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"Click Here  GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L10804, doi:10.1029/2007GL029756, 2007  for  Full Article  Uptake of NO3 on soot and pyrene surfaces Jackson Mak,1 Simone Gross,1 and Allan K. Bertram1 Received 20 February 2007; revised 24 March 2007; accepted 12 April 2007; published 18 May 2007.  [1] The reaction of NO3 with methane soot, hexane soot, and solid pyrene was investigated using a flow tube reactor. The uptake of NO3 on fresh soot was fast (uptake coefficient &gt;0.1). Based on this result and an assumed density of reactive sites on soot, the time to process or oxidize 90% of a soot surface in the atmosphere would take only approximately five minutes. This suggests that NO3 chemistry can rapidly oxidize soot surfaces under atmospheric conditions. After exposing soot films to NO3 for approximately 180 minutes in the laboratory, the uptake reaches a steady-state value. The steady state uptake coefficients (assuming a geometric surface area) were 0.0054 \u00B1 0.0027 and 0.0025 \u00B1 0.0018 for methane and hexane soot, respectively. These numbers are used to show that heterogeneous reactions between NO3 and soot are not likely a significant sink of gas-phase NO3 under most atmospheric conditions. The uptake of NO3 on fresh pyrene surfaces was also fast (uptake coefficient &gt;0.1), and much faster than previously suggested. We argue that under certain atmospheric conditions reactions between NO3 and surface-bound polycyclic aromatic hydrocarbons (PAHs) may be an important loss process of PAHs in the atmosphere. Citation: Mak, J., S. Gross, and A. K. Bertram (2007), Uptake of NO3 on soot and pyrene surfaces, Geophys. Res. Lett., 34, L10804, doi:10.1029/2007GL029756.  1. Introduction [2] Reactions between NO3 and gas-phase species have received a significant amount of attention, and this research has shown that NO3 is an important nighttime gas-phase oxidant [Finlayson-Pitts and Pitts, 2000]. While the chemistry between NO3 and gas-phase species is relatively well understood, the chemistry between NO3 and atmospheric particles remains basically unexplored with a few exceptions. [3] Heterogeneous reactions between NO3 and atmospheric particles may be important for several reasons. For example, these reactions could be a sink of NO3 in the atmosphere, and they may also change the chemical composition and toxicity of aerosol particles [Finlayson-Pitts and Pitts, 2000; Moise et al., 2002]. [4] Heterogeneous reactions between NO3 and aqueous solutions, mineral dust, and organic substrates have previously been studied [Docherty and Ziemann, 2006; Hung et al., 2005; Imamura et al., 1997; Karagulian and Rossi, 2005; Knopf et al., 2006; Moise et al., 2002; Rudich et al., 1996a; Rudich et al., 1996b; Rudich et al., 1998; Schutze and Herrmann, 2005]. Also the uptake of NO3 on decane 1  Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2007 by the American Geophysical Union. 0094-8276/07/2007GL029756$05.00  soot and soot produced with a graphite spark generator was recently explored [Saathoff et al., 2001; Karagulian and Rossi, 2007]. To add to this short list, we have carried out measurements of the reactive uptake coefficient of NO3 on methane and hexane soot surfaces and on solid pyrene surfaces (pyrene is a polycyclic aromatic hydrocarbon consisting of four aromatic rings). The reactive uptake coefficient is defined as the ratio of the molecules removed from the gas-phase by reactions to the total gas-surface collisions. These are the first measurements of the reactive uptake coefficient of NO3 on methane soot, hexane soot, and pyrene. For both soot and pyrene, we measured the uptake coefficient on fresh surfaces, and for soot we also measured the reactive uptake of NO3 as a function of exposed time to NO3 to assess whether or not the NO3 reaction modifies the soot surface. [5] The studies involving solid pyrene were carried out to better understand the chemistry of NO3 with polycyclic aromatic hydrocarbons (PAHs) adsorbed on or absorbed in atmospheric particles. PAHs while in the atmosphere can be adsorbed on or absorbed in atmospheric particles where they can undergo heterogeneous reactions with gas-phase oxidants. There has only been one study that has looked at the reaction of NO3 and PAHs adsorbed on a substrate, but the reactive uptake coefficient of NO3 on the surface was not determined [Pitts et al., 1985]. Our measurements with solid pyrene surfaces are a starting point to understanding the reactivity of NO3 with PAHs adsorbed on and absorbed in atmospheric particles. Our studies with solid PAH surfaces are also related to the soot measurements since soot is believed to contain a significant amount of PAH material. [6] In this manuscript we show that NO3 is efficiently taken up by fresh soot and fresh pyrene surfaces and the possible atmospheric implications are discussed.  2. Experimental [7] The apparatus used in this work was similar to that previously used in our laboratory to study heterogeneous loss processes [Knopf et al., 2005; Knopf et al., 2006]. It consisted of a coated-wall flow tube reactor coupled to a chemical ionization mass spectrometer. The flow tube was constructed of borosilicate glass and included a movable injector through which NO3 was introduced. The main carrier gas, which was a mixture of He (80%) and O2 (20%), was introduced through a port at the upstream end of the flow reactor. The inside wall of a Pyrex tube (1.75 cm inside diameter and 15 cm in length) was coated with either soot or a pyrene film and then inserted into the flow tube reactor. These coatings provided the surfaces for the heterogeneous studies. [8] The reactive uptake coefficients were determined by first measuring the loss of NO3 on the surfaces as a function of injector position. This data was then used to calculate the  L10804  1 of 5  \u000CMAK ET AL.: UPTAKE OF NO3 ON SOOT AND PYRENE  L10804  Table 1. Summary of the Uptake Measurements for Fresh and Oxidized Methane and Hexane Soot and Fresh Pyrene Surfaces Surface  g g (Fresh Surface)  g g (Oxidized Surface)  Methane soot Hexane soot Solid pyrene  &gt;0.1 &gt;0.1 &gt;0.1  0.0054 \u00B1 0.0027 0.0025 \u00B1 0.0018 -  L10804  pressures used in the uptake measurements ranged from 2\u2013 4 Torr with a total flow rate ranging from 450 \u20134000 sccm in the flow reactor.  3. Results and Discussion  observed first-order loss rate coefficient, kobs. Next kobs was corrected for concentration gradients that form close to the flow-tube wall by using the procedure developed by Brown [1978]. The diffusion coefficients of NO3 reported by Rudich et al. [1996a] were used when correcting for concentration gradients. The reactive uptake coefficient, g, was calculated from the corrected kobs using a standard procedure [Knopf et al., 2005]. This procedure assumes that the surface area available for reaction is equal to the geometric surface area of the Pyrex tubes. To indicate that the reactive uptake coefficient was based on a geometric surface area we use the symbol g g. The effect of the porosity of the films on the reactive uptake coefficient is addressed below. [9] The flow tube technique utilized in this research is typically capable of accessing reactive uptake coefficients greater than 10 \u00C06 . However, for our flow rates and pressures, reactive uptake coefficients greater than approximately 0.1 are greatly influenced by gas-phase diffusion to the reactive surface. In this case, a small uncertainty in kobs or the diffusion coefficient results in a large uncertainty in the reactive uptake coefficients. For this reason, when the reactive uptake coefficient is &gt;0.1 we are only able to report a lower limit of 0.1. [10] NO 3 radicals were produced by passing N2O5 through a Teflon coated glass oven held at 423 \u2013 433 K [Knopf et al., 2006]. At the exit of the flow cell, NO3 was detected using chemical ionization with I\u00C0 as the reagent ion. NO3 concentration used in these studies ranged from 0.7 \u00C2 1011 to 4 \u00C2 1011 molecule cm\u00C03 and NO3/N2O5 ratios ranged from 20 to 100. [11] Solid pyrene films were prepared by first melting pyrene crystals on the inside of the Pyrex tubes and then rotating the tubes while the liquid pyrene recrystallized. This resulted in an even distribution of the pyrene and a relatively flat surface \u2013 the surface area was within 2 % of the geometric surface area of the Pyrex tube based on measurements with a profilometer. The profilometer measurements were performed using Pyrex slides rather than Pyrex tubes. However, the slides were prepared using the same techniques that were used to prepare the tubes. For the soot experiments, two types of soot surfaces were used: methane soot and hexane soot. Methane soot was generated by exposing the inner wall of the glass tube to a methane flame produced with a standard torch. Hexane soot was generated by combusting hexane in an open glass container. An inverted glass funnel was placed above the glass container to limit the amount of air during combustion. A glass tube was held above the narrow opening of the funnel until a coating of soot covered the inner wall. The total mass of methane or hexane soot deposited in our experiments ranged from 1.6 to 6.4 mg. [12] Soot and pyrene experiments were conducted at a temperature of 298 K and 293 K respectively. The total  3.1. Soot Studies [13] Shown in the auxiliary material are examples of results from NO3 uptake measurements on soot.1 Displayed are several uptake measurements made on the same sample of hexane soot after exposing the sample to NO3 at a concentration of approximately 1.1 \u00C2 1011 molecule cm\u00C03 for 0 to 110 minutes. The data show that the uptake decreases with NO3 exposure time. [14] The uptake coefficient, g g, on fresh methane and hexane soot in both cases is &gt;0.1 (see Table 1). The reported values take into account the uncertainty in the measurements and the uncertainty in the diffusion coefficient used when calculating g g. Since g g is based on the geometric surface area, it is an upper limit to the true uptake coefficient. However, when g g is &gt;0.1, the correction factor for porous films is typically small (between 1/3 and 1) [Keyser et al., 1991]. Hence, our measured g g is close to the true uptake coefficient. Our results are in good agreement with the reactive uptake coefficients measured by Karagulian and Rossi [2007] for decane soot. [15] Shown in Figure 1 are results from the measurements of g g on a hexane soot film as a function of time of exposure to NO3. Methane soot shows a similar trend. The data show that the NO3 uptake decreases with exposure, suggesting the uptake process is reactive. At long exposure times (approximately 180 minutes) the uptake reaches a steady-state rate. The values for the reactive uptake coefficient associated with the steady-state loss are 0.0054 \u00B1 0.0027 and 0.0025 \u00B1 0.0018 for methane and hexane soot, respectively. These values are based on several measurements at an exposure time of approximately 180 minutes. The steady-state loss values are included in Table 1 for comparison. Karagulian and Rossi [2007] also observed a decrease in uptake with exposure time for decane soot, but they used relatively short exposures, so a direct comparison is not possible. [16] As discussed above, when g g is &gt;0.1, the correction for porosity is small; however, the porosity correction for the steady-state uptake coefficients may be large. The correction depends on the fraction of the soot film sampled by NO3. As a lower limit to the true steady-state uptake coefficient we can assume NO3 samples the entire film and use the BET surface area to calculate a lower limit. Methane soot has a BET surface area of 25 m2 g\u00C01[Tesner and Shurupov, 1995] and hexane soot has a BET surface area of 46 m2 g\u00C01[Choi and Leu, 1998]. Based on these values and the mass of soot used for the experiments, the actual surface areas are 5 and 20 times greater than the geometric areas of methane and hexane soot, respectively. This translates to lower limits to the true steady-state reactive uptake coefficients of 0.00083 \u00B1 0.00046 and 0.000124 \u00B1 0.000006 for methane and hexane soot, respectively. These numbers are consistent with the numbers reported by Saathoff et al. [2001] for soot generated by a graphite spark generator. 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2007GL029756.  2 of 5  \u000CL10804  MAK ET AL.: UPTAKE OF NO3 ON SOOT AND PYRENE  Figure 1. Plot of the NO3 uptake coefficient as a function of exposure time on hexane soot. The soot sample was exposed to a NO3 concentration of approximately 1.1 \u00C2 1011 molecule cm\u00C03 for 0 \u2013 110 minutes.  estimate and include several assumptions. Nevertheless, this calculation does suggest that NO3 chemistry can rapidly oxidize soot surfaces in the atmosphere. This may have implications for the CCN properties and hygroscopic properties of atmospheric particles. [19] We have also carried out preliminary measurements of the surface functional groups produced by this NO3 chemistry on soot using Fourier Transform infrared spectroscopy. Preliminary results show the formation of oxygen and nitrogen containing functional groups on the soot surface after exposure. These functional groups may alter the infrared light absorption properties of soot particles in the atmosphere as well as their health effects. [20] To determine if the reaction between NO3 and soot is a significant sink of gas-phase NO3 in the troposphere, we compare the lifetime of NO3 with respect to heterogeneous loss, t het, with the lifetime of NO3 estimated from field measurements. t het can be calculated with the following equation: t het \u00BC  3.2. Atmospheric Implications of the Soot Studies [17] The time required for the reactive uptake coefficient to reach a steady-state value in the soot experiments was approximately 180 minutes for an NO3 concentration of 1011 molecule cm\u00C03. We assume this corresponds to the time to process or oxidize the soot surface. It is rather difficult to calculate the time to process or oxidize the surface of a soot particle in the atmosphere from our exposure studies. This is because the laboratory experiments were carried out using multiple layers of soot particles and also because the laboratory experiments involve both reaction and diffusion into the pores for all cases except at the very start of the exposure studies. Under these conditions, the NO3 exposure level ([NO3] \u00C2 time) needed for processing in the laboratory will be larger than the NO3 exposure level needed in the atmosphere. [18] Nevertheless, we can estimate the time required for the processing of a soot particle in the atmosphere using our measured NO3 reactive uptake coefficients and the following equation:[Bertram et al., 2001] fraction of \u0012 surface \u0013 processed in the atmosphere g 0 Zt \u00BC exp \u00C0 Ntotal  \u00F01\u00DE  where g 0 represents the reactive uptake coefficient of NO3 on a fresh soot surface, Z represents the collision frequency of NO3 with the surface (molecule cm\u00C02 s\u00C01), t represents time (s), and Ntotal represents the total number of surface sites available for reaction (reactive sites cm\u00C02). We assume g 0 = 0.5, which is consistent with our measurements. For Ntotal we used 7 \u00C2 1014 reactive sites cm\u00C02 which is an average of the Ntotal values reported in the literature from measurements of O3 uptake on soot [Kamm et al., 1999; Lelievre et al., 2004; Poschl et al., 2001]. In these calculations we used an NO3 concentration of 50 ppt, which corresponds to polluted urban conditions. Using these values the time to process 90 % of a soot particle in the atmosphere would take only approximately 5.5 minutes. However, keep in mind that these calculations are an  L10804  4 wgA  \u00F02\u00DE  where w is the average thermal velocity, g is the reactive uptake coefficient and A is the soot area density in the atmosphere. For these calculations we assume g = 3 \u00C2 10\u00C03, which is consistent with the steady-state reactive uptake coefficients determined in our measurements. Here we use the steady-state values since the initial fast uptake likely only applies to very fresh plumes in the atmosphere as mentioned above. For A, we assume $2.5 \u00C2 10\u00C06 cm2 cm\u00C03 based on soot concentrations of 5 mg m\u00C03[Sloane et al., 1991] and a surface to mass ratio of 50 m2 g\u00C01, which is consistent with BET surface areas for methane and hexane soot. Soot surface area densities of $2.5 \u00C2 10\u00C06 cm2 cm\u00C03 correspond roughly to polluted conditions. Using these values we get a t het of &gt;4.5 hours. In contrast, field measurements suggest the lifetime of NO3 in the continental boundary layer is typically much less [see e.g., Geyer et al., 2001; Platt et al., 1984]. Hence, the loss of NO3 on soot is likely not a significant sink of NO3 under most atmospheric conditions except close to soot sources. However, keep in mind that the values for t het discussed above are order of magnitude estimates, which rely on published BET surface areas for similar soot samples. Because soot structure and composition are dependent on the mode of generation, BET surface areas and reactive surface species will be different for different soot sources. 3.3. Pyrene Studies [21] Also shown in Table 1 is the uptake coefficient of NO3 on a fresh pyrene surface. In this case the pyrene surface is not porous so g g should be within a few percent of the true uptake coefficient. [22] To the best of our knowledge this is only the third study of the reactive uptake coefficient of NO3 on organic material. Moise et al. [2002] studied the uptake of NO3 on a range of organics including alkanes, alkenes, an alcohol, and carboxylic acids with conjugated and nonconjugated unsaturated bonds, and they observed uptake coefficients ranging from 1.5 \u00C2 10\u00C02 to 3.8 \u00C2 10\u00C04. The largest uptake coefficients were observed for molecules with unsaturated  3 of 5  \u000CL10804  MAK ET AL.: UPTAKE OF NO3 ON SOOT AND PYRENE  L10804  Table 2. Reactive Uptake Coefficients of NO3 and O3 on PAH Surfaces  Oxidant  Surface  g  Atmospheric Concentration,a [Oxidant]  NO3 O3 O3  Solid pyrene Benzo[a]pyrene on soot [Poschl et al., 2001]b Benzo[a]pyrene on solid organic aerosol [Kwamena et al., 2004]b Anthracene at air-water interface [Mmereki and Donaldson, 2003; Mmereki et al., 2004]c  &gt;0.1 $2 \u00C2 10\u00C05 to 5 \u00C2 10\u00C06 \u00C06 $2 \u00C2 10 to 5 \u00C2 10\u00C07  50 ppt 100 ppb 100 ppb  (12 \u2013 1.1) \u00C2 108 $ (4.9 \u2013 1.2) \u00C2 107 $ (4.9 \u2013 1.2) \u00C2 106  $3 \u00C2 10\u00C07 to 2 \u00C2 10\u00C08  100 ppb  $ (7.4 \u2013 0.5) \u00C2 105  O3  g \u00C2 [Oxidant]/ molecule cm\u00C03  a  Taken from Finlayson-Pitts and Pitts [2000]. The reactive uptake coefficient depended on relative humidity and O3 concentrations. c The reactive uptake coefficient depended on O3 concentration and whether or not the air-water interface was coated with an organic monolayer. b  bonds and also octanol. Knopf et al. [2006] investigated the uptake of NO3 on alkane monolayers and obtained a value of 8.8 \u00C2 10\u00C04. A comparison between the previous measurements and our current results suggests that the heterogeneous reactivity of polycyclic aromatic hydrocarbons is significantly enhanced compared to other organic groups. This suggests that NO3 will preferentially oxidize polycyclic aromatic hydrocarbon material in atmospheric particles. [23] The only other study that we are aware of that investigated NO3 with surface-bound PAHs is the work by Pitts et al. [1985]. These authors carried out a preliminary study of the reaction of NO3 with pyrene adsorbed on glass fiber filters in an environmental chamber. The authors monitored the decay of pyrene rather than the loss of NO3 and concluded that adsorbed pyrene did not react to any observable extent with the NO3 radicals. A possible reason for the apparent discrepancy may be the difference in experimental conditions. The experiments by Pitts et al. were carried out in the presence of large concentrations of N2O5 (2.5 \u00C2 1013 molecule cm\u00C03). These large concentrations may have interfered with the surface reaction between NO3 and pyrene by blocking reaction sites. 3.4. Atmospheric Implications of the Pyrene Studies [ 24 ] Recently the reaction between O 3 and PAHs adsorbed on surfaces has received considerable attention [see e.g., Finlayson-Pitts and Pitts, 2000; Kwamena et al., 2004; Mmereki and Donaldson, 2003; Mmereki et al., 2004; Perraudin et al., 2007; Poschl et al., 2001; Raja and Valsaraj, 2005, and references therein]. This research has shown that under certain conditions, these reactions may be an important loss process of PAHs in the atmosphere [see e.g., Alebicjuretic et al., 1990; Finlayson-Pitts and Pitts, 2000; Mmereki et al., 2004; Poschl et al., 2001, and references therein]. To assess whether or not loss of PAHs by heterogeneous reactions with NO3 is comparable to heterogeneous reactions with O3 we first compare reactive uptake coefficients (see Table 2). Note the reactive uptake coefficient of O3 on solid pyrene has not been measured so instead we have compared our uptake coefficients for NO3 with the O3 uptake coefficients on other PAH surfaces. In Table 2, we have also included approximate atmospheric concentrations of NO3 and O3. In addition we have included the product g \u00C2 [oxidant] where [oxidant] is the approximate atmospheric concentration in units of molecule cm\u00C03. g \u00C2 [radical] is a more relevant parameter for comparing the loss of PAHs by heterogeneous reactions, since the number  of PAH molecules lost by reaction should be proportional to g \u00C2 [radical]. 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Richards (1991), Size-segregated fine particle measurements by chemical-species and their impact on visibility impairment in Denver, Atmos. Environ., Part A, 25(5 \u2013 6), 1013 \u2013 1024. Tesner, P. A., and S. V. Shurupov (1995), Some physico-chemical parameters of soot formation during pyrolysis of hydrocarbons, Combust. Sci. Technol., 105(1 \u2013 3), 147 \u2013 161. \u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0\u00C0  A. K. Bertram, S. Gross, and J. Mak, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1. (bertram@chem.ubc.ca)  5 of 5  "@en
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