PUBLICATIONS. Journal of Geophysical Research: Atmospheres

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1 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Key Points: Occurrences of ns-soot/bc from TEM and SP2 data are generally consistent across the techniques Abundance and mixing state of ns-soot/bc fractions changed after rain and during weak wind period Aggregated-Fe particles were found during high iron oxide particles concentration periods Supporting Information: Supporting Information S1 Correspondence to: K. Adachi, Citation: Adachi, K., N. Moteki, Y. Kondo, and Y. Igarashi (2016), Mixing states of light-absorbing particles measured using a transmission electron microscope and a single-particle soot photometer in Tokyo, Japan, J. Geophys. Res. Atmos., 121, , doi:. Received 28 MAR 2016 Accepted 19 JUL 2016 Accepted article online 23 JUL 2016 Published online 9 AUG American Geophysical Union. All Rights Reserved. Mixing states of light-absorbing particles measured using a transmission electron microscope and a single-particle soot photometer in Tokyo, Japan Kouji Adachi 1, Nobuhiro Moteki 2, Yutaka Kondo 3, and Yasuhito Igarashi 1 1 Meteorological Research Institute, Tsukuba, Japan, 2 Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan, 3 National Institute of Polar Research, Tachikawa, Japan Abstract Light-absorbing atmospheric aerosols such as carbonaceous particles influence the climate through absorbing sunlight. The mixing states of these aerosol particles affect their optical properties. This study examines the changes in the mixing states and abundance of strongly light absorbing carbonaceous particles by using transmission electron microscopy (TEM) and single-particle soot photometer (SP2), as well as of iron oxide particles, in Tokyo, Japan. TEM and SP2 use fundamentally different detection techniques for the same light-absorbing particles. TEM allows characterization of the morphological, chemical, and structural features of individual particles, whereas SP2 optically measures the number, size, and mixing states of black carbon (BC). A comparison of the results obtained using these two techniques indicates that the peaks of high soot (nanosphere soot (ns-soot)) concentration periods agree with those of the BC concentrations determined by SP2 and that the high Fe-bearing particle fraction periods measured by TEM agree with that of high number concentrations of iron oxide particles measured using SP2 during the first half of the observation campaign. The results also show that the changes in the ns-soot/bc mixing states primarily correlate with the air mass sources, wind speed, precipitation, and photochemical processes. Nano-sized, aggregated, iron oxide particles mixed with other particles were commonly observed by using TEM during the high iron oxide particle periods. We conclude that although further quantitative comparison between TEM and SP2 data will be needed, the morphologically and optically defined ns-soot and BC, respectively, are essentially the same substance and that their mixing states are generally consistent across the techniques. 1. Introduction The optical properties of atmospheric aerosol particles influence the solar radiation budget, and the hygroscopicity of these particles influences cloud properties. At the scale of individual particles, these properties depend on the composition, size, shape, and mixing state of the particles, all of which are changed by interactions in the atmosphere such as coagulation, condensation, and evaporation [Adachi et al., 2011; Adachi and Buseck, 2008; Zhang et al., 2008; Shiraiwa et al., 2010; van Poppel et al., 2005]. Thus, a better understanding of how aerosol particles mix with other particles and how their optical and hygroscopic properties change is important for improving climate modeling [e.g., Jacobson, 2001; Bond et al., 2013; Matsui et al., 2013; Kondo, 2015]. This study mainly focuses on strongly light-absorbing carbonaceous aerosol particles. The terminology relating to this material varies depending on the definitions and measurement techniques used [Bond and Bergstrom, 2006; Buseck et al., 2014; Petzold et al., 2013]. In this study, we used two techniques: transmission electron microscopy (TEM) and single-particle soot photometer (SP2). We use the terms nanosphere soot (ns-soot), as defined by Buseck et al. [2014] for soot with a definition for the TEM measurements and black carbon (BC) to refer to strongly light absorbing carbonaceous aerosol particles measured by SP2. TEM allows the morphological, chemical, and structural analysis of materials [Adachi et al., 2007, 2014; Pósfai and Buseck, 2010], and ns-soot is defined by its aggregated morphology of carbon nanospheres with graphitic structures [Buseck et al., 2014]. In contrast, SP2 optically measures aerosol particles using a laser beam and detects scattering and incandescence signals [Moteki et al., 2014; Stephens et al., 2003; Schwarz et al., 2006; Moteki and Kondo, 2007; Gao et al., 2007; Huang et al., 2012; Cross et al., 2010]. SP2 performs continuous measurements of the mass and size of individual BC particles and determines whether BC is present in a nascent form, is coated by other materials, or is attached to other particles (attached BC) [Sedlacek et al., 2012; Moteki et al., 2014; Dahlkötter et al., 2013]. Ns-soot and BC can be the same material in the atmosphere because the ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9153

2 Figure 1. Backward trajectories of the air mass (alt. 500 m) during the sampling period. The end times for each trajectory are shown in local time (hh:00 day). Duration: 48 h. Period 1: Green; Period 2: Pink; Period 3: Dark blue; and Period 4: Light blue. Definitions of the periods are described in the text. graphitic structure of ns-soot causes the incandescent properties of BC [Petzold et al., 2013; Bond et al., 2013]. However, the occurrence and mixing states of ns-soot and BC in the atmosphere as determined using both TEM and SP2 have not been compared in detail. Therefore, because TEM and SP2 measure fundamentally different properties of the same materials, measurements using both techniques are important for an in-depth understanding of the mixing states of the particles and to cross check these instruments. In addition to BC particles, we measured iron oxide particles, which are another type of light-absorbing particles that influence the global climate, cause human health problems by an oxidative stress [Nel, 2005], and become nutrients to microorganism in the ocean [Mills et al., 2004], using both chemical and optical measurements by TEM and SP2, respectively. Mixing states strongly affect the optical properties of the ns-soot/bc particles [Bond and Bergstrom, 2006, 2013; Cappa et al., 2012; Liu et al., 2015; Scarnato et al., 2013], the cloud condensation nuclei (CCN) activity [Wang et al., 2010], and the aerosol lifetime in the atmosphere [van Poppel et al., 2005; Liu et al., 2013]. Ns-soot/BC can form CCN when internally mixed with hygroscopic materials, such as when coated with sulfates [Liu et al., 2013]. Whether the ns-soot/bc particles are coated with light-scattering materials or attached to them has a strong effect on the optical properties of these particles [Adachi and Buseck, 2013; Fuller et al., 1999]. Coated ns-soot/bc particles can efficiently absorb light because coating materials enhance the available cross section and focus the light. Although ns-soot/bc particles do not have the exact core-shell structure described in Mie theory because of their fractal-aggregated structure, coated or embedded ns-soot/bc particles can absorb more light than uncoated particles [Adachi et al., 2010; Liu et al., 2015; Peng et al., 2016]. If ns-soot/bc particles have an ideal core-shell structure, then the cross section could be nearly twice or more comparing to that of uncoated structures [Bond and Bergstrom, 2006; Peng et al., 2016]. In contrast, attached ns-soot/bc particles do not significantly enhance light absorption because the enhancement occurs only when the attached nonabsorbing materials face the incident light [Adachi and Buseck, 2013]. Thus, a better understanding of ns-soot/bc particle mixing states is important for evaluating the optical properties of these particles and radiative forcing. The samples and data used in this study were obtained in Tokyo, Japan (Figures 1 and S1a in the supporting information), in August Prior to the current observation campaign, the Integrated Measurement ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9154

3 Program for Aerosol and oxidant Chemistry in Tokyo (IMPACT) campaign was conducted in 2003 and 2004 [Kondo et al., 2010], and the chemical compositions and mixing states of aerosol particles were analyzed. That campaign revealed that the thickness of the coating around BC particles, which were mainly composed of sulfates and organic material, increased as the photochemical age increased [Shiraiwa et al., 2007]. Moteki et al. [2014] reported detailed theoretical and laboratory experiments using SP2 and showed the mass distribution of BC and the number fraction of attached BC particles in the ambient atmosphere during the current campaign. Our study focuses on comparing TEM and SP2 measurements for ns-soot/bc and iron oxide particles and the mixing states of ns-soot/bc to understand how the mixing states and particle occurrences change in response to interactions with other particles in an urban atmosphere. 2. Materials and Methods 2.1. Sampling Site This observation campaign was conducted at the Hongo campus of Tokyo University, Tokyo, Japan (35.71 N, E) (Figure S1). The metropolis of Tokyo and six nearby prefectures (Kanto area; Figure S1b) constitute the largest megacity in the world, with a population of approximately 42 million in 2008 [Zhu et al., 2012; Gurjar et al., 2008]. The main emission sources of particulate matter with diameters of <2.5 μm (PM 2.5 )in the Kanto area in 2000 were motor vehicles (52%), point sources (29%), and other transportation sources (11%) [Zhu et al., 2012; Kannari et al., 2007] TEM Analysis A 120 kev transmission electron microscope (JEM-1400, JEOL, Tokyo, Japan) was used to analyze the shapes and mixing states of the aerosol particles. A scanning transmission electron microscope (STEM) and an energy-dispersive X-ray spectrometer (EDS; X-Max 80, Oxford Instruments, Tokyo, Japan) coupled with the transmission electron microscope measured the elemental compositions of individual particles. Aerosol particles were collected on collodion carbon substrates coated on copper TEM grids (H2-200 mesh, Okenshoji Co. Ltd. Tokyo, Japan). Two aerosol-impactor samplers (Arios Inc., Tokyo Japan) with a 16 or 24 auto-sample changer were used at an air flow rate of 2 L per minute [Adachi et al., 2014]. The sample collection time was 3 min. Several samples per hour were collected during the day, and at least one sample per hour was collected during the night. The 50% cutoff sizes were between approximately 100 and 700 nm (aerodynamic diameter) for the small and large sizes, respectively. The impactor aerosol samplers were located in the same room with the SP2 unit. Ambient air approximately 20 m above ground level was introduced to the samplers through a conductive tube. The sampling conditions are also described in Moteki et al. [2014]. The elemental compositions of all of the particles within the field of view were measured using STEM-EDS. The analytical details of the STEM-EDS were reported by Adachi et al. [2014]. Magnifications of between 5000 and 20,000 were used for the STEM-EDS analysis, depending on the particle loading, to collect approximately 100 particles within each TEM image. We analyzed 123 TEM samples and a total of approximately 18,000 particles during the campaign. We used EDS intensities to evaluate semiquantitative compositions within particles [Adachi and Buseck, 2015]. The EDS intensities, which depend on particle composition and thickness, were obtained by scanning the electron beam over the particle surface for 20 s. The STEM-EDS was also used to obtain examples of elemental distribution images (mapping images) for selected samples. As an intense electron beam could evaporate beam-sensitive materials [Adachi et al., 2014], we first took TEM images using a relatively weak electron beam and then measured the compositions using STEM-EDS. Volatile materials can be lost while collecting samples on the TEM grids, during the storing, or in the TEM vacuum. However, ns-soot and iron oxide particles are not volatile materials and will not be lost during the TEM analysis. Coating materials can be lost if they are volatile and would be underestimated on the TEM measurement. Ns-soot particles were classified either uncoated one (external mixture) or coated one (internal mixture) from the representative TEM images of each sample. Here we define uncoated ns-soot as that has no coating or only thin coatings (coating thickness < ~10 nm) and has apparent monomer aggregates in the TEM images. Ns-soot particles that are coated or embedded within other materials are defined as internally mixed one SP2 Analysis SP2 determines the mass of each incandescent particle from the height of its incandescence peak. SP2 also determines the scattering number concentrations and sizes of particles from the scattering signals that arise ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9155

4 when the particles pass through the laser beam. The size ranges of the measured particles were nm for BC and nm for all of the nonincandescent, scattering particles. If the BC is coated by or embedded within other nonincandescent materials, then the nonincandescent material evaporates before the BC incandesces [Moteki et al., 2014]. In contrast, if the BC is attached to other materials, the attached material separates from the BC before it begins to incandesce and will not completely evaporate during its transition through the laser beam. As a result, different scattering waveforms are produced relative to those of coated BC [Moteki et al., 2014]. These time-dependent physical processes can be inferred from the scattering waveforms; thus, SP2 can count and measure the sizes of coated and attached BC. We used a BC size range of 185 to 220 nm to measure the BC mixing states. Within the size range, internally mixed BC particles are defined as log(c s-be /C s-bc ) > 0.8, where the C s-be and C s-bc are the measured scattering cross sections at the leading edge of laser beam and of the uncoated BC, respectively [Moteki et al., 2014]. Coated and attached types are distinguished to each other among the internally mixed BC particles [Moteki et al., 2014]. The details of the SP2 algorithm and the threshold values used in this study are described in Moteki et al. [2014]. SP2 can identify light-absorbing iron oxide (FeO x ) particles (e.g., hematite, magnetite, and wüstite) by using the signal ratio of two incandescence channels (color ratio) and the appropriate wavelength bands [Yoshida et al., 2016]. As BC and these iron oxide particles show different color ratios in the incandescence signals, they can be differentiated in the SP2 measurement. The detectable iron oxide particle size range in SP2 is estimated to be approximately nm using magnetite particles. Yoshida et al. [2016] reported that the incandescing iron oxide particles in Tokyo urban air are similar to magnetite, based on the measured timing of the onset of incandescence of iron oxide particles in SP2 laser beam. The algorithm and details of iron oxide particle detection were as described in Yoshida et al. [2016] Meteorology and Air Mass Backward Trajectories The observation campaign was conducted from 2 to 9 August For the intensive TEM analyses, we focused on the period from 0:00 5 August to 18:00 9 August local time (LT) (in UTC, from 15:00 4 August to 9:00 9 August). The meteorological data were monitored at the Tokyo District Meteorological Observatory, which is about 2 km south of the sampling site (Figure S1c). Precipitation was observed between 9 am and 2 pm on 6 August (LT) (total precipitation of 15.5 mm; Figure 2f). The average ambient temperature and relative humidity were 27.5 C (ranging from 23 to 34 C) and 70% (ranging from 47 to 94%), respectively. Backward-trajectory simulations using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model [Draxler and Rolph, 2011] indicated that from 4 to 6 August, the air mass mainly originated from the south or southeast, i.e., from the Pacific Ocean (Figure 1). Between 0:00 7 August and 6:00 8 August, the air mass was stagnant over the Tokyo area. Subsequently, the air mass originated from the northeast (the Pacific Ocean and inland) until the end of the campaign. 3. Results 3.1. Bulk Inorganic Aerosol Compositions Measured by Ion Chromatography Aerosol particles were collected on quartz filters for 24 h for the ion chromatography analysis (Dionex ICS-1100, Thermo Scientific, Yokohama, Japan) during the sampling campaign. The water-soluble ion data showed that (1) sulfate was the dominant anion, and nitrate amounted to approximately 10% of the sulfate by molar ratio and that (2) the sulfate and nitrate were mainly neutralized by ammonium Number Concentrations of the BC Particles and Their Mixing States Measured by Using SP2 An analysis of the number concentrations of the scattering particles measured by SP2 indicated that the concentration began increasing at 23:00 on 6 August LT and remained relatively high between 1:00 and 14:00 on 7 August (in Figure 2a). Then, the concentration gradually decreased until 7:00 on 8 August. The number concentration of BC began increasing at 3:00 on 6 August (Figure 2b), remained relatively high until 13:00 on 7 August, and then decreased until 18:00 on 7 August. The BC number ratio to the scattering particles, which was determined by dividing the BC number concentration by the scattering particle number concentrations (Figure 2c), was relatively high between 13:00 and 22:00 on 6 August. Here the BC number ratio become >1 because the size range of BC (between 70 and 850 nm) is wider than that of measured ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9156

5 Figure 2. Temporal variation of scattering particle number concentration, BC in various mixing states, wind speed, and precipitation during the campaign. The data measured using SP2 are (a e) shown as 30 min averages and (f) shown as 10 min averages. (a) Number concentration of scattering particles between 170 and 850 nm in diameter. (b) Number concentration of BC particles between 70 and 850 nm in diameters. (c) BC ratio to scattering particles: (BC number)/ (scattering particle number). (d) Number fraction of internally mixed BC to all BC. (e) Number concentration of attached BC particles. (f) Wind speed and precipitation during the campaign measured at the Tokyo District Meteorological Observatory ( Different periods are indicated by shading. scattering particles (between 170 and 850 nm), indicating relatively high BC number concentration including those between 70 and 170 nm during the period. The temporal variation of the number fractions of internally mixed BC particles (Figure 2d) was similar to that of the scattering particle number concentrations (Figure 2a). Moteki et al. [2014] also showed the same temporal variations, in which the ratio of the scattering aerosol volume to the BC volume was used. The number of the attached BC particles was relatively high and exhibited several peaks between 6 and 7 August when the BC ratio was high (Figure 2e), but overall, the number fraction of the attached BC within internally mixed particles was small (1.2 ± 1.1%) throughout the present campaign. Moteki et al. [2014] also reported the number fraction of attached BC during the campaign. However, the data-processing algorithm used in that study could not distinguish light-absorbing iron oxide particles from attached BC particles and misclassified most of the detected iron oxide particles to the attached BC particle category. Subsequently, Yoshida et al. [2016] developed an algorithm to classify these particles, and here we report improved data on both attached BC and iron oxide particle fractions. ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9157

6 Table 1. Summary of Sampling Periods Descriptions Start-End Times (LT) Internally Mixed BC Fractions BC Concentrations Ns-Soot Mixing Ratios by TEM a Remarkable Events Period 1 Southern air mass prevailing period 0:00 on 5 August to 2:00 on 6 August Period 2 High BC fraction period 2:00 on 6 August to 22:00 on 6 August Period 3 High internally mixed 22:00 on 6 August BC period to 9:00 on 8 August Period 4 Northern air mass 9:00 on 8 August prevailing period to 18:00 on 9 August Low Low (stable) 0.81 ± 0.08 Peaks on marine aerosol + Fe-bearing particles Low Increase 0.52 ± 0.20 Low wind speed and precipitation High Decrease 0.88 ± 0.05 Increase of oxidant concentration Low Low (slightly increase) 0.82 ± 0.12 High iron oxide event a Averaged values for internally mixed ns-soot particles divided by total ns-soot particles (± standard deviation). Based on the SP2 results (Figure 2) and the backward-trajectory analysis, we divided the sampling timeframe into four periods (Table 1); Period 1, southern air mass prevailing period (between 0:00 on 5 August and 2:00 on 6 August 2012); Period 2, high BC ratio period (between 2:00 and 22:00 on 6 August 2012); Period 3, high internally mixed BC period (between 22:00 on 6 August and 9:00 on 8 August 2012); and Period 4, northern air mass prevailing period (between 9:00 on 8 August and 18:00 on 9 August 2012). Representative TEM images of samples from each period are shown in Figures 3 and S2. Figure 3. Representative TEM images of samples collected during each period. The scale bars represent 1 μm. The samples were collected on collodion carbon substrates. The sampling start times are shown above each image (local time). Arrows and triangles indicate examples of internally and externally mixed ns-soot particles, respectively. ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9158

7 Figure 4. Temporal variations of light-absorbing iron oxide (FeO x ) fraction measured by SP2 (dot blue line) and of Fe-bearing particle fractions measured by TEM (solid black line with red squares). Iron oxide fraction is number concentrations of iron oxide divided by scattering particle number (all nonincandescent particles), and Fe-bearing particle fraction is number fraction of Fe-bearing particles within all measured particles. Different periods are indicated by shading. Figure 5. Temporal variations of the BC number concentrations measured by SP2 and the average carbon intensities within individual particles measured by TEM. The vertical axis indicates normalized values, i.e., measured values divided by maximum values Changes in Mixing States and Number Concentrations of Ns-Soot/BC During Each Period Period 1: Southern Air Mass Prevailing Period The backward trajectory showed that the air mass originated from the Pacific Ocean and passed over the Boso Peninsula during this period (Figure 1). The distributions of the Na- and Mg-bearing particle fractions measured by STEM-EDS showed simultaneous increases during this period (Figure S3), suggesting strong influences of marine air mass [Lewis and Schwartz, 2004; Adachi and Buseck, 2015]. Additionally, fractions of the Fe-bearing particles, which are all particles that contain Fe regardless of its structural form measured by STEM-EDS, and iron oxide particles measured by SP2, exhibited several peaks at the same time (Figure 4). The number concentrations of scattering aerosol particles and BC measured by SP2 were relatively low, and the attached BC fraction was small (Figure 2) Period 2: High BC Ratio Period The BC number concentration measured by SP2 increased throughout this period (Figure 2b) when the wind speed was relatively low (<3 m/s). The BC number concentration began increasing approximately 20 h earlier than the scattering particle concentration. As a result, the number ratio of BC particles was relatively high, and the internally mixed BC fraction was relatively low (Figures 2c and 2d), resulting in high fractions of uncoated, externally mixed ns-soot particles in the TEM images from this period (e.g., TEM images of 07:12 and 14:00 on 6 August in Figures 3 and S2) as indicated by an uncoated ns-soot fraction of 0.48 (Table 1). The STEM-EDS analysis of the individual particles also showed an increase in the average carbon intensity during this period and during later periods of the campaign (between 21:00 on 8 August and 12:00 on 9 August) (Figure 5). Although the carbon intensity includes signals from both the substrate and the organic matter, the carbon signals from ns-soot were approximately 4 and 7 times higher than those of the organic coating and the carbon substrate, respectively (Figure S4); the carbon intensity can be used as an approximate indicator of ns-soot abundance in TEM analysis Period 3: High Internally Mixed BC Period The fraction of internally mixed BC particles increased as the scattering particle number concentration increased (Figures 2a and 2c). These results suggest that the amount of secondary aerosols determines the BC particle coating fraction. Moteki et al. [2014] also showed that thickly coated BC fractions are positively correlated with the production of secondary aerosols, and similar results were obtained in the UK [McMeeking et al., 2011] and in California [Thompson et al., 2012]. In ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9159

8 Figure 6. Temporal variations of the scattering particle number concentration, number fraction of internally mixed BC particles, and oxidant concentration. The scattering particle number concentration and number fraction of internally mixed BC particles were measured using SP2, and the oxidant concentration was measured at the Tokyo Metropolitan Government s monitoring station ( Oxidant refers to photochemical oxidant measured by UV absorption instrument [Akimoto et al., 2015]. These time series generally correlate and increase during the day except for some peaks, for example, on 7 August 03:00 indicated by an arrow. our samples, some major peaks of the number concentration of scattering aerosol particles and number fraction of internally mixed BC particles were correlated with the amount of oxidant measured approximately 2 km south of the site at the Tokyo Metropolitan Government s monitoring station, i.e., the oxidant began increasing on 7 August at 8:00 and peaked at 17:00 on 7 August, 13:00 on 8 August, and 13:00 on 9 August, similar to the particle number concentration (Figure 6). These results suggest that the aerosol particles formed during this period occurred as secondary aerosols through the oxidation of precursor gases [Moteki et al., 2007]. One exception was the data point from 7 August at approximately 3:00, when the scattering particles increased, but the oxidant did not. A possible explanation for this increase is that the air mass came from the northern region where aged aerosols had passed over the Tokyo area in the previous day through a sea-land breeze circulation [Kondo et al., 2006]. The TEM images indicate that the coated ns-soot particles increased during this period (Figures 3 and S2). The TEM mapping image suggests that the organic material occurs around the ns-soot particles (Figure S3) and can be one of the coating materials of the internally mixed BC. The number fractions of the internally mixed ns-soot from the TEM images were estimated using the method of Adachi et al. [2014], which counts the number of internally and externally mixed ns-soot particles from the representative TEM images. Nearly 88% of the ns-soot particles were internally mixed during this period. The fractions varied by period: Period 3 (0.88 ± 0.05) > Period 4 (0.82 ± 0.12) > Period 1 (0.81 ± 0.08) > Period 2 (0.52 ± 0.20) (Table 1). This order of internally mixed ns-soot fractions among periods is consistent with that determined by SP2, although the number fractions measured by TEM were larger than those of SP2, possibly because of the differences in the definitions applied to the internal mixture. It could be probable that some volatile coating materials were lost before the TEM analysis, but the relatively high internally mixed ns-soot fractions measured by TEM imply that the loss of volatile materials around ns-soot was not significant Period 4: Northern Air Mass Prevailing Period The SP2 results from this period (Figure 2) show small peaks on BC and scattering particle number concentrations, but overall, the number ratios of BC and scattering particles did not substantially change. During this period, the air mass moved from the northeast over the ocean and inland areas (Figure 1), where relatively few anthropogenic emission sources exist compared with the Tokyo metropolitan area (Figure S1). The TEM images indicate the presence of internally mixed ns-soot particles throughout the period (Figures 3 and S2). 4. Discussion 4.1. Comparison of TEM and SP2 Data The TEM and SP2 data showed consistencies in (1) the temporal variations of the average carbon intensity and the BC particle abundance, (2) the peaks in the number fractions of Fe-bearing particles (by TEM) and iron ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9160

9 oxide particles (by SP2) during periods 1 and 2, and (3) the trends in the internally and externally mixed nssoot/bc particle fractions. These consistencies suggest that although TEM and SP2 detect different physical properties, they measure essentially the same substances such as ns-soot and BC particles. However, even though TEM and SP2 showed similar temporal variations, the TEM and SP2 results were not identical. Possible reasons for the inconsistencies between the TEM and SP2 data are (1) the difference in the particle collection sizes ( nm for the scattering particles, nm for the BC particle number, nm for the internally mixed BC in SP2, and nm aerodynamic diameter in TEM); (2) possible losses of collected aerosols because of bouncing effects [Virtanen et al., 2010] and the evaporation of volatile materials during TEM sampling and analysis; and (3) differences in the measured parameters, i.e., TEM measures carbon intensity, whereas SP2 analyzes BC number concentration Mixing States of Ns-Soot/BC, Meteorological Conditions, and Photochemical Processes During the Period 2, the BC number concentration increased when the wind speed was relatively low (generally <3 m/s) and it was rainy (Figure 2f). The result is consistent with previous studies; Kondo et al. [2006] showed that high BC events correlated with weak wind events in Tokyo, and Huang et al. [2012] showed an inverse correlation of the BC concentration with the wind speed in an urban site at Shenzhen, China. In contrast, the number concentration of the scattering particles was relatively low during the Period 2. Washout processes likely removed hygroscopic aerosol particles, including sulfates and internally mixed ns-soot/bc particles. In addition, more uncoated ns-soot/bc particles were present after the rain (Figure 3). These results imply that the ns-soot/bc mixing states strongly depend on atmospheric processes. The ns-soot/bc mixing states depended on wind speeds, air mass sources, and photochemical processes. These results differed from those obtained in a remote mountain site in Japan [Adachi et al., 2014] but agreed with samples from the Los Angeles area during the CalNex (California Research at the Nexus of Air Quality and Climate Change) campaign [Adachi and Buseck, 2013] and from Mexico City [Subramanian et al., 2010]. The mixing states of the previously analyzed mountain samples were mainly influenced by long-range transported aerosol particles and were not significantly altered during the campaign. However, the mixing states of the CalNex urban samples exhibited diurnal changes that depended on meteorological conditions and photochemical processes [Adachi and Buseck, 2013]. Subramanian et al. [2010] also showed that BC coating thickness increased as a plume aged over Mexico City. These results suggest that the ns-soot/bc mixing states within urban areas commonly change within a relatively short term by diurnal and meteorological changes Fe-Bearing Particles TEM mapping images with EDS analysis of samples containing relatively high number concentrations of Fe-bearing particles indicate the presence of aggregated, spherical iron oxide particles (Figures 7 and S5). EDS analysis showed the atomic ratios between Fe and O varied and suggested the presence of FeO, Fe 3 O 4, and Fe 2 O 3 (e.g., Figure S5-3). Similar Fe-bearing nanoparticles have been found in other megacities (e.g., Mexico City [Adachi and Buseck, 2010] and Hong Kong [Li et al., 2015]). The TEM results suggest that a portion of the Fe-bearing particles consists of these spherical iron oxide particles in our samples, although other particles, such as mineral dust, are also possible sources of Fe [Buseck and Pósfai, 1999]. In general, such spherical metal oxide particles originate from various anthropogenic sources that generate heat, such as steel plants [Machemer, 2004], traffic [Sanderson et al., 2016], and coal combustions [Linak et al., 2007] and can be transported from both local and long-range sources. The TEM and SP2 analyses of Fe-bearing particles showed similar temporal variation during the Periods 1 and 2 (between 5 and 6 August), whereas the TEM results did not reflect the high iron oxide particle periods during the Period 4 (12:00 to 24:00 on 8 August) observed by SP2 (Figure 4). The reason for the inconsistency in the peaks is unknown, but similar Fe-bearing particles were found in the samples collected during the Periods 1, 2, and 4; the Fe sources during these periods may be the same. It should be noted that the fraction values determined by TEM and SP2 differed greatly (~40 times), possibly because of the different counting definitions applied, i.e., SP2 counts particles containing light-absorbing iron oxides with size between 200 and 2400 nm, whereas STEM-EDS counts particles containing the element iron in any structural form and sizes including nano-sized particles. Thus, the iron oxide particles measured using SP2 could be a small ADACHI ET AL. 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10 Figure 7. (a e) TEM images and (f) carbon, sulfur, oxygen and iron distributions. Iron oxide particles are aggregated spheres with diameters of <100 nm (Figures 7b 7e). Iron oxide particles are embedded within or attached to sulfates (Figure 7a). The sample was collected on 17:20 8 August fraction of iron aerosols in atmosphere although they can have an important influence on the global climate as a light-absorbing aerosol. Nevertheless, TEM analysis could reveal the particle shapes and occurrences of some fractions of iron oxide particles measured using SP2, and the information is useful to evaluate their possible sources and environmental implications. 5. Conclusion This study first reports a comparison of the TEM and SP2 data and how the mixing states and abundance of ns-soot/bc and Fe-bearing particle fractions change with time in response to meteorological conditions and airmass sources in Tokyo Metropolitan Area, Japan. We observed that the abundance and mixing state of nssoot/bc fractions changed within several hours to days in the area, for example, after rain and during stagnant atmospheric conditions. Nano-sized aggregated iron oxide particles were commonly found in the TEM samples and are one of the candidates for the particles that consisted of high iron oxide number concentration periods measured using SP2. There are agreements between TEM and SP2 measurements, e.g., temporal variations of the average carbon intensity measured using TEM and the BC particle abundance, the number fractions of Fe-bearing particles and iron oxide particles, and fractions of the internally and externally mixed ns-soot/bc particle. On the other hand, there are inconsistencies in results obtained from TEM and SP2 (e.g., quantitative variations and uncorrelated peaks) possibly because of differences in collected particle sizes, particle collection and detection efficiencies, and measured parameters. We conclude that occurrences of the ns-soot/bc measured using TEM and SP2 are generally consistent across the techniques even if they measure different properties of the same materials, i.e., TEM analyzes the mixing states, shapes, and compositions of individual particles, whereas SP2 provides continuous measurements of particle mixing states, number, and mass concentrations data. ADACHI ET AL. LIGHT-ABSORBING PARTICLES BY TEM AND SP2 9162

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