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1
Heterogeneous reaction of ozone with hydrocarbon flame soot

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The reaction of ozone with toluene and kerosene flame soot was studied over the temperature range 240 to 350 K using a low pressure (a few Torr) flow reactor coupled to a modulated molecular beam mass spectrometer.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well controlled and adjustable combustion conditions.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

Soot was found to be deactivated in reaction with ozone, the uptake coefficient (γ) being dependent on the time of exposure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The values of (1.8 ± 0.7) × 10⁻⁴ and (3.8 ± 1.5) × 10⁻⁴ independent of temperature in the range 240–350 K were determined for the initial uptake coefficient of ozone on toluene and kerosene soot, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The process of soot ageing (deactivation) was parameterized, the uptake coefficient being expressed as a function of time and gas phase ozone concentration: γ = γ₀/(1 + γ₀k[O₃]t), with temperature independent values of k = (1.1 ± 0.4) × 10⁻¹⁰ and (6.2 ± 2.5) × 10⁻¹¹ cm³molecule⁻¹s⁻¹, for toluene and kerosene soot, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

From the soot surface saturation experiments the following maximum number of ozone molecules taken up were determined: ≃7 × 10¹⁴ for toluene and ≃9 × 10¹⁴ molecule cm⁻² for kerosene soot.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Experiments on soot ageing confirmed that soot deactivation occurs under real ambient conditions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

The present results support current considerations that heterogeneous loss of ozone on soot has negligible impact on ozone concentration throughout the atmosphere.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Introduction

Carbonaceous aerosols, which derive from biogenic sources (organic carbon) and from the incomplete combustion of fossil fuels and biomass (black carbon as well as organic carbon), are now recognized for their potential impact on climate.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Carbonaceous particles directly influence the radiative budget of the atmosphere (an extinction and absorption of solar and terrestrial radiation) and also indirectly as they may serve as condensation nuclei for the formation of clouds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Besides these “radiative” aspects of the carbonaceous aerosol role in the atmosphere, soot particles providing high surface area for heterogeneous reactions may also influence the chemical composition of the atmosphere by reacting with important atmospheric trace gases.^2–4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

The present study addresses this last issue and concerns an experimental investigation of the reaction of ozone with soot.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

Ozone reaction with soot has been studied in several laboratories, as reported in recent data reviews^5,6 (see Discussion section for details), yet the reported values of the uptake coefficient (γ) are highly variable (between 10⁻³ and 10⁻⁸).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

This is due to the dependence of the measured values of γ on the surface area of the soot samples accessible to O₃, soot type and soot deactivation process leading to a dependence of the uptake coefficient on the time scale of experiments and ozone concentrations used.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

In the present study we used a low pressure flow reactor combined with mass spectrometric detection of gas phase species to measure the uptake coefficient of ozone on toluene and kerosene flame soot samples prepared under well controlled and adjustable combustion conditions.

Type: Method | Advantage: None | Novelty: None | ConceptID: Met1

The process of soot ageing (deactivation) was parameterized, the uptake coefficient being expressed as a function of time and gas phase ozone concentration in the temperature range T = (240–350) K.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

Experimental section

The mode of flame soot generation and combustion conditions (particularly the fuel/air ratio) are known to influence the chemical reactivity of soot samples.^7,8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

In the present study we have adapted the flat-flame burner existing in the laboratory (LCSR)⁹ for the preparation and deposition of soot samples from premixed flames of liquid fuels in a reproducible way and under well controlled combustion conditions.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

This soot generation system is shown in Fig. 1.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The liquid fuel pumped with a Gilson pump manufactured for liquid chromatography was first atomized by forcing it through a small orifice by a high pressure (a few atm) nitrogen jet and then vaporized in a heated chamber.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Then oxygen and nitrogen flows were added to this fuel vapor-nitrogen mixture.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

All these gases were mixed in a tubular volume filled with glass beads.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The liquid flow rate was regulated and controlled with Gilson pump and the flow rates of nitrogen and oxygen were measured with mass flowmeters (MKS Instruments).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Resulting mixture with known fuel/oxygen ratio was transported to the flat-flame burner.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The matrix of the burner was made of a brass disc drilled with 0.5 mm diameter holes on a 4 cm diameter circular area.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

It was placed into a Pyrex tube to prevent the flame perturbation by external air movements.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Additional nitrogen was added into the tube surrounding the burner in order to avoid external oxygen diffusion and preserve real premixed flames with well defined richness.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

In order to prevent any fuel condensation all the parts of the system and connection lines were heated at adequate temperature.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The temperature was controlled with thermocouples located at different points of the system.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Toluene and a mixture of hydrocarbons (decane∶propylbenzene∶propylhexane = 74∶15∶11) were used as fuels in the present study.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

This last mixture (which will be referred as kerosene in the paper) was chosen as a proxy of kerosene as it represents well the combustion of kerosene¹⁰ and also facilitates soot preparation due to a small number of hydrocarbon constituents (with lower boiling points) compared with kerosene.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Soot samples from stabilized premixed flames of these liquid fuels were deposited on Pyrex rods (0.9 cm od), which were rotated and moved through the flame as shown in Fig. 1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The soot was sampled at different locations in the flame (from 1 to 4 cm above the burner surface).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The thickness of soot coverage was determined directly by means of an universal optical microscope (Reichert meF2).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The microscope was sequentially focused on the support Pyrex surface and the surface of the soot.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The soot coverage thickness was determined as the difference between two readings.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Repeating this operation in several points along the soot samples, it was found that the procedure of soot deposition used in this study provided a homogeneous soot coverage (within 20%).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The thickness of soot samples used was varied in the range 20–300 μm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

As one could expect, the mass of the soot deposited on the outer surface of the support Pyrex tube depended linearly on the deposition time.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

Examples of such dependencies for the flames with different richness are shown in Fig. 2, where flame richness represents the fuel/oxygen ratio multiplied by stoichiometric coefficient of oxygen.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The flame richness is a key parameter determining the rate of soot formation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The results from Fig. 2 are presented in Fig. 3 as a dependence of the rate of soot deposition (in mg per 1 cm length of the support tube and per minute) on the toluene flame richness.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The line on Fig. 3 corresponds to a power function^e.g.:¹¹ soot deposition rate ≃ (ϕ − ϕ_c)ⁿwhere ϕ_c is the critical richness (ϕ_c ≈ 1.6 for n = 2).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

At ϕ < ϕ_c soot is not formed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

It is clear that from the few data points shown in Fig. 3 it is not possible to determine ϕ_c and n independently (ϕ_c is in the range 1.45–1.7 for the values of n between 3 and 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

However, the important point in the context of the present study was to show existence of the critical richness and strong dependence of the soot formation rate on the flame richness.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The soot samples used in the present study were obtained from the flames with richness between 1.7 and 2.2.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The lower limit of the flame richness was imposed by very low rate of soot formation and the upper one by the difficulties in stabilization of the rich flames over the burner.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Specific (BET) surface area is an important parameter for the quantitative characterization of soot reactivity toward different chemical species.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

In the present study BET surface measurements of the soot samples used in the uptake experiments were performed with ASAP-2000 apparatus and nitrogen as adsorbate gas (soot sample was removed from the support tube prior to these measurements).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The values of 175 ± 25 and 120 ± 20 m² g⁻¹ were found for soot formed in toluene and kerosene flames, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

In the range of the quoted uncertainty and under experimental conditions used no influence on the BET surface area of the flame richness, location in the flame and treatment with ozone was observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Interaction of ozone with soot was studied using the flow tube technique and a modulated molecular beam mass spectrometer as the detection method.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

A Pyrex tube with the deposited soot sample was introduced into the flow reactor along its axis.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The main reactor (Fig. 4) consisted of a Pyrex tube (45 cm length and 2.4 cm id) with a jacket for the thermostated liquid circulation (water or ethanol).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The movable injector consisted of three coaxial Pyrex tubes as shown in Fig. 4.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The tube with the soot coverage could be moved relative to the outer tube of the injector.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

This allowed the variation of the soot sample length exposed to ozone, which was introduced through the side arm of the main reactor.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Thus the reaction time (t) was defined as the soot sample length (l) exposed to O₃ divided by the average flow velocity in the reactor (v): t = l/v.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The inner tube of the movable injector was used for circulation of the thermostated liquid inside the tube with soot sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

This allowed to maintain the same temperature in the main reactor and on the soot surface in the measurements of the temperature dependence of the uptake coefficient.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Fresh soot samples were used in all kinetic experiments.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

Generally, freshly prepared soot sample was introduced into the flow reactor and pumped during 15–20 min before being exposed to ozone.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

It was verified that pumping for a longer time had no influence on the kinetics of ozone loss (see Fig. 5).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

Ozone was produced by an ozonizer (Trailigaz), collected in a liquid nitrogen trap, pumped in order to reduce the O₂ concentration and, finally, diluted in Helium and stored in a glass flask.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The absolute concentration of O₃ was derived using the reaction of ozone with NO with simultaneous detection of ozone consumed and NO₂ formed (Δ[O₃] = Δ[NO₂]): NO + O₃ → NO₂ + O₂k₁ = 2.0 × 10⁻¹²exp(−1400/T) cm³molecule⁻¹s⁻¹⁵.The mass spectrometer was calibrated for NO₂ by letting a known concentration of NO₂ flow into the reactor.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Results

Fig. 5 represents a typical behavior of the ozone concentration when soot sample is moved into the reaction zone.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

Fast initial consumption of O₃ followed by rapid surface deactivation leading to a decrease of ozone loss rate was observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

In additional experiments, the soot sample was withdrawn from the reaction zone at different times after the initial exposure to ozone.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

Under these conditions, when ozone is no longer in contact with soot surface, ozone concentration relaxed rapidly toward its initial value.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

No additional ozone, which could be desorbed from the soot surface was observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

These measurements indicate that ozone loss on soot is an irreversible process.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

The uptake coefficient, γ, was determined as the probability of the irreversible ozone loss per collision of O₃ with soot surface: where k′ (in s⁻¹) is the rate constant of ozone loss, ω the average molecular speed, V the volume of the reaction zone and S is the surface area of the soot sample.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

To calculate the uptake coefficient two parameters have to be determined experimentally: the rate constant of the heterogeneous ozone loss (k′) and the soot surface area (S) accessible for interaction with ozone.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Determination of k′

Kinetics of ozone loss

In order to verify if the first-order loss approximation is applicable for the determination of k′ from ozone loss kinetics, a series of experiments was carried out where similar fresh soot samples of different lengths (2, 4, 6 and 8 cm) were successively exposed to the same initial concentration of ozone.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

Variation of the soot sample length is equivalent, in fact, to variation of the reaction time (see Experimental Section).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The results are displayed in Fig. 6a as the kinetics of ozone decay for different times of exposure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

The straight lines are the exponential fits to the experimental points.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

The conclusion from these experiments is that for given exposure time and initial ozone concentration the kinetics of ozone loss on soot can be treated with the first-order kinetics formalism and the rate constant can be determined as: where t is the reaction time defined by sample length and flow velocity: t = l/v.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

It should be noted at this point that k′ determined in this way is dependent on initial ozone concentration because soot is deactivated during the reaction and the rate of this deactivation is expected to be dependent on the ozone concentration.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

The results from Fig. 6a are presented in Fig. 6b as the dependence of k′ (calculated from eqn. (II)) on the time of soot exposure to ozone.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

The uncertainties on determination of k′ shown in Fig. 6b are between 13 and 25% (lower for higher k′ values) and include the uncertainty on soot mass measurements.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

Good agreement between the values of k′ obtained with soot samples of different length justifies the procedure used for the calculations and shows the good reproducibility of the measurements.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

In addition, the graph well illustrates the behavior described above: the ozone loss rate is high at the beginning of the reaction and rapidly decreases during the exposure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Pressure dependence

The measurements of the rates of heterogeneous loss of gas phase species in flow reactor from the kinetics of the axial decrease of their concentration is complicated by simultaneous occurrence of two processes: diffusion and uptake of the molecules on the reactor wall.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

The treatment of the kinetics of heterogeneous decay is simple only in the case of so called “kinetic regime”, when the uptake of molecules on the wall is much slower than their diffusion^e.g..¹²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

When an efficient heterogeneous loss leads to the important local depletion of the gas phase molecules close to the surface, their diffusion from volume toward the surface becomes a rate limiting step in the heterogeneous reaction and should be taken into account in the processing of the experimental data.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

For the determination of the first order rate constant for the kinetic regime we have used an approach based on the kinetic resistance additivity rule:¹³ where k′obs is first order rate constant measured from heterogeneous decay kinetics as shown above, k′kin and k′dif are the kinetic and diffusion limits of the rate constant, respectively.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

In the present work the heterogeneous interactions of the gas phase species with soot samples were studied in a coaxial reactor with an active central rod (covered with soot) and passive surface of the main reactor (covered with halocarbon wax).

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj2

The radial diffusion problem for this type of reactor configuration was solved by Gershenzon and co-workers.^14–16

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

For k′dif under fast flow conditions (v → ∞) they give an expression similar to that for a cylindrical reactor: where K^d(q) is dimensionless rate constant of radial diffusion, which is a function of ratio q = r/R, where r is the external radius of the coated internal tube and R is the internal radius of the main reactor.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

For our reactor, r = 0.45 and R = 1.2 cm, q = 0.375 and K^d(q) ≈ 4..4¹⁶

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

Thus: and where D₀ is the diffusion coefficient of O₃ in He at 1 Torr pressure (Torr cm² s⁻¹) and P is the total pressure in the reactor (Torr of He).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

A series of experiments has been carried out, where the rate of ozone loss on soot was measured as a function of pressure in the reactor in the range 0.5–5.0 Torr.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

It was observed that the rate of O₃ loss is strongly dependent on the total pressure in the reactor, with a decrease as pressure increased.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

An example of the dependence of 1/k′_obs on pressure measured at T = 240 K is shown in Fig. 7.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

The experimental points measured at different times of soot exposure to ozone can be well fitted with linear regression according to eqn. (IV).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

The intercept provides the value of 1/k′kin for a given time of exposure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

100

The slope of the straight line, α = R²/4.4D₀, defines the correction factor which has to be applied to a measured value of k′_obs in order to obtain the value of k′_kin: For 1 Torr total pressure (generally used in our experiments): One can note that the slopes of the three straight lines corresponding to different exposure times are identical in the range of experimental uncertainty, which indicates that the correction factor α is independent of the magnitude of k′obs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

101

Similar series of experiments have also been carried out at T = 298 and 350 K. The values for the correction factor α determined at T = 240, 298 and 350 K are 2.2 × 10⁻³, 1.7 × 10⁻³ and 1.4 × 10⁻³ s, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

102

As one could expect the correction factor is lower at higher temperature, where the gas phase diffusion is faster.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

103

Eqn. (IV) including the empirically found correction factors α was used for the calculation of k′kin from the experimental values of k′obs for the O₃–soot interaction.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

104

Maximum corrections on k′obs were around 40%.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

105

For the intermediate temperatures T = 265 and 320 K extrapolated values of α were used.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

106

The knowledge of the parameter α = R²/4.4D₀ allows the determination of D₀, the diffusion coefficient of O₃ in He at 1 Torr total pressure.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

107

Thus, for T = 298 K the value of D₀ = (205 ± 15) cm² s⁻¹ (with 2σ statistical uncertainty) can be calculated.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

108

This value is a factor of two lower than recently reported: D₀(O₃–He) = (410 ± 25) Torr cm² s⁻¹.¹⁷

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

109

The reason for the lower value of D₀ of the present study is not clear.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

110

It can be an artifact of our experiments or questionable applicability of the K^d(q)-values^15,16 to our reactor configuration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

111

It is interesting to note that a similar systematic (factor of two) discrepancy was observed between literature values for HO₂ and OH diffusion coefficients in helium and those obtained in our unpublished study applying the kinetic resistance additivity rule to our coaxial reactor.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

112

Additional experiments with variation of the geometry of the reactor (q = r/R parameter) are under way to confirm the applicability of eqn. (III) (more specially of the K^d(q) coefficients¹⁶), at least, to the reactor configuration used in the present study.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

Dependence on flame richness

113

Fig. 8 shows the results of the experiments where the rate of ozone loss on soot was measured on soot samples of similar mass and length, however generated under different combustion conditions (flame richness).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

114

One can note that no dependence of the reaction rate on the flame richness was observed in the range of the quoted experimental uncertainty (14–25%).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

115

Although a rather narrow range of flame richness was used in these experiments, however, as was shown in Experimental section, even a slight variation of this parameter had an important influence on the flame structure and especially on the rate of soot formation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Dependence on soot sampling position

116

The rate of ozone/soot interaction was studied as a function of soot sampling position in the flame.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

117

Experiments were carried out with similar (mass and length) soot samples collected at 1, 2.5 and 4 cm above the burner surface.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

118

As shown in Fig. 9 no influence of the soot ageing in the flame on the reactivity of soot toward ozone was observed under experimental conditions used.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

119

All the experiments described below were carried out with soot sampled at 2.5 cm height above the burner surface.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Surface area

120

Specific experiments have been carried out in order to determine the soot surface area accessible to ozone.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

121

For that, the rate of ozone loss on soot k′ was studied as a function of the mass of soot deposed on the unity length of the support tube.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

122

In fact, this dependence is equivalent to the dependence of k′ on the thickness of soot coating.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

123

Results are shown in Fig. 10.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

124

For all the plots corresponding to a different time of soot exposure to ozone two regimes are observed:¹⁸ the first one, where k′ is linearly dependent on the mass of soot sample and the second one (saturation region), where k′ is independent of the sample mass.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

125

Linear dependence of the reaction rate on mass, hence thickness, of soot coating indicates that the entire surface area of soot samples is involved in the interaction with ozone and, consequently, the BET surface area should be used for calculations of the uptake coefficient.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

126

The soot mass range, corresponding to the linear regime, is different for different exposure times.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

127

This could be expected, since the soot mass probed by ozone is defined by two concurrent processes: diffusion of ozone into soot bulk and ozone loss rate which decreases upon deactivation of soot sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

128

The uptake measurements in the present study were carried out with soot sample masses lower than 0.4 mg cm⁻¹, where linear dependence of the reaction rate on soot mass was observed for exposure times higher than 30 s.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

Uptake coefficient

129

The kinetics of ozone consumption on soot surface was investigated as a function of the initial concentration of ozone.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

130

The results obtained with different initial concentrations of ozone are shown on Fig. 11.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

131

We observe similar behavior with an almost total consumption of ozone in the first seconds of the reaction, followed by a progressive return to the initial concentration of ozone.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

132

This behavior indicates that a deactivation process occurs, the rate of which increases with increasing initial ozone concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

133

Thus the reaction rate (and consequently the uptake coefficient) depends on two parameters: exposure time and ozone concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

134

Considering that soot reactivity is defined by the number of active sites on its surface, which are consumed in the reaction with ozone and in order to describe the process of soot deactivation by only one parameter, we attempted to express the uptake coefficient as a function of the number of ozone molecules taken up per unit surface area of soot.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

135

An example is presented in Fig. 12, where the straight line represents an exponential fit to the experimental points.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

136

One can note that all the data corresponding to the exposure time from 0.5 to 15 min and to the initial ozone concentration varied by an order of magnitude can be represented by a simple expression: γ = γ₀exp(−βΔ[O₃])where γ₀ is the initial uptake coefficient, β a constant characterizing the deactivation processes and Δ[O₃] the number of ozone molecules taken up by 1 cm² surface area.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

137

This approach allows the determination of the initial uptake coefficient by extrapolation of the experimental data to the beginning of the reaction, when Δ[O₃] = 0.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

138

The values of γ₀, derived in this way for kerosene and toluene soot at different temperatures in the range 240–350 K are reported in Tables 1 and 2, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

139

Although eqn. (VI) reproduces well the values of γ at initial rapid stage of the reaction, its application for the calculation of the time dependence of the uptake coefficient at a given concentration of ozone at longer reaction times, for instance for atmospheric application, seems to be rather difficult and most probably incorrect.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

140

That is why we have tried to find a parametric representation of γ as a function of time and ozone concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

141

First, it was observed that reciprocal of the uptake coefficient for given initial concentration of ozone can be well represented by a linear function of the exposure time: where C is a coefficient characterizing the soot deactivation rate (decrease of γ) and depending on the concentration of ozone.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

142

Examples of such plots observed at different initial ozone concentrations are shown in Fig. 13.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

143

The slopes of the straight lines in Fig. 13 provide the values of C, which are presented in Fig. 14 as a function of the concentration of O₃.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

144

It should be noted at this point that in the data analysis the ozone concentration was considered to be constant along the reaction zone (limited by soot sample length) while it was not the case.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

145

In fact, the ozone concentration decreased along the soot sample from its maximum value ([O₃]₀) at the upstream end of the soot sample to the minimum one at the downstream end of the reaction zone.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

146

In the present experiments this ozone consumption ranged from ≈50% to a few percent depending on the exposure time and the mean ozone concentration along the reaction zone was used in the calculations (an uncertainty of near 20% on this ozone concentration scaling was estimated and taken into account in the final results).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

147

Another point is that the uptake coefficient being dependent on [O₃], the ozone concentration profile along the soot sample could lead to the “gradient of soot reactivity” along the soot sample length.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

148

This issue was neglected and not considered in the calculations.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

149

Fig. 14 shows that the coefficient C can be well approximated by linear function of the ozone concentration: C = k[O₃].

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

150

The value of the constant k, which has a dimension of a second order rate constant (cm³ molecule⁻¹ s⁻¹), can be easily determined from the slope of the straight line in Fig. 14.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

151

Thus, the uptake coefficient can be, finally, represented by the following expression: which can be written as: The values of k for ozone reaction with kerosene and toluene soot determined at different temperatures are given in Tables 1 and 2, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

152

One can note that reactivity of soot samples under investigation as well as the rate of soot deactivation were found independent of temperature in the range T = 240–350 K. The following values of γ₀ and k can be recommended from this study for this temperature range: γ₀ = (1.8 ± 0.7) × 10⁻⁴ and k = (1.1 ± 0.4) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ (toluene)γ₀ = (3.8 ± 1.5) × 10⁻⁴ and k = (6.2 ± 2.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (kerosene).Quoted uncertainties are near 40% for both γ₀ and k and represent a combination of statistical and estimated systematical errors, including those on k′, mass of soot samples, specific surface area and absolute ozone concentrations measurements.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

153

Two limiting cases for γ in eqn. (VII) can be noted: γ₀k[O₃]t ≪ 1 resulting in γ = γ₀ and γ₀k[O₃]t ≫ 1 resulting in γ = 1/k[O₃]t.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

154

In the first case (low ozone concentrations) the uptake coefficient is independent of the volume O₃ concentration and exposure time.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

155

In the second case, it is inversely proportional to both [O₃] and time of exposure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

156

The parameters γ₀ and k in eqn. (VII) which describes decrease of soot reactivity with time were obtained from the experimental data corresponding to the first 10–20 min of soot exposure to ozone.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

157

In order to verify that eqn. (VII) still holds when the ratio [O₃]∶t is varied while the product [O₃]t is kept in the sensitivity range of our experiments, soot samples were exposed to relatively low ozone concentration, [O₃]₀ = 5.5 × 10¹¹ molecule cm⁻³, for up to 140 min.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

158

The results obtained for temporal behavior of the uptake coefficient are shown in Fig. 15.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

159

The experimental points were fitted with eqn. (VII), where γ₀ and k were variable parameters.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

160

The curve in Fig. 15 corresponds to the best fit obtained with γ₀ = 4.2 × 10⁻⁴ and k = 4.7 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

161

These values are in good agreement with those reported above considering the experimental uncertainties.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

162

Thus, expression (VII) is applicable for the description of soot deactivation process under largely varied conditions: exposure time from a few minutes up to a few hours, initial concentration of ozone from 5 × 10¹¹ to ≃10¹³ molecule cm⁻³.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

163

In another series of experiments soot reactivity toward O₃ was studied as a function of time of the soot exposure under outdoor conditions.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj1

164

The day-time concentrations of ozone and NO₂ in ambient air were measured to be (75 ± 10) and (2.5–8) ppb, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

165

Soot samples with similar masses (≈0.3 mg cm⁻¹ × 6 cm) were exposed outside for different times (up to 7 h).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

166

The initial uptake coefficient (γ₀) was chosen as a parameter to characterize the outdoor soot ageing process.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

167

This parameter was determined by introduction of the aged soot samples into the flow reactor and measuring their reactivity toward ozone.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

168

Fig. 16 shows the measured values of the uptake coefficient as a function of the number of ozone molecules taken up by the soot surface in the reactor for soot samples being exposed outside for different times.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

169

The time, 2 min, for the fresh soot corresponds, in fact, to the time which is necessary for installation of the freshly prepared soot sample in the flow reactor.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

170

The deactivation of soot under outdoor conditions is clearly observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

171

The values of γ₀ obtained from the data in Fig. 16 are presented in Fig. 17 as a function of the time of soot ageing.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

172

The line in Fig. 17 is a fit to the experimental points according to eqn. (VII).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

173

The best fit is obtained with γ₀ = 4 × 10⁻⁴ and k = 4.3 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

174

The value of γ₀ agrees with that measured above for ozone reaction with kerosene, however the value of k is around two orders of magnitude lower than corresponding value measured in experiments on soot deactivation in the reactor.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

175

This lower rate of soot deactivation under outdoor conditions, compared to that in the low pressure reactor, could be expected since the results obtained in a kinetic regime (reactor) could not be directly applied to the experiments on soot ageing outside, where the diffusion of gas phase species toward our “compact” (not dispersed) soot sample is a limiting factor of the heterogeneous reaction (diffusion regime).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

176

It should be noted that these diffusion limitations do not occur in the case of small atmospheric soot particles (with diameter of a few tens nm) dispersed in the atmosphere and eqn. (VII) may be applicable under such atmospheric conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

Number of active sites

177

Another parameter characterizing the soot reactivity toward ozone is the maximum number of ozone molecules which can be lost on the unity surface area of soot sample.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

178

If reaction is considered as non catalytic (one ozone molecule lost per active site) which seems to be the case at least for the first rapid reaction stage this parameter can be considered as a number of active (toward ozone) sites on the soot surface.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

179

To determine this parameter we measured the total number of ozone molecules consumed on the soot surface from the beginning of the reaction up to the complete soot sample deactivation (defined as an absence of observable ozone loss within experimental accuracy).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

180

Exposure time to reach the total soot surface saturation was in the range 30–60 min depending on soot sample mass and ozone concentration used.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

181

The results obtained for toluene soot are shown in Fig. 18, where the number of ozone molecules taken up is presented as a function of mass of soot samples generated in flames of different richness.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

182

The number of active sites per mg of soot derived from the slope of the straight line of Fig. 18 is 1.2 × 10¹⁸ sites mg⁻¹, independent of flame richness.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

183

Similarly, for the kerosene soot the value of 1.1 × 10¹⁸ sites mg⁻¹ was determined.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

184

Appling the BET surface area (175 and 120 m²g⁻¹, respectively for toluene and kerosene soot), one calculates the respective values of the number of active sites per unity of surface area: 7 × 10¹⁴ sites cm⁻² and 9 × 10¹⁴ sites cm⁻².

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

185

It can be noted that these numbers measured with the flame soot are very close to those reported by Pöschl et al¹⁹. and Kamm et al²⁰. for spark discharge soot aerosol particles: 5.6 × 10¹⁴ and 6.5 × 10¹⁴ sites cm⁻², respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

186

Concerning the results reported in this section it should be noted that if the 1/t-dependence of γ (eqn. (VII)) is assumed to be valid for long exposure times (t → ∞), then there is no surface saturation and the maximum number of ozone molecules taken up by the soot surface can not be determined since the number of consumed ozone molecules does not level off at any value and is going to infinity with time.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

187

Considering this as well as the method applied to determine the total number of consumed ozone molecules (experimental sensitivity limitations in determining the soot surface saturation), the total number of ozone molecules consumed per unit soot surface area determined in the present study has to be considered as a lower limit.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

Discussion

Comparison with literature data

188

Kinetics of ozone reaction with soot has been the subject of numerous previous studies (Table 3).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

189

Stephens et al.,²¹ using a Knudsen cell combined with mass spectrometric detection of gas phase species, have measured the uptake coefficients of ozone on commercial ground charcoal (BET surface area 37 m²g⁻¹): (0.2–4.1) × 10⁻³ and (2.7–11.3) × 10⁻⁵ for initial and steady state uptake coefficients, respectively.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

190

The uptake coefficient was found to be dependent on the carbon sample, flow rate and concentration of ozone.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

191

Using similar experimental apparatus Rogaski et al²². studied the interaction of ozone with a commercial amorphous carbon (Degussa FW-2, BET surface area 460 m²g⁻¹).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

192

The O₃ uptake coefficient (determined considering a geometric surface area) was (1 ± 0.7) × 10⁻³ and slightly decreased with time.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

193

Fendel et al²³. used flow tube technique to investigate the reaction of ozone with carbon aerosol (of 10–100 nm radius, BET surface area 395 m² g⁻¹) produced in a spark discharge generator.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

194

The ozone volume mixing ratio as a function of the aerosol concentration was monitored for (15–30) s reaction time.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

195

The resulting uptake coefficient was found to be strongly dependent on the initial concentration of ozone: γ, calculated using specific surface area, decreased from 3.3 × 10⁻³ to 2.2 × 10⁻⁴, ozone being increased from 160 to 915 ppb.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

196

In the flow tube study of Longfellow et al²⁴. the kinetics of ozone reaction with methane soot was monitored by using chemical ionisation mass spectrometry.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

197

Considering geometric surface area, the authors report the value of 7 × 10⁻² for initial uptake of ozone on 3.5 mg of fresh soot, which drops to ≃4 × 10⁻³ on soot exposed to O₃ (≃3 × 10¹⁰ molecule cm⁻³) for 30 min and to 1.6 × 10⁻⁴ for 7 h of exposure (soot sample of 2.2 mg).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

198

Following the authors and assuming a 30-fold (for 3.5 mg sample) decrease in γ if available surface area instead of geometric one is applied, one gets: γ₀ ≃ 2.3 × 10⁻³, γ₀/γ_{(30 min)} ≃ 18 and γ₀/γ_{(7 h)} ≃ 275.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

199

It is interesting to note that the application of eqn. (VII) for γ derived in the present study (with k = 1.5 × 10⁻¹⁰ cm⁻³ molecule⁻¹ s⁻¹, which is an upper limit of k for toluene soot) to these data gives: γ₀/γ_{(30 min)} ≃ 20 and γ₀/γ_{(7 h)} ≃ 261, i.e. very close to the experimental values.²⁴

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

200

This shows that the empirical eqn. (VII) derived from the experimental data for the first minutes of the ozone + soot reaction and using relatively high initial concentrations of ozone could be applicable at longer reaction times and lower ozone concentrations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

201

All studies cited above dealt with the first rapid stage of ozone–soot interaction.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

202

On contrary, in a aerosol chamber study of Kamm et al²⁰. the experiments were carried out on a time scale of 72 h, where the second slow stage of the uptake process was investigated, the rapid initial reaction stage being not resolved on the time scale of the experiments.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

203

A reaction probability γ = 1.2 × 10⁻⁶ at T = 296 K was observed and was shown to decrease with a characteristic time of about 12 h.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

204

This slow exponential dependence of γ with time differs from the 1/t dependence of γ derived in our study for the fast initial reaction stage.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

205

In another static aerosol reactor study by Disselkamp et al²⁵. the values of ≃10⁻³ and ≃10⁻⁸ were reported, respectively for the initial and steady state uptake coefficients of ozone on carbon black aerosols (Degussa FW1).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

206

Interestingly, if eqn. (VII) with k ≃ 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ is applied for estimation of γ under the experimental conditions of the study by Disselkamp et al.²⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

207

(ozone partial pressure ≃3.5 × 10⁻⁵ atm, characteristic exposure time around 50 min) one calculates γ ≃ 4 × 10⁻⁸ in fair agreement with their experimental value.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

208

Analysis of the data available in the literature (Table 3) shows that the values of the initial uptake coefficient measured in different studies with different soot samples and under different experimental conditions are all near 10⁻³ (with uncertainty factor of 4).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

209

However quantitative data on evolution of γ with time due to deactivation of soot are very scattered: so called “steady state” values of γ vary between 10⁻⁴ and 10⁻⁸.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

210

This could be expected, since the reaction rate depends strongly on two parameters, exposure time and ozone concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

211

Thus the measured values of γ are strongly dependent on the time scale of experiments and concentrations of ozone used.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

212

In this respect, the analytical expression derived in the present study for the uptake coefficient, where this latter is represented as a function of exposure time and concentration of ozone, seems to be a useful tool for determination of the heterogeneous reaction rate under given experimental or ambient conditions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

213

In the present study the uptake coefficient of ozone on kerosene and toluene flame soot surface was found to be independent of temperature in the range (240–350) K. This is in disagreement with the results of previous studies,^20,24,26,27 where relatively strong temperature dependence of the uptake coefficient was observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

214

On methane soot previously exposed to ozone Longfellow et al²⁴. measured γ ≃ 4 × 10⁻³ at T = 298 K and a factor of three lower value, (1.3 ± 0.3) × 10⁻³, at T = 253 K. Il’in et al²⁶. reported a temperature factor of 780 and 1000 K for ozone reaction on fresh and aged flame soot, respectively.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

215

Finally, in the most recent temperature dependence study by Chunghtai et al²⁷. the reaction of ozone with n-hexane flame soot beyond its initial stage was shown to proceed with an activation energy of (12.9 ± 0.5) kJ mol⁻¹.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

216

The reason for the disagreement between our results and those from other studies is not clear.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

217

Although all previous temperature dependence studies dealt mainly with the second slow reaction stage (aged soot), while the results of the present study are relevant rather to the first rapid reaction stage on fresh soot sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

218

In several studies the gas phase products of the ozone reaction with soot surface were detected.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

219

Stephens et al²¹. observed an oxygen molecule formation for each ozone molecule lost on the surface.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

220

In addition, they have found CO and CO₂ in the reaction products with the yield of 20 to 40%.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

221

The formation of O₂ was supported by study of Rogaski et al.,²² where O₂ was observed, however its yield was not quantified.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

222

Disselkamp et al²⁵. measured a stoichiometric coefficient of 2 for CO₂ production in reaction of ozone with soot, however they have not found CO in the reaction products, in contradiction with the results of Stephens et al²¹. and Smith et al.²⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

223

In the present study the possible reaction products CO, CO₂, O₂ were difficult to detect by mass spectrometry because of the presence of very high residual signals at m/z = 28 (CO⁺, N₂⁺) and 32 (O₂⁺) and very low sensibility (very high background noise) of the mass spectrometer at m/z = 44 (CO₂⁺).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

224

Concerning the reaction mechanism, Stephens et al²¹. and Pöschl et al¹⁹. proposed a Langmuir type kinetic model, consisting in reversible adsorption of O₃, followed by slow reaction on the surface.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

225

Kamm et al²⁰. interpreted their experimental results by suggesting a slow catalytic ozone destruction process occurring after the fast initial stage of the surface reaction on fresh soot.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

226

The fast initial catalytic decomposition of ozone was postulated by Smith and Chughtai.²⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

227

However, Disselkamp et al.,²⁵ on the basis of the measurements of CO₂ formation at the initial reaction stage, concluded that ozone loss is not a catalytic process.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

228

In the present study, the ozone loss on soot was observed to be an irreversible process with the reaction rate depending on the number of active sites on the surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

229

From another side, empirical eqn. (VII) derived from the experimental data corresponding to the first 10–20 min of soot exposure to ozone, implies (if it is correct at longer reaction times) a slow catalytic ozone destruction after the fast initial uptake phase.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

230

Finally, our experimental data concerning the measurements of ozone loss rate at the first rapid reaction stage do not allow to draw any conclusion about the reaction mechanism at the molecular level, and about possible catalytic processes which may contribute to ozone destruction on much longer time scales.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

Atmospheric implication

231

Previous two-dimensional model calculations⁴ suggested that a direct ozone loss on soot aerosols has the potential of explaining the lower stratospheric ozone depletion at northern hemisphere middle latitudes.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

232

However the ozone loss on soot was considered as a catalytic process with a reaction probability of 10⁻³, excluding soot deactivation and subsequent decrease of its reactivity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

233

Experimental data from the present study allow for a rough estimation of the potential importance of the direct ozone loss on soot under real atmospheric conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

234

We have determined the number of ozone molecules taken up by the soot surface during the initial rapid stage of the ozone/soot interaction where the measured uptake coefficient drops by a factor of ≃100 from its initial value to γ ⩽ 10⁻⁶.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

235

These numbers were 8.9 × 10¹⁴ and 7.1 × 10¹⁴ molecule cm⁻² for kerosene and toluene flame soot, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

236

Combining these values (we use 10¹⁵ molecule cm⁻² in the calculations below) with the maximum atmospheric soot surface area density of ≃10⁻⁵ cm²cm⁻³ (based on soot concentrations of ≃10 μg m⁻³ measured in urban area near heavy traffic roads^30–32 and surface to mass ratio of 140 m²g⁻¹ which can be derived from the data reported by Blake and Kato³³ for fractal geometry of soot particles) one can calculate the maximum amount of ozone which can be taken up by freshly emitted soot in a very polluted area.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

237

The obtained value, ≃10¹⁰ molecule cm⁻³, is negligible compared with the typical ozone concentrations in the troposphere (⩾10¹² molecule cm⁻³).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

238

Considering that in the upper troposphere and lower stratosphere soot mass density is a factor of ≃10⁴ (^{refs. 33–35}) and in the free troposphere a factor of ≃10–10³ (^{refs. 36–38}) lower than the value used in the above calculations, the direct ozone loss due to the initial rapid reaction with soot can be considered as negligible throughout the atmosphere.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

239

Concerning the second slower stage of the ozone reaction with deactivated soot aerosol (γ ≃ 10⁻⁷–10⁻⁸) the lifetime of ozone toward its reaction with soot can be estimated as: where ω is the mean molecular speed of ozone and σ is the area density of atmospheric soot.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

240

Taking the maximum values for γ, ω and σ (10⁻⁷, 360 m s⁻¹ at T = 298 K and 10⁻⁵ cm²cm⁻³, respectively) a lower limit of τ ⩾ 3.5 years can be determined for these extreme conditions, indicating a negligible contribution of the ozone/soot reaction to ozone loss processes in the atmosphere.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

241

This is in agreement with the conclusions of previous laboratory studies^20,24,25 and stratospheric measurements^35,39.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32