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1 Supporting Information Han et al /pnas SI Materials and Methods Aging of Soot by O 3. Soot particles were deposited on the ZnSe crystal. The sample in the in situ reactor was purged with 100 ml min 1 of high-purity nitrogen for 1 h. Then, 100 ppb O 3 in 100 ml min 1 nitrogen was introduced into the reactor, and in situ IR spectra were recorded under simulated sunlight. The O 3 was generated by irradiation of a mixture of high-purity O 2 and N 2 using a mercury lamp with wavelength of 185 and 254 nm. Soot remained almost unchanged in this concentration of O 2 in mixture flow. Concentration of O 3 was measured with an ozone monitor (Model 202, 2B Technologies). The spectra of soot was recorded (100 scans, 4-cm 1 resolution) using a blank ZnSe as reference. Diffusion Limitations. If photochemical aging process of soot by O 2 is a diffusion-controlled process, reactivities of soot samples with different compositions should not show a difference under the same reaction conditions. Different functional groups in a single soot sample also exhibit different reactivities to O 2, further demonstrating that the photochemical aging process of soot by O 2 is not a diffusion-limited process. Thus, the difference in composition of soot samples results in differing reactivities toward O 2 during irradiation. As a result of these observations, diffusion limitations through particle layers were not considered during the photochemical aging process of soot by O 2. Loss of OC. Compared with the mass of the attenuated total internal reflection infrared (ATR-IR) cell, the change in mass of organic carbon (OC) in soot deposited on the ZnSe crystal was too small to be accurately measured by weighing. To measure the change in mass of the soot, a second set of experiments was performed in which soot that was placed in a small and light crucible was then exposed to O 2 under irradiation. We also did not observe any difference in mass within ±0.01 mg (1.765 mg) between the fresh and oxidized soot. Two possible scenarios can account for the low change in mass of the soot: OC was not lost, and the mass of O 2 added to the soot was too small to detect any change in mass of the soot; the mass of O 2 added to the soot compensated that of OC lost. We think that OC is oxidized but not removed from the soot during the photochemical aging of soot by O 2. As shown in Fig. 1, the absorptions of alkyne and aromatic carbon decreased, whereas bands of carbonyl carbon increased with larger amplitude and remained almost unchanged when purged by high-purity N 2. Previous studies also found that various polycyclic aromatic hydrocarbons, which were the main active components of OC in this work, were oxidized to various aromatic ketones, aromatic aldehydes, aromatic carboxylic acids, and phenol, but not H 2 O and CO 2 (1 5). These results indicate that little of the OC is lost from soot during the photochemical aging process by O 2. We also compared the mass and ATR-IR spectra (Fig. S8) of fresh soot and that exposed to 100 ppb O 3 in the dark and under irradiation. The results were similar to those of soot oxidized by O 2 under irradiation, indicating that there was also little change in the mass of ozonized soot under irradiation. However, Zelenay et al. (6) observed that O 3 led to much more vigorous condensed phase oxidation, resulting in the removal of OC from soot by oxidation and volatilization. Similar results have also been observed by Smith and Chughtai (7), who found H 2 O and CO 2 as products of soot exposed to O 3. The difference between our work and previous studies might originate from the different soot sources. It should be pointed out that in the atmosphere, although O 2 because of its abundance plays a more significant role in the reaction rate of species on soot than O 3,O 3 may have a more important effect on the loss of OC in soot than O 2 under irradiation. 1. Mallakin A, Dixon DG, Greenberg BM (2000) Pathway of anthracene modification under simulated solar radiation. Chemosphere 40(12): Bernstein MP, et al. (1999) UV irradiation of polycyclic aromatic hydrocarbons in ices: Production of alcohols, quinones, and ethers. Science 283(5405): Sotero P, Arce R (2008) Major products in the photochemistry of perylene adsorbed in models of atmospheric particulate matter. J Photochem Photobiol Chem 199(1): Sigman ME, Schuler PF, Ghosh MM, Dabestani RT (1998) Mechanism of pyrene photochemical oxidation in aqueous and surfactant solutions. Environ Sci Technol 32: Barbas JT, Dabestani R, Sigman ME (1994) A mechanistic study of photodecomposition of acenaphthylene on a dry silica surface. J. Photoch. Photobio. A 80: Zelenay V, et al. (2011) Increased steady state uptake of ozone on soot due to UV/Vis radiation. J Geophys Res 116:D Smith DM, Chughtai AR (1997) Photochemical effects in the heterogeneous reaction of soot with ozone at low concentrations. J Atmos Chem 26: of5

2 Fig. S1. Temporal changes of in situ ATR-IR spectra of fresh fuel-rich flame soot purged by N 2. (Inset) Plot shows the changes in spectra of soot irradiated for 12 h in high-purity N 2. Fig. S2. Fitting results for ATR-IR spectra in the range of 1,800 1,520 cm 1 of the oxidized fuel-rich flame soot by airflow with 20% O 2 under simulated sunlight. Fig. S3. Thermal gravimetric curve of the fuel-rich and fuel-lean flame soot in N 2. 2of5

3 Fig. S4. Changes of ATR-IR spectra of the fuel-rich flame soot heated at 300 C as exposed to 20% O 2 for 12 h under simulated sunlight. Fig. S5. Comparison of ATR-IR spectra for fresh fuel-rich and fuel-lean flame soot. 3of5

4 Fig. S6. Temporal changes of peak areas for Ar-C=O, C=O, Ar-H, and C-H during the reaction of fuel-rich flame soot as a function of O 2 content under simulated sunlight. Fig. S7. Kinetics of Ar-C=O, C=O, Ar-H, and C-H on fuel-rich flame soot as a function of O 2 content under simulated sunlight. 4of5

5 Fig. S8. ATR-IR spectra of fresh fuel-rich flame soot and samples exposed to 100 ppb O 3 for 12 h in the dark and under irradiation. Fig. S9. Typical smoothed UV-vis spectra of fresh fuel-rich flame soot and samples exposed to 20% O 2 in air for 12 h under simulated sunlight irradiation. 5of5