N HCl (1:1 mixture) was added for another 24 hrs to remove silicates. Finally, the residues were separated from the supernatant by centrifugation (3500 rpm for 10 min) and rinsed with de-ionized water 3 or 4 times until neutral.
(7) “HCl + H2O2 + HF pretreatment”: Add 2 N HCl, then 30% H2O2 and finally 48% HF: 2 N HCl mixture
After HCl and H2O2 pretreatment as described in (4) above, add HF as in (6) above.
Before being used for sample collection, the blank quartz filters (of 47 mm diameter) were heated in a muffle furnace at 850˚C for 3 hrs to remove any adsorbed organic vapors or previous BC content.
After the pretreatments described above, the sample residues were moved to 500 ml quartz beakers, diluted with 200 ml of de-ionized water, and then drawn through the pre-fired quartz fiber filters. These loaded filters were placed on clean tinfoil and were dried in an oven at 50˚C for at least 12 hrs. All final samples for analysis were kept in individual sample boxes and stored in a refrigerator before optical analysis.
In loess sediments, there are various minerals that may influence the absorption of light at 880 nm passing through the sample filters. It is necessary to consider the contribution of both the BC and the mineral content to the attenuation of transmitted light. The optical attenuation of sample filter is [
where [BC] represents the mass per unit area of black carbon in the sample on the filter, σmineral 1 represents the specific attenuation coefficient of the mineral content in its initial form, and [mineral] represents the mass per unit area of mineral compounds on the filter.
After heating to a high temperature in an oxygen-containing atmosphere, any BC in the sample will be completely burned away [
where σmineral 2 is the specific attenuation coefficient of the mineral content after heating; and which may be different from σmineral 1.
The difference of the two measured ATN values (ΔATN) can be used to determine the BC concentration of the samples.
where
This method was firstly developed by Hansen et al to eliminate the effect of dust on BC measurement of aerosol samples [
Before measuring BC content, it was necessary to investigate the repeatability of the OT-21 instrument in terms of the deviation in measurements between repetitive placements of a given sample filter, and the deviation in measurements of several replicates of the same sample.
Five sample filters (ME4-46, ME4-47, ME4-48, ME4-49, and ME4-50) were derived from a specific paleosol S0 sample (WN05-I-A-32). The sample mass was about 20 mg, and the “HCl + H2O2 pretreatment” (procedure 4) was used.
Precision Analysis (1): Repeatability of OT-21 instrument.
The five sample filters were measured identically. Each was analyzed 5 times in succession to calculate the average measurement value (mean) and the standard deviation (SD). The deviation represented the repeatability of measurement by the OT-21 instrument.
Precision Analysis (2): Deviation in measurement of repetitive placements of a given sample.
Each sample filter after heating was analyzed multiple times by repetitive placement in the OT-21. After each sample was measured, it was taken out, and then put back in again; and the next measurement was performed. Each filter was tested 5 times and the mean and SD were calculated (
Sample filter | Mass (mg) | Precise analysis (1) | Precise analysis (2) | BC (ng∙mg−1) | ||||
---|---|---|---|---|---|---|---|---|
STD | Mean (ng∙mg−1) | S/M (%)a | STD | Mean (ng∙mg−1) | S/M (%) | |||
ME4-46 | 20.27 | 3.88 | 550 | 0.70 | 5.09 | 270 | 1.88 | 280 |
ME4-47 | 20.11 | 1.87 | 479 | 0.39 | 2.70 | 229 | 1.18 | 250 |
ME4-48 | 20.23 | 1.45 | 611 | 0.24 | 3.39 | 332 | 1.02 | 279 |
ME4-49 | 20.32 | 2.31 | 527 | 0.44 | 5.12 | 266 | 1.92 | 261 |
ME4-50 | 20.23 | 1.57 | 467 | 0.34 | 9.40 | 211 | 4.45 | 256 |
Mean = 265 STD = 13.6 S/M = 5.14% |
aS/M (%) =100 × STD/mean. It represented the precise of measurement.
0.70% and the minimum was 0.24%. For Precision Analysis (2), the S/M ratio was the order of 1.02% - 4.45% (
For multiple replicates, the SD of ΔATN was 0.47. The S/M ratio suggests a measurement precision of 5.6% (
For five replicates of the same sample, the mineral composition and grain size should be identical. Thus, the positive value of the measured ΔATN might result from the slight difference of BC distribution, or changes in the mineral properties after heating the samples on these quartz filters. Indeed, Schmidt et al. found that the BC concentration measured for individual soil samples varied over 2 orders of magnitude in multiple measurements [
Whether or not the mineral content changes its optical properties after heating, is a central issue. If we use the assumption that the absorption of light at 880 nm by the mineral content of the sample is unchanged by heating, we may represent this as σmineral 1 = σmineral 2. Thus, based on Equation (5), ΔATN can be used to calculate the BC concentration directly [
Results of BC measurements using this assumption are shown in
It is noticeable that some measured BC values, especially those obtained using HF pretreatment, were smaller than zero (Figures 1(a)-(d)). This indicates that the measured value of ATN for a sample filter after heating is larger than before heating.
Comparison between different pretreatment procedures
The BC values measured for samples undergoing the seven different pretreatments were compared with each other in order to reveal the possible effects of differing pretreatment procedures on the BC result.
1) Comparison between “(NaPO3)6 pretreatment” and “HCl pretreatment”
In most cases, the measured BC value of samples given pretreatment procedure 1 (adding (NaPO3)6 as dispersant) is a little larger than those given procedure 2 (adding HCl for carbonate removal), as shown in
2) Comparison between “HCl pretreatment” and “HCl + H2O2 pretreatment”
Most of the results obtained using pretreatment procedure 2 (adding HCl) are lower than those obtained using pretreatment procedure 4 (adding HCl + H2O2 to remove carbonate and OC), as shown in
Similar to the above situation (2), the BC results using pretreatment procedure 2 were higher than those using procedure 4 for the L1-1 sample.
3) Comparison between “(NaPO3)6 pretreatment” and “HF acid pretreatment”
The result obtained using pretreatment procedure 1 were compared with those using procedure 7 (adding HCl + H2O2 + HF) with OC removal, and procedure 6 (adding HCl + HF) without OC removal. Most of the BC measurements using procedure 1 are far greater than those using either HF procedure, as shown in
4) Comparison between “HCl pretreatment” and “HF pretreatments”
In order to analyze the effect of acid pretreatments on measurements of the BC content of sediments, the result of the HCl pretreatment was compared with that of the HF pretreatments, as shown in
the S0 paleosol sample.
5) Comparison between “(NaPO3)6 pretreatment” and “H2O2 pretreatment”
6) Comparison between “H2O2 + HCl pretreatment” and “H2O2 pretreatment”
7) Comparison between “H2O2 + HCl pretreatment” and “HCl + H2O2 pretreatment”
To analyze the possible effect of the sequential order of H2O2 and HCl pretreatments, the BC results after the H2O2 + HCl pretreatment were compared with those after the HCl + H2O2 pretreatment.
We may summarize the above results to state that an H2O2 pretreatment may usually result in a positive bias in BC measurement; while acid pretreatments, especially HF pretreatment, may lead to a negative bias on the BC measurement.
1) Trends of optical attenuation (ATN) of different samples before and after heat treatment
2) The ΔATN results and the determination of BC content
The process of heating the sample filter to a high temperature may change the absorption of light at 880 nm by the mineral content. This is represented in Equation (6) as the relation σmineral 1 ≠ σmineral 2.
The relationship between ΔATN and sample mass, shown in
where the intercept C of the regression lines is a constant, which can represent the effect of heating to the optical absorption of the sample filter.
Therefore, the equation (6) is rewritten as the following
Let P represents the slope σBC × a. Then, the BC content can be gotten from the formula a = P/σBC.
The equation describes results shown in
pretreatment procedures or a possible tiny negative bias caused by HF pretreatment procedures. It is because no minerals but BC were left on the quartz-fiber filter in HF pretreatments. Therefore, it can be concluded that Δσmineral is low in most cases (i.e., Δσmineral ≈ 0) and can be neglected for the measurement of BC content on 880-nm wavelength light.
3) Comparison of BC contents of various pretreatments
In order to eliminate the impact of minerals on BC concentration, the slope of ΔATN will be used to determine BC concentration, because the change in specific attenuation coefficient of minerals after heating is very tiny (Δσmineral ≈ 0). The result is shown in
It may be concluded that the (NaPO3)6 pretreatment is a relatively simplified procedure which does not materially change the properties of the samples. In the (NaPO3)6 pretreatment (
When the sample mass is at the 200 mg level, BC values obtained using HF pretreatment (with and without OC removal) estimated by Equation (6), have the same order of magnitude as those obtained using other pretreatment methods, estimated by the slope of Equation (9). Some quartz-fiber filters of 200 mg sample mass cannot be measured because these filters are too thick for the 880-nm transmitted light. This suggests that samples of mass 50 - 150 mg may be suitable for BC measurement using the OT-21 optical method after HF pretreatment. However,
4) Comparison of the intercepts of ΔATN lines of various pretreatments
Most of the values of the ΔATN intercepts after different pretreatment procedures are negative, and the trends are also similar. However,
Hansen et al. had assumed that the optical properties of the mineral content were not changed by the heating process [
There may be a possible influence of some minerals on the results after different pretreatments, except after HF pretreatment (see
Compared with the (NaPO3)6 pretreatment, H2O2 pretreatment may usually result in a positive bias in the BC determination by according to Equation (7) based on the assumption [
In the HF pretreatment process, some BC particles may be lost during the centrifugation and rinsing procedures, leading to an underestimation of BC. It was noticed that some of the ATN results before heated are negative for the HF pretreatment (see
An analysis method based on optical transmission using the OT-21 instrument was developed to measure the BC content of sediment samples.
1) Our progress was the improvement of optical method for BC measurement of dust aerosol samples. An experimental procedure for the BC optical measurement was developed in this study. Pretreated samples were added to de-ionized water and collected on quartz fiber filters. The optical attenuation of this filter was then measured. The subtraction ΔATN of the two measured attenuation values―before and after the sample filters were heated at 850˚C for 3 hours―was used to determine the BC concentration.
2) Various pretreatment processes of the loess samples were studied. The variations of measured BC in the loess
section are similar for all the pretreatments and the BC value of different samples was comparable if the samples were pretreated by the same procedure. The (NaPO3)6 pretreatment is the simplest and best method for the BC optical measurement.
3) ΔATN was always linear with sample mass for all pretreatment procedures. The slope of the regression line of ΔATN with sample mass can be interpreted in terms of the BC concentration. Thus, using this slope to determine the BC concentration is the best optical measurement method.
4) Replicate analyses of BC for loess samples showed a deviation less than 5.6%.
We would like to thank Liu Yuan and Lu Haiyan for technical assistance with the OT-21 instrument, Ning Bo for sampling, Wang Xu for useful discussion, Gu Zhaoyan and Xu Bing for providing the pretreatment laboratory, respectively. We also particularly acknowledge Tony Hansen for his important suggestions and the revision in English grammar. This work is financially supported by projects 411,721,158 and 40,772,212 of the National Natural Science Foundation of China, the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. 241KZCX2-YW-Q1-03) and 973 Program (2010CB950200) of the National Basic Research Program of China.