Light-absorbing Carbon Research Articles

Diurnal variation of atmospheric aerosol during the wood combustion season in Northern Sweden

A set of aerosol measurements was conducted in the residential area of Forsdala in Lycksele, Northern Sweden, during winter 2005/2006. This article describes the temporal and diurnal variation of the aerosol physical properties (concentrations of PM10, PM1, light-absorbing carbon, and particle number, and number size distributions), and the relationship among aerosol concentrations and meteorological variables. A large day-to-day and hour-to-hour variability in aerosol concentrations was observed during the intensive study period. Evening aerosol concentrations were statistically significantly higher on weekends than on weekdays. On weekdays, particle size distribution and concentrations varied diurnally with small particles (diameter <30nm) associated mainly with morning motor vehicle emissions. The results suggest that a combination of emissions from residential wood combustion and traffic sources might explain the high evening concentrations of PM10, PM1, particle number, and light-absorbing carbon as well as large geometric mean diameters observed during weekdays and weekends. Strong correlations of PM10 and PM1 with particle size distributions are found in the diameter range 130–500nm and are remarkably high on weekend evenings when larger particles are sampled. The correlation between light-absorbing carbon mass concentration and particle size distribution is high regarding both particle number and mass for particle diameters >95nm. High aerosol concentrations were associated with low air temperatures and very stable atmospheric conditions close to the ground.

Changes in Lung Function and Airway Inflammation Among Asthmatic Children Residing in a Woodsmoke-Impacted Urban Area

Fine particulate matter (PM2.5) is associated with respiratory effects, and asthmatic children are especially sensitive. Preliminary evidence suggests that combustion-derived particles play an important role. Our objective was to evaluate effect estimates from different PM2.5 exposure metrics in relation to airway inflammation and lung function among children residing in woodsmoke-impacted areas of Seattle. Nineteen children (ages 6–13 yr) with asthma were monitored during the heating season. We measured 24-h outdoor and personal concentrations of PM2.5 and light-absorbing carbon (LAC). Levoglucosan (LG), a marker of woodsmoke, was also measured outdoors. We partitioned PM2.5 exposure into its ambient-generated (Eag) and nonambient (Ena) components. These exposure metrics were evaluated in relation to daily changes in exhaled nitric oxide (FENO), a marker of airway inflammation, and four lung function measures: midexpiratory flow (MEF), peak expiratory flow (PEF), forced expiratory volume in the first second (FEV1), and forced vital capacity (FVC). Eag, but not Ena, was correlated with combustion markers. Significant associations with respiratory health were seen only among participants not using inhaled corticosteroids. Increases in FENO were associated with personal PM2.5, personal LAC, and Eag but not with ambient PM2.5 or its combustion markers. In contrast, MEF and PEF decrements were associated with ambient PM2.5, its combustion markers, and Eag, but not with personal PM2.5 or personal LAC. FEV1 was associated only with ambient LG. Our results suggest that lung function may be especially sensitive to the combustion-generated component of ambient PM2.5, whereas airway inflammation may be more closely related to some other constituent of the ambient PM2.5 mixture.

On the diurnal variability of particle properties related to light absorbing carbon in Mexico City

Abstract. The mass of light absorbing carbon (LAC) in individual, internally mixed aerosol particles was measured with the Single Particle Soot Photometer (SP2) in April of 2003 and 2005 and evaluated with respect to concentrations of carbon monoxide (CO), particle bound polycyclic aromatic hydrocarbons (PPAH) and condensation nuclei (CN). The LAC and CO have matching diurnal trends that are linked to traffic patterns and boundary layer growth. The PPAH reaches a maximum at the same time as CO and LAC but returns rapidly back to nighttime values within three hours of the peak. The number of particles containing LAC ranges between 10% to 40% of all particles between 150 nm and 650 nm and the mass is between 5% and 25% of the total mass in this size range. The average LAC equivalent mass diameter varies between 160 and 230 nm and the thinnest coating of non-light absorbing material is observed during periods of maximum LAC mass. The coating varies between 10 nm and 30 nm during the day, but is a strong function of particle size. The mass absorption cross sections, σabs, derived from the SP2, are 5.0±0.2 m2g−1 and 4.8±0.2 m2g−1, dependent on the optical model used to describe LAC mixtures. The LAC contributes up to 50% of the total light extinction in the size range from 100 nm to 400 nm. The estimated emission rate of LAC is 1200 metric tons per year in Mexico City, based upon the SP2 measurements and correlations between LAC and CO.

An Inter-Comparison of Two Black Carbon Aerosol Instruments and a Semi-Continuous Elemental Carbon Instrument in the Urban Environment

Aerosol absorption coefficients were obtained using two versions of the Magee Scientific Aethalometer and a Particle Soot Absorption Photometer (PSAP) in Riverside, California during July and August of 2005. These measurements were subsequently compared to each other and to hourly elemental carbon (EC) mass concentrations as determined by a Sunset Labs semi-continuous OCEC analyzer. Measurements from all four instruments were shown to be highly correlated (R 2 = 0.83 to 0.92). Differences between absorption values measured by the PSAP and the Aethalometer were found to be dominated by differences in the filter media used by the respective instruments. Comparison of optical and thermal measurements revealed that the specific attenuation cross section (σ ATN ) of light absorbing carbon (LAC) varied as a function of the time of the day, most notably during weekdays. Minimum σ ATN values were observed during morning rush hour when EC concentrations were at their greatest and maxima were seen in the late after...

Fine Particulate Chemical Composition and Light Extinction at Meadview, AZ

The concentration of fine particulate nitrate, sulfate, and carbonaceous material was measured for 12-hr day-night samples using diffusion denuder samplers during the Project Measurement of Haze and Visibility Effects (MOHAVE) July to August 1992 Summer Intensive study at Meadview, AZ, just west of Grand Canyon National Park. Organic material was measured by several techniques. Only the diffusion denuder method measured the semivolatile organic material. Fine particulate sulfate and nitrate (using denuder technology) determined by various groups agreed. Based on the various collocated measurements obtained during the Project MOHAVE study, the precision of the major fine particulate species was ±0.6 μg/m3 organic material, ±0.3 μg/m3 ammonium sulfate, and ±0.07 μg/m3 ammonium nitrate. Data were also available on fine particulate crustal material, fine and coarse particulate mass from the Interagency Monitoring of Protected Visual Environments sampling system, and relative humidity (RH), light absorption, particle scattering, and light extinction measurements from Project MOHAVE. An extinction budget was obtained using mass scattering coefficients estimated from particle size distribution data. Literature data were used to estimate the change in the mass scattering coefficients for the measured species as a function of RH and for the absorption of light by elemental carbon. Fine particulate organic material was the principal particulate contributor to light extinction during the study period, with fine particulate sulfate as the second most important contributor. During periods of highest light extinction, contributions from fine particulate organic material, sulfate, and light-absorbing carbon dominated the extinction of light by particles. Particle light extinction was dominated by sulfate and organic material during periods of lowest light extinction. Combination of the extinction data and chemical mass balance analysis of sulfur oxides sources in the region indicate that the major anthropogenic contributors to light extinction were from the Los Angeles, CA, and Las Vegas, NV, urban areas. Mohave Power Project associated secondary sulfate was a negligible contributor to light extinction.

Limitations in the enhancement of visible light absorption due to mixing state

Absorption by light‐absorbing carbon (LAC) particles increases when the carbon is mixed with other material, and this change affects climate forcing. We investigate this increase theoretically over a realistic range of particle sizes. Perfect mixing at the molecular level often overestimates absorption. Assuming that LAC is coated by a concentric shell of weakly absorbing material, we calculate absorption by a range of realistic particle sizes and identify regimes in which absorption behaves similarly. We provide fits to amplification in five regions: (1) small cores and (2) intermediate cores, both with large shells; (3) small to intermediate cores with intermediate shells; (4) cores with growing shells; and (5) intermediate to large cores with large shells. Amplification in region 1 is highest but is physically implausible. Amplification in region 5 is constant at about 1.9 and represents an asymptote for particles with broad size distributions. Because absorption by aggregates is amplified by about 1.3 above spherical particles, and that factor is lost when particles are coated, we suggest that absorption by aged aerosol is about 1.5 times greater than that of fresh aerosol. The rate at which particles acquire sufficient coating to increase their original diameter by 60% is important in determining total absorption during their atmospheric lifetimes. Fitted amplification factors are not very sensitive to assumed refractive index of LAC and can be used even in simple models.

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

Abstract. Although the definition and measurement techniques for atmospheric "black carbon" ("BC") or "elemental carbon'' ("EC") have long been subjects of scientific controversy, the recent discovery of light-absorbing carbon that is not black ("brown carbon, Cbrown") makes it imperative to reassess and redefine the components that make up light-absorbing carbonaceous matter (LAC) in the atmosphere. Evidence for the atmospheric presence of Cbrown comes from (1) spectral aerosol light absorption measurements near specific combustion sources, (2) observations of spectral properties of water extracts of continental aerosol, (3) laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and (4) indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that brown carbon may severely bias measurements of "BC" and "EC" over vast parts of the troposphere, especially those strongly polluted by biomass burning, where the mass concentration of Cbrown is high relative to that of soot carbon. Chemical measurements to determine "EC" are biased by the refractory nature of Cbrown as well as by complex matrix interferences. Optical measurements of "BC" suffer from a number of problems: (1) many of the presently used instruments introduce a substantial bias into the determination of aerosol light absorption, (2) there is no unique conversion factor between light absorption and "EC" or "BC" concentration in ambient aerosols, and (3) the difference in spectral properties between the different types of LAC, as well as the chemical complexity of Cbrown, lead to several conceptual as well as practical complications. We also suggest that due to the sharply increasing absorption of Cbrown towards the UV, single-wavelength light absorption measurements may not be adequate for the assessment of absorption of solar radiation in the troposphere. We discuss the possible consequences of these effects for our understanding of tropospheric processes, including their influence on UV-irradiance, atmospheric photochemistry and radiative transfer in clouds.

CCN Activation of Pure and Coated Carbon Black Particles

The CCN (cloud condensation nucleus) activation of pure and coated carbon black particles was investigated using the University of Vienna cloud condensation nuclei counter (Giebl, H.; Berner, A.; Reischl, G.; Puxbaum, H.; Kasper-Giebl, A.; Hitzenberger, R. J. Aerosol Sci. 2002, 33, 1623-1634). The particles were produced by nebulizing an aqueous suspension of carbon black in a Collison atomizer. The activation of pure carbon black particles was found to require higher supersaturations than predicted by calculations representing the particles as insoluble, wettable spheres with mobility equivalent diameter. To test whether this effect is an artifact due to heating of the light-absorbing carbon black particles in the laser beam, experiments at different laser powers were conducted. No systematic dependence of the activation of pure carbon black particles on laser power was observed. The observations could be modeled using spherical particles and an effective contact angle of 4-6 degrees of water at their surface. The addition of a small amount of NaCl to the carbon black particles (by adding 5% by mass NaCl to the carbon black suspension) greatly enhanced their CCN efficiency. The measured CCN efficiencies were consistent with Kohler theory for particles consisting of insoluble and hygroscopic material. However, coating the carbon black particles with hexadecanol (a typical film-forming compound with one hydrophobic and one hydrophilic end) efficiently suppressed the CCN activation of the carbon black particles.

Light Absorption by Carbonaceous Particles: An Investigative Review

The optical properties of the light-absorbing, carbonaceous substance often called “soot,” “black carbon,” or “carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp 2-bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m2/g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for strongly-absorbing carbon particles, of which the highest is 1.95–0.79i. Our refractive indices are consistent with most measurements reported in the literature, and values used in present-day climate modeling are in error. Realistic refractive indices underpredict measured absorption by about 30% when used with common theories for spherical particles or aggregates. Field programs since about 1970 have measured quantities relevant to light absorption, but have only recently made enough measurements to isolate the light-absorbing carbonaceous component and determine its absorptive properties.

Source Apportionment of Indoor, Outdoor, and Personal PM2.5 in Seattle, Washington, Using Positive Matrix Factorization

As part of a large exposure assessment and health-effects panel study, 33 trace elements and light-absorbing carbon were measured on 24-hr fixed-site filter samples for particulate matter with an aerodynamic diameter <2.5 μm (PM2.5) collected between September 26, 2000, and May 25, 2001, at a central outdoor site, immediately outside each subject's residence, inside each residence, and on each subject (personal sample). Both two-way (PMF2) and three-way (PMF3) positive matrix factorization were used to deduce the sources contributing to PM2.5. Five sources contributing to the indoor and outdoor samples were identified: vegetative burning, mobile emissions, secondary sulfate, a source rich in chlorine, and a source of crustal-derived material. Vegetative burning contributed more PM2.5 mass on average than any other source in all microenvironments, with average values estimated by PMF2 and PMF3, respectively, of 7.6 and 8.7 μg/m3 for the outdoor samples, 4 and 5.3 μg/m3 for the indoor samples, and 3.8 and 3.4 μg/m3 for the personal samples. Personal exposure to the combustion-related particles was correlated with outdoor sources, whereas exposure to the crustal and chlorine-rich particles was not. Personal exposures to crustal sources were strongly associated with personal activities, especially time spent at school among the child subjects.

Spatial patterns of major aerosol species and selected heavy metals in the United States

This paper focuses on the spatial variability of aerosol measurements from the IMPROVE (Interagency Monitoring of Protected Visual Environments) monitoring network. Spatial concentration trends of fine aerosol mass (PM 2.5) and five major aerosol species that comprise the bulk of fine mass for 3 years spanning 1993 through 1995 at 43 IMPROVE sites are presented. Using data from 62 monitoring sites spatial trends for select heavy metals for the years 1995 through 1998 are presented with the goal of lending insight to local versus regional sources of fine aerosol mass. The trace elements examined are arsenic, copper, zinc, selenium, lead, bromine, vanadium, and sodium that can be associated with industrial, agricultural, and mobile sources. Fine mass concentrations tend to be highest in the eastern United States and southern California with the lowest concentrations in the inter-mountain west. High concentrations of fine mass in the East are driven by ammonium sulfate concentrations, while in southern California ammonium nitrate is dominant; elsewhere carbonaceous species (light-absorbing carbon and organic mass) are about on par with sulfate as a contributor to fine mass. The trace element arsenic has its highest concentrations in the desert southwest. Zinc and copper show high concentrations in the desert southwest but have high concentrations in the eastern United States. Bromine concentrations are lowest in the Pacific Northwest with a local hot spot in southern California and an increasing trend to the eastern United States peaking in the Mid-Atlantic region. Lead and vanadium tend to be highest in the urbanized areas of the United States, which leads to elevated levels in southern California and much of the eastern United States.

Spatial and Temporal Patterns in Particle Data Measured During the MOHAVE Study

Project MOHAVE (Measurement of Haze and Visual Effects) was conducted during 1992 to assess the contribution of the Mohave Power Project, other point sources, and regional emissions to visibility impairment in Grand Canyon National Park. One objective of Project MOHAVE was to determine the spatial and temporal patterns of aerosol concentrations during summer and winter in the region. Patterns in the fine particle data were examined by time plots, spatial contour maps, and empirical orthogonal function (EOF) analysis, which reduces all observed spatial patterns to a few that explain most of the variance. Results obtained during the summer differed from those obtained in the winter. However, differences were not uniform across species. During the summer there was evidence of transport of sulfur, organic and light-absorbing carbon, and several trace elements from southern California into the study area. Winter concentrations of several species, including sulfur, selenium, and organic and light-absorbing carbon, were higher within the canyon than on the rim, probably because emissions from local sources stagnate within the canyon during the winter. Sources in and near Las Vegas, NV, influenced the concentrations of several species, especially at sites to the north and east.

Source apportionment of organic and light-absorbing carbon using receptor modeling techniques

An intensive field-monitoring program designed to attribute visibility reducing aerosols to their sources or source regions was carried out in the western half of Washington state. Because of the perceived concern over the contribution of carbonaceous material from prescribed and wildfires to visibility impairment, this paper focuses on attribution of organic and light-absorbing carbon to known sources and source regions. A variety of receptor-oriented models was successfully exercised and reconciled with each other. It is shown that much of the organic and light-absorbing carbon is associated with urban emissions of the Seattle-Tacoma area. Fire tracers were not strongly linked to either organic or light-absorbing carbon; however, the fire-related tracers were shown to be primarily associated with transport from south of the Washington-Oregon state border.

The reconstruction of aerosol light absorption by particle measurements at remote sites: An independent analysis of data from the IMPROVE network — II

The author's mutual validation of the IMPROVE measures of light absorption — the light absorption coefficient σ a and the TOR carbon measures — at remote sites in the western United States, has identified more light-absorbing carbon (LAC) than the current interpretation of TOR admits. Further comparison of σ a with the new determination of LAC allows us to identify fine soil as the remaining significant contributor to light absorption at these remote sites, and thus to fully reconstruct σ a there. This reconstruction also confirms the accuracy of the blank corrections to the carbon measurements. Using σ a or the new reconstruction of it given herein as the appropriate measure of light absorption allows more correct reconstructions of aerosol light extinction σ e and of organic mass; the latter provides evidence that the newly identified LAC is also essentially elemental carbon (EC). The new interpretation of the TOR carbons for the remote western sites also reveals apparently much less pyrolysis than previously though occurring during TOR analysis, for most of the aerosol samples collected at these sites. A very small minority population, comprising less than 5% of the samples and occurring mostly in the summer and autumn, is also identified, containing a larger proportion of supposed pyrolyzable organics. The differences in apparent makeup between the two populations strongly suggest that the majority population represents a widespread background of aerosol light absorption which averages 5 Mm −1 and is probably due primarily to diesel fuel emissions transported from urban areas and highways, while the minority population is probably due to wood fires. A number of possible explanations are offered for why the newly identified EC is not currently recognized in the TOR analysis. In particular, it is claimed that sample darkening during thermal analysis is not a reliable quantitative indication of pyrolyzable organics, particularly in the remote aerosols considered; and that optical monitoring of the sample during thermal analysis should be corrected for expected strong multiple scattering effects, as the IMPROVE measurement of σ a is already corrected. It is also hypothesized that internal mixing of some EC with an oxygen-containing species, most probably sulfate, during aerosol transport is the source of the oxygen that allows some of the EC to evolve at a lower temperature in TOR analysis than previously thought, and in a pure helium atmosphere. Further tests are suggested to confirm the new interpretations offered here. The primary importance of the present results, besides unifying a number of IMPROVE analyses and removing major discrepancies in the data, is to highlight what the author believes are the two most important measurement problems in aerosol research today: (1) the failure to recognize and eliminate multiple scattering effects in the optical measurement of light absorption and in optical monitoring of a sample during thermal carbon analysis; and (2) the failure to accurately distinguish between light-absorbing and non-light-absorbing carbons in thermal analysis.

Comparison of the light absorption coefficient and carbon measures for remote aerosols: An independent analysis of data from the IMPROVE network—I

Using the IMPROVE network aerosol data from rural or remote sites across the United States, the ratio of the optically measured light absorption coefficient ( σ a) to the elemental carbon measured by Thermal/Optical Reflectance (TOR) analysis consistently indicates an absorption efficiency that is twice the accepted value of 10m 2g −1. Correlations between σ a and the TOR carbon strongly suggest that the discrepancy is due to an underevaluation of light-absorbing carbon rather than to an overestimation of σ a or a real, higher value of the absorption efficiency. In particular, past doubts about the accuracy and precision of the IMPROVE σ a measurement are here shown to be unsupported by the IMPROVE data. The large empirical correction that is applied to this σ a measurement, for multiple scattering effects due to filter mass loading, is given a new explanation as the effect of an increasing forward scattering fraction as sample thickness increases. The old explanation of shadowing by overlying particles in the sample is rejected as having just the opposite effect to that needed to explain the correction. The use of a diffuse source rather than a laser beam is indicated as a way to avoid the large empirical correction of σ a. Modelling of the light absorption by TOR carbon measurements, at twelve remote sites over a wide portion of the western United States, suggests the following errors in the current interpretation of TOR analysis for these sites: (1) The pyrolysis correction, based upon optical reflectance monitoring, appears to be largely wrong; and (2) The carbon evolving between 450 and 550°C in a pure helium atmosphere, currently interpreted as organic and therefore non-light-absorbing, appears to be as strongly light-absorbing as elemental carbon. However, the present analysis indicates that for a large majority (∼90%) of samples the light-absorbing carbons, as reinterpreted herein, are not only measured accurately by TOR, they are also reasonably well separated from the non-absorbing carbons evolving at and below 450°C. σ a and the TOR carbons, newly interpreted, are seen to be consistent both with one another and with the widely accepted absorption efficiency of elemental carbon. These results are statistical observations, independent of considerations about the form of the aerosol, particularly whether or not it is internally mixed. Multivariate regression of σ a vs the TOR carbons, properly physically constrained, is indicated to be a particularly useful and important analytical tool for distinguishing light-absorbing and non-absorbing carbons measured by thermal methods.

The dri thermal/optical reflectance carbon analysis system: description, evaluation and applications in U.S. Air quality studies

The thermal/optical reflectance method of carbon analysis developed by Huntzicker et al. (in Particulate Carbon, Atmospheric Life Cycle, edited by Wolff G. T. and Klimisch R. L., pp. 79–88, Plenum Press, New York, 1982) has been adapted by several laboratories for the quantification of organic and elemental carbon on quartz-fiber filter deposits. While the principle used by these laboratories is identical to that of Huntzicker et al., the details differ with respect to calibration standards, analysis time, temperature ramping and volatilization/combustion temperatures. This paper reports a variation on this method which has been applied to over 27,000 samples taken in more than a dozen urban and regional air quality studies in the U.S.A. In this variation, a 0.5 cm 2 punch from a dozen urban and regional air quality studies in 120, 250, 450 and 550°C in a pure helium atmosphere, then to combustion at temperatures of 550, 700 and 800°C in a 2% oxygen and 98% helium atmosphere. The carbon which evolves at each temperature is converted to methane and quantified with a flame ionization detector. The seven separate carbon fractions facilitate evaluation of the method and increase the information content concerning the samples. The reflectance from the deposit side of the filter punch is monitored throughout the analysis. This reflectance usually decreases during volatilization in the helium atmosphere owing to the pyrolysis of organic material. When oxygen is added, the reflectance increases as the light-absorbing carbon is combusted and removed. Organic carbon is defined as that which evolves prior to re-attainment of the original reflectance, and elemental carbon is defined as that which evolves after the original reflectance has been attained. By this definition, “organic carbon” is actually organic carbon that does not absorb light at the wavelength used (632.8 nm) and “elemental carbon” is light-absorbing organic and elemental carbon. Assumptions underlying the procedure are discussed, as well as comparisons with other methods and recommendations for further work.