GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 12, 1616, doi:10.1029/2003GL017076, 2003
Radiative Absorption Capability of Asian Dust with Black Carbon Contamination Charles C.-K. Chou, Tze-Kuang Chen, Shu-Hui Huang, and Shaw C. Liu Environmental Change Research Project, Institute of Earth Sciences, Academia Sinica, Nankang, Taipei, Taiwan Received 7 February 2003; revised 30 April 2003; accepted 9 May 2003; published 19 June 2003.
[1] Radiative forcing by mineral dust is one of the major uncertainties in assessing the impact of aerosols on the climate. Coagulation and condensation of pollutants on dust particles can further complicate the problem by altering their optical properties. During four Asian dust events in the spring 2002, concentration and size distribution of aerosols were measured in Taipei, Taiwan. Black carbon (BC), organic carbon (OC) and optical properties of the dust aerosols were measured. We found that both the mass specific absorption efficiency and imaginary refractive index of the aerosols decreased significantly during the dust events because of reduced black carbon concentrations. In addition the black carbon in aerosols was found to have significantly higher absorption efficiency than that adopted in current climate models. We suggest that the higher absorption efficiency is due to the formation of an internal mixture of INDEX TERMS: 0305 BC and other particulate matter. Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry. Citation: Chou, C. C.-K., T.-K. Chen, S.-H. Huang, and S. C. Liu, Radiative Absorption Capability of Asian Dust with Black Carbon Contamination, Geophys. Res. Lett., 30(12), 1616, doi:10.1029/2003GL017076, 2003.
1. Introduction [2] Asian dust storms in the arid and semi-arid areas over northwestern China, Mongolia, and other Central Asia regions can lift dust particles high into the atmosphere and generate ‘‘dust clouds’’ in springtime [Husar et al., 2001, and references given there]. The dust particles are usually transported by westerlies to eastern China, Korea, Japan, and sometimes across thousands of miles to north Pacific and even the American Continent [e.g., Braaten and Cahill, 1986]. During dust storms, extremely high concentrations of aerosols were frequently observed over East Asia [e.g., Murayama et al., 2001]. Even in North America, significant amounts of dust were occasionally found to raise environmental concerns [e.g., Jaffe et al., 1999]. One of the major issues of the dust particles is their radiative forcing of climate change [Sokolik and Toon, 1996]. Important implications of tropospheric aerosols to climate change have been proposed for a long time [e.g., Chylek and Coakley, 1974; Houghton et al., 1996]. However, accurate estimate of the radiative forcing of tropospheric aerosols is still difficult to reach. Previous studies of the climate forcing of sulfate and carbonaceous aerosols indicated that the Copyright 2003 by the American Geophysical Union. 0094-8276/03/2003GL017076
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radiative forcing of green house gases could be offset partially by Mie scattering effect of aerosols [e.g., Charlson et al., 1992; Haywood et al., 1999]. During the past decade, the direct radiative forcing by dust also attracted significant attention [e.g., Sokolik and Toon, 1996; Kaufman et al., 2001]. However, there has been relatively little discussion about the radiative forcing of dust particles mixed with anthropogenic pollutants until recently [e.g., Liao and Seinfeld, 1998; Jacobson, 2001].
2. Experiment [3] This field measurement campaign was conducted in Taipei, Taiwan during four Asian dust events in the spring 2002. Aerosol samples of PM10 and PM2.5 (particulate matter with aerodynamic diameter less than 10 mm and 2.5 mm, respectively) were collected using a four-channel sampler, two channels were equipped with 10 mm-cut-size inlet and the other two were equipped with 2.5 mm-cut-size inlet. Each sample set includes one collected on PTFE membrane filter for gravimetric mass measurement and another collected on pre-fired quartz filter for carbonaceous composition analysis and optical measurement. In addition, sizesegregated aerosol samples were collected on quartz-fiber filters using a MOUDI2 sampler (Model 110, MSP Co., MN). [4] Aerosol absorption coefficient (Sa) of the PM10 samples on quartz filters was determined by laser transmission method [Rosen and Novakov, 1983]. The apparent transmission (Ta) of a 632.8 nm laser beam through the quartz filter is given by: Ta ¼ Ipf =If ;
ð1Þ
where Ipf and If are the light intensities passing through the sampling filter and a blank filter, respectively (In practice, an average If of ten blank filters was used in this work to account for the differences in filters). We assume that the attenuation of the laser beam follows the Beer-Lambert law: Ipf ¼ If esaX ¼ If eAa ;
ð2Þ
where X is the optical path length, which is determined from the volume of sampled air and the effective crosssection area of the filter. We can then define the apparent attenuation (Aa) caused by the particulate matter on the filter and measure it as the negative logarithm of Ta, i.e., Aa = ln(Ta). Finally, the value of Sa is obtained from equation (2) with the knowledge of Aa and X. [5] The imaginary refractive index (K) was also calculated from the measured Aa according to the relationship derived by Lindberg et al., [1999]. To characterize light absorbing - 1
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CHOU ET AL.: RADIATIVE ABSORPTION OF BC IN DUST
3. Results and Discussion
Figure 1. Trajectories of air masses at 500 m above ground level over Taipei at 0000Z on dust-event days. Each trajectory line is labeled by the event date with Julian date in the parentheses. All air parcels backtracked to the desert and loess areas over Mongolia and northwestern China and past through industrialized area of China before arriving Taipei.
capability of the dust particles, mass specific absorption efficiency (Aa) of the particulate matter is calculated from the ratio of Sa and mass concentration of PM10 (CPM10): aa ¼ sa =CPM10 :
ð3Þ
By assuming BC is the sole light-absorbing component of the aerosols, mass specific absorption efficiency of BC (ABC) is defined by a similar equation: aBC ¼ sa =CBC ;
ð4Þ
where CBC is the mass concentration of BC. This assumption tends to lead to an overestimate of ABC. Nevertheless, for simplicity it is a reasonable approach because the absorption coefficient of BC is substantially higher than other species at the wavelength used in the measurement. The discrepancies between the measured optical parameters of filter samples and those of the ‘‘free aerosols’’ should also be kept in mind. The fundamental assumption of the experimental method is that the collection process does not change the optical properties of the particles. However, some features of aerosols, such as the distance between individual particles and mixing state of chemical species, are unavoidably changed when they are collected on filters. Inter-comparison studies showed that the values of Sa measured with filter samples are generally higher than the true optical values by a factor of 20– 40% [Horvath, 1993a, 1993b; Reid et al., 1998]. For externally-mixed aerosols and light-loading filter samples, the bias might be further increased to some degree. [6] Carbonaceous species, i.e. organic carbon (OC) and elemental carbon (EC), of the aerosol samples were analyzed following the IMPROVE protocol [Chow et al., 2001]. Because the definition of ‘‘EC’’ in the IMPROVE protocol is actually based upon the light reflectance of the filter sample at 632.8nm, it is reasonable to assume to be equivalent to ‘‘BC’’.
[7] Trajectories of the air parcels studied in this work were analyzed using NOAA’s HYSPLIT model. Figure 1 shows that the air parcels passed through metropolitan areas like Beijing and Shanghai before arriving in Taipei. Such trajectories suggest the likelihood of mixing between dust and pollutants. Figure 2 shows time series of the mass concentrations of PM10 and PM2.5, and the ratio of coarse to fine mode (C/F) during the dust events. Size distribution of ambient aerosol is characterized by its bimodal feature and is generally divided into a fine mode (2.5 mm). The components of fine mode are usually associated with air pollution, whereas the coarse mode consists mostly of dust and mineral particles. Normally, the concentration of PM10 is around 50– 70 mg/m3 in Taipei and fine particles predominate in the ambient aerosols. When the air mass with the Asian dust arrived, concentration of PM10 usually rose drastically and C/F also climbed up by a factor of 2 to 4. The sharp increase of C/F suggests that dust particles were overtaking the pollution aerosols as the dominant component. In contrast, a high PM10 event (Mar. 19– 20) without C/F anomaly that was characterized by local pollution was also observed during the campaign. [8] While the size distribution of PM10 shifted toward larger particles during a dust event that of BC particles shifted even more. Figure 3 shows the evolution of the mass median aerodynamic diameter (MMAD) of PM10, OC, and BC during a typical dust event (March 5 –10, 2002). Because it is quite unlikely that BC particles can grow up to 1.5 mm by themselves, coagulation with dust particles during transport is the most reasonable explanation for the ‘‘growth’’ of BC particles. This finding of size distribution shift of BC is consistent with the modeled results of Jacobson [2001, 2002], which show that coagulation can internally mix fine BC particles and large particles like soil and sea spray to form large multi-component particles within a short period. The concurrent variation of carbonaceous components and PM10 also suggests that carbonaceous material was incorporated in the dust and transported together. [9] Figure 4a illustrates the time series of mass specific absorption efficiency of the PM10 samples. The value of Aa
Figure 2. Concentrations of aerosols observed during dust events in the field campaign. The sampling period of each sample was 12 hours. Day-time (D) samples were collected from 0800 to 2000 LST. Night-time (N) samples were collected from 2000 to 0800 LST of next day.
CHOU ET AL.: RADIATIVE ABSORPTION OF BC IN DUST
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Figure 3. Evolution of mass median aerodynamic diameters (MMAD) of PM10, OC, and BC during a typical dust event (March 5 – 10, 2002). dropped significantly during dust events. Figure 4b shows the relationship between K and the mass fraction of BC (fc) of PM10 samples. The linear correlation indicates that the imaginary refractive index of ambient aerosols indeed follows the dilution model: light-absorbing component (BC) is diluted with non-absorbing components (dust, sea salts). Intersect of the linear regression with zero fc gives a K value of about 0.006, which is a typical value of desert minerals [Lindberg and Laude, 1974]. In contrast, K is expected to reach 0.74 when fc approaches unity. This value is consistent with the imaginary refractive index of elemental carbon suggested by Jacobson [2000], who scaled the K of crystalline graphite (1.34) with density down to a density of 1.25 g/cm3 for ambient soot. [10] Figure 5 shows that aerosol absorption coefficient is proportional to the concentration of BC in the atmosphere. The slope of the regression line of our measurements gives a value of 23.7 m2/g for the average absorption efficiency of BC in aerosols (ABC). This value is near the high end of the range (2 – 28 m2/g) obtained in previous field measurements [e.g., Liousse et al., 1993; Martins et al., 1998; Sharma et al., 2002] and is also larger by about a factor of 2 than the value (10 m2/g) generally accepted for soot particles. Studies on BC from biomass burning usually reported high values of ABC due to formation of internally mixed aerosols of BC and organics [e.g., Liousse et al., 1993; Martins et al., 1998]. In contrast, ABC measured in remote and urban areas scatters across an order of magnitude and was suggested to be a ‘‘site-specific’’ parameter [Liousse et al., 1993; Petzold et al., 1997; Sharma et al., 2002]. The variation was suggested to be related to the variability in composition, size distribution, as well as mixing state of the black carbon in aerosols. Low ABCs were usually obtained for fresh aerosols collected at urban sites where a large portion of BC particles was in external mixing state with other particulate matter. Despite the mixing state, agglomeration of primary BC aerosols could occur near the emission sources. Consequently, the size of BC became larger and resulted in the decrease of ABC. To date there is no consensus about the explanation for high ABCs observed at urban and remote sites. Petzold et al., [1997] investigated factors causing the variability of ABC and showed that the value of ABC was strongly dependent on the mixing level of
Figure 4. (a) Variations of PM10 concentration (triangle) and mass specific absorption efficiency (open circle) of the particulate matter during four dust events. ‘‘D’’ and ‘‘N’’ denote daytime and nighttime samples, respectively. (b) Linear regression fit of the imaginary refractive index (k) and the fraction of black carbon (fc) of aerosols. scattering component in the aerosols but no size dependence of ABC for pure BC aerosol was found. Accordingly, one could expect a high ABC for aerosols that consist mostly of light-scattering material and a small mass fraction of BC
Figure 5. Aerosol absorption coefficient as a function of the black carbon concentration. A linear regression line (solid line) of data from this work (solid square) gives an average value of 23.7 m2/g for the mass specific absorption efficiency of black carbon (aBC) in the aerosols. Data of ACE-Asia (open circle and dash line) are plotted for comparison.
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CHOU ET AL.: RADIATIVE ABSORPTION OF BC IN DUST
(fc). However, this could not explain the calibration value (19 m2/g) for aethalometer that was obtained from roadside aerosols with high fc [Ruellan and Cachier, 2001]. [11] In Figure 5, values of ABC reported here are compared to the values derived from airborne measurements of ACE-Asia, which was conducted over the western Pacific, upwind of this study [Mader et al., 2002]. Our values are about a factor of 2 larger than theirs (11 ± 5 m2/g). However, it should be noted that the values of ABC in the data set of ACE-Asia scatter over a factor of eight (5 – 40 m2/g). A major uncertainty in determining ABC may be due to imprecise measurement of aerosol absorption coefficient (as described in section 2). Because similar measurement techniques were used in the two studies, similar uncertainties were expected. The detection limit of the carbon analysis system in this work is 0.2 mgC/cm2 in terms of filter loading, which is about 4– 17% of the field levels of BC concentration during our observation. However, it can contribute significantly to the airborne measurements where concentrations are relatively small. This could be one of the reasons that contributes to the large scattering of the airborne data. [12] The large absorption efficiency deduced from our ground-based measurements suggests that the light absorbing capability of aerosols consist of dust and BC might be stronger than previous estimates. We hypothesize that the large ABC value is due to the formation of aggregates of BC and the dust (a non-ideal internal mixture) during their longrange transport. Local BC might also be involved in the coagulation process when the dust was mixed with local pollutants. This is consistent with the results of studies on the optical properties of aerosols, which indicated that BC internally mixed (or coated) with light-scattering material can absorb more light than externally-mixed aerosols [Jacobson, 2000, and references given there]. This also agrees with the observations by Liousse et al. [1993] in Western Mediterranean: an ABC of 18 m2/g was attributed to formation of internal mixture of BC and sulfate during their transport. [13] In summary, we found that the imaginary refractive index of BC in dust particles was identical to that calculated by scaling the index proportionally with density to that of crystalline graphite. When mixed with light-scattering materials (dust, sea salts, sulfates), the absorption efficiency of BC became significantly higher than values used in current climate models [Chung and Seinfeld, 2002, and references given there]. However, this work was conducted at a single station over a short period, it may not be representative of the value for the dust/BC mixture in this region. Finally, this study shows that our quantitative knowledge of optical property of dust/BC aerosols is rather limited and uncertain. Given the large area affected by the Asian dust, more comprehensive investigations of the radiative property of BC/dust mixture are imperative. [14] Acknowledgments. This work was supported, in parts, by grants from Academia Sinica under theme project ‘‘Particulate Matter and its Environmental Impacts in Taiwan’’ and the National Science Council grant NSC-91-2111-M-001-001.
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C. C.-K. Chou, T.-K. Chen, S.-H. Huang, and S. C. Liu, Environmental Change Research Project, Institute of Earth Sciences, Academia Sinica, P. O. Box 1 – 55, Nankang, Taipei, 115, Taiwan. (
[email protected]. tw;
[email protected];
[email protected]; shawliu@ earth.sinica.edu.tw)
