Aerosol particles from megacities influence the regional and global climate as well as the health of their occupants. We used transmission electron microscopes (TEMs) to study aerosol particles collected from the Los Angeles area during the 2010 CalNex campaign. We detected major amounts of ns-soot, defined as consisting of carbon nanospheres, sulfate, sea salt, and organic aerosol (OA) and lesser amounts of brochosome particles from leaf hoppers. Ns-soot-particle shapes, mixing states, and abundances varied significantly with sampling times and days. Within plumes having high CO2 concentrations, much ns-soot was compacted and contained a relatively large number of carbon nanospheres. Ns-soot particles from both CalNex samples and Mexico City, the latter collected in 2006, had a wide range of shapes when mixed with other aerosol particles, but neither sets showed spherical ns-soot nor the core-shell configuration that is commonly used in optical calculations. Our TEM observations and light-absorption calculations of modeled particles indicate that, in contrast to ns-soot particles that are embedded within other materials or have the hypothesized core-shell configurations, those attached to other aerosol particles hardly enhance their light absorption. We conclude that the ways in which ns-soot mixes with other particles explain the observations of smaller light amplification by ns-soot coatings than model calculations during the CalNex campaign and presumably in other areas.
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 Atmospheric aerosol particles exert important influences on global climate through scattering and absorbing sunlight and by acting as the nuclei of cloud droplets. They affect human health when inhaled. Both the climate and health effects of aerosol particles depend on their compositions, sizes, shapes, and mixing states with other particles, all of which vary particle-by-particle [Buseck and Pósfai, 1999; Adachi and Buseck, 2010; Davidson et al., 2005]. Thus, knowledge of these properties for significant numbers of individual particles from various places is necessary for understanding and evaluating their influence on climate and human health. Here we focus on the aerosol particles that we call ns-soot (short for nanosphere soot) [Buseck et al., 2012] to indicate that they consist of nanospheres of concentrically wrapped, graphene-like layers of carbon and with grape-like (acinoform) morphologies. If ns-soot particles are coated or embedded within other types of materials, then they are called internally mixed ns-soot particles. Buseck et al.  introduced the term ns-soot to clearly define the particles, which have loosely been called black carbon, elemental carbon, or soot in the literature. Although we cannot be certain that prior aerosol papers discussing soot were concerned with ns-soot, we shall assume that for simplicity.
 The aerosol particles for this study were collected during the CalNex (California Research at the Nexus of Air Quality and Climate Change) campaign from 15 May to 15 June 2010. The campaign emphasized the interactions between air quality and climate-change issues and aimed to understand the emissions, atmospheric chemistry, climate effects, and transport of anthropogenic atmospheric pollutants that included both gases and aerosol particles. We collected samples at the Pasadena ground site, 16 km northeast of downtown Los Angeles (LA) [Adachi et al., 2011].
 During the MILAGRO (Megacity Initiative: Local and Global Research Observations) campaign in March 2006 [Molina et al., 2010], we investigated aerosol particles from Mexico City (MC) and regional biomass burning and found a wide range of particle types, among which ns-soot was prominent [Adachi and Buseck, 2010, 2011; Yokelson et al., 2007, 2009, 2011]. Both LA and MC are megacities and emit large amounts of atmospheric pollutants [Molina and Molina, 2004]. During MILAGRO, we collected over and around MC from aircraft, and thus the samples had various ages and sources. In contrast, during CalNex, our samplers were on the ground, and samples were collected regularly at roughly hourly intervals. Thus, the CalNex samples are especially useful for detecting diurnal changes of aerosol particles from a megacity, including changes produced during aging.
 Ns-soot comprises a major type of aerosol particle and its particle shapes and mixing states determine the magnitude of its light absorption and radiative forcing [Adachi et al., 2007; 2010; Bond and Bergstrom, 2006; Jacobson, 2001], but these parameters change during atmospheric aging. For example, Metcalf et al.  showed aircraft measurements of refractory black carbon using a single particle soot photometer and indicated the changes of its coating thickness over the Los Angeles basin. Diurnal changes of light absorption of bulk CalNex aerosol samples were measured by Thompson et al. , who also observed a lack of light absorption enhancement by the coatings of ns-soot. In the current study, we focus on aerosol particles at the single particle scale using transmission electron microscopes (TEMs), which are powerful instruments for observing the details of shape and mixing states of internally mixed ns-soot. Our goal is to understand the changes in shape and mixing states of ns-soot aerosol particles and to determine the effect of these changes on the ns-soot optical properties.
2 Samples and Methods
2.1 Sampling, Meteorological Conditions, and Emission Inventories
 Aerosol particles were collected using three-stage impactor samplers (MPS-3, California Measurements, Inc.) placed on the roof of a building at the California Institute of Technology (Caltech), Pasadena, CA (34.138°N, 118.124°W). Particles were captured onto lacey-carbon TEM substrates consisting of fibers resembling spider webs. They produced minimal interactions with the trapped particles. The 50% cutoff aerodynamic diameters of the samplers were 2.0, 0.3, and 0.05 µm. This study used TEM grids on the smallest impactor stage (0.05 to 0.3 µm). Sampling times were between 3 and 30 min, and most were ~5 min. In total, 465 TEM samples were collected.
 The same Caltech sampling site had previously been used for the Pasadena Aerosol Characterization Observatory (PACO) campaign between May and August 2009 [Hersey et al., 2011]. The meteorological conditions during CalNex were similar to those of PACO. Briefly, at 20:00–06:00 (LT), the wind was weak (<0.5 m/s) and generally from the N/NE. Local emissions accumulated in the LA Basin during these times. During daytime, the wind direction changed to W/SW. When sea breeze developed, fresh emissions with ages between 1 and 2 h were transported from the western LA Basin toward downwind areas, including the sampling site.
 The California Air Resources Board Emission Inventory reported the major sources of particulate matter <2.5 µm (PM2.5) in LA in 2008 (available from California Air Resources Board, http://www.arb.ca.gov). On-road motor vehicles contributed 17%, and other mobile sources such as aircraft and off-road vehicles contributed 19%. Other major sources included paved-road dust (16%) and cooking (13%). The largest SOx source in the LA area was ships followed by petroleum refining. The dominant sources of black carbon in California were wildfires, on- and off-road vehicles, managed burning, waste disposal, and residential fuel combustion [Chow et al., 2010].
2.2 TEM Analyses
 A CM 200 TEM (Philips Corp.) operated at an accelerating voltage of 200 kV was used for both imaging and energy-dispersive X-ray spectroscopy analyses to determine particle compositions [Adachi and Buseck, 2010; Buseck and Adachi, 2008]. A 200 kV Tecnai F20 TEM (FEI Corp.) was used for imaging, electron energy-loss spectrometry (EELS), and electron tomography. We obtained images from >20 areas (~56 µm2 for each area) on each TEM grid.
3 Results and Discussion
3.1 Aerosol Particles
 The CalNex samples contain ns-soot, sulfate, organic aerosol (OA), sea salt, mineral dust, and metal particles (Figure 1), although the relative abundances change significantly from one sample to the next. Biological (brochosome) particles also occur in 32 of our samples (~7% of all TEM samples) (auxiliary material).
 Here we focus on the ns-soot particles. In the CalNex samples, they consist of aggregated carbon nanospheres ranging in diameter from 20 to 80 nm, with a median value 43 ± 11 nm. The sizes of carbon nanosphere are similar to those from Mexico City (44 nm) [Adachi and Buseck, 2008] but larger than those from laboratory-generated diesel exhaust (31.9 ± 7.2 nm) [Park et al., 2004]. Ns-soot particles contain poorly ordered, curved graphitic layers and display a sharp π* peak in EELS analyses, features consistent with the ns-soot from MILAGRO and other areas [Adachi and Buseck, 2008; Buseck et al., 2012]. Many ns-soot particles are mixed with OA, sulfate, sea salt, or their combinations (Figure 1). The OA that coats or embeds ns-soot consists of amorphous carbonaceous material, and its occurrence on ns-soot surfaces suggests that it mainly formed through condensation in the atmosphere.
3.2 Diurnal Changes of Aerosol Type and Ns-Soot Mixing State
 TEM observations indicate that diurnal variations of shapes, mixing states, and relative abundances of aerosol particles depend on the meteorological conditions and emission rates. May 22 provides a good example (Figures 2). We collected three samples per hour between 6:00 and 18:00 local time and one sample per hour for the rest of the day. Based on TEM observations, these samples are divided into four periods: 1:00 to 6:00, 7:00 to 11:00, 12:00 to 18:00, and 19:00 to 24:00.
 Ns-soot particles are relatively abundant between 1:00 and 6:00, are larger than those from other periods, and consist of >100 carbon nanospheres (e.g., 3:00; arrows). Some ns-soot particles are partly embedded within OA or sulfate (e.g., 2:00), but many are free of coatings and have open shapes (e.g., 4:00). Sulfate particles are relatively abundant in samples collected between 7:00 and 11:00 (e.g., 7:00). The ns-soot particles consist of several tens of carbon nanospheres, and many are partly embedded within OA or sulfate (e.g., 11:00). Mixtures of OA and sulfate are abundant from 12:00 to 18:00 (e.g., 16:00). Many of these particles coagulate with each other and embed ns-soot (e.g., 12:00). Between 19:00 and 24:00, aggregates of sulfate, OA, and sea salt are abundant (e.g., 23:00). Ns-soot particles are relatively large (~100 carbon nanospheres), and many tend to be attached to other particle types but not be fully embedded (e.g., 22:00).
 These results suggest that mixing states of ns-soot, as well as the number of its constituent nanospheres, can change significantly within several hours, and such changes tend to be related to the occurrences of other aerosol particles. Extensive photochemical processing occurs during the afternoon, producing much OA [Williams et al., 2010]. Since, TEMs generally measure only nonvolatile materials, much of the OA that we observed on ns-soot surface in the afternoon is either nonvolatile or only slightly volatile. The result suggests that both gas-to-particle condensation and heterogeneous oxidation [Jimenez et al., 2009] likely contribute to its formation. In contrast, during nights and mornings, we did not observe much OA, probably because this material was more volatile than in the afternoons and would have evaporated before or during TEM measurements.
3.3 Ns-Soot Shapes and Mixtures With Other Particle Types
 Many urban ns-soot particles are mixed with OA, sulfate, and sea salt, presumably through atmospheric condensation, coagulation, and cloud processing. Ambient air measurements in the LA basin [Geller et al., 2006] using a differential mobility analyzer and aerosol particle mass analyzer show that ns-soot changes shape as it ages, commonly to more compact forms. Laboratory studies [Pagels et al., 2009] showed that ns-soot particles become more compact when sulfuric acid and water condense onto them. These changes in ns-soot mixing state and shape affect its light absorption [Adachi et al., 2010]. Some CalNex ns-soot particles coagulate with or are attached to other aerosol particles rather than being embedded or coated (Figure 3a; arrows). Other ns-soot particles are partly or fully embedded within other aerosol particles (Figure 3b; arrows). In addition, relatively large, compact ns-soot particles occur with apparently little coating (Figure 3c; arrows). Their compact shapes with little coating material extended along the substrate fibers indicates that coatings, had they been present, evaporated before or during TEM measurements.
3.4 Large, Compact Ns-Soot Particles and CO2 Concentrations
 The ns-soot particles were relatively large and abundant when the CO2 concentration was high (>430 ppmv), which occurred more than 10 times at the site during the campaign (Figure 4). CO2 was measured using a pressure-controlled, nondispersive, infrared absorption instrument based on a LI-COR model 6262 analyzer [Nicks et al., 2003; Peischl et al., 2010]. TEM samples collected during these events consist mainly of ns-soot particles containing >100 carbon nanospheres and are essentially free of significant coatings. Forty-two of our samples (9% of all TEM samples) consist mainly of such large, compact ns-soot particles (Figure 4), and 95% of these occur within the high CO2 plumes. Back-trajectory models [Draxler and Rolph, 2011] suggest that these high CO2 plumes came from the downtown LA area and were relatively fresh. These results suggest that coagulation of ns-soot into large particles containing many nanospheres is enhanced in pollution plumes, and their compaction could have occurred during evaporation of volatile materials (secondary organic aerosol or water).
3.5 Comparison of Ns-Soot and Other Aerosol Particles from CalNex and MILAGRO
 Most ns-soot particles from both CalNex and MILAGRO are mixed with OA, but their occurrences differ. Although the MILAGRO OA that coats ns-soot did not change significantly during electron microscopy [Adachi et al., 2010], that from CalNex decomposed within ~30 s. The TEM electron beam produces heating and ionization damage, which can result in mass loss for organic materials [Egerton et al., 2004]. The sensitivity difference between the two sample sets reflects compositional differences, presumably in part because the CalNex samples were collected closer to their sources and were thus less oxidized than those from MILAGRO. The latter, collected from aircraft, had aged for an average of ~8 h prior to collection [Adachi and Buseck, 2008].
 Many CalNex ns-soot particles are more compacted than those from MILAGRO. The atmospheric relative humidity (RH) in LA was between 40 and ~100%, suggesting that many CalNex ns-soot particles once existed within droplets when the RH was higher than the deliquescence values of ammonium sulfate (80%) and sodium chloride (75%). In contrast, the RH values during MILAGRO were mostly <70%, indicating many had not deliquesced in the atmosphere. Evaporation of water as well as other volatile materials in the atmosphere or after collecting samples could be one of the causes of the observed compaction.
3.6 Shapes, Mixing States, and Light Absorption of Ns-Soot Particles
 The light absorption of ns-soot particles is influenced by their shapes and mixing states [Adachi et al., 2010; Bond and Bergstrom, 2006; Fuller et al., 1999]. Using these parameters, we grouped the CalNex ns-soot particles into the following: (1) open and uncoated, (2) coated or embedded within other particles, (3) compact and relatively uncoated, and (4) attached to other particles. Adachi et al.  discussed the influences of ns-soot shape and position within MILAGRO OA using types 1, 2, and 3, but ns-soot type 4 was not observed in those samples. Here we discuss the effects of ns-soot shapes, focusing on ns-soot that is attached to other particles (type 4).
 Semivolatile organic materials or water that might have been on the surfaces of some CalNex aerosol particles would have been lost before or during TEM analysis. The following calculations assume that there was either no such volatile coating material or it occurred in negligible amounts. These situations will be possible when ns-soot particles that were attached to solid particles (e.g., sea salt, sulfate, tar balls, or mineral dust) occur in dry air, although that was not always the case for the CalNex samples. Coatings of semivolatile or nonvolatile materials, if any, would increase the ns-soot light absorption [Adachi et al., 2010]. As discussed below, the calculations thus provide limiting conditions.
 We simulated shapes of fractal ns-soot particles attached to (a) cubic, (b) plate-like, and (c) spherical particles using the discrete dipole approximation (DDA) of Draine and Flatau [1994, 2003; DDSCAT 6.1] (Figure 5). Cubic and plate-like shapes approximate sea salt and mineral dust particles such as micas and clays, and the spherical shape simulates tar balls [Adachi and Buseck, 2011; Hand et al., 2005; Pósfai et al., 2004]. We assumed that (1) the attached particles have the same volume as the ns-soot particles and (2) particles rotate randomly in the atmosphere. Assumption (2) was implemented by using 100 incident light angles. The models used the same parameters as in Figure 4 of Adachi et al. , which contains ns-soot particles 37 nm in diameter containing 50 spherules and with fractal dimensions between 2.1 and 2.5. The light absorption of 39 simulated particles was calculated.
 When these modeled ns-soot particles are attached onto the surfaces of other particles (e.g., Figures 5a to 5c), on average they absorb 1.04 times more light than for particles without attached particles. In other words, there is no significant difference in absorbance for such attached ns-soot particles. In contrast, the modeled ns-soot embedded within other aerosol particles, and the core-shell particle (Figure 5d) absorbed, respectively, 1.10 (from Figure 4 in Adachi et al. ) and 1.23 times more than that without coating. We include the hypothetical core-shell model only because it is widely used in the aerosol literature. However, we have never observed such configurations nor did we observe spherical ns-soot cores. Thus, even had there been a liquid coating in the atmosphere so that the outer shape was spherical prior to electron microscopy, it would not have been concentric to the core.
 Our calculations indicate that ns-soot particles do not significantly enhance light absorption when attached to other aerosol particles, which is in contrast to when ns-soot particles are embedded within other particles. These calculations plus those of Adachi et al. , explain the results of Thompson et al.  and Cappa et al.  during CalNex, both of which implied that the core-shell configuration overestimates light enhancement.
 Enhancement of light absorption by internally mixed ns-soot from ambient air during events encountered during CalNex is clearly less than predicted from model calculations. Based on TEM observations and DDA calculations, this result can be explained by the ways in which the ns-soot is attached to, coated by, or embedded within other particles, all of which differ from the modeled core-shell configuration. For determining accurate absorption data, it is desirable to know the physical details of the mixing states of particles.
 Ns-soot particles from CalNex showed a wide range of abundances, shapes, and mixing states even within a day. In plumes having CO2 concentrations >430 ppmv, much ns-soot was compacted and contained a relatively large number of carbon nanospheres. Since ns-soot shape and mixing state exhibited diurnal variations, the light absorption of the individual particles would change as well. Although both LA and MC are megacities, the aerosol particles differed significantly. Meteorology, geography, regulation, emission sources, and particle age can account for the differences. Regional climate models should consider the differences in aerosol particle shapes and mixing states in each area to evaluate the scattering and absorption.
 The diurnal changes of particle shapes and mixing states through condensation, coagulation, and cloud processing significantly influence how ns-soot particles mix with others. In contrast to embedded ns-soot particles, those attached to other aerosol particles hardly affected the light absorption. The theoretical core-shell configuration is unrealistic for the internally mixed ns-soot particles in our samples and overestimates their light enhancement. We conclude that the ways ns-soot is attached to, coated by, or embedded within other particles need to be considered when measuring aerosol light absorption and modeling climate.
 We thank J. Surratt, J. Seinfeld, J. Jimenez, J. de Gouw, and J. Stutz for help with CalNex sample collections, J. Peischl, T. Ryerson, and R. Washenfelder for providing their CO2 data, and B. Draine and P. Flatau for providing the DDSCAT 6.1 code. The authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT model and READY website (http://www.arl.noaa.gov/ready.php) and the use of TEM facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. This study was supported by NSF grants ATM0531926 and ATM1032312. K. Adachi acknowledges support from the global environment research fund of the Japanese Ministry of the Environment (A-1101).