We have analyzed several different soot samples. Kerosene lamp soot was produced by burning Coleman fuel in a wicked lantern (Walmart). Soot produced by this technique is known to be largely uncoated and composed of primary particles of approx. 40 nm diameter [12] . We also obtained soot from the chimney of a residential wood-burning fireplace (here called fireplace soot). This soot sample was scraped from the chimney wall, and collected into a small plastic vial for analysis. Diesel soot samples were collected from the tailpipe of active municipal buses in Lubbock, TX (Citibus). All buses burn the same fuel (low sulfur #2), but have different engines and manufacturers. One sample was collected from a 2006 Gillig Cummins Diesel using a plastic brush. A second sample was scraped from tail pipe of a 2001 Nova Cummins Diesel by a metal scraper. A third sample was scraped from the tail pipe of a 2000 RTS Detroit Diesel bus. A fourth sample was removed from a metal diesel particulate filter. The soot on this filter was produced from burning low sulfur #85 Diesel. After collection, the samples were filtered through a 250 μm sieve to remove any large debris. Each sample was stored in a small plastic bottle (under air) until analysis.

Soot pellets were prepared by manually compressing a small amount of a soot sample between sheets of aluminum foil prior to exposure with reactive gases. All experiments were carried out at laboratory pressure (0.91 atm) and temperature (295 K).

Soot samples were exposed to ozone within a 3.6 m³ chamber lined with FEP. Ozone was produced from zero air (flow rate is 2.25 L/min) using a photolytic ozone generator, and then added to the chamber. Meanwhile, filtered air at a flow rate of 1 L/min was also added into the chamber to dilute and circulate/mix the ozone. The concentration of ozone was monitored with a 2B Technologies ozone sensor. The soot pellets were placed into the chamber when the ozone concentration reached 5000 ppbv, and the concentration of ozone was recorded every 10 minutes to determine the dose experienced by the soot pellet. The concentration of ozone typically increased during the experiment and reached levels as high as 14,000 ppbv. The soot samples were not directly illuminated during the experiment. Dose of ozone, nitric acid, and sulfuric acid vapor (ppm sec) were calculated by multiplying exposure time (sec) by observed concentrations (ppm).

In a separate set of experiments, pelletized soot samples were exposed to HNO₃ (g) within a glass dessicator placed within a fume hood. Nitric acid vapor was produced from outgassing of a few mL volume of a concentrated 68% - 70% w/w solution of nitric acid also placed within the dessicator. The dessicator lid was left slightly ajar to allow a capillary sample inlet tube for a residual gas analyzer (RGA) to enter the dessicator. The concentration of vapor phase HNO₃ was estimated through use of a residual gas analyzer (RGA, Extorr Inc.). This was accomplished by monitoring the peak for at m/z = 46. This peak has previously been observed as the base peak for the electron impact spectrum of HNO₃ [13] and is believed to result from fragmentation (HNO₃ → OH + NO₂) in the source. We then ratio the m/z = 46 peak to that observed by the RGA for molecular oxygen at m/z = 32 and correct for oxygen’s fractional abundance (e.g. 20% v/v) to report a concentration of HNO₃ in ppm units. Typical indicated concentrations were in the 5 - 70 ppm range. The indicated concentrations were much lower than the equilibrium vapor pressure of 2 - 4 mm Hg (approx. 2 - 5 ppth) reported by Taylor [14] for a 70% HNO₃ solution at room temperature. This discrepancy likely results from several factors including the lack of equilibration between liquid/gas phases due to dessicator lid being slightly open and adsorption of nitric acid to surfaces. NO₂ (g) would represent an isobaric interference with the peak at m/z = 46 so the concentrations of HNO₃ reported within are subject to uncertainty. Given the HNO₃ vapor pressure is roughly 100 - 1000× greater than measured, if error does exist, the HNO₃ dose administered to the soot samples may be higher than what has been reported within this work.

A third set of conditions exposed soot pellets to sulfuric acid vapor within a sealed glass dessicator. Briefly, a 96% wt/wt sulfuric acid solution was placed in the bottom of the dessicator and the soot pellet was placed within the head-space of the solution such that the soot was only exposed to the vapor. The dessicator was sealed, and the vapor-liquid equilibrium condition was sought over 3 h periods of time. At each 3 h time increment, the water contact angle was measured. Sulfuric acid is far less volatile than nitric acid (sat. vapor pressure of 1.8 × 10⁻⁸ bar or 17.7 ppbv) [15] and the concentration could not be directly measured by techniques available in our laboratory. Instead, we use the saturation vapor pressure for estimating dose of H₂SO₄ (g) vapor applied to soot samples. Since, the dessicator seal was broken every 3 hours to remove samples for analysis it is likely our reported doses are slightly high since the vapor would be diluted with room air when the seal is broken, and some amount of time would be needed to reestablish phase equilibrium. As a result, the sulfuric acid vapor doses reported within are also subject to significant uncertainty.

Soot pellets were placed on the surface of a small aluminum cube and a drop of water was carefully added onto the surface of the soot pellet. Then, photographs of the soot surface and drop were quickly acquired with a digital camera (Panasonic, DMC-FX 48) mounted on a tripod. Only the initial contact angle was reported. Image J freeware (http://rsb.info.nih.gov/ij/index.html) was used to determine contact angle (angle can be directly read from this software). Enthalpy of immersion for each soot sample was then computed using Equation (1).

Figure 2(b) illustrates a photograph of a water drop on a soot pellet, and the resulting contact angle. Figure 2(a) reports initial contact angle data and immersion enthalpy data for several soot samples tested and comparison data from previous literature [16] . The samples tested exhibited a variety of initial contact angles. Both kerosene soot and fireplace soot always exhibited initial contact angles near 100˚. However, the diesel engine soot samples were far more diverse yielding contact angles from > 100˚ to approx. 65˚. This is particularly interesting because three of the four samples were obtained from engines that burn identical fuel.

The range of contact angles observed is very similar to values Persiantseva et al. reported in 2004 for several different soot samples [16] . Four of the six samples tested exhibited contact angles similar to graphite.

The American Society for Testing and Materials (ASTM) standard ASTM D7334- 08 (2013) defines several classes for materials based on water contact angle exhibited.

Materials exhibiting contact angles < 20˚ have excellent wetting properties, contact angles < 45˚ indicate hydrophilic surfaces. Contact angles between 45˚ - 90˚ are of intermediate range as materials transition from mostly hydrophilic to hydrophobic. A contact angle of 90˚ or greater indicates a hydrophobic surface. All soot samples tested without further chemical treatment fell within either the hydrophobic or intermediate range. Corresponding enthalpy of immersion values ranged from 5 - 54 mJ/m². The very large range of contact angles observed for the different bus samples suggests the wetting behavior of soot may be heavily influenced by the source of emission.

Figure 3 reports contact angle and H_imm data for all soot samples as a function of the extent of ozone treatment. As observed, most samples exhibited a decrease in contact angle (and corresponding increase in H_imm) when treated with large doses of ozone. The only exception to this trend was the diesel soot from a Gillig bus. This sample exhibited a slight initial decrease in contact angle, but as the ozone dose increased the trend reversed and the final contact angle was even larger than the initial value. The cause of this result is not clear. However, after extensive treatment with ozone many of the soot

pellets became hydrophilic and exhibited contact angles < 45˚. Two soot samples did not support formation of a droplet (full wetting) after ozone treatment. These observations are largely consistent with the hypothesis that soot surface modifications can drive surface wetting. We note that the dose of ozone used in these experiments was generally far in excess of what might typically be encountered in the atmosphere. A second x-axis has been added to Figure 3 that lists the projected atmospheric aging time required to achieve the dose used in the experiment assuming a typical daylight tropospheric concentration of ozone (40 ppb). Since a typical soot particle may have an atmospheric residence time of < 10 days, and diurnal mean O₃ concentration will likely be less than 40 ppbv, it is unclear whether the action of gas-phase ozone is sufficient to extensively modify the soot surface and dramatically affect wetting during a soot particles atmospheric lifetime.

A similar set of experiments has also been carried out in which soot pellets were treated with gas-phase HNO₃ produced from the headspace vapors of a ca. 70% solution of nitric acid. Figure 4 reports contact angle and H_imm data for the soot samples as a function

of applied dose. As observed, most samples (exception being Gillig bus soot) exhibited a decreased contact angle with extensive nitric acid treatment. Comparing Figure 3 and Figure 4 leads to the conclusion that the nitric acid treatment was, on average, slightly less effective than ozone at increasing the soot wetting. Interestingly, very similar trends were observed between the two chemical treatments. The shape of curves for the Nova bus soot is virtually identical in both figures. The curves for the RTS bus soot and kerosene soot also show similar shapes. Gillig bus soot is largely unresponsive to either chemical treatment. The fireplace soot sample showed the largest difference between treatments-exhibiting a large change in contact angle during ozone processing, but only a moderate change when treated with HNO₃ (g). Another x-axis is added in Figure 4 in an effort to quantitate projected atmospheric age to reaction with HNO₃ (g) at 5 ppb assumed concentration. Again, mega-doses of HNO₃ have been used in this experiment, projected to be far greater than what an atmospheric soot particle may experience during its lifetime in the atmosphere. Only very marginal changes in contact angle are observed during the modeled first few hundred days of exposure.

Another set of experiments investigated whether treating pelletized soot with sulfuric acid vapor affects wetting behavior. In this set of experiments the soot was placed within a sealed dessicator containing 97% sulfuric acid solution at 20˚C - 25˚C. The partial pressure of sulfuric acid in the vapor phase used to treat the soot could not be directly measured using technology our laboratory possesses. Because of this, we must estimate the vapor phase concentration through literature reports of the sulfuric acid saturation vapor pressure. W. Roedel [17] reported the vapor pressure above a 99% solution of sulfuric acid to be about 32 ppbv (2.5 × 10⁻⁵ torr, at 23˚C) while the work attributed to Perry [15] suggests 17.7 ppbv as the saturation vapor pressure of a 96% solution at 20˚C. We have adopted the 17.7 ppb estimate since it matches our experimental concentration and temperature conditions. Since atmospheric levels of H₂SO₄ (g) are believed to often be < 10⁷ cm⁻³ with levels never exceeding 5 × 10⁷ cm⁻³ or approx. 2 ppt [18] [19] , treating the soot samples with the vapor in equilibrium with the concentrated liquid still represents a very large dose compared to the typical atmospheric case. Figure 5 reports contact angle and H_imm data for various soot samples treated with sulfuric acid vapor. Again, the dose reported should be construed as being very uncertain. While a slight downward trend in the contact angle is noted as applied dose increases, by in large, the sulfuric acid treatment increases hydrophilicity of the soot surface only marginally.