# Time resolved investigation of the nucleation process initiated by the reaction of OH radicals with terpenes

Nicolas Daugey, Pierre-Marie Flaud, Robert Lesclaux

► To cite this version:

Nicolas Daugey, Pierre-Marie Flaud, Robert Lesclaux. Time resolved investigation of the nucleation process initiated by the reaction of OH radicals with terpenes: Contribution to the NUCVOC project, Final Report, July 2000. [Research Report] ENV4-CT97-0391, Université Bordeaux 1 Sciences et Technologie; CNRS; Environment Institute - European Community Joint Research Centre, EC JRC. 2000. hal-02515908

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# Time resolved investigation of the nucleation process initiated by the reaction of OH radicals with terpenes

## Final Report, July 2000

Leading scientist: R. Lesclaux

Scientific staff: N. Daugey and P.M. Flaud

Address :

Laboratoire de Physico-Chimie Moléculaire
Université Bordeaux I
33405 Talence Cedex - France

Tel : 05 56 84 63 06

Fax : 05 56 84 66 45

E-mail : lesclaux@cribx1.u-bordeaux.fr

## I. OBJECTIVES FOR THE REPORTING PERIOD

* To explore a new way of initiating the formation of secondary aerosols, using pulsed laser photolysis and to monitor in real time the nucleation and condensation processes by recording time resolved absorption signals arising from light scattering by particles ; to investigate the relevant parameters of the nucleation process : nature, concentration and vapour pressure of the condensing species, concentration of formed particles, ...

* To model the dynamics of the observed nucleation process using the theories of nucleation, in collaboration with other groups, principally the group of Strasbourg ; this simple way of forming particles should provide a good test for nucleation theories.

## II. MAIN RESULTS OBTAINED:

### 1. Introduction

Modelling nucleation and condensation processes leading to the formation of secondary aerosols in the gas phase is a difficult task [1,2], as many parameters must be known : nature and concentration of condensing species, vapour pressures, thermodynamic parameters, etc... One of the difficulty is to establish the production rate of condensing species, which determines their concentration. The production rate is itself determined by emission rates of precursors (terpenes in the present case), by transport and by the kinetics and reaction mechanisms of oxidation processes leading to the formation of condensing species. It is known that the overall dynamics of those processes are very difficult to establish with enough accuracy, even in smog chambers where they can significantly be simplified. The principal objectives of the present work were to use a laboratory experiment to produce instantaneously the precursors of nucleation, i.e. in a much shorter time than the time of nucleation and of condensation processes. Then the dynamics of nucleation and condensation processes can be
observed in real time, without being perturbed by the production rate of the condensing species.

It has been observed in our laboratory that, when OH radicals were generated by laser flash photolysis, in the presence of terpenic compounds, particles were formed within a time scale of a few milliseconds after the flash. This could be recorded by observing the scattered light generated by the particles and thus, the formation of particles could be easily monitored in real time. Thus, flash photolysis can provide a simple way of analysing the dynamics of nucleation process leading to aerosol formation. The project was to explore this way of forming particles with the final aim of modelling the particle formation process, using nucleation theories, in collaboration with the Strasbourg group.

## 2. Experimental procedure

Our laser flash photolysis set-up was described previously [3,4]. Briefly, it consists in a cylindrical reaction cell (2 cm inside diameter, 100 cm long) in which the gas mixture is flowing continuously. The carrier gas is usually air at 1 atm pressure. OH radicals are produced by laser photolysis of H₂O₂ at 248 nm. H₂O₂ is introduced in the gas flow by bubbling part of the gas in a 50 - 70 % solution of H₂O₂ in water and its concentration is controlled by UV absorption spectrometry at 220 nm (σ = 2.58 × 10⁻¹⁹ cm² molecule⁻¹). Terpenic compounds are introduced in the same way, the concentrations being evaluated from vapour pressure values. Typical concentrations are (units of molecule cm⁻³): [H₂O₂] = 3 - 6 × 10¹⁵; [terpene] = 2 - 10 × 10¹⁵; [OH]ᵢinitial = 3 - 10 × 10¹³, [H₂O] = ≈ 1 × 10¹⁷.

Both the laser beam (from an excimer laser) and the analysing beam (from a deuterium lamp) are directed through the cell and time resolved absorption signals are recorded with a digital oscilloscope. The signals are then transferred to a personal computer for averaging and analysis. A typical signal, obtained with γ-terpinene at 270 nm, is shown in Figure 1A. The first decay at short time is characteristic of peroxy radical recombination and the subsequent increase in absorption is assigned to particle formation, the analysing beam being attenuated by light scattering.

<figure><img src="image_1.png" /><figcaption>**Figure 1** : Reaction OH + γ-terpinene in air. (A) : Typical trace obtained by UV absorption; (B) : by observation of the scattered light from the He-Ne laser beam, at right angle (dotted line : same as 1A).</figcaption></figure>
A second analysing beam has been added for the specific observation of particles, using the scattered light at a 90° angle. This second beam was provided by a He-Ne laser at 632.8 nm. This additional way of observing particles has yielded signals of excellent quality which were not perturbed by the absorption of peroxy radicals, as shown in Figure 2B. Comparing the two types of signals (Figure 2A and 2B), shows clearly that they have the same origin. Note that the peroxy radical absorption signal provides an internal calibration of initial radical concentration, the absorption cross section being determined comparatively to that well known of C₂H₅O₂ : σ₂₇₀(C₂H₅O₂) = 2.2 × 10⁻¹⁸ cm² molecule⁻¹ [5], giving, for example, for the RO₂ radicals derived from the OH + α-pinene reaction : σ₂₇₀ = 2.3 × 10⁻¹⁸ cm² molecule⁻¹.

The number of particles formed after each laser pulse was measured using a condensation particle counter (TSI 3022A) placed at the outlet of the reaction cell. The number of particles formed were generally around 10⁷ - 10⁸ cm⁻³.

### 3. Results

Several series of experiments were performed under various conditions with α-pinene, *d*-limonene and γ-terpinene. In all cases, traces typical of particle formation, shown in Figure 1, were obtained. Traces obtained by UV absorption are composed of two parts, occurring at different time.

- Immediately after the flash, the characteristic absorption of the peroxy radical (see reaction mechanism below) appears in less than 10 µs, as expected from the kinetics of peroxy radical formation, and then decays according to radical recombination. Peroxy radicals generally absorb between 200 and 300 nm, with a maximum at 240 - 250 nm. Consequently, this first part of the trace is not seen at wavelengths longer than about 300 nm. Note that signals could not be recorded between 235 and 265 nm, due to the absorption of dichroic mirrors used to reflect the laser beam into the reaction cell. Similar signals of peroxy radicals are obtained with any organic compound which reacts with OH (or Cl atoms) in the presence of oxygen.

- During the peroxy radical recombination, a new absorption builds up, more slowly than the preceding one (Figure 1A). This absorption does not exhibit any defined spectrum, its intensity decreasing regularly when going towards longer wavelengths. This is typical of an attenuation of the beam intensity by light scattering. Similar signals are observed with most terpenic molecules, whereas they are not observed with smaller molecules, which have less propensity of forming aerosols. Thus, this second part of the signal, also clearly seen by light scattering at 90° angle, is assigned to the nucleation process leading to particles.

#### Dependence of the signal shape and intensity with various parameters

3.1. *Concentration of terpene* - There is no significant dependence of signal shape and intensity with the concentration of terpene, as long as the concentration is large enough to scavenge all OH radicals. However, when the concentration is decreased until a competition occurs between the OH reaction with the terpenic molecule and the OH + H₂O₂ reaction, yielding HO₂ radicals, the peroxy radical RO₂ reacts preferentially with HO₂ and dramatic changes are observed, as described below.
### 3.2. Presence of NO
The presence of NO does not change significantly the reaction mechanism, it essentially converts the peroxy radicals $RO_2$ into oxy radicals RO more rapidly than in the absence of NO (see reaction mechanism below):

$$RO_2 + NO \rightarrow RO + NO_2$$

The principal change is the suppression of the $RO_2 + RO_2$ reaction. This latter reaction also form oxy radicals RO and, to some extent, molecular products such as alcohols and carbonyl compounds. The presence of excess NO also suppresses the $RO_2 + HO_2$ reaction. Nevertheless, the products are essentially the same and indeed, particles were formed as in the absence of NO, with similar growing rates. As a result, most experiments were performed without NO.

### 3.3. Concentration of $H_2O_2$
For a given fluence of the laser beam, changing the concentration of $H_2O_2$ is equivalent to changing the initial concentration of OH radicals and, consequently, to changing the initial concentration of $RO_2$ radicals and of condensing species. It was verified that the initial concentration of peroxy radicals was roughly proportional to the $H_2O_2$ concentration and it can reasonably be assumed that it was the same for the concentration of condensing species.

<figure><img src="image_2.png" /><figcaption>**Figure 2:** Traces observed for three different initial concentrations of $RO_2$ radicals, either by absorption at 270 nm or by light diffusion at 90° angle; $[RO_2] = 1.0, 0.71$ and $0.47 \times 10^{14}$ molecule cm⁻³, for A, B and C, respectively; the corresponding concentration of particles were 8.8, 4.8 and $2.6 \times 10^7$ cm⁻³, respectively.</figcaption></figure>

The effect of changing the concentration of $H_2O_2$ is illustrated in Figure 2 in the case of α-pinene, where the initial $H_2O_2$ concentration was reduced by 0.6 and 0.3 factors. It is seen that the initial concentration of $RO_2$ is roughly divided by the same factors and that the shape of the particle build-up signal is strongly affected. The formation of particles is delayed when decreasing the initial $RO_2$ concentration and the build-up is much slower. This represents a good example of the dependence of the nucleation process dynamics on the concentration of condensing species. It provides a good base for testing the nucleation theories.

This example shows clearly that the intensity and shape of the observed signals are strongly dependent on the initial concentration of radicals. This explains the difficulties we had at the beginning of the work to obtain reproducible signals, as emphasised in our first periodic report. Those difficulties have been solved by having the laser repaired, the laser pulse energy being now much more reproducible than before.
4. Reaction mechanism and nature of the condensing species.

In the particular case of our experiments, the OH radicals react very rapidly with the terpenic molecule (in less than 10 µs) and thus the subsequent oxidation reactions occur without any new reaction of OH radicals nor any photolytic processes. It results that the oxidation process stops with the formation of the first carbonyl compound, according to the following simplified reaction mechanism :

$$
\mathrm{OH}+\mathrm{terpene}+\mathrm{O}_{2} \rightarrow \mathrm{RO}_{2}(\text { formation of peroxy radicals })
$$

In the case of terpenes, R is a radical generally resulting from the addition of OH to a double bond. It may also result, to a lesser extent, from abstraction of an allylic hydrogen atom. In air, it combines instantaneously with O₂ to the peroxy radicals RO₂. The subsequent reactions are :

$$
RO_2 + RO_2 \rightarrow 2 RO + O_2 \text{ (formation of oxy radicals)} \quad (2)
$$

$$
RO + O_2 \rightarrow \text{aldehyde or ketone} + HO_2 \quad (3)
$$

If the addition of OH occurs on a double bond included in a C₆-ring, this latter step is generally accompanied by ring opening. In the case of α-pinene, ring opening lead to the formation of pinonaldehyde, by reaction of the new radical formed with O₂ and production of HO₂ [8].

<figure><img src="image_3.png" /><figcaption>RO₂</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>RO</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>ring opening</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>pinonaldehyde</figcaption></figure>

Thus, pinonaldehyde should be the principal species giving rise to the formation of particles. However, other species can be formed and in particular, hydroperoxides formed in the reactions of peroxy radicals with HO₂ :

$$
RO_2 + HO_2 \rightarrow ROOH + O_2 \quad (4)
$$

The participation of side oxidation products, like hydroperoxides, diols, gem-diols, ..., is
discussed below.

Formation of hydroperoxides

It has been postulated that hydroperoxides ROOH might contribute to the formation of
secondary aerosols in the atmosphere. Our experimental method is well adapted to generate
hydroperoxides from a well controlled chemical system and thus, we have tested the above
hypothesis.

As emphasised above (§ 3.1 - *Concentration of terpene*), reaction (4) can be favoured
in our system by lowering the terpene concentration until a competition occurs between
reaction (1) and reaction (5) :

$$
\mathrm{OH}+\mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{HO}_{2}
$$
In this way, we can produce HO₂ and RO₂ concentrations of the same order of magnitude, so that, in the absence of NO, reaction (4) is the fastest reaction that HO₂ and RO₂ undergo : k₄ ≈ 2 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for RO₂ radicals deriving from terpenes (measured in this work, see below), compared to k < 3 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for self and cross reactions of most RO₂ radicals [6,7].

Concerning the formation of particles, dramatic changes are observed when the concentration of terpenes is decreased in order to favour reaction (4) and the formation of hydroperoxides. Signals from scattered light (either seen by absorption or by diffusion at 90° angle) are strongly decreased and rapidly disappear whereas the concentration of particles indicated by the counter increases (strongly - up to 10⁹ cm⁻³ - in the case of γ-terpinene, to a lesser extent - above 10⁸ - in the case of α-pinene). This clearly shows that hydroperoxides are efficient precursors for nucleation. The disappearance of the scattered light signals reflects that the size of particles is smaller, since this signal is proportional to d⁶ (d being the diameter of particles).

The formation of particles is so efficient in such conditions that, even when the flow of terpene has been shut down, the counter shows that particles are still formed at significant concentrations - more than 10⁶ cm⁻³ - the small amount of terpene adsorbed on the walls and then released into the gas phase being sufficient to produce those particles (note that no more signals from scattered light, nor from RO₂ absorption, can be seen in those conditions). Unfortunately, we had no means for measuring the low terpene concentration in those experiments, in order to estimate whether the concentration was of the order of magnitude of that found in the atmosphere, but such a measurement may be possible in the future.

## 5. Modelling particle formation

The results obtained in the present work provide new data on the dynamics of particle formation, since it has been possible to record the growing of particles in real time. Our results have been modelled by the group of Strasbourg, using the theories of nucleation. The results are presented and discussed in the case of α-pinene, on the base of the particular example presented in Figure 2.

The first important point is that pinonaldehyde, the principal oxidation product of α-pinene in our experiments, cannot be the precursor of nucleation in this particular system. Indeed, the vapour pressure measured in Strasbourg is much larger than that expected to initiate nucleation in our experiments. Thus, only side products, having much lower vapour pressures, can be the precursors of nucleation. One of the possible product is the hydroperoxide since, as shown above, it can initiate particle formation at fairly low concentration. Another possibility is the diol formed from the terminating molecular channel of RO₂ self reactions. However, in the presence of NO, both the formation of hydroperoxides and diols is normally suppressed, whereas particle formation is still observed. In the absence of more information, we have assumed that the precursor is the gem-1,1-diol formed by complexation of pinonaldehyde with water (H₂O is always present beside H₂O₂ at a partial pressure of ≈ 3 mbar) :

$$R-CHO + H_2O \rightleftharpoons R-CH(OH)_2 \quad (6)$$
Calculations in Strasbourg have shown that the vapour partial pressure of the nucleation precursor should be a little less than $10^{-4}$ mbar, which seems to be a likely value for the possible candidates cited above.

The part of the process relevant to our observations is the condensation of pinonaldehyde, the main oxidation product, on the nucleus formed from the nucleation precursor. Condensation of pinonaldehyde and coagulation of particles have been modelled by the Strasbourg group and the results compared to our experimental observation. The details are given in the report of the Strasbourg group.

Comparing our results (Figure 2) and the results of modelling (Figure 9 of the Strasbourg group report) shows that the model can reproduce the experimental data quite satisfactorily. The growing rate and the delay between the laser pulse and the appearance of particle formation are well reproduced. In addition, the rapid increase of the delay with decreasing the initial radical concentration is also well predicted by the model.

The principal remaining problem is the identification of the species responsible of the nucleation process. This is apparently a general problem in the understanding of secondary aerosol formation. Those species are generally minor products and they are very difficult to identify among the bulk of products which have condensed.

# 6. Additional results obtained during the present work

## A new method for measuring the kinetics of $RO_2 + HO_2$ reactions

The experiments described above, performed at low terpene concentration in order to favour the formation of hydroperoxide, have led us to find out that a similar method can be used for measuring rate constants for $RO_2 + HO_2$ reactions, simply and accurately.

The kinetics of recombination cross reaction between radicals are generally difficult to investigate [6,7]. The first reason is that at least three reactions are in competition : the self reaction of both radicals and the cross reaction. In addition, in the case of peroxy radicals, $RO_2$, which reactions are generally investigated by flash or laser flash photolysis combined to UV absorption, the $RO_2$ (and $HO_2$) UV spectra generally overlap. This introduces an additional difficulty since absorption signals result from the contribution of all species, including reactants and products. It must also be taken into account that $RO_2$ radical self reactions may initiate complex reaction mechanisms, since new peroxy radicals are often generated.

In this new method, rate constant for $RO_2 + HO_2$ reactions are measured using laser flash photolysis, under conditions such that $HO_2$ is in large excess compared to the $RO_2$ radical. This is possible because our experimental set-up allows us to measure the very small signals corresponding to $RO_2$ radicals at low concentrations. This presents two significant advantages :

i. such a condition is similar to pseudo-first order condition in which the $RO_2$ concentration has not to be known accurately (nor its absorption spectrum) ; only the $HO_2$ concentration must be well characterised ; the $RO_2$ absorption cross section at 270 nm, where the radical is monitored, must be known, but only approximately.
ii. the reaction mechanism is extremely simplified since RO₂ radicals only react with HO₂ to form unreactive molecular products, essentially the hydroperoxide ROOH.

Peroxy radicals are monitored at 270 nm, where the absorption of HO₂ is negligible and the concentration of HO₂ is determined at 210 nm, using its well characterised absorption cross section [5]. A typical signal is shown in Figure 3, which includes the simulation of experimental traces, based on a reaction mechanism including all possible reactions but in which, only the HO₂ + HO₂ and RO₂ + HO₂ reactions play a significant role.

**Figure 3** : Typical decay traces obtained for measurements of RO₂ + HO₂ reaction rate constants (particular case of C₂H₅O₂ + HO₂). The dotted line represents what would be the C₂H₅O₂ decay in the absence of HO₂, showing that C₂H₅O₂ reacts almost exclusively with HO₂.

<figure><img src="image_7.png" /></figure>

A series of rate constants have been measured (or re-measured) for RO₂ radicals originating from various hydrocarbons (from methane to terpenes). It must be emphasised that this way of measuring rate constants for RO₂ + HO₂ reactions is very simple, reproducible and quickly done. The only difficulty we have encountered is that measurements of HO₂ concentrations were generally too small by 20 to 40 %. This was determined by comparing the values of rate constants measured in this work to those determined previously for some RO₂ radicals. This is probably a problem linked to the homogeneity of the laser beam, a problem which will be solved. Despite this difficulty, measurement were quite reproducible

All the results, obtained for a large variety of RO₂ radicals are listed in Table 1. The table includes, the raw data (k measured), the rate constants to which a systematic correction has been applied (reduction of the raw data by a factor of 1.3, in order to correct the problem linked to the HO₂ concentration, see above), and the available data from the literature for comparison.
<table><caption>**Table 1** - Rate constants for RO₂ + HO₂ reactions (298 K, units of 10⁻¹² cm³ molecule⁻¹ s⁻¹).</caption><thead><tr><th>RO<sub>2</sub> radical</th><th>k measured <sup>a</sup></th><th>k corrected <sup>a,b</sup></th><th>Literature value <sup>a</sup></th><th>Reference</th></tr></thead><tbody><tr><td colspan="5"><em>Radicals obtained by hydrogen abstraction</em></td></tr><tr><td>CH<sub>3</sub>O<sub>2</sub></td><td>6.6</td><td>5.0</td><td>5.2</td><td>5</td></tr><tr><td>C<sub>2</sub>H<sub>5</sub>O<sub>2</sub></td><td>10.0</td><td>7.7</td><td>7.8</td><td>5</td></tr><tr><td>cyclohexyl-O<sub>2</sub></td><td>25</td><td>19</td><td>17</td><td>7</td></tr><tr><td>neopentyl-O<sub>2</sub></td><td>16</td><td>12.3</td><td>15</td><td>7</td></tr><tr><td colspan="5"><em>Radicals obtained by OH addition to unsaturated compounds</em></td></tr><tr><td>HO-(C<sub>2</sub>H<sub>4</sub>)-O<sub>2</sub></td><td>14</td><td>10.8</td><td>13</td><td>7</td></tr><tr><td>HO-(C<sub>2</sub>H<sub>2</sub>Cl<sub>2</sub>)-O<sub>2</sub></td><td>15</td><td>11.5</td><td>10<sup>d</sup></td><td>9</td></tr><tr><td>HO-(TME)-O<sub>2</sub><sup>c</sup></td><td>22</td><td>17</td><td>≈20</td><td>10</td></tr><tr><td>HO-(cyclohexene)-O<sub>2</sub></td><td>35</td><td>27</td><td></td><td></td></tr><tr><td>HO-(α-pinene)-O<sub>2</sub></td><td>27</td><td>21</td><td></td><td></td></tr><tr><td>HO-(γ-terpinene)-O<sub>2</sub></td><td>26</td><td>20</td><td></td><td></td></tr><tr><td>HO-(d-limonene)-O<sub>2</sub></td><td>29</td><td>22</td><td></td><td></td></tr></tbody></table>

a) units of 10⁻¹² cm³ molecule⁻¹ s⁻¹; b) see text; c) TME for tetramethylethylene; d) for Cl-(C₂H₂Cl₂)-O₂; note that the RO₂ radicals originating from terpenes are not unique.

It is seen that corrected values agree with literature values within uncertainties, which are at least 20 % for this type of reactions. The trend, already observed, indicating that rate constants increase with the number of carbon atoms of the RO₂ radical, from 5 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ to an upper limit of about 2 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ [11], is confirmed by the present measurements. The interesting point, here, is that all RO₂ radicals originating from terpenes seem to exhibit rate constant values close to this upper limit. The higher value observed for the radical resulting from OH addition to cyclohexene is not understood. It should be confirmed, although the result was found to be reproducible. Nevertheless, the high rate constant values of 2 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ found in the case of terpenes for RO₂ + HO₂ reactions show that formation of hydroperoxides must be an efficient process in the case of terpene oxidation in the atmosphere.

### III. DEVIATIONS FROM TECHNICAL ANNEX AND REASONS:

There is no particular deviation from the initial programme. More data could have certainly been obtained if we did not have difficulties with the laser during the first year, which resulted in irreproducible results. However, it would not have significantly added to the conclusions. Note the interesting side result on the new way of measuring RO₂ + HO₂ reaction rate constants.

### IV. CONCLUSIONS

The principal issue of our work is to show that time resolved techniques, such as laser flash photolysis, can be used to monitor in real time the formation process of secondary
aerosols. In particular, it has been shown that such an experimental approach can provide valuable data for testing theories of nucleation.

Most of our data have been obtained with α-pinene. This compound was chosen because the oxidation mechanism is now fairly well established and vapour pressure data for pinonaldehyde, the principal oxidation product formed under our conditions, are available (measured in Strasbourg within the present project). Our collaboration with the group of Strasbourg, to model our experimental data has been quite fruitful. The model developed by the group has been able to reproduce our experimental data quite satisfactorily, showing thereby the validity of the model, despite the assumptions which had to be made. At the same time, it demonstrates that our experimental approach can provide valuable data for the validation of models.

This work, together with that of the Strasbourg group, must be considered as a preliminary work which has to be developed further in order to fully demonstrate the capacities of this new approach. This will be done in the future by analysing in more details the effects of various parameters (temperature, concentrations, relative humidity) and attempting to analyse the nature of nucleation precursors.

A consequence of the present work, is the development a new simple way of measuring rate constants for RO₂ + HO₂ reactions, which are known to be very important in the atmosphere. Using this method, we have shown that peroxy radicals originating from terpenes react very efficiently with HO₂, probably to form hydroperoxides. This was suspected from relationships between structure and reactivity which were recently established [7,11] and is now confirmed. We have also shown that hydroperoxides are very efficient precursors for particle formation, at least under the conditions used in the present work.

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## Publication of the work

The present work should result in two publications :

* Modelling the time resolved observation of secondary aerosol formation.

* A simple method for measuring rate constants for RO₂ + HO₂ reactions.