Anal. Chem. 1983, 5 5 , 1569-1572
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Determination of Elemental Carbon in Rainwater John A. Ogren" and Robert J. Charlson Envlronmental Engineering and Science Program, Department of Civil Englneering, FC-05, University of Washington, Seattle, Washington 98 195
Peter J. Grobllckl Environmental Science Department, General Motors Research Laboratory, Warren, Michigan 48090
A three-step procedure for separatlng elemental carbon from organic and biogenic carbon filtered from rainwater samples has been developed. S(amplesare flrst passed through a flne nylon mesh to remove large partlcles, such as insects, leaves, and pollen, and then 8118 flltered through a quartz-fiber fllter. Biogenic carbon on the filter is removed by oxldatlon in a basic peroxide solution, and any remalnlng nonelemental carbon Is removed by volatiilratlon in an Inert atmosphere at 900 OC. Elemental carbon Is then determined by oxldation to CO, followed by detection In a nondispersive infrared analyzer. The method Is capable of measuring masses of eiemental carbon as low as 10 pg with an accuracy better than &:50%. The accuracy of the method Is limlted by the uncertalntles In the approach used to determlne the flltratlon efficiency of the quartz-fiber fllters. Measurements of rainwater samples from Sleattle and rural Sweden yielded EC concentrations in the range 20-600 pg L-'.
Elemental carbon (EC) has been identified as an important constituent of the submicrometer atmospheric aerosol (I),has been shown to be responsible for much of the absorption of solar radiation by airborne particles (2),and is suggested to provide a catalytic surfitce within cloud and rain droplets (3). In order to understand the behavior of EC in the atmosphere, and hence its effects, it is necessary to study the manner in which it is incorporated into atmospheric liquid water and removed from air. The removal mechanisms for EC are expected to be either dirlect contact with surfaces (dry deposition) or incorporation into rain or snow (wet deposition); chemical reactions are not expected to act as a sink for EC because of its low reactivity (4). As a fiist step toward studying these mechanisms of removal from the atmosphere, a method has been developed for determining EEC in rainwater (or melted snow) samples, as well as in samples of dry deposition. The approach is designed to exclude EC particles larger than 5 bm diameter, which have sufficiently high gravitational settling velocities (and thus short atmospheric residence times) that their contribution to atmospheric chemical and physical processes is thought to be minor in comparison with smaller EC particles. Measurement of elemental carbon filtered from rainwater samples uses a technique which volatilizeel organics in a helium atmosphere to separate them from elemental carbon (5). One drawback of this approach is that some compounds, particularly natural products, tend to char during the volatilization step and are detected as elemental carbon. Precipitation samples, particularly those collected over an extended period of time, often contain 1,argeamounts of biogenic carbon (e.g., bacteria, algae, pollen, insects) which are prone to charring. The method described here uses two approaches to remove Present address: De artment of Meteorology, University of Stockholm, Arrhenius Laioratory, S-10691, Stockholm, Sweden. 0003-27 00/83/0355-1569$0 1.50/0
this interference: prefiltering the sample through a nylon mesh to remove particles greater than about 5 pm diameter, and oxidation of organic carbon with hydrogen peroxide in a basic solution (6). This combined three-step procedure (prefiltration, digestion, volatilization) effectively removes organic carbon from a rainwater sample without affecting the submicrometer elemental carbon. This approach has the potential to remove charring compounds from ambient aerosol samples as well, although this has not been proven as part of the present study. EXPERIMENTAL SECTION Sample Collection. Samples are collected in glass containers that have been cleaned with a solvent (l,l,l-trichloroethane)prior to use. The size of the collector depends on the amount of rainfall expected during the sampling period; cross-sectional areas of 0.0045 and 0.022 m2have been used successfully for monthly and weekly sampling periods, respectively. If the sample cannot be refrigerated shortly after collection, a biocide may be used to hinder the growth of bacteria, algae, or fungi. No completely suitable agent has yet been found, although 5 mg of pentachlorophenol dissolved in 1 mL of methanol prevented visible biological growth in rain samples collected over a week's time. Filtration. Liquid samples, as well as all collector washes and rinses, are dispersed in an ultrasonic bath for 15 min and then passed through a 5-hm nylon mesh prefilter (Nitex HD 3-5, Tetco, Monterey Park, CA) to remove large particles such as leaves, insects and pollen grains. After the sample volume is recorded, the collector is washed carefully with a dilute surfactant (0.01 g L-l sodium lauryl sulfate)and a rubber policeman, and then rinsed with filtered, deionized water. In some locations, the walls of the collector may be coated with gummy materials which are not removed by the surfactant washes. In such cases, methanol is used to clean the container. Collectors of dry deposition are washed in the same fashion as wet samplers and processed identically thereafter. The combined sample, wash water, and rinse water are filtered three times through a prebaked, 25 mm diameter quartz-fiber filter (2500 QAST, Pallflex Corp., Putnam, MA). The filtration apparatus is custom-fabricated entirely of borosilicate glass and is a scaled-down version of a comercially available apparatus (P.N. XX1504700, Millipore Corp., Bedford, MA). Quartz-fiber filters are not completely efficient collectors of submicrometer EC. Accordingly, the filtrate is next passed through a polycarbonate membrane filter (0.4 W m pore diameter, Nuclepore Corp., Pleasanton, CA) to collect any remaining EC. While this substrate is incompatible with the combustion analysis for carbon, it can be analyzed for optical absorption (7-IO), from which the amount of EC on the filter is estimated by using an assumed absorptivity of 3 m2g-'. Filtration efficiencies determined from this backup filter are generally 50-80%. (Note: absorbance and absorptivities reported in this paper are calculated by using base 10 logarithms for consistency with the chemical literature. These values must be multiplied by 2.3 to convert them to the optical depth and specific absorption units used in the radiative transfer literature.) On occasion the quartz-fiber or membrane filter becomes plugged during the filtration step. In this case, a plugged quartz-fiber filter is replaced with a fresh one, and the two (or more if needed) filters are combined in the oxidation step. If the membrane fiiter plugs, the fraction of the total sample that passed 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
through it is recorded and its absorbance is scaled accordingly. Oxidation of Organic Carbon. The procedure for removing organic carbon from the quartz-fiber Yiters is based on the method reported by Smith et al. (6). The filter is placed in a glass vial, and 4 mL of a solution of equal parts of 30% HzOzand 2 M KOH is added. The vial is loosely capped t o allow gaseous reaction products to escape and is placed in a room temperature water bath to dissipate the heat of reaction. After a reaction time of 20 h, the vial is placed in a boiling water bath for 30 min to decompose any remaining hydrogen peroxide. The fiial step prior to refiltration of the sample is dispersion of the entire original filter and the unknown amount of EC in an ultrasonic bath for 30 min. The digested sample is passed three times through an 11mm diameter quartz-fiberfilter mat along with the water used to rinse the digestion vial and filtration apparatus after each filtration step (50-100 mL total). This mat is made by grinding a 25 mm diameter filter into a pulp with a glass rod, diluting with water, and passing the pulp through a stainless steel support screen in the filtration apparatus. The resulting pellet is small enough to fit directly in the carbon analyzer (5). A second membrane backup filter is used to collect any EC which passes through the quartz-fiber mat. Analytical. The quartz-fiberfilters are analyzed for elemental carbon by using the method of Cadle et al. (5). This method involves heating the sample to 900 "C in a helium atmosphere to remove volatile carbon, followed by introduction of oxygen to oxidize elemental carbon. This step is needed even after the digestion step because 50-100 pg of volatile carbon is generally found on the digested filters, perhaps due to bicarbonate ions sorbed on the quartz-fiber surface in the strongly basic digestion solution. The evolved gases pass through a catalyst to complete the oxidation to COz, which is detected with a nondispersive infrared analyzer. Filters are analyzed for optical absorption by using the integrating plate method (7-9) or integrating sandwich method (IO), operating at a wavelength of 550 nm. These techniques utilize optical configurations that eliminate the effects of scattering by the particles on the filter, such that the change in transmittance of the filter after sampling can be attributed solely to absorption by the particles. Results from both of these optical techniques are reported in terms of the absorbance of the sample, which is the absorbance of the deposit on the filter adjusted for the fraction of the total sample which passed through the filter. An empirical multiplier of 0.38 is applied to absorbances measured on quartz-fiber fiiters to correct for multiple-scattering and absorption within the filter. This value is slightly different from the factor of 0.35 reported by Sadler et al. (11) and is based on reanalysis of Sadler's measurements and a similar (but more extensive) set of measurements obtained by Edwards (12). Reagents. Reagent-grade l,l,l-trichloroethane, potassium hydroxide, hydrogen peroxide, sodium lauryl sulfate, pentachlorophenol, and methanol are used. All liquids are fiitered twice prior to use through 0.4 pm pore Nuclepore membrane filters, with the exceptions of the sodium lauryl sulfate (0.45 pm pore Millipore HA filter) and the l,l,l-trichloroethane (quartz-fiber filter). Filtered, deionized water is used for all sample processing. Standards. A commercial carbon black (Monarch 71, Cabot Corp., Billerica, MA) is used to prepare standards for validating the method. This carbon black is reported to have an average primary particle diameter of 0.016 pm, a volatile content of 4.2%, and an absorptivity of 4.2 m2 g-' (13). Assay of the carbon black yielded a carbon content of 91%, determined by weighing out about 1mg on a Cahn 4100 electrobalance and measuring the mass of total carbon with a Carlo Erba 1106 elemental analyzer. Calibration standards are prepared by weighing out several milligrams of the black on a Cahn 4700 electrobalance and adding it to about 500 mL of filtered, deionized water. About 2 mg of surfactant (sodium lauryl sulfate) is added to the sample, which is then dispersed for about 1day in an ultrasonic bath. Adequate dispersal is confirmed by passing the colloid through a 5-pm nylon mesh and observing no darkening of the mesh. Prior to each use, the standard is redispersed in the ultrasonic bath for several hours.
RESULTS AND DISCUSSION Calibration, Precision, a n d Detection Limit. Calibra-
tion of the method was accomplished by using 10-mL aliquots of a standard suspension containing 5.52 mg L-l of Monarch 71 carbon black (carbon content 5.04 mg L-l). The aliquots were subjected to the complete treatment described above; for comparison another set of aliquots were subjected to the same procedure without the basic peroxide digestion. Four undigested samples yielded an average concentration of 5.34 mg L-' and an absorptivity of 4.5 f 1 m2g-l, while four digested samples had an average concentration of 5.58 mg L-l. This is considered to be excellent agreement given the &lo% accuracy reported for the carbon analyzer (5). High filtration efficiencies were achieved for these heavily loaded samples, so that the uncertainties associated with the filter blanks and with the assumed absorptivity of the EC on the backup filters did not have much effect on the results. The precision of the method for filtering EC from water was evaluated by determining the filter absorbance (assumed to be proportional to EC mass loading) of multiple aliquots taken from both the calibration standard and actual rain samples. The relative standard deviation of absorbances measured on five membranes filters taken from a rain sample was 5%, while relative standard deviations of absorbances obtained from the calibration standards were generally below 10%. The relative standard deviation of the EC concentration in the calibration standard, measured using the full filtration and digestion procedure above, was 6% for four samples. The detection limit of this technique is limited by variations in the carbon content of blank samples. Two clean glass sample collectors, treated in the same manner as actual samples, yielded 0.1 and 0.5 pg of apparent elemental carbon. Six blank filters, processed as if they were samples, yielded 1-3 pg of apparent elemental carbon, with a mean value of 1.8 pg. Filter samples of the reagents used in sample processing were all below these blank values, although a bulk sample of pentachlorophenol crystals had 10% of its mass appear as elemental carbon. This charring did not appear when the pentachlorophenol was dissolved in a water sample, probably because that fraction that was not dissolved in the water was captured by the nylon mesh. Accordingly, a blank value of 1.5 f 1.0 pg is subtracted from all measurements of EC using this method. Validation. Losses to the walls of the sample collectors were evaluated by washing the vessel with the surfactant solution, followed by repeated washes of the collectors with l,l,l-trichloroethane. The wash solutions were filtered through quartz-fiber filters, which were subsequently analyzed for optical absorption. This approach revealed that surfactant washes remove as little as 25% of the sample absorbance from the walls of some high-density polyethylene buckets, but that over 90% is removed from Nalgene high-density polyethylene bottles. Similar tests with stainless steel and glass beakers indicated that over 80% and 98%, respectively, of the sample absorbance is removed by the surfactant washes. Accordingly, glassware was chosen for sample collection and processing. All glassware, however, requires extensive cleaning with a solvent prior to use to remove absorbing particles deposited on the surface during the manufacturing process. Generally, three washes and rinses are required before no visible deposit is observed when the cleaning solvent is passed through a quartz-fiber filter. Losses of submicrometer particles to the nylon mesh prefilter were assessed by repeatedly passing an aliquot of the calibration standard through the mesh and periodically filtering extracts of the sample through membrane filters. The absorbance of these filters is plotted in Figure 1as a function of the number of passes through the mesh. Analysis of this figure indicates that less than 2% of the absorption is removed by each pass through the mesh. Losses of submicrometer EC
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
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Table I. Recovery of Elemental Carbon from an Aged Rain Sample amt of c
0
1
m
/
slope of least-squares line corresponds to 1.3 i 0.4 percent loss per (pass
1 1
1
EC added itg 0.0 5.3 10.7 16.0 21.4
(XI,
amt of EC determined (Y),f i g
arnt of EC added
amt of EC determined
(X),Pg
(Y),k g
8.5 12.6 14.8 15.7 25.3
26.7 37.4 42.7 48.1 53.4
27.8 42.5 46.1 61.8 12.8
least-squares regression line: y = (2.5 p g i 14.7 p g ) t (1.16 I 0.23), n = 10, r 2 = 0.94; uncertainties are 95% confidence limits.
0
5 10 15 20 Numbisr of Passes Through Mesh
Flgure 1. Loss of elemental carbon to mesh prefilter. Error bars indicate the precision of the method for filtration and absorbance measurement (f5%).
by attachment to larger particles that are removed by the mesh are thought to be minor, as mesh prefilkrs from rain samples are observed to be much less discolored than the corresponding quartz-fiber filters. Furthermore, the coloration of mesh prefilters is usually tan or brown, whereas the quartz-fiber filters tend to show thle grayish color typical of EC. The approach to determining the collection efficiency of the quartz-fiber filters was developed by first comparing the absorbance of rain samples measured from an extract on a membrane filter with that measured from an extract on a quartz-filter with a backup membrane filter. Sample absorbances determined with these two methods were found to be equivalent, as evidenced by a least-squares regression line with a slope of 1.00 f 0.11 and an intercept of 0.01 f 0.15 (uncertainties are 95% confidence limits, n = 2 5 , 3 = 0.93). Next, evaluation of the absorptivity of 10 rain samples collected on the University of Washington campus yielded values in the range 2.1-3.3 m2 g-l, with a mean value of 2.6 m2 g-l. On the basis of this result, the measured absorptivity of the carbon black calibration standiard, and absorptivity measurements of 3-5 m2 g-l for EC aerosols reported in the literature (8, 14-16), a value of 3 m2 g-' k50% was chosen as a representative value for estimating the amount of EC on the backup membrane filters. The error in total sample mass introduced by assuming this consi;ant value for the absorptivity of EC depends on the filtration efficiency of the quartz-fiber filter but is generally below 15% for samples clontaining more than 10 pg of EC. The presence of other absorbing substances would produce an overestimate of the amount of EC on the backup filter, resulting in as much as a 50% overestimate in EC mass for samples with poor filtration efficiency of the quartz-fiber filter. Two tests were performed to evaluate the effectiveness of the method in removing biogenic carbon without affecting the elemental fraction. Algae cultures (Selenastrurn capricornaturn) grown under laboratory conditions were evaporated to dryness and the residue was treated with the basic peroxide solution. Five samples yielded a mean value of 0.3 pg of apparent elemental carbon, indistinguishable from blank sample values, as compared to a mean value of 34 pg of apparent elemental carbon for two untreated algae samples of the same volume. This demonstrates that algae, of which 5%
of the mass chars, is completely removed by the digestion procedure. As a test of the overall method, aliquots of a well-aged rain sample containing large amounts of visible algae were spiked with various amounts of the EC calibration standard and then extracted and analyzed. The results of this test are presented in Table I. The fact that the slope of the least-squares line is close to one indicates that the treatment does not alter the EC to a large extent, while the intercept of 2.5 pg, corresponding to an EC concentration in the rainwater of 100 pg L-l, is a reasonable value for samples collected at the University of Washington (see below). The mean deviation from the least-squares line of 4.4 pg provides another indication of the precision of the overall method. On the basis of these results, the overall method is seen to be capable of determining EC concentrations in biologically contaminated rain samples within f50% provided that at least 10 pg of EC is collected. Alternative Digestion Techniques. A number of other processes for removal of biogenic carbon were tested and rejected because either they removed optical absorption from carbon black on quartz-fiber filters or they did not remove all the algae used for the tests. Treatments in the former category include 6 M and concentrated nitric acid (25 OC and 100 "C), as well as sodium hypochlorite. The latter category included hydrolysis with 6 M hydrochloric and sulfuric acids and dissolution of the cellulose associated with biogenic carbon by acetylizatiort with glacial acetic acid and acetic anhydride. The acetylization treatment, which was catalyzed with a few drops of sulfuric acid ( I 7), was effective in removing all the visible absorption produced by the algae but produced erratic results in the carbon analysis. This could be due to variations in the degree of substitution of acetyl groups for OH groups on the cellulose, in which case a more carefully controlled process might yield consistent results without disturbing the EC or the quartz-fiber filter. Such an approach would allow analysis of the absorbance of the quartz-fiber filters following digestion, eliminating the need to assume a value for the absorptivity of the EC on the backup filter. Results of Ambient Measurements. This method has been used to measure EC concentrations in rainwater on the University of Washington campus in Seattle (an urban location) and at 12 rural sites in Sweden. Concentrations of EC in weekly samples collected during the winter of 1980-1981 in Seattle range from 30 to 400 pg L-, with a median of 60 pg L-I (seven samples). Monthly samples collected from April to August, 1981, in Sweden had a range of 20-600 pg L-l, with a median of 180 pg L-l (58 samples). These measurements will be discussed in detail in subsequent publications. ACKNOWLEDGMENT We wish to thank Jost Heintzenberg (University of Stockholm) for suggesting the use of Monarch 71 carbon black as a calibration standard, John Hedges (University of Washington) for determining the carbon content in the carbon
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Anal. Chem. 1983, 55, 1572-1575
black, and Carolina Ang (General Motors Research Laboratory) for performing the carbon analyses. Registry No. Carbon, 7440-44-0; water, 7732-18-5.
(10) Clarke, A. D. Appl. Opt. 1982, 2 1 , 3011-3019. (11) Sadler, M.; Charlson, R. J.; Rosen, H.; Novakov, T. Atmos. Environ. 1981, 15, 1265-1268. (12) Edwards, J. D., Master's thesis university of Washington, Seattle,
