Heterogeneous Reaction of NO2 on Diesel Soot Particles

Apr 20, 2001 - Reaction 1 suggests that reduced organic species are consumed in ... revolutions per minute, 1100, 3000, 3000, 3000, 3000 ..... The ins...
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Environ. Sci. Technol. 2001, 35, 2191-2199

Heterogeneous Reaction of NO2 on Diesel Soot Particles FRANK ARENS,† LUKAS GUTZWILLER,† URS BALTENSPERGER,† H E I N Z W . G A¨ G G E L E R , † , ‡ A N D M A R K U S A M M A N N * ,† Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland, and Department of Chemistry and Biochemistry, University of Bern, CH-3012 Bern, Switzerland

terials in laboratory experiments. In particular, HONO might be important because of its potential role in initiating daytime photochemistry by its rapid photolysis to NO and OH (15). The mechanisms by which HONO is formed in the atmosphere are not completely understood at the present, and the main production pathways for HONO are thought to be by heterogeneous reactions of NO2 on liquid or solid surfaces. Several laboratory studies suggested that soot aerosol particles might provide a reactive surface for HONO formation via the overall reaction mechanism (eq 1) (2-4, 6-8) where R(red) and R(ox) are reduced and oxidized species or functionalities on the surface of soot particles, respectively. H2O

Soot particles were collected from a diesel engine using a procedure that realistically mimics exhaust gas conditions in tailpipes and during dilution at room temperature. After being sampled, the particles were exposed to NO2 concentrations and relative humidity in ranges relevant for the troposphere using 13N as tracer. Gas-phase nitrous acid (HONO) and irreversibly bound (i.e., chemisorbed) species were the main reaction products with initial yields of 8090% and about 10%, respectively. Neither NO nor HNO3 were detectable. The HONO formation increased with increasing engine load (i.e., with a decreasing air to fuel ratio, λ). The reaction rates of HONO and chemisorbed NO2 increased with increasing NO2 concentration and did not depend on relative humidity. At the beginning of reaction, the uptake coefficient averaged over 3 min ranged from 5 × 10-6 to 10-5 for NO2 concentrations between 2 and 40 ppb. The HONO formation rates decreased with time, indicating consumption of reactive surface species, while the chemisorption rates remained almost constant. The total HONO formation potential of the particles was estimated to about 1.3 × 1017 molecules/mg of diesel soot or to about 4.7 mg/kg of diesel fuel, indicating that the reaction between NO2 and diesel soot particles does not provide a significant secondary HONO source in the atmosphere. A LangmuirHinshelwood type reaction mechanism was proposed that adequately describes the observed results and also allows discussing important general features of reactions on soot.

Introduction The heterogeneous reactions between oxidized nitrogen compounds and carbonaceous particles have attracted considerable interest in atmospheric chemistry during the past years. Nitrogen oxides largely control ozone levels in most areas of the troposphere, and between 10 and 50% of the tropospheric particles are carbonaceous, with particularly high levels in the urban troposphere (1). There was an increasing interest to investigate heterogeneous reactions between nitrogen dioxide (NO2) and soot particles, two concurrent combustion products, because nitrous acid (HONO) (2-8), nitrogen monoxide (NO) (9-14), or even nitrous oxide (N2O) (12) were observed as products from interactions between NO2 and different carbonaceous ma* Corresponding author phone: ++41/56/310-4049; fax: ++41/ 56/310-4435; e-mail: [email protected]. † Paul Scherrer Institute. ‡ University of Bern. 10.1021/es000207s CCC: $20.00 Published on Web 04/20/2001

 2001 American Chemical Society

NO2 + R(red) 98 HONO + R(ox)

(1)

Other mechanisms including the classical disproportionation reaction (16, 17) or N2O3 as intermediate species (18) were consistently excluded from being important on soot surfaces. Reaction 1 suggests that reduced organic species are consumed in this process, and its significance for the atmosphere depends on the kinetics and on the total amount of species available for reaction. A considerable debate on the interpretation of the laboratory studies cited above has been invoked. The three most critical parameters are as follows: the soot source, the sample structure and mass, and the way kinetic results are transferred to atmospheric conditions in models (19). Soot from a number of laboratoryscale soot sources such as diffusion flames using various hydrocarbon fuels, commercial carbon black samples, or even spark discharge between graphite electrodes was investigated. In these sources, the formation of reactive species responsible for reaction 1 might be rather different as compared to typical atmospheric soot sources such as biomass burning or combustion engines. Furthermore, sampling soot from a combustion source must include a separation of the soot from the complex exhaust gas mixture; therefore, the sampling conditions might significantly affect the experimental results, a point that has not been adequately treated in previous studies. Therefore, in the present study, we investigated the heterogeneous reaction of NO2 on soot particles sampled from a diesel engine operated at different engine loads. For this typical anthropogenic soot, an extraction procedure was chosen mimicking as best as possible the exhaust emission and dilution at the end of a tailpipe. The sample mass was kept in the low microgram range. Working with such small samples and atmospherically relevant NO2 concentrations and humidity was only possible by exposing the samples to NO2 labeled with the radioactive tracer 13N and observing NO2 adsorption on the soot surface as well as HONO and NO formation using denuder techniques and γ-spectroscopy. The results show that on diesel soot particles not enough HONO can be formed to significantly influence atmospheric HONO concentrations. A surface chemical model gives further insight into the reaction mechanism of NO2 on diesel soot particles.

Experimental Section A diesel electrical power generator (Kubota GL-4500S) without exhaust treatment devices was used. Loads up to 4.5 kW could be attained. Table 1 characterizes the engine with respect to O2, CO2, H2O, CO, SO2, NOx, and particulate matter emissions. Air to fuel ratios (λ) of currently used diesel engines vary between 1.3 at high load and 11 at very low load (20). The NO2 to NOx volume ratios of diesel exhaust vary between VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Emissions of the Diesel Engine Used (Kubota GL-4500S)a engine load (kW) revolutions per minute fuel consumption (L/h) exhaust volumetric flow (m3/h), (norm, dry) exhaust temp (°C) O2 (vol %)b CO2 (vol %)b H2O (vol %) air to fuel ratio, λ CO (mg/m3)b SO2c (mg/m3)b NO (as NO2) (mg/m3)b NO2d (mg/m3)b NOx (as NO2) (mg/m3)b ratio NO2 to NOx (%)e total no. of particles (no./m3)e total surface of particles (cm2/m3)e particle mass concn (mg/m3)e

idling 1100 0.52 22.0 93 16.2 3.3 3.8 4.5 305 20 421 93 514 18.1 1.0E+13 1.5E+3 3.0

1 3000 0.93 29.7 154 14.7 4.4 4.7 3.5 444 27 158 92 250 36.8 1.5E+13 3.7E+3 7.4

2 3000 1.13 29.1 195 13.3 5.4 5.5 2.8 324 33 257 68 325 20.9 2.2E+13 6.5E+3 12.9

3 3000 1.43 29.4 250 11.5 6.7 6.6 2.3 294 41 370 39 409 9.5 2.8E+13 9.2E+3 18.4

4 3000 1.70 28.8 308 9.6 8.2 7.7 1.9 397 50 454 10 464 2.2 3.7E+13 1.7E+4 34.2

a Conversion factors: 1 ppm CO ) 1.25 mg/m3 CO; 1 ppm NO ) 2.05 mg/m3 NO ; 1 ppm SO ) 2.86 mg/m3 SO . b All concentration values x x 2 2 are based on dry exhaust at normal conditions (1013 h Pa-1, 273 K). c Calculated from fuel consumption. d Valid in the area of the dilution unit e (at about 75 °C). Estimated from SMPS behind the denuder series, in consideration of the dilution ratio (∼1/20) and the particle loss (∼60%).

FIGURE 2. Number size distributions of diesel soot particles after an exhaust dilution by a factor of 20 behind the denuder series for different engine loads.

FIGURE 1. Setup of extraction and sampling of diesel soot particles. Part of the exhaust was extracted by a dynamic dilution device. After passing a 85Kr neutralizer in order to establish a new equilibrium charge distribution, all charged particles were removed by an electrostatic precipitator. After gas-phase stripping by a series of denuders, the remaining neutral particles were sampled on glass fiber filters. The aerosol size distribution was characterized by a DMA and a CPC. 2% at high load and 30% at low load (21), whereas in on-road remote sensing measurements, about 8% was found (22). A similar NO2 to NOx volume ratio of 9.5% and an air to fuel ratio of 2.3 were attained using our engine at 3 kW load where most of the soot samples for analysis were taken. Aerosol Sampling and Characterization. Figure 1 shows the setup for soot aerosol sampling. Part of the exhaust with a temperature of 90-310 °C depending on λ was fed through a heated tube (total travel time from combustor about 1 s) to a dynamic dilution device (MD19-1E, Matter Engineering AG, Switzerland) operated at 75 °C. Therein, the exhaust was diluted by dry synthetic N2 by a factor of about 20. Under these conditions, the exhaust gas remained well above the dewpoint of water, and the maximum relative humidity (RH) attained was about 20%. To ensure a representative sampling and to avoid uncontrolled losses of charged particles in the 2192

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overall sampling setup, a 85Kr source was used to establish an equilibrium charge distribution at room temperature before all charged particles were removed in an electrostatic precipitator. A series of diffusion denuders (silica gel, charcoal, cobalt oxide) removed all gaseous water, SO2, VOC, and NOx at 25 °C. Behind these denuders, NO and NO2 levels were always below 1 and 0.050 ppb, respectively, as measured using a chemiluminescence detector (CLD77AM, ECO Physics, Switzerland). As soon as the NO levels reached 1 ppb, the whole denuder train was exchanged. As the capacity of the denuders to retain NO2 was much better than for NO, this procedure ensured that background NO2 remained always below the limit stated above and that no undesirable reactions occurred on the filter during sampling of soot particles. This technique ensured representative and reproducible samples. Passing the soot particles through the denuder train (total travel time about 2 s) resulted in an additional dilution of the aerosol with respect to all reactive gas-phase species of at least 3 orders of magnitude. This procedure therefore approximates the dilution process at 25 °C at the exhaust pipes of automobiles, where the exhaust is diluted by a factor of 1000 or more within seconds (23). Part of the resulting gas flow was used to continuously monitor the particle size distribution (Figure 2) using a scanning mobility particle sizer (SMPS) consisting of a differential mobility analyzer (DMA, TSI 3071) coupled to a condensation particle counter (CPC, TSI 3022). All kinetic

FIGURE 4. Setup for analysis. Diesel soot containing filters were exposed to 13N-labeled NO2. The decays of 13N molecules were measured by moving two coincident γ-detectors along the filter and denuder assembly. The amount of surface-bound 13NO2 was directly detected at the filter; the amounts of gaseous HO13NO, 13NO2, and 13NO downstream of the filter were measured at the compoundspecific traps.

FIGURE 3. Images obtained with a high-resolution scanning electron microscope (HRSEM) of diesel soot particles collected on a glass fiber filter from diesel exhaust at 3 kW engine load. The soot surface area is 7.6 cm2 cm2 of filter cross-sectional area, resulting in 70% reduction of optical filter reflectance. results were referred to the mobility diameter-based particle surface area. The rest of the aerosol flow was drawn through four glass fiber filters fixed either in aluminum or in special Perfluoralkoxy Teflon (PFA) filter holders in parallel. After being sampled, the filters were stored in an argon atmosphere in the dark at 4 °C until analysis. The glass fiber filters (Schleicher & Schu ¨ ll, GF6) had an effective diameter of either 2.0 or 0.4 cm. The aerosol loading was kept as low as possible, with soot surface areas of 6.48.0 and 80-160 cm2/cm2 of filter cross-sectional area for kinetic studies on gas-phase products and for quantification of soot-bound products, respectively, as described below. The lower coverage corresponded to 70% reduction of optical filter reflectance. Figure 3 shows a high-resolution scanning electron microscopy (HRSEM) picture of diesel soot particles on a glass fiber filter for this case. Kinetic Experiments. The filters containing the soot were placed in a filter holder and exposed to 13N (β+ decay, T1/2 ) 9.96 min) labeled NO2 in the dark at 22 °C in an atmospheric pressure flow system (Figure 4). The high analytical sensitivity of the radioactive tracer technique allowed experiments at atmospherically relevant NO2 concentrations and relative humidity. This configuration is therefore especially suited for kinetic studies of very slow reactions and becomes mass transfer limited only for uptake coefficients above 10-3, which is far above the net uptake on diesel soot observed over longer time scales. The facility for production of 13N is described in detail elsewhere (24, 25). It delivered a small flow of 13NO in a 20% O2 in He mixture to the laboratory where it was mixed with

inactive NO in synthetic air to give defined NO mixing ratios from 1 to 40 ppb. The final He content was about 10%. The ratio of 13N to 14N was about 10-6. This gas flow was humidified, and NO was oxidized to NO2 over CrO3 (26). The total NO2 concentration was monitored with the chemiluminescence detector. The decay of 13N results in the coincident emission of two γ-rays (511 keV) in opposite directions. Using a coincident counting configuration with two opposite γ-detectors, a high spatial resolution and low background was obtained for 13N detection. Downstream of the filter holder, a series of differently coated traps absorbed the corresponding species. The coatings were sodium carbonate to absorb HONO, a mixture of [N-(1-naphthyl)ethylene diamine dihydrochloride] (NDA) and KOH to absorb NO2, and cobalt oxide to absorb NO (3). Eventually, an additional trap coated with NaCl was used to absorb HNO3. This flow system was mounted on a linear device along which the detection system was scanning. The radioactivity at each position of the flow system is directly related to the number of 13N molecules absorbed at this position per unit time; the radioactivity caused by molecules simply passing by with the bulk gas flow is negligible. The radioactivity was measured every 3 min at each position. Spatial integration along the filter area and along the different specific trap sections gives the compound-specific 13NOy fluxes (molecules per unit of time). Multiplying the 13NOy fluxes with the ratio between nonlabeled NO2 molecules and 13NO molecules gives the compound-specific NO fluxes. 2 y The HONO measurements using the sodium carbonate coating were complicated by a small interference from NO2 (27) and by eventual HONO formation on the surfaces other than soot also exposed to NO2, e.g., filter holder, tubing, etc. Therefore, the signals obtained with a blank filter were subtracted from the signals obtained with soot on the filter. This technique for analysis is extremely sensitive to labeled molecules adsorbed on any surface of the system, e.g., also on tubing and connectors, etc. It was observed that a small fraction of HONO produced on the soot resulted in some signal from reversible adsorption on the surfaces downstream of the filter (supporting grid and outlet), which made it difficult to properly separate this signal from labeled NO2 associated with the soot. Therefore, to make exact measureVOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Typical record of adsorbed 13NO2, HO13NO(g) (peak at the beginning of the NaCO3 trap), and 13NO2(g) when a blank and a diesel soot-containing filter were mounted, respectively. The filters were fixed in a PFA filter holder.

FIGURE 6. Typical temporal signal of NO2(g), HONO(g), and chemisorbed NO2 when a blank and a diesel soot-containing filter were mounted, respectively. The filters were fixed in a PFA filter holder.

ments of chemisorbed NO2 possible, the 0.4-cm glass fiber filters with the higher soot loading were fixed between two PFA tubes without any further supporting grid. On PFA alone, no signal from any NOy species was observed. The remaining experiments related to the kinetics of HONO formation were performed with the standard filter holders, allowing lower soot loadings on larger filters.

Results Figure 5 shows a typical record of 13N detected along the denuder setup for an experiment with a PFA filter holder (Leff,filter ) 0.4 cm, high filter loading). The three different zones correspond to 13NO2 adsorbed on the sample filter as well as the two gas-phase species of HO13NO and 13NO2 trapped in their respective denuders. The blank curve gives the corresponding signal with a blank filter inline. About 90% of the NO2 loss appears as HONO (i.e., peak at the beginning of the NaCO3 trap), while only about 10% of the NO2 loss is found to be retained at the position of the soot. In all experiments, the amount of NO and HNO3 formed was below the detection limit (data not shown). The activity associated with soot can either come from physisorbed NO2 (i.e., in equilibrium with gas-phase NO2) or from NO2 reacting on the surface leading to a surface-bound product (i.e., chemisorbed NO2). So-called “purge experiments” were performed in order to resolve these two contributions. Diesel soot containing filters (PFA filter holder, Leff,filter ) 0.4 cm, high filter loading) were exposed for 30 min to the gas flow containing 13N-labeled NO2. Thereafter, the samples were purged with synthetic air. Activity from chemisorbed 13NO2 molecules would decrease according to the lifetime of 13N, whereas activity from physisorbed 13NO2 molecules should decrease much faster due to desorption from the soot surface. In all purge experiments, the decrease of activity associated with soot was in exact agreement with the 13N decay. Therefore, the 13NO2 associated with the soot surface was interpreted as 13NO2 chemisorption rate. For samples taken at 3 kW load and exposed to 10 ppb NO2, the chemisorption rate was 1.5 × 109 (molecules per cm2 soot surface area and s) and increased linearly with the NO2 concentration, independent of the relative humidity varied between 5 and 75%. However, at relative humidity below 5%, a significantly increased chemisorption rate was measured. Figure 6 shows a typical temporal behavior for the HONO(g), NO2(g), and chemisorbed NO2. Each data point in Figure 6 corresponds to a spatially integrated peak of a typical scan as shown in Figure 5. Whereas the HONO formation 2194

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FIGURE 7. HONO production integrated over 50 min of reaction time at 6 ppb NO2 and 30% RH as a function of the engine load (left axis). The corresponding air-to-fuel ratio (λ) of the diesel engine is plotted on the right axis. rate clearly decreased with time, indicating consumption of reactive species on the soot surface, the chemisorption rate remained almost constant within the observation period. Exposing the samples to humidified synthetic air prior to an experiment did not result in a significant change of the reactivity, indicating that the presence of oxygen during the experiments did not cause the time-dependent HONO formation rate. As mentioned in the Experimental Section, quantification of the NO2 chemisorption rate was only possible for the special PFA filter holder with high soot loading on a small filter area (without supporting grids, etc.). On the other hand, when increasing the soot loading on the larger filters by a factor of 10, the soot surface-specific HONO formation rate and the corresponding NO2 loss decreased by 30-40%. We assume that this reduction was due to the smaller surface area accessible for gaseous molecules in the case of more densely packed soot. Therefore, all results regarding the HONO formation rate were obtained using the low filter loading on the larger filters (Leff,filter ) 2.0 cm, standard filter holders). Figure 7 shows the total amount of HONO formed on soot particles within the first 50 min of exposure at 6 ppb NO2 as a function of engine load. HONO formation increased with increasing engine load. In an analogous experiment at 34 ppb NO2, a similar behavior was found. For comparison, Figure 7 shows the corresponding air to fuel ratio (λ) of the engine as well. A lower λ value might indicate a less oxidizing exhaust atmosphere.

FIGURE 8. HONO formation rate (averages of the first 3 min of reaction) as a function of the NO2 mixing ratio at 30% RH. All soot samples (low filter loading, standard filter holders) were taken at an engine load of 3 kW. Soot samples collected in parallel are marked with the same symbol. The insert shows the linearly fitted data according to eq 9. The solid lines represent the fit results in both parts of the figure.

FIGURE 10. HONO formation rates as a function of time for different NO2 mixing ratios at 30% RH. All soot samples (low filter loading, standard filter holders) were taken at an engine load of 3 kW. The solid lines were calculated on the basis of the equations and parameters explained in the text. FIGURE 9. HONO formation rate (averages of the first 3 min of reaction) as a function of the relative humidity at 3 ppb NO2. All soot samples (low filter loading, standard filter holders) were taken at an engine load of 3 kW. Soot samples collected in parallel are marked with the same symbol. In further experiments, diesel soot samples collected at 3 kW load were exposed to different NO2 concentrations and varying relative humidity. While the experiments lasted typically 30-50 min, Figure 8 shows data corresponding to the HONO formation rate averaged over the first 3 min of NO2 exposure. At 30% RH, this rate increased with increasing NO2 concentration but does not follow a true linear trend. A similar NO2 dependence was observed at other humidity and at other engine loads. On the other hand, the smaller the NO2 concentration, the slower was the decrease of HONO formation rates with time (see also Figure 10), consistent with a consumption of the involved reaction partner. Figure 9 shows the initial HONO formation rate as a function of the relative humidity at 3 ppb NO2. From these experiments and experiments at higher NO2 mixing ratios, it seems that within the time scale of 30-50 min no influence of the relative humidity on the HONO formation rate or on the integrated amount of HONO formed occurred. A few samples were analyzed up to several hours in order to get an estimate of the full HONO formation potential, giving about 2.5 × 1014 (molecules per cm2 soot surface). This value corresponds to about 1.3 × 1017 molecules/mg of diesel soot or 4.7 mg of HONO/kg of diesel fuel (see Table 1 for conversion factors). Modeling. A kinetic model was developed to obtain parameters describing the mechanism underlying reaction

1. The model should adequately describe HONO formation on soot as a function of NO2 concentration and time. NO is not considered in this kinetic analysis as the amount of NO formed was always below the detection limit (≈3 × 108 molecules per cm2 soot surface and s). Water is not treated explicitly as no humidity dependence was observed. It is beyond the scope of this work to model the NO2 dependence of the chemisorption rate at very low humidity. Similarly to the approach applied by Tabezadeh and Turco (28) and Carslaw and Peter (29) and in agreement with the observed nonlinear increase in the rate of HONO formation with increasing NO2 concentration (Figure 8), it was assumed that the NO2 uptake on the surface is limited by an adsorption-desorption equilibrium preceding a surface reaction. Simple noncompetitive Langmuir adsorption was assumed for this equilibrium: kads

NO2(g) + S(s) y\ zNO2‚S(s) k

(2)

des

where S and NO2‚S denote empty and occupied surface sites with respect to physisorbed NO2, while kads and kdes denote the adsorption constant (ppb-1 s-1) and the desorption constant (s-1), respectively. The equilibrium surface coverage is given by

θ)

KXNO2 [NO2‚S] NS - [S] ) ) NS NS (1 + KXNO )

(3)

2

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XNO2 denotes the NO2 mixing ratio in the gas phase (ppb), and K (ppb-1) is the adsorption equilibrium constant defined as

K)

kads kdes

(4)

As the HONO formation rate decreased with time (see Figures 6 and 10), a corresponding surface-bound reactant is obviously consumed, presumably oxidized, consistent with the findings of most previous studies cited above. However, the time-dependent decrease of the HONO signal cannot be properly represented by a single-exponential function but rather by a superposition of at least two exponential functions, i.e., at least two competing surface reactions. Thus, two different functionalities (R1H and R2H) were introduced, which seems reasonable considering the complex organic composition of soot. A third partner (R3H) transforms physisorbed NO2 into chemisorbed NO2 (NO2R3H). This chemisorption lumps together a variety of reactions forming stable organic nitrites, nitrates, etc. (30). For further procedure, the three independent surface reactions 5-7, treated as a noncompetitive Langmuir-Hinshelwood mechanism, were introduced. This mechanism assumes that S‚NO2 is mobile on the surface and, once lost through reaction, is rapidly replaced from the gas phase through equilibrium reaction 2. (H2O) k1

NO2‚S(s) + R1H(s) 98 S(s) + HONO(g) + R1(s) (5)

justifies the Langmuir-Hinshelwood approach for this case. Provided that the Langmuir preequilibrium according to reaction 2 is fast in comparison to the subsequent surface reactions 5 and 6 and that these two reactions are independent, a separate integration of the two parts in eq 8 is possible. Thus, the amount of surface-bound reactants available at time t is

[(

)]

KXNO2 t for i ) 1, 2 [RiH](t) ) [RiH](t)0) exp - NSki (1 + KXNO2) (12) The sum of [R1H](t)0) and [R2H](t)0), i.e., the total amount of reactants available at the beginning of the reaction, was estimated from the total HONO yield of those samples that were exposed to NO2 for several hours, giving 2.5((1.0) × 1014 (molecules per cm2). Using this value and K and B as boundary conditions, the time-dependent experimental data shown in Figure 10 were simultaneously fitted, and the initial reactant concentrations and the first-order reaction rates for θ ) 1 were obtained:

[R1H](t)0) ) 1.15((0.31) × 1013

(

)

molecules cm2

and

(NSk1) ) 4.50((1.75) × 10-3 [R2H](t)0) ) 2.39((0.99) × 1014

(

)

molecules cm2

(H2O) k3

NO2‚S(s) + R3H(s) 98 S(s) + NO2R3H(s)

(7)

The constants k1, k2, and k3 are second-order surface reaction rate coefficients (in units of cm2 s-1). Equation 8 describes the total HONO formation rate (molecules per cm2 soot surface and s) as a function of NO2 mixing ratio and time from both HONO formation pathways (eqs 5 and 6):

{

KKNO2 d[HONO] ) {k1[R1H](t) + k2[R2H](t)}NS dt (1 + KXNO2)

} (8)

This equation, evaluated at t ) 0, allowed fitting the NO2dependent first 3-min average of HONO formation as shown in Figure 8. The reciprocal form of eq 8

1 d[HONO] dt

[

]

(t)0)

[ ]

1 1 1 ) + KB XNO2 B

(9)

with

B ) {k1[R1H](t)0) + k2[R2H](t)0)}NS

(NSk2) ) 1.10((0.40) × 10-4

( )

kads 1 K) ) 2.32((0.85) × 10-2 kdes ppb

and

B ) 7.80((2.85) × 1010

(

)

molecules (11) cm2s

with R2 ) 0.97. In this notation, B is the total initial reaction rate at full coverage (θ ) 1). The quality of this linear relation 2196

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()

1 (14) s

This set of equations (eqs 8 and 12) and constants (eqs 11, 13, and 14) are sufficient to calculate the HONO formation on diesel soot particles as a function of NO2 mixing ratio and time. Figure 10 shows for some examples a comparison between model calculations and measured HONO formation rates. The overall relative error associated with the model predictions of HONO formation based on soot mass amounts to (50% at the beginning of the reaction and (75% after 2 h at 40 ppb NO2. This estimate includes the uncertainty of the regression analysis, of the mass specific soot surface area, and of the errors associated with the radioactivity measurement and NOx calibration. The fact that the radioactivity associated with soot was dominated by the chemisorption reaction as observed in the purge experiments can be used to estimate an upper limit for NS according to

NS
6.6((4.2) × 1016

( ) cm2 s

and

k2 > 1.6((1.0) × 10-17

( )

cm2 (17) s

A comparison between eq 16 and the total HONO formation potential for diesel soot mentioned above shows that the maximum number NS of adsorption sites S must be at least 15-105 times smaller than the total number of reactive sites (i.e., than the sum of [R1H](t)0) and [R2H](t)0)). This result is consistent with the two-step reaction mechanism involving adsorption into NO2‚S followed by reaction.

Discussion Hot primary diesel exhaust gas is a complex mixture of nitrogen oxides (including NO, NO2, HONO), sulfur dioxide, water, gas-phase and condensed-phase organic compounds, and elemental carbon. While the exhaust travels through the tailpipe at a decreasing temperature, the particles are processed, e.g., by reaction with NO2 or condensation of organic compounds. At the end of the tailpipe, the exhaust gas is rapidly diluted by a factor of 1000 or more within seconds and cooled to ambient temperature, which induces a similar processing once more. This moment (after dilution) sets the starting point of secondary reactions, which are the focus of this study. The samples collected here at room temperature are representative of soot particles emitted and diluted at the tailpipe of diesel engines, because they have experienced the same processing including condensation of organic material. As diesel engines emit soot particles in the submicron size range (31) and atmospheric measurements in this size range are widely performed using differential mobility analysis, the mobility-based surface area estimates are ideal for relating the present results to atmospheric measurements. In their study on the reactivity of soot with O3, Kamm et al. (32) developed and tested models to relate the mobility diameter-based surface area to other surface areas. Applying their model (33) to our samples, using a volume fractal dimension of 2.0, a primary particle diameter of 27 nm, and a primary particle density of 1.7 g cm-3, the mobility-based surface area density becomes 50 m2 g-1, and the surface area accessible to gas molecules becomes 87 m2 g-1. On the other hand, the total surface area of the primary particles is 131 m2 g-1. These estimates provide conversion factors for comparing surface area-related quantities reported here with those of other studies. The low soot loadings on our filters assured that the soot surface exposed in our experiments was very close to the surface of the same particles exposed in gas suspension. This is consistent with the observed decrease of the reaction rate with a 10-fold larger soot loading. The main result of the present work was that, during the first hour of interaction of diesel soot particles with NO2, HONO was the main gas-phase product and that the HONO formation rates decreased with time, consistent with a surface reaction in which surface-bound organic reactants are consumed. The total amount of HONO formed over longer time scales (1.3 × 1017 molecules/mg of soot) was somewhat higher than in other studies on flame soot (4, 6, 7) but lower than in an aerosol study using soot from spark discharge between graphite electrodes (3). In both cases, soot composition might explain the differences. The amount of organic species condensed on soot critically depends on the sampling temperature, a parameter that was not under control when soot was sampled from above diffusion flames (4, 6, 7). Our samples were collected at 25 °C. However, this temperature was not systematically varied, and any consequences of especially lower temperatures, which could eventually increase the amount of condensable organic material, was beyond the scope of the present study. The variation of the HONO formation capacity with engine load (Figure 7) provides a measure of the variations to be expected under realistic conditions. A look at other power output related exhaust parameters (Table 1) might further help to understand this behavior. The exhaust temperature

as well as the water and SO2 contents of the exhaust increased with increasing engine load. The NO2 and O2 contents of the exhaust as well as the air to fuel ratio (λ) decreased with increasing engine load. SO2 is in equilibrium with hydrogen sulfite in adsorbed water and may remain there until analysis, so that it might interfere with the proposed reactions with the organic species (34) and be the cause of the observed engine load dependence. However, according to a recent study on soot, SO2 undergoes a reversible adsorption even at low temperatures of about -100 °C (35), and any adsorbed SO2 or hydrogen sulfite (if it had not been oxidized to sulfate) would have been removed in the denuders. In the exhaust pipe, the particles were exposed for about 1 s to high NO2 concentrations in the ppm range. After the dilution unit, a further interaction time of 2 s remained during which the particles were exposed to between 0.25 and 2.25 ppm NO2 before the whole NO2 was removed in the denuder series. However, as the reaction rate seemed to saturate already at much lower NO2 mixing ratios due to the reversible reaction (eq 2), it is likely that the samples taken at different loads must have been affected by the same amount of S‚NO2, independent of the NO2 contents of the exhaust. We conclude that the long-term reactivity of the soot samples is not affected by the power-dependent NO2 and SO2 concentrations but rather by a power-related variation of the organic composition, which is influenced by the air to fuel ratio, λ (20). Qualitatively, this is consistent with the results of flame soot studies where samples under fuel-rich and lean conditions were sampled (7) or the sampling position in the flame was varied (4). From previously reported studies (5, 6), in which DONO was observed when D2O instead of H2O was supplied to vary the humidity, it was concluded that water is necessary for HONO formation on soot particles. According to the present study, HONO formation was independent of the water vapor concentration (Figure 9) in the atmospherically relevant range of 4-77% RH (≈0.1-2.0 vol% H2O(g)). If water provides the proton for HONO, even at a low relative humidity of 4%, a sufficient amount of water would be available for this; therefore, the overall reaction rate seemed to be limited by another process. We therefore considered adsorbed water as a nonlimiting reaction partner and did not include it explicitly in the proposed reaction mechanism. When using the emission parameters given in Table 1 for soot and NOx, secondary HONO generation through the reaction studied in an air mass dominated by diesel engine emissions would lead to a maximum HONO to NOx ratio of 4 × 10-4 after sufficiently long time. However, the average primary HONO emission from diesel engines already amounts to a fraction of HONO of 7 × 10-3 of total NOx (36). Furthermore, typical nighttime HONO to NOx ratios often exceed 3 × 10-2 (37, 38). Therefore, the numbers reported here, obtained in an atmospherically relevant range of humidity and NO2 concentration, clearly indicate that HONO formed on such diesel soot particles cannot account for secondary HONO formation in the atmosphere. This is consistent with modeling results obtained by Aumont et al. (19) for the case in which a reactant on the soot surface is consumed. While the NO2 diesel soot interaction seems not to significantly supply HONO to the atmosphere, the soot particle surface is oxidized by this reaction. Furthermore, the measured chemisorption does not decrease as rapidly as the HONO formation does but becomes the dominant reaction after several hours. It includes a wider ensemble of reactions lumped in reaction 7 and leading to different organic nitration products (12, 30, 39). While they are probably not of importance for affecting gas-phase chemistry of ozone or nitrogen oxides, both processes slowly alter the soot surface chemical composition. They are part of the VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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important aging process in the atmosphere making the soot particles increasingly hydrophilic. It is therefore worth to discuss in more detail the kinetics within the framework of the reaction mechanism proposed. The kinetics of a heterogeneous reaction is usually described in terms of the uptake coefficient, γu, which is the ratio of the number of molecules lost from the gas phase to the number of gas-kinetic collisions. Similarly, if the rate of product formation is the main observable in the experiment, the reaction probability, γr, is defined by

γr,HONO ) number of NO2 - surface collisions resulting in HONO ) total number of NO2 - surface collisions d[HONO] dt (18) νPo XNO2 4kBT where d[HONO]/dt is the HONO formation rate (molecules per cm2 soot surface and s), and XNO2 denotes the NO2 mixing ratio in the gas phase (ppb). The total atmospheric pressure (Pa) and the absolute temperature (K) are denoted as Po and T, respectively, kB is the Boltzmann constant (1.381 × 10-23 J K-1), while ν denotes the mean thermal velocity of NO2 molecules in the gas phase (ν ) 3.7 × 104 cm s-1). As the present results show, the NO2 loss, the HONO formation, and the chemisorption rates are represented by a complex mechanism. The elementary steps involved are an accommodation on the surface into the adsorbed state (S‚NO2), desorption back to the gas phase, and the surface reactions 5-7. A similar mechanism has also been proposed by Kleffmann et al. (6). The probability that an NO2 surface collision results in accommodation can be very high. This might explain why in experiments reported in the literature, in which this process could be resolved, very high initial uptake coefficients of up to 0.1 were observed on soot (2, 4, 5, 7-10). However, according to the present mechanism, as soon as reaction 2 reaches equilibrium, the net loss of NO2 from the gas phase becomes equal to the rate of reactions 5-7. In our experiments, we were not able to resolve the kinetics of the fast equilibrium (eq 2) due to mass transfer limitations at atmospheric pressure. Considering the upper estimate of NS derived from the experiments and assuming that every NO2 molecule supplied to the system was taken up, the equilibrium was reached in much less than 1 min at the present NO2 concentrations. Therefore, the NO2 loss and product formation rates reported as 3-min averages were clearly limited by the surface reactions. When these rates were converted according to eq 18, we obtained values of γu ranging from 5 × 10-6 to 1 × 10-5, γr from 4 × 10-6 to 8 × 10-6 for HONO, and from 5 × 10-7 to 1 × 10-6 for the chemisorption for the first 3 min of reaction. The uptake coefficient is in reasonable agreement with the most recent study by Kleffmann et al. (6). Exposing a bulk sample of several milligrams to NO2 in the ppm range, these authors obtained an initial uptake coefficient of 10-6 when referring to the BET surface. Beside very high initial uptake coefficients which are consistent with the mechanism proposed here, also high uptake coefficients over longer time periods had been observed in low-pressure reactors (4, 5, 9, 10) with considerably larger sample masses and using the geometric sample surface to calculate the collision rate. In a recent study, Stadler et al. (7) report a strongly decreasing uptake coefficient with time on decane soot when γu refers to the BET surface. AlAbadleh and Grassian (39) came to the same conclusion when recalculating γu for a number of previously published studies. 2198

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With respect to the kinetics of product formation, some authors reported HONO yields (i.e., ratios of HONO formed per NO2 lost), some others have reported γr. Earlier experiments performed in our lab (2, 3) in which γr was measured on suspended spark discharge soot particles yielded values on the order of 10-2 for the first seconds and 10-4 for the first minute of reaction. Such fast reactions were not observed on the present diesel soot samples, and if ever very fast reactions occur, they might be important for processing of soot in the tailpipe but not for secondary processes in the atmosphere as addressed here. In the present study, HONO yields between 80 and 90% were found at the beginning of reaction, which is similar to values obtained by Gerecke et al. (4) but significantly higher than in some other studies where HONO yields below 50% and high NO yields were observed. As in our study, Hind and Grassian (39) observed a decreasing HONO yield with time. Considering the reaction mechanism proposed by Kleffmann et al. (6), which includes a HONO to NO decomposition pathway on the soot surface, the kinetics of this decomposition process must be very slow because we were not able to observe any NO as product. It is likely that again the differing soot sources, sampling conditions, and experimental procedures have affected these results. For the formation of chemisorbed NO2, the observed reaction probabilities are within the range reported by Kirchner et al. (30) for diesel engine soot (10-9 to 10-4). The surface reaction mechanism and its parametrization presented here simply assumes a reversible Langmuir adsorption followed by surface reactions, without including more elementary, physical, or chemical processes. Nevertheless, it demonstrates several major features of surface reactions on soot. The fast adsorption equilibrium between NO2(g) and S‚NO2 is key to understanding why high initial uptake coefficients are rapidly decreasing with time, when the surface reaction becomes rate limiting and can therefore account for part of the discrepancies in observed uptake coefficients or reaction probabilities reported in the literature. In addition, as the number of sites for S‚NO2 is also limited (far below a formal monolayer), the reaction rates do not linearly increase with increasing NO2 concentration; therefore, uptake coefficients decrease with increasing NO2 concentration, already above 20 ppb. The model also correctly describes the time dependence of the reaction rate caused by the consumption of organic reactants. It seems, therefore, that HONO formation on soot and the overall chemisorption reactions are both reliably assessed over several hours using this model. Furthermore, the behavior of the reaction rates or uptake coefficients observed and explained here seems characteristic of many other processes on soot or other solid surfaces.

Acknowledgments The help of E. Ro¨ssler, D. T. Jost, D. Piguet, M. Wachsmuth, M. Bru ¨ tsch, L. Tobler, M. Kalberer, and E. Weingartner is greatly appreciated. We thank the staff of the Philips cyclotron at Paul Scherrer Institute for providing stable proton beams during many days and nights, and we thank the ECO SWISS company for providing several devices for emission measurements. This work was supported by the Kommission fu ¨r Technologie und Innovation (KTI).

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Received for review September 5, 2000. Revised manuscript received February 26, 2001. Accepted February 27, 2001. ES000207S

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