Optical-chemical relationships for carbonaceous aerosols observed at Jeju Island, Korea with a 3-laser photoacoustic spectrometer

Transport of aerosols in pollution plumes from the mainland Asian continent was observed in situ at Jeju, South Korea during the Cheju Asian Brown Cloud Plume-Asian Monsoon Experiment (CAPMEX) ﬁeld campaign throughout August and September 2008 using a 3-laser photoacoustic spectrometer. Transport of mixed sulfate, carbona- 5 ceous, and nitrate aerosols from various Asian pollution plumes to Jeju accounted for 76% of the deployment days, showing large variations in their measured chemical and optical properties. Our analysis of eight distinct episodes, spanning a wide range of chemical composition, optical properties, and source regions, reveals that at episodes with higher OC/SO 2 − 4 and NO − 3 /SO 2 − 4 composition ratios exhibit lower single 10 scatter albedo at shorter wavelengths ( ω 405 ); signiﬁcantly lower [ ω meas405 = 0.79 ± 0.06, ω calc 405 = 0.86 ± 0.01] than predicted by an optical model that assumes constant com-plex index of refraction with wavelength (an optical model of soot). We attribute this discrepancy to enhanced absorption by organic material. Organic carbon absorption accounts for up to 50% of the measured aerosol absorption at 405 nm for the high 15 OC/SO 2 − 4 episode. Coatings of elemental carbon aerosol cores are hypothesized to increase absorption by factors up to 6 at visible wavelengths. Carbonaceous aerosol absorption can alter global radiative forcing estimates substantially, underscoring the need to understand and predict chemical composition e ﬀ ects on optical properties.

with age as it is transported over the Pacific (Dunlea et al., 2009) Process level understanding of the optical properties of Asian aerosol outflow during its transport across distances up to thousands of kilometers lasting several days is essential to evaluate its radiative forcing in the region.
The CAPMEX field campaign was designed to monitor continental outflow from Asia 10 and to observe the effect of Asian emissions and ABCs on atmospheric radiation especially during enforced air quality periods surrounding the summer 2008 Beijing Olympics (Ramanathan, 2009) The Gosan Observatory is located on the western side of Jeju Island, a resort island approximately 1100 km southeast of Beijing; and the Gosan Observatory is not affected by major local industrial pollution sources and has 15 been used to observe continental outflow from Asia for several years (Chen et al., 1997;Clarke et al., 2004;Topping et al., 2004) In this paper we show correlations between simultaneous and independent measurements of aerosol optical properties and chemical composition at the Gosan site, in plumes undergoing transport from Asia and demonstrate some effects of chemical composition on aerosol light absorption. Introduction

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The β sca at 405 and 781 nm were used to calculate the intensiveÅngström exponent of light scattering (Å 405 sca ), which depends on particle size, where λ 1 = 405 nm and λ 2 = 781 nm. Smaller particles exhibit largerÅ 405 sca . In order to diagnose the absorbing component of the aerosol, theÅngström exponent of β abs (defined in complete analogy with (1) but for the absorption coefficient β abs ) at 405 relative to 532 nm,Å abs (405/532), as well asÅ abs (405/781), were also calculated. The absorption wavelength dependence depends on carbonaceous particle composition 20 because strong wavelength dependence (Å abs >1.6) can indicate organic carbon absorption (Bond and Bergstrom, 2006;Gyawali et al., 2009;Lack and Cappa, 2010) Additionally the intensive single scatter albedo (ω λ ) were calculated using, where λ refers to any one of the PASS-3 laser wavelengths. NOAA HYSPLIT five-day back-trajectories for each day of the CPAMEX deployment are included in the supplementary information (http://www.atmos-chem-phys-discuss. net/10/9369/2010/acpd-10-9369-2010-supplement.pdf)to trace the path(s) the air mass had taken to Jeju and to assist in determination of the origin, evolution, and 5 duration of each pollution transport episode (Draxler and Rolph, 2003) We discuss only the episodes in which we have both optical property and composition data.

Optical properties
We expect aerosols undergoing transport from Asia to the Gosan Observatory to be 10 well-mixed in aged air masses in which atmospheric processing over several days likely increases internal mixing of the aerosol components hereby producing more uniform intensive optical properties. The continuous observations of β sca and β abs at 405, 532, and 781 nm, the calculated (Å 405 sca ) and ω 405 for the CAPMEX deployment are shown in Fig. 1 and are used, along with back trajectories, to separate Asian transport from 15 local pollution episodes.

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Printer-friendly Version Interactive Discussion 9 Mm −1 while mean β sca varies between 23 and 280 Mm −1 and at 781 nm, mean β abs varies between 3 and 5 Mm −1 while the mean β sca varies between 17 and 188 Mm −1 . We observe theÅngström exponent of absorption (Å abs (405/781)) is nearly constant (mean values ∼2.0) for all transport episodes, indicating significant presence of absorbing organic material in each episode. To further diagnose the absorbing organic 5 component, we use theÅngström exponent of absorption (Å abs (405/532)), where the wavelength dependence is strongly influenced by organic content and we expect to see strong increases in the wavelength dependence at short wavelengths corresponding to absorption by organic material. TheÅngström exponents over both wavelength ranges are included with the optical properties in Table 1.

Chemical composition
Independent mass and chemical information was assembled from simultaneous analysis of aerosol collected on quartz filters from 24-hour periods during most of the campaign. The measured PM 2.5 total mass temporal profile (in µg m −3 ) included in Fig. 1 agrees very well with the β sca profile, independently separating Asian transport from 15 local input periods. Also shown in Fig. 1 is the temporal profile of sulfate (SO 2− 4 ), ammonium (NH + 4 ), OC, and EC as well the temporal profile of the OC/SO 2− 4 ratio. We focus our discussion on OC/SO 2− 4 and NO − 3 /SO 2− 4 ratios as metrics for aerosol composition that vary significantly due to changes in source regions and the transport trajectory of the plumes (see Table 2).

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The individual pollutants are enriched during transport episodes, though by varying amounts. The SO 2− 4 mass (from the PM 2.5 fraction) changed the most, mean levels varied between 2.8 and 12.3 µg m −3 , while there was less variation in OC (1.1-1.9 µg m −3 ) and NO − 3 (0.7-1.9 µg m −3 ) in transport episodes. Episode 3 has the largest sulfate mass (12.3 µg m −3 ) and the lowest OC/SO 2− 4 and NO − 3 /SO 2− 4 ratios. This is consis- the Sea of Japan, traversing over Japan and Korea, as well as remote continental Asian locations (see Table 2). In the next section our analysis will show that enhancements in particle organic carbon and nitrate relative to sulfate lead to increased light absorption at short wavelengths, underscoring the importance of treating such chemical effects on optical properties explicitly in climate and chemistry models.

Optical property and composition correlations
We show the relationships between mean chemical composition (OC/SO 2− 4 and NO − 3 /SO 2− 4 ) and the intensive optical properties ω 405 andÅ abs (405/532) for episodes 3-8 in Fig. 2 ratios. Nitrate rich plumes originate from NO x source regions where subsequent heterogeneous processing enriches particle phase nitrate. If organics are present in the plumes, there is potential to form organic nitrates in the particle phase. Organic nitrates are known to absorb light at short wavelengths and are likely processed to the particle 5 phase during LRT of pollution plumes rich in organics and nitrogen species such as episode 8 (Jacobson, 1998) The changes in mass of OC and NO − 3 are relatively small across the transport episodes, what changes significantly is the relative SO 2− 4 percent composition (86% in episode 3, 44% in episode 8, see Table 2). The changes in composition influence 10 the optical properties in pollution transport episodes observed during of CAPMEX. The decrease in ω 405 could be attributed in part to reduction in particle size thereby decreasing β sca and changing particle composition can also have significant effects. To separate these effects, bothÅ 405 sca andÅ abs (

Wavelength dependence of optical properties
Enhanced absorption by organic and nitrate rich aerosols is evident in the observed wavelength dependence of single scatter albedo, ω(λ) in Fig. 3. We have separated the data into three groups based on the measured OC/SO 2− 4 ratio: high (episode 8), intermediate (episodes 4, 5, 6) and low (episodes 3, 7). The measured ω(λ) val-5 ues are plotted using ±1 σ standard deviations as error bars. This represents the real variability of ω for each episode, a quantity much larger than instrumental uncertainty underlying our ω measurements. The figure also shows Mie scattering theory calculations of the expected dependence of ω(λ) assuming a wavelength independent index of refraction (to neglect the absorption of organics at short wavelengths).

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The Mie calculations were performed using lognormal size distributions with variable (free parameters) mean particle diameters and geometric standard deviations and by changing the value of the real and imaginary parts of the index of refraction. To constrain the model outputs, we forced the calculation to approach the measuredÅ 405 sca and the measured ω 781 such that the square root of the summed square of the rel-15 ative differences between model and measurements were below 2.5%. The choice of a 2.5% threshold was dictated by an error propagation analysis of the instrumental relative uncertainty (random errors only) at the mean optical signals measured during the different episodes and represents a conservative estimation at a 2σ level. These constraints resulted in the following ranges for the mean diamteter of the par- where uncertainties represent half of the interval between maximum and minimum value of the sensitivity study. The full lines in Fig. 3

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Interactive Discussion the maximum and minimum deviations. The lines indicate how ω(λ) may behave for an absorbing aerosol with an imaginary index of refraction that remains constant with wavelength. The deviations of the measurements from the modeled curves are in agreement with an increase in imaginary index of refraction with wavelength, especially at 405 nm; this could be due to the presence of absorbing organic compounds. 5 We measure ω 405 that is 10 % less than predicted (or 4x darker at 405 nm than black carbon) for Asian transport aerosol with high OC/SO 2− 4 and NO − 3 /SO 2− 4 ratios, providing direct in situ evidence for a relative absorption enhancement at 405 nm. The enhancement is insignificant for medium and low OC/SO 2− 4 and NO − 3 /SO 2− 4 ratios, which indicates that ω(λ) is controlled by the amount of OC, organic nitrates, NO − 3 and SO 2− 4 10 in mixed carbonaceous aerosol, primarily at shorter wavelengths. Our in situ field ω(λ) observations provide a chemical basis for reports of enhanced short wave absorption in remote column radiance data made in Mexico City (Barnard et al., 2008) and laboratory vegetation combustion studies reported by (Lewis et al., 2008). We then postulate that OC absorption is negligible at 781 and empirically determine a factor that best matches β est abs to our measured β abs (781 nm); (β est abs = f ×β meas abs ). We interpret f as the absorption enhancement factor for the coating and find this ranges from 3-6 (at 781 nm) for the transport episodes in CAPMEX and is attributed to enhanced absorption by clear and absorbing coatings on elemental carbon aerosol cores. Our 5 values are much higher than the enhancement reported in limited observational studies (Cross et al., 2010;Schwarz et al., 2008) We hypothesize that large enhancements observed in CAPMEX can result from increasing coating thickness during aerosol transport and also the mixed carbonaceous aerosol can contain materials that absorb in the red (Garland et al., 2008;Gyawali et al., 2009). Using β est abs calculated at each 10 PASS-3 wavelength, we also determine percent contributions of elemental carbon and elemental carbon coating to the overall β meas abs at each wavelength. We found that elemental carbon absorption and the coating effect accounted for 73% of β abs (405 nm) in episode 7 and 99% of β abs (532 nm) in episodes 5 and 8. The smallest coating effect was observed during episode 8 (33 % at 405 nm). The difference between our β meas abs 15 (405 nm) and β meas abs (532 nm) compared to β est abs (at 405 and 532 nm) is attributed to brown carbon absorption (additional absorption than accounted for by the coating of elemental carbon cores) and determine MAC BrC by dividing (β meas abs −β est abs ) by OC mass . We infer MAC BrC that range between 2.1±0.1 and 3.4±0.1 m 2 g −1 at 405 nm and between 0±0.1 and 1.0±0.1 m 2 g −1 at 532 nm and that brown carbon accounts for up to 20 50% of β abs (405 nm) in episode 8 and up to 20% of β abs (532 nm) in episode 2. Table 3 exhibits the MAC BrC and absorption fractions for each episode. Similar MACs for OC have been reported (Favez et al., 2009) and similar EC and OC contributions to total aerosol absorption have been observed (Barnard et al., 2008;Clarke et al., 2004) Our analysis describes how light absorption is enhanced by brown carbon directly and 25 by coatings on elemental carbon cores, with the coating effect being pronounced at 405 and 532 nm for all episodes except for the fire-impacted episode 8. Our MAC results for brown and coated elemental carbon shoule be valuable for chemistry/transport models and to quantitatively determine their impacts on climate radiative forcing and

Conclusion
Transport of absorbing carbonaceous aerosols accounted for 74% of the observation time on Jeju, South Korea in August and September 2008. We sampled a range of polluted air masses with varying amounts of particle carbon, sulfate, organics, and 5 nitrate. Measured intensive optical properties are correlated with chemical composition observations. These empirical correlations provide clues to how aerosol chemical composition influences and moderates aerosol optical properties that will help develop prognostic treatments for models. Episodes with high OC/SO 2− 4 and NO − 3 /SO 2− 4 ratios exhibit lower than expected ω 405 , when compared to a size resolved optical model 10 assuming constant index of refraction with wavelength. Our findings underscore the manner in which OC can enhance light absorption by coating and mixing with EC and that OC absorption can account for a significant fraction of aerosol absorption at short visible wavelengths.
A recent study shows that a small reduction in global mean aerosol single scat- episodes. Chemical composition (both absorbing and non-absorbing, organic and inorganic components) significantly affects aerosol optical properties particularly at shorter wavelengths. Our results underscore the need to explicitly parameterize aerosol composition effects on optical properties in regional chemistry and climate models, particularly in the Asian outflow, whose composition is changing due to energy growth and clean air policies. Our chemical-optical relations and derived MACs for carbonaceous aerosols provide empirical parameterizations that enable such analysis by    Temporal profile of aerosol scattering coefficient (β sca ), absorption coefficient (β abs ), scatteringÅngström exponent (Å 405 sca ), single scatter albedo at 405 nm (ω 405 ), PM 2.5 mass, scaled sulfate and ammonium, and elemental and organic carbon masses (µg m −3 ) for the CAPMEX campaign. The periods highlighted in ash are Asian aerosol transport episodes and their diagnosis as such is discussed in the text. The blue lines represent the 405 nm data, the green line represents the 532 nm data, and the red line represents the 781 nm data.