Uptake of HNO3 on Aviation Kerosene and Aircraft Engine Soot

May 17, 2013 - Uptake of HNO3 on Aviation Kerosene and Aircraft Engine Soot: Influences of H2O or/and H2SO4 ... Citation data is made available by par...
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Uptake of HNO3 on Aviation Kerosene and Aircraft Engine Soot: Influences of H2O or/and H2SO4 Ekaterina E. Loukhovitskaya,†,‡,§ Ranajit K. Talukdar,*,†,‡ and A. R. Ravishankara† †

National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, Colorado 80305, United States Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States § Moscow State University, 119992 Moscow, Russia, and Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia ‡

S Supporting Information *

ABSTRACT: The uptake of HNO3 on aviation kerosene soot (TC-1 soot) was studied in the absence and presence of water vapor at 295 and 243 K. The influence of H2SO4 coating of the TC-1 soot surface on HNO3 uptake was also investigated. Only reversible uptake of HNO3 was observed. HONO and NO2, potential products of reactive uptake of HNO3, were not observed under any conditions studied here. The uptake of nitric acid increased slightly with relative humidity (RH). Coating of the TC-1 soot surface with sulfuric acid decreased the uptake of HNO3 and did not lead to displacement of H2SO4 from the soot surface. A limited set of measurements was carried out on soot generated by aircraft engine combustor (E-soot) with results similar to those on TC-1 soot. The influence of water on HNO3 uptake on E-soot appeared to be more pronounced than on TC-1 soot. Our results suggest that HNO3 loss in the upper troposphere due to soot is not significant except perhaps in aircraft exhaust plumes. Our results also suggest that HNO3 is not converted to either NO2 or HONO upon its uptake on soot in the atmosphere.

1. INTRODUCTION Heterogeneous interactions of gas-phase species with aerosols play important roles in the chemistry of Earth’s atmosphere. Nitric acid is one of the most important nitrogen-containing species in the atmosphere. Atmospheric models do not reproduce the measured ratio of HNO3 to NOx (NOx = NO + NO2) in the troposphere.1,2 This discrepancy could be due to an inadequate accounting for HNO3 washout in convective processes,3,4 and/or to not including all relevant chemical processes that remove HNO3 or convert HNO3 to NOx. Similarly, any conversion of HNO3 to NOx, or sequestration of HNO3, in the stratosphere would also be important to the chemistry of that region. Therefore, the potential role of heterogeneous reaction and uptake of HNO3 on soot is of interest. Soot is common in the atmosphere and it can be expected to take up HNO3 or be reactive toward it. In the lower troposphere the principal sources of soot are the partial combustion of fossil fuels and biomass.5 In the upper troposphere and lower stratosphere the primary source of soot is jet aircrafts.6 Lofting of biomass burning can also be a source of soot in the upper troposphere.7 Pyro-cumulous © XXXX American Chemical Society

events have also been known to inject soot all the way into the lower stratosphere.8,9 Soot particles in the atmosphere might be altered by uptake of O3, H2SO4, HNO3, and H2O. Coating by sulfuric acid can be particularly important for soot from aviation in aircraft plumes. The amount of H2SO4 in such plumes strongly depends on the composition of the fuel.10 For example, it has been estimated that H2SO4 at the engine nozzle exit of a B-747 aircraft fitted with RBB211-524B engines varied from 1.9 × 1011 cm−3 to 1.3 × 1012 cm−3 when the fuel sulfur content was changed from 0.04% to 0.3%.10 The role of aerosols in heterogeneous atmospheric chemistry is still poorly understood and many questions remain. They include questions such as the following: Do NOy species (particularly HNO3) convert to NOx on soot surface? How is HNO3 interaction with soot altered by H2SO4 and/or H2O that are inevitably present in the exhaust plume and in the atmosphere? Received: February 18, 2013 Revised: May 15, 2013

A

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Figure 1. (a) Schematic diagram of the flow tube used to study the uptake of HNO3 on soot samples inside the reactor. It shows a cylindrical Pyrex tube coated on its inside wall with TC-1 soot made by burning aviation kerosene in a lantern. (b) The bottom panel shows the placement of a pellet of soot collected from a jet engine combustor inside the reactor. The noted components are as follows: (1) movable injector; (2) a cylinder coated with TC-1 soot; (3) glass cylinder made to hold a pellet of E-soot; (4) pellet of E-soot prepared pressing collected soot with a pellet press; and (5) Teflon mesh used to hold the pellet in place.

temperature-controlled fluid from a temperature-controlled bath was flowed through the inner jacket that was surrounded by an evacuated outer jacket to minimize heat loss. The temperature of the flow tube could be varied from 223 to 420 K. A retractable glass tube containing a chromel-alumel thermocouple was inserted from one end of the flow tube. Temperature in the region where HNO3 was exposed to soot was measured under flow conditions identical with those in the experiments. The measured temperature was constant, to within 1 K, along the entire length of the coated tube. During the uptake measurements, the thermocouple was retracted from the NFT such that the reactants and products were not exposed to the glass tube containing the thermocouple. The soot samples were inserted into NFT in 10 cm long cylindrical Pyrex tubes (internal diameter ∼1.8 cm) that were either coated on their inside surface with soot (Figure 1a) or contained a pellet of soot (Figure 1b). Further details on the soot samples are given later in this section. The effluents of the NFT passed through a Pyrex throttle valve into the chemical ionization mass spectrometer consisting of an ion flow tube (IFT) and quadrupole mass spectrometer. The flow entered the IFT ∼50 cm downstream of the region where reagent ions were produced. The Pyrex throttle valve controlled the gas flow rate out of the NFT. The pressure in the NFT was adjusted by changing the gas flow rate into and out of

In our previous work, the uptake of HNO3 on aviation kerosene soot was studied in detail.11 In this paper, we report the extent of HNO3 uptake on aviation kerosene soot (TC-1) that is coated with H2SO4, H2O, or both. We also investigated if HNO3 is converted to NOx upon its uptake by these soot samples. A few similar studies were also carried out using aircraft engine soot (E-soot) to assess if there are major differences between TC-1 soot made by burning aviation kerosene in the laboratory and that collected from engines. Our studies were carried out at concentrations of nitric acid, relative humidity, and temperature comparable to those in the atmosphere.12−14

2. EXPERIMENTAL SECTION The method of investigation of HNO3 interaction with aviation kerosene soot (TC-1) was very similar to that previously used.11 As in the previous study, the apparatus (Figure 1) consisted of a neutral flow tube (NFT) where the soot samples were exposed to HNO3 and a chemical ionization mass spectrometer (CIMS) where the reactant and products were ionized and the ion abundances were measured. Only a brief description of the apparatus is given here. 2.1. Neutral Flow Tube (NFT). The NFT reactor was a 35 cm long, 2 cm inner diameter, double-jacketed Pyrex tube maintained at a constant temperature (within 1 K). A B

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10−9 cm3 molecule−1 s−1 by using the methodology outlined by Su and Chesnavich.17 2.3. Soot Samples. Two different types of soot samples were used in the present investigations: (1) laboratory soot prepared from burning aviation kerosene (TC-1) and (2) Esoot collected from an aircraft combustor. The methods of their preparation are described below. 2.3.1. TC-1 Soot. Aviation kerosene (TC-1) was burned in a lantern at 1 atm of air to generate soot. The fuel/air ratio was determined by the fuel coming out of the lantern and 1 atm of air in which it burned. This was highly reproducible. [Note: It is a diffusion flame and not a premixed flame; therefore the fuel/ air ratio varies from the flame front to the center and not a fixed number. However, the kerosene lantern flame is considered to be a fuel lean flame.] An inverted Pyrex funnel was held above the flame. Soot exiting the stem of the funnel was collected on the inside wall of a 10 cm long, 1.8 cm inside diameter, cylindrical Pyrex tube. To obtain a uniform coating, the Pyrex tube was rotated manually around its cylindrical axis and occasionally flipped lengthwise to introduce the soot stream from the other end. The soot appeared to be uniformly coated. Soot sample mass for each coating was determined from the measured differences in the masses of the soot coated and uncoated Pyrex tube. Total mass of the sample was measured subsequent to the uptake experiments to be ∼4 mg in one case and 5.7 mg in another case. The mass of the pellet was 19 mg. The results from all the samples were consistent. The coated tube was placed inside the flow tube for the uptake measurements (Figure 1a). BET surface area of soot similarly prepared has been previously shown to be roughly 90 m2 g−1. The possible sulfate content of this soot was not measured but is expected to be very small given the low temperature of combustion for its preparation. To investigate the influence of sulfuric acid on HNO3 uptake the soot sample was coated with H2SO4 and placed inside the NFT. Helium was flowed through/over concentrated sulfuric acid at 60 °C and the ensuing stream was passed over the soot sample held at room temperature for ∼12 h. The amount of H2SO4 deposited on the soot surface was estimated by heating the coated soot sample to 393 K and measuring the total amount of desorbed H2SO4 (roughly 5 × 1016 molecules or ∼2.1 × 10−2 mmol/g of soot). 2.3.2. Aircraft Engine Soot (E-Soot). The method for the preparation of aircraft engine soot (E-soot) was described in Popovicheva et al.18 The combustor of a gas turbine engine, D36KU, at the Central Institute of Aviation Motors in Moscow, Russia, was operated to simulate the cruise conditions with an air/fuel ratio of ∼4 and a pressure up to 7 atm. Aviation kerosene TC-1 was burned and the produced soot was collected on a water-cooled copper probe located at the combustor exit. The E-soot is predominantly black carbon that is embedded with metals as well as CH3COO−, SO42−, HCOO−, (COO)22−, Cl−, NO3−, K+, Na+, and NH4+.18,19 In addition, it also contains carbonyl groups. These embedded components, which are distributed homogeneously throughout the soot particles, have been shown to be responsible for the hygroscopicity of E-soot.18 Further, Popovicheva et al. have contended that soot from burning TC-1 fuel in a burner, or a lantern such as the one used here, is a good surrogate for real engine soot.18 The E-soot powder was pressed into a circular-shaped pellet with use of a pellet press used in IR investigations of powder samples. The E-soot pellet (7 mm in diameter, ∼2 mm thick,

the NFT. The pressure in the NFT (3−6 Torr) was significantly higher than that in the IFT (0.2−0.4 Torr); it was measured at the two ends of the NFT with use of calibrated capacitance manometers. A small (calibrated) flow of He was bubbled through water maintained at room temperature to saturate the He with water vapor (∼23 Torr at 298 K). The water vapor concentration in the neutral flow tube was calculated from the total gas flow rate, the pressures, and temperatures in the bubbler and NFT. The water vapor concentration was adjusted to obtain the desired relative humidity in the flow tube. HNO3 was introduced into the flow tube by flowing ultra high purity (UHP) helium over solid HNO3 kept in a reservoir that was maintained at a constant temperature in the range of 195−213 K. The eluting HNO3/He mixture was added to the NFT through a 46 cm long, 0.4 cm inner diameter, moveable Pyrex injector. The position of the injector in the NFT could be varied along the length of the 10-cm long cylinder either containing the soot pellet or coated with soot. Flow rates of UHP He were between ∼500 and ∼1200 STP cm3 min−1 in the NFT and led to linear flow velocities between 800 and 1500 cm s−1 through the soot-coated cylinder. The HNO3 content of the gas flowing through the NFT was measured at various positions of the injector tip inside the sootcoated tube or over soot pellet. The time dependence (adsorption−desorption profile) of the HNO3 signal at a fixed injector position was measured when the soot was exposed to HNO3 as well as when it was not exposed to soot. In addition to changes in HNO3 signal, potential products of a reactive uptake, such as NO2, HONO, N2O5, and NO3, were also monitored simultaneously by using the chemical ionization mass spectrometer consisting of the IFT and the quadrupole mass spectrometer. The ion chemistry used for these detections is given below. 2.2. Schemes for Detection of Reaction Products. The reagent ion for chemical ionization, SF6−, was produced by the reaction of thermalized electrons with SF6 in the IFT. A small fraction of the reagent ions reacted with the reactant and product molecules of interest from the NFT to generate the ions that were detected by the quadrupole mass spectrometer. The relative product ion intensities reflected the relative concentrations of the neutral species. The concentration of the ion produced exclusively from a species of interest was proportional to the product of its concentration and the rate coefficient for its reaction with the reagent ion. SF6− was used as the reagent ion to detect HNO3, NO2, HONO, N2O5, H2SO4, and SO3, via the following ion− molecule reactions, with rate constants in units of cm3 molecule−1 s−1:15,16 HNO3 + SF6− → NO3− ·HF + SF5 → −

NO3−

+ product

−

NO2 + SF6 → NO2 + SF6 HONO + SF6− → NO2−·HF + SF5 −

N2O5 + SF6 →

NO3−

k1 = 2.0 × 10−9

(1a) (1b)

−10

k 2 = 1.4 × 10

(2)

k 3 = 6.0 × 10−10

(3)

−10

(4)

+ SF6 + NO2 k4 = 7.5 × 10

−

−

−9

(5)

−

−

−9

(6)

H 2SO4 + SF6 → HSO4 + HF + SF5 k5 = 2 × 10 SO3 + SF6 → SO3·F + SF5

k6 = 2 × 10

The rate coefficients for reactions of SF6− with H2SO4 to give HSO4− and with SO3 to give SO3·F− were estimated to be 2 × C

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weighing 62 mg) was covered with a Teflon mesh to prevent the pellet from flying out and was placed in a cylindrical Pyrex tube with a circular inset made to retain the pellet (Figure 1b). The cylindrical Pyrex tube with the sample was inserted into the NFT. Experiments were carried out at 243 K at a few different relative humidities (RH). After the experiments, the pellet was removed from the flow tube reactor and a set of “blank experiments” were conducted with only the holder inside. Signals from the “blank experiments” were subtracted from the ones with the soot sample to account for the contributions of the Teflon mesh and its holder. The signals from the “blank experiments” were usually 10−30% of those with the soot sample in place. 2.4. Experiments and Experimental Conditions. UHP He was used as the carrier gas. Pressure and flow velocities in the flow tube were 3 Torr and 950−1300 cm s−1 for experiments with TC-1 soot and about 6 Torr and 300−500 cm s−1 for experiments with E-soot. The partial pressure of water vapor in the reactor was controlled by flowing a portion of the carrier gas through/over water kept in a bubbler.20 At 243 K, the RH ranged from ∼0 to ∼96% while at 295 K it was roughly 0.8%. The concentration of HNO3 was varied from ∼1010 to ∼1011 cm−3. The concentration of HNO3 in the gas flow exiting the HNO3 container was measured by absorption at 184.9 nm (Hg pen ray lamp) in a 50-cm long absorption cell before the flow was introduced into the flow tube.11 HNO3 was synthesized by the reaction of KNO3 with concentrated H2SO4 (1:3 ratio) and HNO3 was collected in a Pyrex trap at 77 K.11 The HNO3 sample was degassed and stored at 195 K before use. 2.5. Uncertainty Analysis. Uncertainties in the measured amounts of HNO3 taken up by the soot samples arise from the uncertainties in the absolute HNO3 concentrations in the NFT as well as calibration of ion signal due to HNO3, variation of relative humidity, and uncertainties in the numerical integration of the adsorption−desorption time profile of HNO3. [HNO3] in the flow tube was determined by UV absorption at 184.9 nm before the gas mixture entered the neutral flow tube. The uncertainty in HNO 3 concentration in the NFT was determined by the uncertainty in absorption cross-section of HNO3 at 184.9 nm (±5%), variations and uncertainties in the flow rates of diluent gas (±2%), and pressure (±1%) in the absorption cell and flow tube. We estimate the overall uncertainty at twice the standard deviation (2σ) to be 30% by adding all of these uncertainties in the quadrature. In the case of sulfuric acid coated soot, the overall uncertainty is estimated to ∼40% at the 2σ level.

Figure 2. Typical adsorption−desorption profiles of HNO3 on TC-1 soot at higher (1011 cm−3) and lower (1010 cm−3) concentrations and different temperatures. Adsorption−desorption profiles for lower concentrations of HNO3 were multiplied by 5 to display them both on the same scale. (a) Upper panel: T = 295 K; black line, [HNO3] = 1.7 × 1011 molecules cm−3; red line, [HNO3] = 2.9 × 1010 molecules cm−3. (b) Lower panel: T = 243 K; black line, [HNO3] = 1.9 × 1011 molecules cm−3; blue line, [HNO3] = 3.7 × 1010 molecules cm−3. Measured signals for HONO (red line) and NO2 (green line) are shown in the lower panel. The background signals were due to either impurities or reactions of walls prior to exposure of HNO3 to soot and background ion-noise.

equilibration. At ∼305 s (for the black line) or ∼400 s (for the red line), the injector was again moved downstream of the TC-1 soot (i.e., the soot was no longer exposed to HNO3, but the same concentration of HNO3 was exiting the injector). Clearly, nitric acid desorbed from the soot sample. Desorption of HNO3 was slower than adsorption as shown by the longer time it took for the concentration of nitric acid in the NFT to reach the pre-exposure levels. The same behavior was seen when a lower concentration of HNO3 (red line) was used. Figure 2b shows the data for similar experiments carried out at 243 K. As seen in the figure, the behavior at 243 K is qualitatively the same as that at 295 K, except that larger amounts of HNO3 were taken up. The total number of HNO3 molecules taken up by soot was determined by integrating the time-dependent uptake profile of HNO3 concentrations shown in Figure 2, and by using the flow velocity and the cross-sectional area of the flow tube. The concentrations of HNO3 in the gas phase and the total amounts of HNO3 taken up by the soot sample are listed in Table 1. The number of HNO3 molecules that desorbed was ∼15% and ∼40% lower than that taken up at 295 and 243 K, respectively. However, when the soot sample was heated to 303 K, most of the absorbed HNO3 desorbed as shown in the Supporting Information (see Figure SI-1). When this additional desorption is included, the total HNO3 desorbed was within 5% of that taken up. Thus, the amount of HNO3 taken up was the same as that desorbed, within our measurement uncertainty. We looked for NO2, HONO, NO3, and N2O5, which are possible products of reactive uptake of HNO3. Figure 2 also shows the signal levels for NO2 or HONO with and without

3. RESULTS AND DISCUSSION 3.1. HNO3 Uptake on TC-1 Soot and E-Soot. The uptake of HNO3 on laboratory prepared TC-1 soot was measured at two ranges of HNO3 concentrations (∼1010 and ∼1011 cm−3) at 295 and 243 K. Figure 2a shows the concentration of HNO3 in the gas phase flowing out of the NFT as a function of time when exposed to 4 mg of soot in the cylindrical tube at 295 K. The black line is for HNO3 concentrations that are about five times larger than that for the red line. Initially, the injector carrying the gas phase HNO3 was positioned beyond the sootcoated cylinder so that soot was not exposed to HNO3. At ∼114 s the injector was quickly withdrawn to expose soot to HNO3. Clearly, nitric acid was taken up by soot as seen by the reduction in the HNO3 signal. The uptake slowly decreased until there was negligible further uptake indicating an D

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thus even the HNO3 that was retained longer did not lead to reactive uptake. This conclusion is consistent with our previous work on TC-1 soot.11 The extent of reversible HNO3 uptake measured here is roughly a factor of 2 smaller than that reported earlier from our laboratory, if we assume that the surface area of soot in this study is the same as that previously measured (STC‑1 = 90 m2 g−1), that the same number of HNO3 molecules are needed for a monolayer coverage, and that our previously measured equilibrium constant is valid.11 This difference of roughly a factor of 2 at 295 K is within the combined uncertainties in the two studies, especially since the surface area per unit weight of the sample used here could be less than that used previously.11 A few other points are worth noting. First, the equilibration process is faster at 295 K than at 243 K (see Figure 2). Second, the amount of HNO3 taken up at 243 K is more than an order of magnitude larger than at 295 K. Third, desorption of HNO3 at 243 K is much slower than at 295 K. Fourth, it required heating to desorb all of the HNO3 that was taken up. It is likely that the desorption rate gets so slow as the surface coverage decreases that it would have taken a very long time for complete desorption. Heating the sample allowed a more rapid desorption. All of these observations are consistent with physical uptake of HNO3 involving no reactive channels, as noted previously.11 The uptake of HNO3 on E-soot was measured at only 243 K. Qualitatively, the profile of HNO3 uptake was similar to that on TC-1 soot. The amount of HNO3 taken up was much smaller (Table 1) than on TC-1 soot, at least in part because there was much less E-soot (surface area ∼1.2 cm2) exposed to the flow than the TC-1 soot (which covered 25 cm2 of the inside surface of the tube). Analogous to TC-1 soot, we did not observe the production of products such as HONO and NO2 in the case of E-soot. Therefore, we conclude that HNO3 uptake on E-soot was similar to that on TC-1 soot. The E-soot used here was measured to contain roughly 0.035 g of water-soluble sulfate per gram of E-soot; the determination of water-soluble sulfate in our soot sample is given in the Supporting Information. In contrast to E-soot, the TC-1 soot prepared in a manner similar to that done here has been shown to contain only ∼0.0015 g of sulfate per gram of soot.18,19 We briefly investigated the possibility of volatilization of H2SO4 due to HNO3 uptake from both soot samples. We heated the nitric acid exposed soot up to 323 K and looked for H2SO4 and/or SO3 in the gas phase. There was no detectable increase in the signals for either H2SO4 or SO3; only HNO3 desorbed. We conclude that the interaction of HNO3 did not convert sulfate in soot to H2SO4 or SO3. 3.1.1. Influence of Water on HNO3 Uptake. A few experiments were carried out to investigate the influence of water on HNO3 uptake on TC-1 soot by adding different amounts of H2O to the flow tube reactor. We could introduce at most 5 × 1015 cm−3 of water because of interference with our chemical ionization mass spectrometry method. At room temperature, such a concentration of water led to RH of only 0.8%. However, this water addition at 243 K led to a relative humidity of 84%. Two sets of experiments with RH 13% and 84% were carried out at 243 K. Figure 3 shows HNO3 uptake profiles at 243 K with RH of 13% and 84%. Uptake of nitric acid increased by ∼25% when the RH was increased from 13% to 84%. The uptake at 13% RH was essentially the same as on dry soot. When water addition was stopped, the uptake clearly went back down to

Table 1. Total Number of HNO3 Molecules Taken up by TC-1 and E-Soot Samples soot exposure conditions

T, K

RH, %

TC-1 (4 mg)

243 243 243 295 295 295 295 295 243 243

[HNO3], 1011 cm−3

total no. of HNO3 molecules taken up ±2σ,a 1016 molecules

TC-1 soot (4 mg) coated with 5.4 × 1016 molecules of H2SO4

243 295

0

TC-1 soot (5.7 mg) coated with 2.1 × 1017 molecules of H2SO4 TC-1 soot (5.7 mg) coated with 2.0 × 1017 molecules of H2SO4 TC-1 soot (4 mg) coated with 5.4 × 1016 molecules of H2SO4

295

0

0.37 1.91 1.55 0.29 1.71 1.51 0.63 2.5 2.6 1.75 1.41 1.70 2.6 2.4 2.4 0.4 1.63 1.0 0.32 2.8

295

0

2.21

1.25 ± 0.35

243

81 13 96 0.8

0.424 1.7 1.57 0.34 1.4

0.91 5.1 7.5 0.17 0.34

TC-1 (5.7 mg) E-soot (62 mg) TC-1 exposed to H2O (4 mg soot) E-soot (62 mg) exposed to H2O

295 243

295

4.1 7.3 12.0 0.25 0.64 1.2 1.0 3.5 0.06 7.4 9.3 0.63 0.06 0.54 0.65 2.4 4.1 0.17 0.13 1.85

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0 0 0 0 0 0 0 0 0 13 84 0.8 0 39 99 0

± ± ± ± ±

1.2 2.2 3.6 0.08 0.2 0.3 0.3 1.1 0.02 2.2 2.8 0.2 0.02 0.16 0.20 0.9 1.4 0.07 0.05 0.35

0.36 2.0 3.0 0.07 0.10

a

Each data point in this table is the average of 3 uptake measurements. Error bars are twice the standard deviation of the mean as discussed in the uncertainty analysis (see text).

the exposure of HNO3 to soot at 295 and 243 K. Clearly, there was no detectable increase in signals of these species upon exposure of HNO3 to soot, in agreement with our previous results from our laboratory.11,21 Further, there was no change in the ion signals due to possible products when HNO3 desorbed from soot. Thus, we did not see these products either when HNO3 was taken up by the soot surface or when it desorbed from the soot sample. The signal of NO3− was also monitored to check for a possible conversion of HNO3 to N2O5. Even though the detected product ion, NO3−, could be produced from both HNO3 and N2O5, the ratio of signals of NO3− (the major ion from N2O5) to that of HF·NO3− (predominant ion signal from HNO3) would be different for HNO3 and N2O5. This ratio was essentially constant (∼3%) at all injector positions, suggesting that N2O5 was not formed as a product either when HNO3 was taken up or when it desorbed. This latter ratio is consistent with the source of these ions being HNO3 and not N2O5, as observed for the reaction of SF6− with HNO3.15 We conclude that HNO3 was not converted to NO3 or N2O5 on soot surface and that the NO3− signal that was seen arose from nitric acid alone. These observations are essentially the same as that reported previously.11 On the basis of the above observations, we conclude that the uptake of HNO3 on soot surface was reversible, with no detectable reactive uptake. There was no measurable desorption of NO2 or HONO even when the soot sample was heated, and E

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3.1.2. Influence of H2SO4 on HNO3 Uptake. TC-1 soot coated with H2SO4 was exposed to different concentrations (∼1010 and ∼1011 cm−3) of nitric acid at 295 and 243 K. We refer to this sulfuric acid on soot as “irreversibly” bound even though heating the sample to higher temperatures led to desorption of H2SO4. Figure 4 shows the adsorption−

Figure 3. Adsorption and desorption of HNO3 exposed to TC-1 soot in the presence of water at 243 K. Red line: [HNO3] = 1.75 × 1011 cm−3; RH = 13% or [H2O] = 1.5 × 1015 cm−3. Black line: [HNO3] = 1.41 × 1011 cm−3; RH = 84% or [H2O] = 9.5 × 1015 cm−3. As described in the text, the integration of the profiles yielded the number of absorbed and desorbed molecules. Also, the productions of NO2 and HONO products are shown to be immeasurably small.

Figure 4. Typical adsorption−desorption profiles of nitric acid on TC1 soot coated with sulfuric acid (red line) and without H2SO4 coating (blue line) at T = 243 K. The concentration of [HNO3] was 1.63 × 1011 and 1.55 × 1011 molecules cm−3, respectively, for coated and uncoated soot. The total amount of HNO3 taken up was 4.1 × 1016 and 12.0 × 1016 molecules for coated and uncoated soot, respectively. The total amount of H2SO4 on soot sample was 5.4 × 1016 molecules (see text). Desorption of H2SO4 (dashed black line) or SO3 (green line) was undetectable as shown in the lower panel.

that on dry soot. There was no measurable production of NO2 or HONO when the RH was increased. As in the case of dry soot, heating the soot sample from 243 to 303 K led to complete desorption of HNO3 and there was no production of NO2 or HONO. The extents of HNO3 uptake are listed in Table 1. On the basis of this limited set of observations, we conclude that H2O did not influence HNO3 uptake on TC-1 laboratory prepared soot significantly either in terms of the nature of the uptake or the extent of the uptake (5 × 1017 sites for a monolayer coverage in 4 mg of soot assuming a BET surface area of roughly 90 m2 g−1 that was measured previously for such soot.11] Clearly, our experiments explored the influence of only irreversibly bound H2SO4 on soot. We could not carry out experiments where the soot was continuously exposed simultaneously to H2SO4 and HNO3 such that they could compete for the same sites. [High concentrations of H2SO4 cause problems with the flow tubes and the ion detection schemes.] We argue that H2SO4 coverage used here surpasses those in the atmosphere such that our results are upper limits for H2SO4 coverage in the atmosphere. As in the case of uncoated soot, or soot in the presence of H2O, we did not see any increase in the signals of potential products such as HONO, NO2, and N2O5. Similarly, we did not see any products even when we heated the soot sample previously exposed to HNO3. Therefore, we conclude that the presence of H2SO4 on soot did not change its reactivity toward HNO3. We explored the possibility that HNO3 could displace the irreversibly bound H2SO4 by looking for ion signals due to H2SO4 and SO3. We did not see any increases in these signals, as shown in Figure 4 (bottom panel). Therefore, we conclude F

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that HNO3 could not displace the bound H2SO4 from the soot surface or convert it to SO3. Unfortunately, we could not study the influence of sulfuric acid on HNO3 coated soot since it would require gas phase concentrations of H2SO4 that could not be used in our apparatus, as noted above. In the absence of such experiments, we cannot definitively say how the binding of HNO3 differs from that of H2SO4. As in the previous experiments, we heated the H2SO4-coated soot sample exposed to HNO3 to 303 K. HNO3 left over on soot desorbed upon heating, as in previous experiments. We did not see any changes in ion signals due to HONO, NO2, H2SO4, and SO3 (see Figure SI-3). Clearly, we did not see any decomposition of H2SO4 or dehydration of HNO3 to its acid anhydride, N2O5 (or NO2 or NO3). It is not surprising that H2SO4 did not undergo decomposition since such a process requires much higher temperatures.22 It appears that even in the presence of H2SO4, a strong dehydrating agent, HNO3 uptake is due only to nonreactive reversible uptake. 3.1.3. Influences of H2SO4 and H2O on HNO3 Uptake. The influence of RH on HNO3 uptake on sulfuric acid coated TC-1 soot was studied at 295 K (0 < RH < 0.8%) and at 243 K (0 < RH < 96%). The measured extents of HNO3 uptake are listed in Table 1. Clearly, nitric acid uptake at T = 295 K on H2SO4 coated soot surface was enhanced by a factor of roughly 1.3 upon addition of water even at the low RH of 0.8%. At 243 K, the enhancement was roughly a factor of 1.8 in going from a RH of 13% to 96%. As in the case of H2SO4 alone, HNO3 uptake on H2SO4 coated soot in the presence of H2O did not show the formation of products of reactions such as HONO, NO2, and N2O5. When the sample was heated to 303 K, we did not see formation of HONO, NO2, H2SO4, and SO3. Thus, it appears that the uptake of HNO3 on H2SO4 coated surface is reversible and nonreactive even in the presence of H2O. 3.2. Comparison with Previous Studies of H2SO4 Coating of Soot. Decomposition of HNO3 to give NO2 and HONO on soot surfaces at HNO3 partial pressures greater than 5 × 10−4 Torr has been reported in some previous studies.21,23−31 However, at low concentrations, comparable to those observed in the atmosphere, HNO3 does not undergo reaction on the soot surface to produce NO2 and/or HONO. We have discussed in detail such processes in our previous paper.11 The uptake of H2SO4 on soot and the effect of H2SO4 coating on soot’s reactive and optical properties have been studied recently at 298 K.32−37 Sulfuric acid uptake was irreversible at 295 K on different soot samples (kerosene, hexane, and methane soot),32 as we observed here. However, we see that heating the coated sample to 393 K released H2SO4. Sulfuric acid coating has been reported to increase the hygroscopicity of soot, thereby altering their size and the optical scattering.34 To our knowledge there was no study of HNO3 uptake on coated soot surface.

estimate that much less than 1% of HNO3 would be taken up on soot. In contrast, in aircraft plumes where the soot surface area could be as large as 10−3 cm2/cm3,39 about 25% of HNO3 could be taken up on soot. However, this HNO3 would be desorbed as soon as the plume is diluted into the atmosphere. Even if we were to use the ten-times-larger uptake we see on Esoot in the presence of water, the uptake on atmospheric soot away from aircraft plumes would be negligibly small. Based on our measurements presented here, it appears that HNO3 is taken up reversibly so that soot is only a temporary reservoir for HNO3. We do not see any reactive uptake of HNO3 even when there is water or H2SO4. Thus, HNO3 uptake on soot does not appear to reduce nitric acid to NOx (NO2 + NO). Similarly, formation of HONO, a very photolabile molecule that could serve as a source of OH radicals and of NOx, does not appear to be facilitated by soot. At the concentration of HNO3 available in the atmosphere, it appears that H2SO4 coating or mixing on the soot does not affect the fate of HNO3 in the atmosphere. Lastly, given the high variability of the nature of soot and the limited kinds of soot examined here, we cannot rule out an influence of soot on HNO3 in the atmosphere. The influence of solar UV radiation on the reactive uptake of HNO3 and O3 has been found to be significant. UV radiation enhanced photolysis of HNO3 adsorbed on Pyrex surfaces by a factor ∼2−3 compared to the gas phase photolysis or the liquid phase photolysis and produced HONO, NO2, and NO.40 UV light was also observed to enhance O3 photolysis on soot surface and the reactivity of NO2 to make HONO; they were also reported to reactivate the deactivated surfaces after reaction.41,42 Thus HNO3 adsorbed on soot could conceivably continuously produce HONO and NOx in the presence of solar UV radiation without deactivating the soot surface; such a process is worthy of consideration for further study.

4. ATMOSPHERIC IMPLICATIONS The main conclusions from our observations are the following: (1) HNO3 uptake on soot coated with H2SO4 and H2O, as it would be in the atmosphere, does not lead to reactive NOx formation; and (2) there are some changes in HNO3 uptake when soot is coated with H2SO4 and/or H2O. We estimate the extent of HNO3 adsorption by soot in the upper troposphere based on the measured uptake on TC-1 soot at 243 K. For an atmospheric soot loading of roughly 5 ng m−3 (equivalent to BET surface area of 5 × 10−9 cm2/cm3),38 we

ACKNOWLEDGMENTS The authors thank Dr. O. B. Popovicheva (Moscow State University, Russia) for providing the aircraft engine and TC-1 kerosene flame soot. This work was funded in part by NOAA’s Climate Program Office.

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ASSOCIATED CONTENT

S Supporting Information *

Figures describing the temperature programmed desorption to obtain the total HNO3 after each set of the experiments and other products during the desorption process and a description of the determination of the water-soluble sulfate fraction in engine soot (E-soot). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin str., 119991 Moscow, Russia. Notes

The authors declare no competing financial interest.

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REFERENCES

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