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\documentclass[acpd, online, hvmath]{copernicus}

\begin{document}\sloppy

\title{Consecutive reactions of aromatic--OH adducts with NO, NO$_{2}$ and O$_{2}$:
benzene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol
and aniline}

\author[1,2]{R.~Koch}
\author[1]{R.~Knispel}
\author[1]{M.~Elend}
\author[1,2]{M.~Siese}
\author[1,2]{C.~Zetzsch}

\affil[1]{Fraunhofer-Institute of Toxicology and Experimental Medicine,
Hannover, Germany}
\affil[2]{Atmospheric Chemistry Research Laboratory, University of Bayreuth,
Germany}



\correspondence{C.~Zetzsch (cornelius.zetzsch@uni-bayreuth.de)}

\runningtitle{Reaction of the OH-adducts of aromatics with NO, NO$_{2}$ and O$_{2}$}
\runningauthor{R.~Koch et al.}

\received{26 June 2006}
\accepted{26 July 2006}
\published{}


\firstpage{1}

\maketitle


\begin{abstract}

Consecutive reactions of adducts, resulting from OH radicals and aromatics,
with the tropospheric scavenger molecules O$_{2}$, NO and NO$_{2}$ have
been studied for benzene, toluene, m- and p-xylene, hexamethylbenzene,
phenol, m-cresol and aniline by observing decays of OH at temperatures
where the thermal back-decomposition to OH is faster than 3\,s$^{{-}1}$,
typically between 300 and 340\,K. The experimental technique was resonance
fluorescence with flash photolysis of water as source of OH. Biexponential
decays were observed in the presence of either O$_{2}$ or NO, and
triexponential decays were obtained in the presence of NO$_{2}$. The kinetic
analysis was performed by fitting the relevant rate constants of the
reaction mechanism to whole sets of decays obtained at various
concentrations of aromatic and scavenger. In the case of hexamethylbenzene,
the biexponential decays suggest the existence of the ipso-adduct, and the
slightly higher necessary temperatures show that it is even more stable.

In addition, smog chamber experiments at O$_{2}$ concentrations from
atmospheric composition down to well below 100\,ppm have been carried out for
benzene, toluene and p-xylene. The drop of the effective rate constant of
removal by OH occurs at reasonable O$_{2}$ levels, given the FP/RF results.
Comparison of the adduct reactivities shows for all aromatics of this study
that the reaction with O$_{2}$ predominates over that with NO$_{2}$ under
all tropospheric conditions, and that a reaction with NO may only occur
after the reaction with O$_{2}$.
\end{abstract}

\introduction

Aromatic hydrocarbons are a major class of anthropogenic compounds emitted
to the troposphere. Despite emission control measures, they still account
for 20 to 30{\%} of the non-methane hydrocarbons in urban air (see Chapter
I of the monograph by Calvert et al.~( 2002). As they react rapidly with OH
radicals, they have a strong influence on urban air quality. Their high
ozone creation potential is not yet fully understood and justifies further
work on their degradation mechanism (Jenkin et al., 2003; Bloss et al., 2004). This
paper summarises our work on the consecutive steps of OH-initiated
degradation of aromatic compounds in the presence of the typical atmospheric
radical scavengers NO, NO$_{2}$ and O$_{2}$.

The hydroxyl initiated degradation of aromatics and the respective reaction
mechanisms have been studied using various experimental techniques (see
reviews by Atkinson, 1989, 1994; and Atkinson and Arey, 2004). The kinetics
attributed to the formation of the addition complex, the equilibrium between
the adduct, aromatic--OH, and the educts, and the abstraction of an H atom
from the aromatic ring or from a substituent have been observed over a wide
range of temperature and pressure. They have been studied in our laboratory
as well (Wahner and Zetzsch, 1983; Rinke and Zetzsch, 1984; Witte et al., 1986)
and are fairly well understood.

In tropospheric chemistry, however, the competition of the consecutive
reactions of the adducts with scavenger molecules, such as NO, NO$_{2}$, O$_{2}$ and other atmospheric constituents, the possible
reaction pathways and resulting product distributions are of major interest.
These aspects have been studied by laser photolysis/laser absorption and
laser photolysis/UV absorption by Zellner et al.~(1985); Bohn and Zetzsch
(1999); Bohn (2001); Johnson et al.~(2002, 2005); Raoult et al.~(2004) and Grebenkin and
Krasnoperov (2004), by discharge-flow/laser induced fluorescence (Goumri et al.,
1990); in flash photolysis/resonance fluorescence studies by Perry et al.~(1977),
Zetzsch et al.~(1990) and Knispel et al.~(1990) for benzene, toluene and phenol; and in
product studies by Atkinson et al.~(1989), Atkinson and Aschmann (1994), Bethel
et al.~(2000), Berndt and B\"{o}ge (2001a, 2001b, 2006) and Volkamer et al.~(2002) and
Klotz et al.~(2002) for benzene, toluene, o- and p-xylene, hexamethylbenzene (HMB)
and 1,2,3- and 1,2,4-trimethylbenzene.

In the following, we present a kinetic study of the consecutive reactions of
aromatic--OH adducts in the presence of variable amounts of either NO,
NO$_{2}$ or O$_{2}$. Most of the results on benzene, toluene, p-xylene,
phenol and m-cresol were obtained using the VUV flash photolysis/resonance
fluorescence (FP/RF) technique, introduced by Stuhl and Niki (1972). The
data on benzene, toluene and phenol have been reported previously (Knispel
et al., 1990), but some of those measurements (benzene--OH and toluene--OH + O$_{2}$
and phenol--OH + NO$_{2})$ have been re-evaluated using improved
software-tools. The data on p-xylene, m-xylene, m-cresol and aniline have
been briefly reported by Koch et al.~(1994) and Zetzsch et al.~(1997) and are described
in more detail in the present study. The adduct reaction was furthermore
examined by a smog chamber technique in N$_{2}$ with continuous addition of
O$_{2}$ (Elend and Zetzsch, 1992) for benzene, toluene and p-xylene.

\section{Methods}

\subsection{Flash photolysis/resonance fluorescence setup}

The basic components are a flash lamp, a resonance lamp and a fluorescence
detector mounted at right angles to each other. In the following
description, emphasis is on improvements of both the set-up and evaluation
procedure over what has been used by Wahner and Zetzsch (1983) and Witte et al.~(1986).

\subsubsection{Data acquisition and hardware}

A multichannel scaler board (EG{\&}G Ortec, model ACE MCS) with 4096
channels is used on a PC, which also controls the triggering of the flash
lamp and the flows, pressures and temperatures of the experiment. The PC is
equipped with a multi-I/O card (Meilhaus PC30, 16x 12-bit A/D, 4x D/A, 24
bit I/O) supplemented by a card with 8 D/A converters (Bockstaller, PC8408).
Since multiplexing the temperature signals at the sensor level proved to be
unreliable, separate converter modules (Analog Devices, 5B34) for each Pt100
resistor were multiplexed at the 5-V level (Analog Devices, 5B02).

\subsubsection{Software running the experiment}

A TurboPascal program was developed featuring: low-level control of the
hardware; calculation of the settings necessary for intended concentrations
in the cell and \textit{vice versa}, applying calibration data for each device as well as
Antoine constants for the partial pressure in the saturators; graphical
presentation of the decay while it is summed up, together with a preliminary
exponential fit; a user interface allowing the operator to change all the
settings interactively; a batch language with one command for each menu
entry of the user interface and directives for calling subroutines, i.e.~other
batch files, for the convenient programming of measurement tasks. Batch
processing can be interrupted at any level of nested subroutines and can be
resumed after a problem has been fixed or continued with the current batch
file adapted to the latest findings.

\subsubsection{Precautions for semivolatile compounds and a wider temperature
range}

To avoid condensation of low-reactivity, low-vapour-pressure compounds, the
glass tubing from the thermostated saturator to the cell was heated up to
and including the needle valve where the gas expands from atmospheric to
cell pressure. The black anodized surface of the cell and the flanges has
been removed where it prevented gas-tight contact to o-rings, lowering the
leakage rate of the cell to 0.25\,mbar\,ml/min, 0.2\,ppm of the typical
buffer-gas flow. Viton o-rings, degassing reactive compounds at high
temperatures, were replaced by odorless silicone-rubber o-rings (Busak +
Luyken) leading to longer lifetimes of OH.

\subsubsection{Modifications of the flash lamp}

A quartz lens (Heraeus, Suprasil, $f_{\rm vis}$=50\,mm) mounted 9\,cm from the
spark focuses the VUV flash to a point well before the MgF$_{2}$ entrance
window. The aim was not to increase the radical concentration (see below)
but to widen the beam inside the cell: The volume, where H$_{2}$O is
photolysed, is wider than the field of view of the multiplier so that the
influence of diffusion and the slow flow through the cell on the decay rates
can be neglected during the first second after the flash at the typical
total pressure of 130\,mbar. A cuvette may be placed at the intermediate
focus in order to filter the VUV flash-light. This option was used in the
case of p-xylene where an interfering fluorescence signal lasting several
milliseconds after the flash was diminished by a 2-cm preabsorbing layer of
p-xylene vapour. Most experiments were performed using a flash energy of 2\,J, a few experiments (on m-xylene) were performed using an excimer laser
(EMG102, Lambda Physics) at 193\,nm with an energy fluence of 10\,mJ/cm$^{2}$
instead of the flash lamp, photolysing N$_{2}$O and converting the resulting
O($^{1}$D) to OH by the rapid reaction with H$_{2}$O. The experiments with
HMB were performed with a flash lamp of 0.5\,J, omitting the defocusing
quartz lens.

\subsubsection{Measures to increase the signal intensity}

The shape of the glass tube of the resonance lamp was modified to widen just
above and below the $\lambda $/4 resonator, which has been silver plated for
reducing losses of microwave power. The spreading plasma is concentrated
towards the point of efficient light collection by increasing the pressure
to several mbar, and the aperture of the lamp is approximately doubled. The
four collimating and focusing lenses, two for the lamp and multiplier each,
are antireflection-coated for 300\,nm. The interference filter towards the
multiplier has been replaced by one with slightly wider transmission curve
(FWHM, 9\,nm) but considerably higher peak transmission (47{\%} at 309\,nm,
Anders, Nabburg). The total effect was an increase of signal intensity from
40 to 190$\times$10$^{3}$ counts/s. Although the background increased
even more (from 5 to 40), the signal-to-noise ratio improved by a factor of
1.7, allowing us to decrease the concentration of water from 3 to
2$\times$10$^{15 }$\,cm$^{-3}$, which decreases the signal to
160$\times$10$^{3}$ counts/s. The initial OH concentration is now well
below 10$^{10 }$\,cm$^{-3}$ estimated by comparing signal intensities
(corrected for fluorescence quenching by H$_{2}$O): By titrating
6$\times$10$^{11}$\,cm$^{-3}$ NO$_{2}$ with excess H atoms generated by
193-nm photolysis of NH$_{3}$, OH is visible over 6~orders of magnitude.
Thus, build-up of reaction products, depletion of aromatics and
radical--radical-reactions are all negligible.

\subsubsection{Measures to stabilize the signal}

A microwave generator (Muegge, Reichelsheim, MW-GPRYJ1511-300-01, 2.45\,GHz,
300\,W) with a switching-mode power supply for the magnetron eliminated
100-Hz ripple, and intermittent flicker was remedied by supplying the
microwave power, typically 200\,W, to the lamp via a circulator (Philips,
Type 2722 163 02071), absorbing the reflected power. The influence of
possibly remaining low-frequency noise and drift is reduced by determining
the background as close as possible to where it is needed, i.e.~at the end of
the decay (recorded up to 5\,s). Typically, 50 to 150 scans are accumulated
for a decay curve. If necessary, repeated recordings are coadded after
visual inspection. The first one or two channels have to be omitted due to
an afterglow of the flash lamp, lasting about 1.5 milliseconds.

\subsubsection{Data reduction and evaluation}

During the slow decay into the noise of the already subtracted background,
binning the photons into the millisecond time slots of the multichannel
scaler is neither necessary nor appropriate. The irrelevant high-frequency
noise that is characteristic of the photon-counting technique would obscure
both, low-frequency noise and signal. Even worse, in a semi-log plot, where
negative values have to be omitted and small values are largely expanded,
the highest density of data points would appear above the mean value.
Therefore, the data is rebinned in a manner appropriate for decay curves
that are sums of exponentials: The interval width is doubled whenever the
elapsed time is 12 times the actual width, i.e.~every sixth interval. By this
procedure, the original 4096 data points are compressed into 62 values which
are roughly equidistant on a logarithmic time scale. It has been tested by
evaluation of synthetic, noisy, biexponential decays that even much coarser
binning would not lead to systematic deviations. This enables us to
demonstrate a dynamic range of three orders-of-magnitude in a clear plot. It
should be noted that, for the fitting procedure, it is not the plotted
function values of modelled decay curves that are compared to the coarsely
binned data, but the integrals of the curves over the very same time
intervals.

Reaction mechanism and kinetic analysis (FP/RF): The following reaction
mechanism, where A is the aromatic, AOH the adduct and S the scavenger, is
considered in the resonance fluorescence experiments:
\renewcommand{\theequation}{\arabic{equation}/--1}
\setcounter{equation}{0}
\begin{equation}
\chem{OH} + \chem{A} \leftrightarrow  \chem{AOH}
\end{equation}
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{1}
\begin{equation}
\chem{OH} + \chem{A} \rightarrow
\end{equation}
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{2}
\begin{equation}
\chem{OH} + \chem{S} \rightarrow
\end{equation}
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{3}
\begin{equation}
\chem{OH} \rightarrow
\end{equation}
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{4}
\begin{equation}
\chem{AOH} \rightarrow
\end{equation}
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{5}
\begin{equation}
\chem{AOH} + \chem{S} \rightarrow
\end{equation}
The differential equation governing the reversible formation of an adduct,
AOH, with loss processes on either side of the equilibrium is given as Eq.~(I):
\renewcommand{\theequation}{\Roman{equation}}
\setcounter{equation}{0}
\begin{eqnarray}
\lefteqn{\frac{{d}}{{{d}}t}[{\rm OH}]
=
-a[{\rm OH}]+b[{\rm AOH}]}\\ \nonumber
\lefteqn{\frac{{d}}{{{d}}t}[{\rm AOH}]
=
c[{\rm OH}]-d[{\rm AOH}]}
\end{eqnarray}
The coefficients $c$ and $b$ are the rate constants for adduct formation and
back-decomposition, Reaction~(1/--1), while $a$ and $d$ additionally contain the
irreversible loss processes, Reactions~(2) to (6):

$a$=$k_{4}$+($k_{1}$+$k_{2})$[A]+$k_{3}$ [S], $b$=$k_{-1}$$c$=$k_{1}$
[A] and $d$=$k_{1}$+$k_{5}$+$k_{6 }$[S], where [A] and [S] are the
aromatic and the scavenger concentration, respectively, and [AOH] is the
concentration of the thermolabile adduct, aromatic--OH. Note that the formal
Reaction~(2), which represents the part of $a$--$c$ which is proportional to
[A], contains the abstraction channel as well as the reaction of OH with
possible impurities of the aromatic. Reaction~(3) is used for NO or NO$_{2}$
as scavenger only, since O$_{2}$ has been tested to be pure enough not to
decrease the lifetime of OH. Reaction~(4) is called diffusion loss but also
contains reactions of OH with background impurities of the cell as well as
an offset error in [A]. Similarly, Reaction~(5) may comprise a unimolecular
isomerisation of the adduct as well as an offset error of the flow
controllers employed for the scavenger or a background of O$_{2}$ from
leaks.

The analytic solution of Eq.~(I), already given by Wahner and Zetzsch
(1983), is the sum of two exponential components $I{\times}{\rm {e}}^{-t/\tau
}$ for each of [OH]($t)$ and [AOH]($t)$ with common parameters $\tau
_{1}^{-1}$ and $\tau _{2}^{-1}$:
\renewcommand{\theequation}{\Roman{equation}a}
\setcounter{equation}{1}
\begin{equation}
\tau _{1,2}^{-1}  = \frac{a+d}{2}\pm \sqrt {\left( {\frac{a+d}{2}}
\right)^2-bc}
\end{equation}
The ratio of the amplitudes, $I_{1}$/$I_{2}$, for the OH part of the solution
is given by
\renewcommand{\theequation}{\Roman{equation}b}
\setcounter{equation}{1}
\begin{equation}
I_1 /I_2
=
\frac{(\tau _1^{-1} -d)^2}{bc}
\end{equation}
while for the adduct it is simply --1.

The coefficients $b$ and $c$ occur in Eq.~(II) only as a product, see Appendix.
This has no effect on the evaluation of the title reactions.

Note that while the dependence of $\tau _{2}^{-1}$ on the concentration
of the scavenger may appear linear for a given data set, an evaluation
according to $\tau _{2}^{-1}{=}k_{5}{+}k_{6}$[S] (implying the
approximation $\tau _{2}^{-1}{\approx}d)$ can be considerably in
error, especially in the case of NO as scavenger, due to Reaction~(3). The
influence of $k_{3}$ on $\tau _{2}^{-1}$ becomes marginal as
$I_{1}$/$I_{2}$ exceeds 100, but then, however, $\tau _{2}^{-1}$ is barely
visible in decays of OH.

\subsubsection{Fitting strategy}

The two exponentials are fitted simultaneously, using as fit parameters
either $\tau _{1}^{-1}$, $\tau _{2}^{-1}$ and $I_{1}$/$I_{2}$; or
$a$, $d$ and \textit{bc} (the former set can be converted into the latter by inverting Eqn.
(II), starting with $\tau _{1}^{-1}+\tau _{2}^{-1}$ and $\tau
_{1}^{{\-}1}\tau _{2}^{-1})$. We call such fits ``e-fits''. Sets of
biexponential decays obtained at various concentrations of aromatic and
scavenger lead to sets of $a$, $d$ and \textit{bc,} varying linearly with the concentrations.

We took a further step (Koch, 1992): Sets of biexponential decays were
analysed simultaneously by fitting the slopes (and diffusion offsets) of
$a$, $d$ and \textit{bc}, see Appendix, directly to the data. The Levenberg-Marquardt
algorithm (see Press et al., 1992) is employed in a comfortable Pascal program,
that allows to (i) adjust the time ranges for fitting either the background
or decaying parts, to (ii) e-fit decays in a batch with individual start
parameters calculated from [A] and [S], and to (iii) select subsets of
decays for k-fitting a bunch of model functions to them (in fact, the
functions may be watched ``dancing'' around the data if the PC is not too
fast). The result can be displayed on the screen as succession of residua;
written to a protocol file as set of rate constants with their variances and
covariances, and as table of exponential parameters to be compared to the
results of e-fits; and written as time profiles to a spreadsheet to be
plotted. All settings are saved for the next run of the program.

The benefit of this simultaneous fit (k-fit) is greatest for decays that are
not distinctly biexponential, i.e., if $\tau _{1}^{-1}$ and $\tau
_{2}^{-1}$ are separated by a factor of less than, say, 3. In these
cases, the exponential parameters would have large mutual covariances, i.e.,
different sets of parameters would describe the decay almost equally well.
The k-fit skips this shaky ground (see the Results section for an example).
Another benefit is the ease of including proper weights (the number of scans
divided by the modelled intensity including background integrated over each
time interval) with the result of realistic statistical error limits of the
rate constants obtained. Care has to be taken to ensure that the decays
differ only in parameters included in the model. Changes in the strongly
temperature-dependent back-decomposition, for example, are not treated as
such, since $k_{-1}$ is a common fit parameter. Instead, other fit parameters
would be influenced due to the covariances between them. On the other hand,
the k-fit copes with an unintended variation of reactant concentration,
which would be a problem for the classical two-step approach (analysis of
e-fit results obtained at constant [R] and varied [S]).

In the presence of NO$_{2}$, its rapid reaction with H atoms originating
from the photolysis of H$_{2}$O leads to an additional OH source-term and
thus to a third exponential component in the decays (see Results section).

\subsubsection{Chemicals}

Deionized and bidestilled water is used. Aromatics, except benzene and
toluene (Rathburn, $>$99.8{\%}, glass distilled grade), are obtained from
Aldrich (m- and p-xylene and HMB for analysis, $>$99{\%} (GC), phenol
redistilled, $>$99{\%}, m-cresol, 99{\%}, aniline, glass distilled grade,
$>$99.5{\%} (GC)) and are dispensed by the saturator method (Wahner and
Zetzsch, 1983), usually from the liquid phase except benzene and HMB.

Since impurities are transported and concentrated by a slowly progressing
solid-liquid phase-boundary, benzene had to be further purified by
freeze-pump-thaw cycles, until $k_{\rm OH}$ becomes stable, before it is
rapidly frozen for the last time. Concentrations of the aromatics were
chosen, depending on their $k_{\rm OH}$, for $\tau _{1}^{-1}$ to be between
100 and 200\,s$^{{-}1}$, and it has been checked that photolysis is
negligible by varying the flash energy between 1.6 and 5\,J, without a
considerable effect on decay rates.

All gases were obtained from Messer Griesheim. Oxygen ($>$99.998{\%}) is
used without further purification up to some 10$^{17 }$\,cm$^{-3}$. For the
lower concentrations, needed for the phenolics, mixtures of about 2000\,vpm
O$_{2}$ in He were produced using flow controllers and stored in a 2-l
cylinder. Up to 5${\times}$10$^{13}$\,cm$^{-3}$ NO is added from a 2000-vpm
mixture with Ar (specified by the manufacturer to be within $\pm $2{\%}) and
is purified from NO$_{2}$ impurities by a FeSO$_{4}{\times}$7H$_{2}$O
scrubber. As determined by H--to--OH conversion, the NO$_{2}$/NO ratio
decreases from about 2$\times$10$^{-2}$ to below 10$^{-4}$. To prevent the
formation of OH from peroxy radicals (the oxidation of background impurities
may proceed with less O$_{2}$ than of the aromatics), the buffer gas,
usually 130\,mbar of Ar ($>$99.998{\%}), is purified from traces of oxygen
using Oxisorb (Messer Griesheim). Up to 7$\times$10$^{11 }$\,cm$^{-3}$
NO$_{2}$ is added using permeation tubes (Vici Metronics) calibrated to $\pm
$2{\%} by monitoring their loss of mass.

\subsection{Smog chamber}

The removal of the aromatic--OH adducts by O$_{2}$ is investigated in a smog
chamber, described by Behnke et al.~(1988), with 2450\,l volume at 300\,K in N$_{2
}$ at atmospheric pressure at O$_{2}$ levels between about 50\,ppm and 21{\%},
as briefly reported by Elend and Zetzsch (1992). The N$_{2}$ gas, taken from
liquid nitrogen (Messer Griesheim), was determined by GC-ECD analysis to
contain less than 10\,ppm O$_{2}$. Low levels of O$_{2}$ are reached in the
smog chamber by flushing the O-ring seals with N$_{2}$ from outside. OH
radicals are produced by photolysis of H$_{2}$O$_{2}$ using 7
medium-pressure arc lamps (Metallogen HMI, Osram, 1200\,W). H$_{2}$O$_{2}$
(Peroxid-GmbH, $>$80{\%}, stabilised) is kept constant at about 500\,ppb by
continuously adding 1\,ppm/h from an impinger, containing 98\,wt-{\%}
H$_{2}$O$_{2}$ and flushed continuously to keep H$_{2}$O$_{2}$ constant.
Starting from different levels, O$_{2}$ is added continuously to the chamber
and determined at 30-min intervals by GC/ECD analysis of 0.2-ml gas samples
(Chrompack CP Sil 8 CB, 25\,m, 0.32\,mm inner diameter, 1.27\,$\mu $m film
thickness). The O$_{2}$ response is calibrated by chamber runs with
exponential dilution, checked for stability against an electrochemical
analyser (Oxanal, Gerhard GmbH, Blankenbach) before and after each chamber
run.

The aromatics and the reference compounds (n-butane, tetramethylbutane,
n-heptane and the inert perfluorohexane), initially 5--40\,ppb each, are
determined every 30\,min from 20-ml gas samples using cryogenic trapping
(Nolting et al., 1988) and GC/FID (Chrompack Al$_{2}$O$_{3}$ PLOT).

Peak areas are divided, sample per sample, by the area of the
perfluorohexane peak to correct for dilution of the chamber content during
the run. The remaining decrease is obviously consumption by OH, as the [OH]
profiles obtained from the reference compounds agree with each other within
5{\%}, if the $k_{\rm OH}$ values 1.10, 2.56 and 7.30$\times$10$^{-12
}$\,cm$^{3}$\,s$^{-1}$ are used for tetramethylbutane, n-butane and n-heptane,
respectively, which are well-established in our laboratory (Behnke et al., 1988).
These values meet the recommendations of Atkinson (1994) at 300\,K. Using the
average [OH]($t$), the apparent OH rate constant of the aromatic is then
calculated as a function of time and plotted against [O$_{2}$].

\section{Results}

\subsection{Aromatic--OH + O$_{2}$}

At the high O$_{2}$ levels of 10$^{17 }$\,cm$^{-3}$ needed to observe any
reaction with benzene--OH, part of the VUV is absorbed between entrance
window and detection volume, adding to the gradient of [OH]$_{t{=}0}$ due to
the divergence of the beam. It has been checked that, in the absence of
benzene, [OH]($t)$ does not depend on [O$_{2}$] except for its amplitude.
Oxygen atoms and ozone are formed in concentrations estimated to be
negligible.

For benzene--OH, the reaction rate constant with O$_{2}$ has been obtained
at four temperatures in the range 299 to 354\,K. In order to keep the slow
exponential component at a detectable level with decreasing temperature, the
rate of adduct formation (${\approx}\tau _{1}^{-1})$ must be reduced
with the rate of its thermal back-decomposition, thus limiting the range of
variation of $\tau _{2}^{-1}$. For this reason and due to the quenching
of the resonance fluorescence by O$_{2}$, the absolute error is higher at
299\,K, see Table~1. How the k-fit helps to include decays with $\tau_{2}^{-1}{\approx}\tau _{1}^{-1}$, becomes more obvious with
the stronger signals in the case of m-cresol where 10$^{15 }$\,cm$^{-3}$ of
O$_{2}$ are sufficient: Fig.~1 shows decays obtained at 334\,K with curves
representing the result of the k-fit. Note that the two steepest decays
differ considerably and therefore contain information about the target
reaction, but that the steepest one would be impossible to evaluate by an
e-fit due to instability (see Methods section). Figure~2 shows the case of
hexamethylbenzene, for which the temperature had to be raised by about 40\,K
compared to the other monoaromatics, indicating a higher stability of the
ipso-adduct. Results on the eight aromatic compounds of this study are listed in
Table~1.

Experiments had also been made with naphthalene (Koch and Zetzsch, 1994) at
temperatures well above 370\,K, where the OH-adduct starts to decompose on
the FP/RF time-scale. These suffer, however, not only from low signal
intensities due to quenching of the OH-fluorescence by O$_{2}$ but also from
sticky reaction products, which increase the background reactivity of the
cell and are photolysed by the flash to produce OH and other radicals (OH
decays were observed with only He flowing through the cell). Recent
measurements with a different flash lamp (10 instead of 50 nF capacitor,
spectrum limited by the MgF$_{2}$ window instead of a Suprasil lens for a
more effective photolysis of water compared to organics) and further
improved detection limit (polished microwave resonator, He instead of Ar in
the lamp) yielded apparent O$_{2}$ reactivities well below the former result
of 1.1$\times$10$^{-16 }$\,cm$^{3}$\,s$^{{-}1}$ and show a marked dependence on
H$_{2}$O concentration, i.e. radical and product level. Thus,
$k$(naphthalene--OH + O$_{2})$ at 400\,K may well be less than 10$^{-17
}$\,cm$^{3}$\,s$^{{-}1}$, in accordance with the negative temperature
dependence observed in experiments at 336 and 298\,K (Koch and Zetzsch, 1994)
employing cycling of radicals (Koch et al., 1994 and 1997).

\subsection{Aromatic--OH + NO$_{2}$}

In the presence of NO$_{2}$, the rapid reaction of H atoms (originating from
the photolysis of H$_{2}$O) with NO$_{2}$ generates additional OH and leads
to a third exponential component. The model is supplemented by the
respective rate constant, 1.3$\times$10$^{-10 }$\,cm$^{3}$\,s$^{-1}$ (Wagner and
Welzbacher, 1976; Michael et al., 1979), plus an H-atom diffusion term, and the
triexponential solution is used in the analysis. Note that in the case of
NO$_{2}$ as scavenger, coefficient $a$ contains a term proportional to
[NO$_{2}$]. For the respective rate constant (OH + NO$_{2 }$+ M),
measurements in the absence of aromatic yielded 2.4$\times$10$^{-12
}$\,cm$^{3}$\,s$^{-1}$ at 133\,mbar in Ar at 319\,K, in good agreement with the
literature (Paraskevopoulos and Singleton, 1988).

For most aromatics, the model matches the shape of the observed OH time
profiles quite well, see for example Fig.~2 in our previous paper (Knispel
et al., 1990). In the case of p-xylene, shown in Fig.~3, the deviations during the
first 30 milliseconds indicate a higher initial H-atom concentration. It
appears that the VUV photolysis of p-xylene leads to the formation of H
atoms. At $t{>}$60\,ms, however, the [OH]/[AOH] ratios, which influence the
effective loss rate depending on losses on both sides of the equilibrium,
are thought not to be distorted by the fast H--to--OH conversion, and that
part of the decays is used for a k-fit (with the parameters describing the
scavenger-free system held fixed). As can be seen in Fig.~3, the model
adequately describes the [NO$_{2}$]-dependence of $\tau _{2}^{-1}$. The
results on the six aromatic compounds studied with NO$_{2}$ are listed in
Table~1.

\subsection{Behaviour of the adducts in the presence of NO}

For the case of NO as scavenger, the rate constant of the interfering
reaction with OH is determined in the absence of aromatics to be 0.95$\times
$10$^{-12 }$\,cm$^{3}$\,s$^{-1}$ at 320--343\,K with 133\,mbar of Ar as third body,
in good agreement with literature data of Anastasi and Smith (1979) and
reviewed by Paraskevopoulos and Singleton (1988). This loss of OH increases
mainly $\tau _{1}^{-1}$ but also $\tau _{2}^{-1}$ and
$I_{1}$/$I_{2}$. As a compromise between reduced influence on, and
visibility of, the 2nd exponential, the concentration of the aromatic
is chosen, at any given temperature, for the adduct--to--OH ratio being about
10.

The exponential parameters with their mutual covariances are skipped by the
k-fit, from which 2-$\sigma $ upper limits for most of the adduct+NO
reactions of 3$\times$10$^{-14 }$\,cm$^{3}$\,s$^{-1}$ are obtained at about 330\,K.
The uncertainty is about threefold higher in the cases of phenol, due to
the higher branching ratio for its abstraction channel limiting the dynamic
range of $\tau _{2}^{-1}$, and p-xylene, due to the photolysis and
fluorescence of the aromatic. The cycling experiments with naphthalene (Koch
and Zetzsch, 1994) showed such a high yield of HO$_{2}$ that the radical
loss due to adduct+NO cannot be faster than 10$^{-13 }$\,cm$^{3}$\,s$^{{-}1}$.

\subsection{Smog chamber}

Five runs with all three aromatics (benzene, toluene and p-xylene) together,
two runs with benzene and one each with toluene and p-xylene were performed
at OH levels ranging from 5 to 10$\times$10$^{6}$\,cm$^{-3}$. Figure~4 shows a
run with p-xylene, where OH is produced from photolysis of H$_{2}$O$_{2}$ at
420\,ppb. The logarithmic scale applies to the organics while the
continuously added oxygen is plotted to the linear scale at the right. After
an initial, linear increase of O$_{2}$ by 7.6\,ppm/h (from very minor leaks
and decomposition of H$_{2}$O$_{2})$, O$_{2}$ is increased by 35\,ppm/h. The
exponential consumption of the reference compounds indicates [OH] to be
fairly constant while the consumption of p-xylene accelerates with
[O$_{2}$]. The quantitative evaluation yields a time profile of [OH]($t$),
reaching a stable level within less than a sampling period and slowly
increasing from 8.3 to 8.6$\times$10$^{6}$\,cm$^{-3}$ during the following 20\,h.

\conclusions[Discussion]

As can be seen in Table~1, the obtained adduct+NO$_{2}$-reaction rate
constants are not far from collision frequency and, therefore, do not show a
significant temperature dependence. On the other hand, the obtained O$_{2}$
reactivities are much lower and span a range of three orders of magnitude,
with each methyl substituent increasing the reactivity by about a factor of
3 (comparing benzene with toluene, m-xylene and finally HMB). The only cases
with a significant temperature dependence are the two extreme ones, benzene
and HMB, with positive and negative activation energy, respectively. The
factor of 3 also occurs between the adduct reactivities of phenol and
m-cresol, which are, however, more reactive than the corresponding
non-phenolic cases by a factor of 200. The following comparisons with
literature data are sorted by scavenger species.

\subsection{Reaction of the adduct with NO}

For the NO reactivity of benzene--OH (hydroxycyclohexadienyl, HCHD), Zellner
et al.~(1985) obtained a value of 1$\times $10$^{-12 }$\,cm$^{3}$\,s$^{-1}$ which
should be two orders of magnitude lower to fit within our upper limit and is
probably caused by NO$_{2}$ as impurity in the NO mixture, that was used
without purification. This applies as well to earlier results of Witte and
Zetzsch (1988) on both, benzene--OH and toluene--OH, of 6$\times$10$^{-13
}$\,cm$^{3}$\,s$^{-1}$, which were also not corrected for the influence of the
OH+NO reaction.

\subsection{Reaction of the adduct with NO$_{2}$}

Zellner et al.~(1985) studied also the NO$_{2}$ reaction of HCHD, fitted
exponentials to absorption profiles (of the adduct and possibly subsequent
products at 308.5\,nm) and obtained a rate constant which is a factor of 3
below the result of this study. Goumri et al.~(1990) report a value of (4$\pm
$2)$\times$10$^{-11 }$\,cm$^{3}$\,s$^{-1}$ for toluene--OH at 297\,K, and
Bjergbakke et al.~(1996) a value of (1.1$\pm $0.2)$\times$10$^{-11
}$\,cm$^{3}$\,s$^{-1}$ for HCHD at 338\,K. In the latter study, the radical level
was so high that almost half of the initial 0.2\,mbar NO$_{2}$ were consumed
and their stated observation of the peroxy radical has already been disputed
(Koch, 1997).

\subsection{The adduct reaction with O$_{2}$}

In an additional measurement, Zellner et al.~(1985) switched the bath gas (20\,mbar) from N$_{2}$ to O$_{2}$ and found a slight decrease of the slope of
$\tau _{2}^{-1}$ vs.~[NO] which could be explained by cycling of OH via
HO$_{2}$ from benzene oxidation. However, instead of an increased offset,
$\tau _{2}^{-1}$ at [NO]=0, hinting at an O$_{2}$ reaction, they mention
very slow decays even at 50\,mbar of O$_{2}$. Based on that paper, Atkinson
et al.~(1989) estimated an upper limit of 2$\times$10$^{-16 }$\,cm$^{3}$\,s$^{-1}$,
just including our result. Another experiment less than a factor of 2 off
the goal to detect a slow O$_{2}$ reaction is that of Perry et al.~(1977), who
came up with the rate constant for toluene--OH being less than or equal
1.0$\times$10$^{-15 }$\,cm$^{3}$\,s$^{-1}$.

More recently, several authors studied the transient absorption of HCHD in
the presence of O$_{2}$. Bohn and Zetzsch (1999) provided first evidence of
a fast equilibrium (7/--7) in the gas phase, predicted theoretically by Lay
et al.~(1996) and Ghigo and Tonachini (1998) but already observed in water by Pan
and Von Sonntag (1990),
\renewcommand{\theequation}{\arabic{equation}/--7}
\setcounter{equation}{6}
\begin{equation}
\chem{HCHD} + \chem{O_{2 }}\leftrightarrow  \chem{RO_{2 }}
\end{equation}
by observing a decrease of the HCHD signal with O$_{2}$ and a saturation of
Reaction~(6), which is now explained by assuming a loss process competing
with either Reaction~(7) or (--7). Johnson et al.~(2002) obtained an effective
$k_{6}$ of 5.5$\times$10$^{-16 }$cm$^{3}$s$^{-1}$, considerably higher than
observed in this work, 1.6$\times$10$^{-16 }$\,cm$^{3}$\,s$^{-1}$, and by Bohn
and Zetzsch (1999), 2.1$\times$10$^{-16 }$\,cm$^{3}$\,s$^{-1}$. Improvements of
the apparatus of Johnson et al.~(2002) by Raoult et al.~(2004) allowed the use of lower radical
concentrations which resulted in slower decays, corresponding to $k_{6}$=2.5$\times$10$^{-16 }$\,cm$^{3}$\,s$^{-1}$. These discrepancies are obviously
due to radical--radical reactions. Very recently, Grebenkin and Krasnoperov
(2004) obtained similarly shaped and timed decays, but proposed a
radical--radical Reaction~(8) instead of Reaction~(6) to explain the
increased loss rate with added O$_{2}$:
\renewcommand{\theequation}{\arabic{equation}}
\setcounter{equation}{7}
\begin{equation}
\chem{HCHD} + \chem{RO_{2 }}\rightarrow \quad {\rm products}
\end{equation}
Although they do not rule out Reaction~(6), they denigrate it for the lack
of a plausible mechanism for a reaction being both, rather slow and almost
independent of temperature. Their loss mechanism, however, would lead to a
strongly negative temperature dependence of the effective loss rate under
our experimental conditions on the left-hand side of (7/--7), in
contradiction to the slightly positive temperature dependence obtained for
benzene, see Table~1.

Note also that a loss due to Reaction~(8) would not saturate at high O$_{2}$
concentrations, as observed by Bohn and Zetzsch (1999), but would go through
a maximum at the point where [HCHD] = [RO$_{2}$]. Although Grebenkin and
Krasnoperov (2004) reach such conditions by their variation of temperature,
they did not vary the O$_{2}$ concentration. They are right in stating that
Reaction~(6) is not needed to model their data, but this is not a result but
a limitation of their experiment: It is hard to estimate to what extent
their decays, recorded not far enough into the uncertain background, are
hyperbolic due to Reaction~(8) or exponential due to a pseudo-first-order
reaction as Reaction~(6).

The parameter describing the possible influence of radical-radical
reactions on the evaluation of $k_{6}$ is not the radical level itself but
the radical-to-oxygen ratio. That ratio is much lower in our FP/RF
experiments than in the experiments by Grebenkin and Krasnoperov (2004) and
by Johnson et al.~(2002) and Raoult et al.~(2004) but even lower in the absorption experiments of our
laboratory (Bohn and Zetzsch, 1999). In that experiment, however, an
underlying background absorption with unknown formation kinetics has been
subtracted, influencing the simultaneous determination of $k_{6}$ and the
equilibrium constant (7/--7). The result obtained by Bohn (2001) for
toluene, (6.0$\pm $0.5$)\times$10$^{{-}16 }$\,cm$^{3}$\,s$^{-1}$, is in excellent
agreement with this work.

\subsection{Smog chamber experiments and product studies}

The radical-to-oxygen ratio is difficult to estimate for our chamber
experiments. In Fig.~5, the apparent rate constant for removal of the
aromatic by OH, $k_{\rm app}$, is plotted for all the runs of this study together
with model curves calculated according to Eq.~(III), where the addition
channel is reduced by the back-decomposition competing with O$_{2}$,
\renewcommand{\theequation}{\Roman{equation}}
\setcounter{equation}{2}
\begin{equation}
k_{\rm app}=k_{2}+k_{1} /(1 + k_{-1}/k_{6}[\chem{O_{2}}])
\end{equation}
using FP/RF results for adduct formation and back-decomposition and either
(solid lines) reasonable values for the abstraction channel derived from our
FP/RF data plus literature data at higher temperatures, or (dotted lines)
the total loss observed in the FP/RF experiments exceeding reasonable values
for diffusion (see Appendix).

Although the downward trend of $k_{\rm app}$ with decreasing O$_{2 }$ follows the
model curves and reaches, in the case of benzene, a fairly low value, the
data is obviously less reproducible than could be expected from the accurate
sample analysis. While the determination of [O$_{2}$] apparently contributes
to the scatter at low concentrations of O$_{2}$, this is unlikely to be
significant above 10$^{16 }$\,cm$^{-3}$. The additional loss processes may
well be reactions of the adduct with peroxy radicals. While the level of OH
radicals is only slightly higher than outdoors, the level of peroxy radicals
may exceed the value of 10$^{8 }$\,cm$^{-3}$ observed in the remote boundary
layer (Monks et al., 1998) by a factor of ten, since in our chamber, background
NO$_{\rm x}$ is even lower. Thus, the ratio may be similar as in our FP/RF
experiments, but another factor of about three should be applied to compare
the steady-state concentration in the chamber with the initial concentration
of the pulsed experiments. While our chamber experiments do show the drop of
the apparent OH reactivity at the right range of O$_{2}$ concentrations, a
heated chamber would allow experiments at higher O$_{2}$ concentrations or
with more reactive adducts than benzene--OH, due to the accelerated thermal
back-decomposition of the adduct.

The competition of O$_{2}$ with NO$_{2}$ takes place on an even faster time
scale. While it is negligible in the tropospheric environment, it becomes
important in high-NO$_{\rm x}$ chamber experiments. One can estimate the level
of NO$_{2}$, [NO$_{2}$]$_{1/2}$, where half of the adducts,
aromatic--OH, are scavenged by NO$_{2}$ instead of O$_{2}$. Table~2 compares
the [NO$_{2}$]$_{1/2 }$-levels calculated from Table~1 with values fitted to
NO$_{2}$-dependent product yields of chamber experiments of Volkamer et al.~(2002,
yields of phenol from benzene) and Bethel et al.~(2000, yields of
3-hexene-2,5-dione from p-xylene), respectively. The agreement is
reasonable. Very recently, the large yield of phenol (53{\%}) from benzene
in the absence of NO$_{2}$, reported by Volkamer et al.~(2002), has been confirmed
by Berndt and B\"{o}ge (2006), reporting 61{\%}. The observation of a yield
of 24{\%} phenol in the presence of NO$_{2 }$ at 9.6\,ppm by Atkinson et al.~(1989)
is not too far from the expected decrease with NO$_{2}$. Furthermore,
Atkinson and Aschmann (1994) re-analyzed the earlier data of Atkinson et al.~(1989) on nitrobenzene from benzene +OH and m-nitrotoluene from toluene + OH
as a function of NO$_{2}$ using the rate constants of Knispel et al.~(1990)
and found reasonable agreement of the experiment with calculated curves. In
the same study, Atkinson and Aschmann (1994) observed a decrease of the
yield of 2,3-butanedione from o-xylene with the expected curvature and a
[NO$_{2}$]$_{1/2 }$ value of about 2\,ppm, confirmed by an experiment with
fivefold decreased level of O$_{2}$. The decreasing yields of
2,3-butanedione from 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene with
increasing NO$_{2 }$ observed by Bethel et al.~(2000) can be taken as a hint
at a similar competition of NO$_{2 }$ with O$_{2}$ in the reaction with the
OH-adducts.

To conclude, the present work shows that the only relevant atmospheric
scavenger for aromatic--OH adducts is O$_{2}$. While the adduct+NO reaction
can be neglected even under high-NO$_{\rm x}$ simulation-chamber conditions,
NO$_{2}$ levels near 1\,ppm should be avoided in chamber experiments aimed at
the mechanism of tropospheric aromatic oxidation.

The mechanism of the effective Reaction~(6) is still unclear. It has to be
either an O$_{2}$ reaction competing with Reaction~(7), with phenol and
HO$_{2}$ as products as calculated ab initio by Estupinan et al.~(2003), or a unimolecular
reaction in competition to (--7), which may be decomposition to phenol and
HO$_{2}$ or an isomerisation to another intermediate. The intermediate must
immediately form HO$_{2}$ (possibly in a fast reaction with a further
O$_{2}$ molecule) as a high yield of prompt HO$_{2}$ is observed in our
FP/RF experiments employing cycling of radicals (Koch et al., 1994 and 1997). In
the tropospheric environment in the presence of NO$_{\rm x}$, the RO$_{2}$
radical formed from Reaction~(7) is known (Bohn and Zetzsch, 1999; Klotz et
al., 2002) to react rapidly with NO.


\appendix
\section{}

The coefficients $a$, $d$ and $b$ $\cdot $ c can be split into three terms
according to their concentration dependence:
\begin{equation}
 a =  a_{0 }+  a_{\rm A} {\rm [A]} +  a_{\rm S} {\rm [S]}
\end{equation}
\begin{equation}
 b \cdot   c = { bc_{\rm A}}{\rm [A]}
\end{equation}
\begin{equation}
 d = { d_{0 }}+  d_{\rm A} {\rm [A]} + d_{\rm S} {\rm [S]}
\end{equation}
To each group of biexponential decay curves (p,T=const; [A], [S] variable),
a set of effective rate constants: $a_{0}$, $a_{\rm A}$, $a_{\rm S}$, $bc_{\rm A}$,
$d_{0}$, $d_{\rm S }$ is fitted simultaneously. Thereby $a_{0}$=$k_{4}$,
$a_{S}$=$k_{3}$ and $d_{S}$=$k_{6}$ result directly from the evaluation;
any reaction of the adduct with the aromatic ($d_{\rm A})$ turns out to be
insignificant. The three constants $a_{\rm A}$=$k_{1}$+$k_{2}$, $bc_{\rm A}$=$k_{1}$ $k_{-1}$ and $d_0$=$k_{-1}$+$k_{5}$ contain four degenerate rate
constants. This degeneracy can be lifted by considering the limiting cases
at low and high temperature, and assuming Arrhenius behaviour for the
abstraction, the addition and the unimolecular decay ($k_{1}$, $k_{-1}$,
$k_{2})$, for the diffusion of OH ($k_{4})$ and for the diffusion of the
adduct ($k_{5})$. The quantity $k_{5}$, termed adduct diffusion, includes
any other thermal loss of the adduct, AOH, such as isomerisation and other
reaction channels not leading back to OH. Likewise, the quantity $k_{2}$,
termed abstraction, may include any impurity of the aromatics.

The coefficients $b$ and $c$ occur only as a product in Eq.~(II) with the
consequence that, by observing OH alone, there remains an uncertainty about
the assignment of observed radical loss to the two sides of the equilibrium
connected with a tradeoff between $b$ and $c$ (the product \textit{bc} is determined by the
data): If all the loss is due to the processes denoted Reaction~(2),
decreasing the contribution of $c$ to the slope of coefficient a versus [aromatic],
then $c$ is small and $b$ is as large as possible, $b=d$ in the absence of a
scavenger. On the other hand, the loss may be assigned to processes
competing with the back-decomposition of the adduct, i.e. Reaction~(5) denoted
adduct diffusion. In this case, the contribution of $b$ to $d$ decreases while $c$
increases. Note that in the case of added scavenger, $b$ contributes to the
$offset$ of $d$ only, while the evaluation of the target reaction ($k_{6 }$=$slope$ of $d)$ is
not affected. Note also that the coefficients $b$ and $c$, and thus the
equilibrium constant, would only be separable if one could measure absolute
concentrations of both OH and the adduct ($c$ occurs without an accompanying
factor $b$ in the adduct--to--OH ratio).

\begin{acknowledgements}
Support of this study by the Federal Ministry for Research and Technology
(grant 07 EU 705), and the Commission of the European Communities (grants
STEP 0007-C and RII3-CT-2004-505968-EUROCHAMP) and the Federal Ministry of
Environment is gratefully acknowledged.
\end{acknowledgements}

\begin{thebibliography}{}
\bibitem{}
Anastasi, C. and Smith, I. W. M.: Rate measurements of reactions of OH by
resonance absorption. Part 6. -- Rate constants for OH + NO(+M)$\rightarrow $HNO$_{2}$(+M)
over a wide range of temperature and pressure, J. Chem. Soc.
Faraday Trans.~II, 74, 1056--1064, 1978.
\bibitem{}
Atkinson, R.: Kinetics and mechanisms of the gas-phase reactions of the
hydroxyl radical with organic compounds, J. Phys. Chem. Ref. Data, Monograph
No. 1, 1989.
\bibitem{}
Atkinson, R.: Gas-phase tropospheric chemistry of organic compounds. A
review, Atmos. Environ., 24A, 1--41, 1990.
\bibitem{}
Atkinson, R.: Gas-phase tropospheric chemistry of organic compounds, J.
Phys. Chem. Ref. Data, Monograph No. 2, 1994.
\bibitem{}
Atkinson, R. and Aschmann, S. M.: Products of the gas-phase reactions of
aromatic hydrocarbons: Effect of NO$_{2}$ concentration, Int. J. Chem.
Kinet., 26, 929--944, 1994.
\bibitem{}
Atkinson, R. and Arey, J.: Atmospheric degradation of volatile organic
compounds, Chem. Rev., 103, 4605--4638, 2003.
\bibitem{}
Atkinson, R., Aschmann, S. M., Arey, J., and Carter, W. P. L.: Formation of
ring-retaining products from the OH radical-initiated reactions of benzene
and toluene, Int. J. Chem. Kinet., 21, 801--827, 1989.
\bibitem{}
Behnke, W., Holl\"{a}nder, W., Koch, W., Nolting, F., and Zetzsch, C.: A
smog chamber for studies of the photochemical degradation of chemicals in
the presence of aerosols, Atmos. Environ., 22, 1113--1120, 1988.
\bibitem{}
Berndt, T. and B\"{o}ge, O.: Gas-phase reaction of OH radicals with
benzene: products and mechanism, Phys. Chem. Chem. Phys., 3, 4946--4956,
2001a.
\bibitem{}
Berndt, T. and B\"{o}ge, O.: Rate constants for the gas-phase reaction of
hexamethylbenzene with OH radicals and H atoms and of 1,3,5-trimethylbenzene
with H atoms, Int. J. Chem. Kinet., 33, 124--129, 2001b.
\bibitem{}
Berndt, T. and B\"{o}ge, O.: Formation of phenol and carbonyls from the
reaction of OH radicals with benzene, Phys. Chem. Chem. Phys., 8, 1205--1214,
2006.
\bibitem{}
Bethel, H., Atkinson, R., and Arey, J.: Products of the gas-phase reactions
of OH radicals with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene: Effect
of NO$_{2}$ concentration, J. Phys. Chem., A, 104, 8922--8929, 2000.
\bibitem{}
Bjergbakke, E., Sillesen, A., and Pagsberg, P.: UV spectrum and kinetics of
hydroxycyclohexadienyl radicals, J. Phys. Chem., 5729--5736, 1996, Comment by
Koch, R., J. Phys. Chem. B, 101, 293, 1997.
\bibitem{}
Bloss, C., Wagner, V., Bonzanini, A., Jenkin, M. E., Wirtz, K.,
Martin-Reviejo, M., and Pilling, M. J.: Evaluation of detailed aromatic
mechanisms (MCMv3 and MCMv3.1) against environmental chamber data, Atmos.
Chem. Phys. Discuss., 4, 5683--5731, 2004, Interactive comment by Referee
{\#}2, ibid., S2391--2395, 2004.
\bibitem{}
Bohn, B.: Formation of peroxy radicals from OH--toluene adducts and O$_{2}$,
J. Phys. Chem. A, 105, 6092--6101, 2001.
\bibitem{}
Bohn, B. and Zetzsch, C.: Gas-phase reaction of the OH--benzene adduct with
O$_{2}$: Reversibility and secondary formation of HO$_{2}$, Phys. Chem.
Chem. Phys., 1, 5097--5107, 1999.
\bibitem{}
Calvert, J. G., Atkinson, R., Becker, K. H., Kamens, R. M., Seinfeld, J. H.,
Wallington, T. H., and Yarwood, G.: The mechanisms of atmospheric oxidation
of aromatic hydrocarbons, Oxford Univ. Press, 2002.
\bibitem{}
Elend, M. and Zetzsch, C.: The influence of oxygen on the apparent rate
constant for the reaction of OH with aromatics in smog chamber experiments
in nitrogen at one atmosphere, in: Laboratory studies on atmospheric
chemistry, air pollution research report 42 (Proc. CEC/EUROTRAC workshop,
York 1991), CEC, Brussels,  123--126, 1992.
\bibitem{}
Estupinan, E., Villenave, E., Raoult, S., Rayez, J. C., Rayez, M. T., and
Lesclaux, R.: Kinetics and mechanism of the gas-phase reaction of the
cyclohexadienyl radical c-C$_{6}$H$_{7}$ with O$_{2}$, Phys. Chem. Chem.
Phys., 5, 4840--4845, 2003.
\bibitem{}
Ghigo, G. and Tonachini, G.: Benzene oxidation in the troposphere:
theoretical investigation of the possible competition of three postulated
reaction channels, J. Amer. Chem. Soc. 120, 6753--6757, 1998.
\bibitem{}
Goumri, A., Sawerysyn, J.-P., Pauwels, J.-F., and Devolder P.: Tropospheric
oxidation of toluene: Reactions of some intermediate radicals, in:
Physico-chemical behaviour of atmospheric pollutants, edited by: Restelli, G. and Angeletti, G., Kluwer Acad. Publ., Dordrecht, 315--319, 1990.
\bibitem{}
Grebenkin, S. Y. and Krasnoperov, L. N.: Kinetics and thermochemistry of
the hydroxycyclohexadienyl radical reaction with O$_{2}$: C$_{6}$H$_{6}$OH +
O$_{2}{\rightarrow}$ C$_{6}$H$_{6}$(OH)OO, J. Phys. Chem. A, 108, 1953--1963, 2004.
\bibitem{}
Jenkin, M. E., Saunders, S. M., Wagner, V., and Pilling, M. J.: Protocol for
the development of the Master Chemical Mechanism, MCM v3 (Part B):
Tropospheric degradation of aromatic volatile organic compounds, Atmos.
Chem. Phys., 3, 181--193, 2003.
\bibitem{}
Johnson, D., Raoult, S. , Rayez, M.-T., Rayez, J.-C., and Lesclaux, R.: An
experimental and theoretical investigation of the gas-phase benzene--OH
radical adduct + O$_{2}$ reaction, Phys. Chem. Chem. Phys., 4, 4678--4686,
2002.
\bibitem{}
Johnson, D., Raoult, S., Lesclaux, R., and Krasnoperov, L.N.: UV absorption
spectra of methyl-substituted hydroxy-cyclohexdienyl radicals in the gas
phase, J. Photochem. Photobiol. A, 176, 98--106, 2005.
\bibitem{}
Klotz, B., Volkamer, R., Hurley, M. D., Sulbaeck Andersen, M. P., Nielsen,
O.J., Barnes, I., Imamura, T., Wirtz, K., Becker, K. H., Platt, U.,
Wallington, T. J., and Washida, N.: OH-initiated oxidation of benzene, Part
II. Influence of elevated NO$_{\rm x}$ concentrations, Phys. Chem. Chem.
Phys., 4, 4399--4411, 2002.
\bibitem{}
Knispel, R., Koch, R., Siese, M., and Zetzsch, C.: Adduct formation of OH
radicals with benzene, toluene, and phenol and consecutive reactions of the
adducts with NO$_{\rm x}$ and O$_{2}$, Ber. Bunsenges. Phys. Chem., 94,
1375--1379, 1990.
\bibitem{}
Koch, R.: Kinetische Untersuchung der Folgereaktionen der OH-Addukte mit NO,
NO$_{2}$ und O$_{2}$ mit simultaner Auswertung von Kurvenscharen, PhD
thesis, Univ.~of Hannover, 1992.
\bibitem{}
Koch, R., Bohn, B., and Zetzsch, C.: Cycles of HO$_{\rm x}$ as a tool to
investigate the tropospheric chemistry of aromatics in kinetic experiments,
in: The oxidizing capacity of the troposphere, physico-chemical behaviour of
atmospheric pollutants, edited by: Larsen, B., Versino, B., and Angeletti, G.,
European Commission, Brussels, 217--221, 1997.
\bibitem{}
Koch, R., Knispel, R., Siese, M., and Zetzsch, C.: Absolute rate constants
and products of secondary steps in the atmospheric degradation of aromatics,
in: Physico-chemical behaviour of atmospheric pollutants, edited by: Angeletti, G.
and Restelli, G.,  European Commission, Brussels/Luxemburg, 143--149,
1994.
\bibitem{}
Koch, R. and Zetzsch, C.: Cycling of OH radicals in the system
naphthalene/O$_{2}$/NO studied by FP/RF and LP/RF, presented at the
13th Int. Sympos. on Gas Kinetics, Dublin, 1994.
\bibitem{}
Lay, T. H., Bozzelli, J. W., and Seinfeld, J. H.: Atmospheric photochemical
oxidation of benzene: Benzene + OH and the benzene--OH adduct
(hydroxyl-2,4-cyclohexadienyl) + O$_{2}$, J. Phys. Chem., 100, 6543--6554,
1996.
\bibitem{}
Michael, J. V., Nava, D. F., Payne, W. A., Lee, J. H., and Stief, L. J.:
Rate constant for the reaction atomic hydrogen + nitrogen dioxide from 195
to 400\,K with FP-RF and DF-RF techniques, J. Phys. Chem., 83, 2818--2823,
1979.
\bibitem{}
Monks, P. S., Carpenter, L. J., Penkett, S. A., Ayers, G. P., Gillett, R.
W., Galbally, I. E., and Meyer, C. P. (Mick): Fundamental ozone
photochemistry in the remote marine boundary layer: The SOAPEX experiment,
measurement and theory, Atmos. Environ., 32, 3647--3664, 1998.
\bibitem{}
Nolting, F., Behnke, W., and Zetzsch, C.: A smog chamber for studies of the
reactions of terpenes and alkanes with ozone and OH, J. Atmos. Chem., 6,
47--59, 1988.
\bibitem{}
Pan, X. M. and von Sonntag, C.: Hydroxyl-radical-induced oxidation of benzene
in the presence of oxygen: R$\cdot $ + O$_{2}{\leftrightarrow} $RO$_{2}\cdot$ equilibria in aqueous solution. A pulse radiolysis study,
Zeitschrift fuer Naturforschung, B: Chemical Sciences, 45, 1337--1340, 1990.
\bibitem{}
Paraskevopoulos, G. and Singleton, D. L.: Reactions of OH radicals with
inorganic compounds in the gas phase, Rev. Chem. Intermed., 10, 139--218,
1988.
\bibitem{}
Perry, R. A., Atkinson, R., and Pitts, Jr. J. N.: Kinetics and mechanism of
the gas phase reaction of OH radicals with aromatic hydrocarbons over the
temperature range 296--473\,K, J. Phys. Chem., 81, 296--304, 1977.
\bibitem{}
Press, W. H., Teukolsky, S. A., Vetterling, W. T., and Flannery, B. P.:
Numerical recipes in FORTRAN: The Art of Scientific Computing, Cambridge
Univ. Press, 1992.
\bibitem{}
Raoult, S., Rayez, M.-T., Rayez, J.-C., and Lesclaux, R.: Gas-phase
oxidation of benzene: Kinetics, thermochemistry and mechanism of initial
steps, Phys. Chem. Chem. Phys., 6, 2245--2253, 2004.
\bibitem{}
Rinke, M. and Zetzsch, C. : Rate constants for the reactions of OH-radicals
with aromatics: benzene, phenol. aniline and 1,2,4-trichlorobenzene, Ber.
Bunsenges. Phys. Chem., 88, 55--62, 1984.
\bibitem{}
Stuhl, F. and Niki, H.: Flash photochemical study of the reaction OH+NO+M
using resonance fluorescent detection of OH, J. Chem. Phys., 57, 3677--3679,
1972.
\bibitem{}
Volkamer, R., Klotz, B., Barnes, I., Imamura, T., Wirtz, K., Washida, N.,
Becker, K. H., and Platt, U.: OH-initiated oxidation of benzene, Part I.
Phenol formation under atmospheric conditions, Phys. Chem. Chem. Phys., 4,
1598--1610, 2002.
\bibitem{}
Wagner, H. G., Welzbacher, U., and Zellner, R.: Rate measurements for the
reactions H + NO$_{2}{\rightarrow}$ OH + NO and H + NOCl $\to $ HCl + NO by
Lyman-alpha fluorescence, Ber. Bunsenges. Phys. Chem., 80, 1023--1027, 1976.
\bibitem{}
Wahner, A. and Zetzsch, C.: Rate constants for the addition of OH to
aromatics (benzene, p-chloroaniline and o-, m- and p-dichlorobenzene) and
the unimolecular decay of the adduct. Kinetics into a quasi-equilibrium, J.
Phys. Chem., 87, 4945--4951, 1983.
\bibitem{}
Wallington, T. J., Neuman, D. M., and Kurylo, M. J.: Kinetics of the gas phase
reaction of hydroxyl radicals with ethane, benzene and a series of
halogenated benzenes over the temperature range 234--438\,K, Int. J. Chem.
Kinet., 19, 725--739, 1987.
\bibitem{}
Witte, F., Urbanik, E., and Zetzsch, C.: Temperature dependence of the rate
constants for the addition of OH to benzene and to some monosubstituted
aromatics (aniline, bromobenzene and nitrobenzene) and the unimolecular
decay of the adducts. Kinetics into a quasi-equilibrium 2, J. Phys. Chem.,
90, 3251--3259, 1986.
\bibitem{}
Witte, F. and Zetzsch, C.: Annual report of the steering committee of
LACTOZ, a joint EUROTRAC/COST 611 project, EUREKA/CEC, Brussels, 62--66,
1988.
\bibitem{}
Zellner, R., Fritz, B., and Preidel, M.: A cw UV laser absorption study of
the reactions of the hydroxy-cyclohexadienyl radical with NO$_{2}$ and NO,
Chem. Phys. Lett., 121, 412--416, 1985.
\bibitem{}
Zetzsch, C., Koch, R., Siese, M., Witte, F., and Devolder, P.: Adduct
formation of OH with benzene and toluene and reaction of the adducts with NO
and NO$_{2}$, in: Physico-Chemical Behaviour of Atmospheric Pollutants, edited by: Restelli, G. and Angeletti, G.,
Kluwer Acad. Publ., Dordrecht, 320--327,
1990.
\bibitem{}
Zetzsch, C., Koch, R., Bohn, B., Knispel, R., Siese, M., and Witte, F.: Adduct
formation of OH with aromatics and unsaturated hydrocarbons and consecutive
reactions with O$_{2}$ and NO$_{\rm x}$ to regenerate OH, in: Chemical Processes
in Atmospheric Oxidation, edited by: Le Bras, G., Springer, Berlin, 1997, 247--256.
\end{thebibliography}

\clearpage

\begin{table}[t]
\caption{Rate constants for reactions of adducts, aromatic--OH, with O$_{2}$
and NO$_{2}$ obtained by FP/RF at various temperatures (T in K), employing
flash photolysis of H$_{2}$O at 130\,mbar in Ar.}
\vskip4mm
\begin{center}
\scalebox{.67}[.67]{
\begin{tabular}{p{59pt}lllllllp{68pt}}
\tophline
$k$ (cm$^{3}$\,s$^{-1})$&
Benzene$^{\rm a}$&
Toluene$^{\rm a}$&
m-Xylene$^{\rm c}$&
p-Xylene&
HMB$^{\rm d}$&
Aniline&
Phenol$^{\rm a}$&
m-Cresol \\
\hhline
\raisebox{-1.50ex}[0cm][0cm]{$\mbox{k}_{\mbox{O2}}^{\mbox{add}}/10^{-16}$}&
1.6$\pm $0.6$^{\rm b}$ (299) \newline
&
5.6$\pm $1.5$^{\rm b}$ (299) \newline
&
18$\pm $5 (303) \newline
&
8.8$\pm $2.5 (298) \newline
&
&
6$\pm $2 (320) \newline
&
300$\pm $70 (323) \newline
&
640$\pm $150 (317) \newline
 \\
 &
2.1$\pm $0.4$^{\rm b}$ (314) \newline
&
5.6$\pm $0.6$^{\rm b}$ (321) \newline
&
16$\pm $3 (323) \newline
&
8.3$\pm $2.0 (308) \newline
&
&
15$\pm $5 (340) \newline
&
260$\pm $60 (333) \newline
&
750$\pm $150 (319) \newline
 \\
&
3.0$\pm $0.3$^{\rm b}$ (333)
&
5.6$\pm $0.6 (339)

&
&
8.7$\pm $1.5 (319)
&
1800$\pm $300&
9.6$\pm $3 (350)
&
270$\pm $50$^{\rm e }$ (337)
&
$>800$ (324)
 \\
 &&&&&(355)&&&\\
 \\
&
3.7$\pm $0.4$^{\rm b}$ (354)
&
5.3$\pm $0.7 (347)
&
&
7.1$\pm $1.0 (327)
&
&
&
290$\pm $60 (343)
&
820$\pm $100 (334)
\\
 \\
&
&
5.9$\pm $0.8 (354)
&
&
8.8$\pm $1.5 (332)
&
1200$\pm $200
&
&
270$\pm $60 (353)
&
800$\pm $100 (350)
 \\
 &&&&&(385)&&&\\
 \\
&
&
&
&
&
&
&
360$\pm $50$^{\rm e }$(363) \newline
&
 \\
 \hhline
\raisebox{-1.50ex}[0cm][0cm]{$\mbox{k}_{\mbox{NO2}}^{\mbox{add}}/10^{-11}$}&
2.75$\pm $0.4 (305) \newline
&
3.6$\pm $0.5 (300) \newline
&
&
3.5$\pm $0.5 (317) \newline
&
&
5$\pm $2 (330) \newline
&
3.4$\pm $0.6$^{\rm b}$ (331) \newline
&
4$\pm $1 (317) \newline
 \\
 &
2.45$\pm $0.3 (320) \newline
&
3.6$\pm $0.4 (311) \newline
&
&
2.9$\pm $0.5 (330) \newline
&
&
&
4.1$\pm $0.7$^{\rm b}$ (354) \newline
&
4$\pm $1 (325) \newline
 \\
&
2.5$\pm $0.4 (333) \newline
&
3.6$\pm $0.4 (320) \newline
&
&
&
&
&
&
 \\
 \\
&
2.5$\pm $0.4 (349) \newline
&
4.0$\pm $0.6 (338) \newline
&
&
&
&
&
&
 \\
\bottomhline
\end{tabular}}
\end{center}
$^{\rm a}$Published before (Knispel et al., 1990),\\
$^{\rm b}$re-evaluated (Koch, 1992),\\
$^{\rm c}$laser photolysis of N$_{2}$O in He,\\
$^{\rm d}$250\,mbar of He,\\
$^{\rm e}$30 to 300\,mbar Ar.
\end{table}



\clearpage

\begin{table}[t]
\caption{ Scavenging by NO$_{2}$ calculated from FP/RF data and observed in
smog chambers for the OH-adducts of benzene, toluene and p-xylene.}
\vskip4mm
\begin{center}
\begin{tabular}{cccc}
\tophline
\raisebox{-1.50ex}[0cm][0cm]{Aromatic}&
\multicolumn{3}{c}{[NO$_{2}$]$_{1/2 }$(ppm)}  \\ [1mm]
\cline{2-4} \\ [-2.5mm]
 &
FP/RF&
\multicolumn{2}{c}{Chamber}  \\
\hhline
Benzene&
1.3&
${\sim}2^{\rm a}$&
Volkamer et al.~(2002) \\
Toluene&
3.3&
$^{\rm a}$&
 \\
p-Xylene&
5.1&
4.3&
Bethel et al.~(2000) \\
\bottomhline
\end{tabular}
\end{center}
$^{\rm a}$See also discussion by Atkinson and Aschmann 1994.
\end{table}

\clearpage

\begin{figure}
\vspace*{2mm}
\center\includegraphics[width=12cm]{acpd-2006-0191-f01}
\caption{Biexponential decays of OH in the presence of m-cresol and O$_{2}$
in 127\,mbar of Ar at 334\,K. The curves represent the result of a fit of the
title reaction to 12\,OH decays (these 6 and 6 similar ones), where the
parameters describing the O$_{2}$-free system are fixed to values determined
from another set of decays with and without m-cresol.}
\end{figure}

\clearpage
\begin{figure}
\vspace*{2mm}
\center\includegraphics[width=12cm]{acpd-2006-0191-f02}
\caption{Biexponential decays of OH in the presence of HMB and O$_{2}$ in
250\,mbar of He at 385\,K. The curves represent the result of a fit of the
title reaction to 10\,OH decays (these 6 and 4 similar ones at [HMB] =
1.0$\times$10$^{12 }$\,cm$^{-3})$.}
\end{figure}

\clearpage
\begin{figure}
\vspace*{2mm}
\center\includegraphics[width=12cm]{acpd-2006-0191-f03}
\caption{Triexponential decays of OH in the presence of p-xylene (5$\times$10$^{12}$\,cm$^{-3})$ and NO$_{2}$ in 130\,mbar of Ar at 317\,K. The
triexponential model curves represent the result of a global fit. Obviously,
the [NO$_{2}$]-dependence of the final slopes is well described by the model
while the earlier parts of the decays had to be excluded from the fit (see
text).}
\end{figure}

\clearpage
\begin{figure}
\vspace*{2mm}
\center\includegraphics[width=12cm]{acpd-2006-0191-f04}
\caption{Chamber experiment on p-xylene. Dilution-corrected time profiles
of the organic compounds (to the logarithmic ppb scale) and [O$_{2}$]
(linear ppm scale). Lights were switched on at $t$=2.4\,h, and continuous
addition of O$_{2 }$ starts at 4.5\,h. Note the curvature observed for
p-xylene, indicating an O$_{2}$-dependent rate constant, in contrast to the
alkanes.}
\end{figure}
\clearpage
\begin{figure}
\vspace*{2mm}
\center\includegraphics[width=12cm]{acpd-2006-0191-f05}
\caption{Rate constants for the removal of aromatics by OH in nitrogen as a function of [O2] up to
atmospheric composition, 300\,K, 1\,bar. Model curves are calculated according to Eq.~(III) using rate
constants obtained with FP/RF (see text).}
\end{figure}


\end{document}
