Simultaneous measurements of CH3O2 radical concentrations have
been performed using two different methods in the Leeds HIRAC (Highly
Instrumented Reactor for Atmospheric Chemistry) chamber at 295 K and in
80 mbar of a mixture of 3:1He/O2 and 100 or 1000 mbar of synthetic
air. The first detection method consisted of the indirect detection of
CH3O2 using the conversion of CH3O2
into CH3O by
excess NO with subsequent detection of CH3O by fluorescence assay by
gas expansion (FAGE). The FAGE instrument was calibrated for CH3O2
in two ways. In the first method, a known concentration of CH3O2
was generated using the 185 nm photolysis of water vapour in synthetic air
at atmospheric pressure followed by the conversion of the generated OH
radicals to CH3O2 by reaction with CH4/O2. This
calibration can be used for experiments performed in HIRAC at 1000 mbar in
air. In the second method, calibration was achieved by generating a near
steady state of CH3O2 and then switching off the photolysis lamps
within HIRAC and monitoring the subsequent decay of CH3O2, which
was controlled via its self-reaction, and analysing the decay using second-order kinetics. This calibration could be used for experiments performed at
all pressures. In the second detection method, CH3O2 was
measured directly using cavity ring-down spectroscopy (CRDS) using the
absorption at 7487.98 cm-1 in the A←X (ν12) band
with the optical path along the ∼1.4 m chamber diameter.
Analysis of the second-order kinetic decays of CH3O2 by
self-reaction monitored by CRDS has been used for the determination of the
CH3O2 absorption cross section at 7487.98 cm-1, both at 100 mbar of air and at 80 mbar of a 3:1He/O2 mixture, from which σCH3O2=(1.49±0.19)×10-20 cm2 molecule-1 was determined for both pressures. The absorption spectrum
of CH3O2 between 7486 and 7491 cm-1 did not change shape when
the total pressure was increased to 1000 mbar, from which we determined that
σCH3O2 is independent of pressure over the pressure range
100–1000 mbar in air. CH3O2 was generated in HIRAC using either
the photolysis of Cl2 with UV black lamps in the presence of CH4 and O2 or the photolysis of acetone at 254 nm in the presence of O2. At 1000 mbar of synthetic air the correlation plot of
[CH3O2]FAGE against [CH3O2]CRDS gave a
gradient of 1.09±0.06. At 100 mbar of synthetic air the FAGE–CRDS correlation plot had a gradient of 0.95±0.024, and at 80 mbar of
3:1He/O2 mixture the correlation plot gradient was 1.03±0.05.
These results provide a validation of the FAGE method to determine
concentrations of CH3O2.
Introduction
Methyl peroxy (CH3O2) radicals are important intermediates during
atmospheric oxidation (Orlando and Tyndall, 2012) and
combustion chemistry (Zador et al., 2011), and they are
produced mainly by the oxidation of CH4 and larger hydrocarbons
followed by the termolecular reaction between the CH3 radical, O2 and a third body M (Reaction R1).
CH3+O2+M→CH3O2+M
In environments influenced by anthropogenic NOx emissions,
CH3O2 predominantly reacts with NO to produce NO2 and
CH3O (Reaction R2).
CH3O2+NO→CH3O+NO2
CH3O subsequently reacts with O2 (Reaction R3) to generate HO2, which in turn oxidises another NO molecule to NO2 (Reaction R4). The subsequent photolysis of NO2 leads to the formation of tropospheric ozone, an important constituent of photochemical smog.
R3CH3O+O2→CH2O+HO2,R4HO2+NO→OH+NO2.
In remote, clean environments, i.e. under low-NOx levels,
CH3O2 is significantly removed by its self-reaction (Reaction R5)
and the cross-reactions with HO2 and other organic peroxy radicals
(RO2) (Tyndall et al., 2001).
R5aCH3O2+CH3O2→CH3OH+CH2O+O2,R5bCH3O2+CH3O2→CH3O+CH3O+O2.
Recently the reaction of CH3O2 with OH was measured to be fast (Fittschen, 2019; Yan, 2016) and provides an
additional loss route for CH3O2 under low-NOx conditions (Fittschen et al., 2014; Assaf et al., 2017). As CH3O2 is formed
by the oxidation of CH4, one of the most abundant tropospheric trace
gases, as well as by the oxidation of other volatile organic compounds, it
is predicted by numerical models to be the most abundant RO2 species in
the atmosphere. Although CH3O2 has not (yet) been selectively
measured in the atmosphere, its concentration has been estimated using
atmospheric models to peak at ∼107–108 molecule cm-3 during the daytime (Whalley et al., 2010, 2011, 2018).
At present, CH3O2 is not measured selectively in the atmosphere by
any direct or indirect method. The sum of HO2 and all RO2 species,
HO2+∑i[RO2,i], and,
separately, the sum of RO2, ∑i[RO2,i], have been measured in the
atmosphere using a range of indirect methods. Onel et al. (2017a) presents
an overview of these methods, such as the peroxy radical chemical amplifier
(PERCA) (Cantrell et al., 1984; Hernandez et al., 2001; Green et al.,
2006; Miyazaki et al., 2010; Wood et al., 2017), ROx chemical conversion – CIMS (chemical ionisation mass spectrometry) (ROxMAS) (Hanke et
al., 2002) and ROx chemical conversion – LIF (laser-induced
fluorescence) (ROxLIF) (Fuchs et al., 2008; Whalley et al., 2013).
ROxLIF uses LIF detection of OH at low pressure, known as fluorescence
assay by gas expansion (FAGE) and has been employed for partially speciated
RO2 detection, distinguishing between the sum of alkene, aromatic and
long-chain alkane-derived RO2 radicals and the sum of short-chain
alkane-derived RO2 radicals (Whalley et al., 2013, 2018).
CIMS methods using reagent ions such as H3O+(H2O)n,
NO3- and NH4+ have been employed in the simultaneous and
selective detection of RO2 in a number of recent studies (Noziere
and Hanson, 2017; Noziere and Vereecken, 2019; Hansel et al., 2018; Jokinen et al., 2014). Volatile small RO2 radicals such as CH3O2 have
been selectively measured in CIMS laboratory experiments with detection
limits between ∼1×108 and 1×109 molecule cm-3 (Noziere and Hanson, 2017). CIMS with
NO3- reagent ion has been employed in field measurements to record
diurnal profiles of some highly oxygenated low-vapour-pressure RO2
radicals produced in the ozonolysis of monoterpenes peaking at a few
107 molecule cm-3 (Jokinen et al., 2014).
Many of the early laboratory studies of the CH3O2 radical
reactions employed UV-absorption spectroscopy to monitor the B←X band centred around 240 nm, which is common to alkyl RO2 species (Wallington et al., 1992; Tyndall et al., 2001). The similarity of the
broad featureless UV-absorption spectra of RO2 radicals made it
challenging to distinguish between the individual RO2 species,
particularly in a mixture (Orlando and Tyndall, 2012). The
sensitivity of UV-absorption spectroscopy is quite low; for example, a
minimum detectable absorption of 5×10-3, corresponding to 4×1012 molecule cm-3CH3O2, was reported (Sander and Watson, 1980). The A←X electronic transition of RO2 in the near IR (NIR) displays more
structured spectra than the UV region, allowing for a selective identification
of RO2 radicals. However, the A←X transition is weaker than
the B←X transition, and multipass arrangements have been used to
improve the detection sensitivity. A step-scan Fourier transform infrared
spectrometer (Huang et al., 2007) operated using a
multipass White cell has been used to detect a number of RO2 species,
including CH3O2, with a typical minimum detectable absorbance of
∼1×10-4, corresponding to a limit of
detection (LOD) of ∼1×1013 molecule cm-3
for most RO2 species studied. The use of cavity ring-down spectroscopy
(CRDS) further improves the sensitivity of the RO2 detection due to the
significantly longer path lengths that can be realised and to the coupling of
high-performance NIR lasers, detectors and optical components. For example,
an absorbance detection limit of less than 1×10-6 has been
obtained by using cavity mirrors of a maximum reflectivity of 99.995 % (Atkinson and Spillman, 2002).
The CRDS technique has been used under both ambient and jet-cooled
conditions to provide insight into the molecular structure of
CH3O2 and more complex RO2, as well as to selectively measure
[RO2] in the laboratory (Sharp et al., 2008; Kline and Miller,
2014; Pushkarsky et al., 2000; Farago et al., 2013; Atkinson and Spillman,
2002; Sprague et al., 2013). Good agreement has been found between the
experimental spectrum of CH3O2 in the range between
∼7200–8600 cm-1 (∼1.18–1.40 µm)
measured using pulsed CRDS at typically 200 mbar of N2/O2=1.5:1.0 and theoretical predictions (Chung et al., 2007; Sharp et al.,
2008). The origin band of the A←X transition has been located at
7382.8 cm-1 and a value of 2.7×10-20 cm2 molecule-1 has been estimated for the absorption cross section at
this wavenumber (Pushkarsky et al., 2000; Chung et al., 2007). A weaker
absorption band has been found at 7488 cm-1 and assigned to a
transition involving the methyl torsion (ν12) (Pushkarsky et
al., 2000; Chung et al., 2007). By using the CH3O2 spectrum
measured by Pushkarsky et al. (2000) from 7300 to 7700 cm-1, which covers
both the origin band and the band involving the methyl torsional mode, a
value of ca. 1.0×10-20 cm2 molecule-1 is
estimated for the maximum cross section for the ν12 transition,
σmax(ν12). A few years later, Atkinson
and Spillman (2002) measured σmax(ν12)=(1.5±0.8)×10-20 cm2 molecule-1 at 27 mbar N2/O2=4:1 using continuous-wave (cw) CRDS. Very recent cw-CRDS
studies reported σmax(ν12)=2.2×10-20 cm2 molecule-1 at 67 mbar of a He+O2 mixture (Fittschen, 2019) and no dependence of σmax(ν12) on pressure over the range from 67 to 133 mbar (Farago et al., 2013).
Recently we have developed a new method for the selective and sensitive
detection of CH3O2 using the conversion of CH3O2 to
CH3O with excess NO followed by CH3O detection by FAGE with laser
excitation at ca. 298 nm (Onel et al., 2017b). The LOD for
the method whilst sampling from atmospheric pressure is ∼4.0×108 molecule cm-3 for a signal-to-noise ratio of 2 and
5 min averaging time; the LOD is reduced to ∼1.0×108 molecule cm-3 by averaging over 1 h. Therefore, the method
has potential to be used in the measurement of atmospheric levels of
CH3O2 in clean environments where [CH3O2] has been
calculated to be of the order of 108 molecule cm-3 (Whalley et al.,
2010, 2011). As LIF is not an absolute method of detection,
FAGE instruments require calibration. Two methods of calibration for
CH3O2 have been used (Onel et al., 2017b):
the 184.9 nm photolysis of water vapour in the presence of excess CH4
and the kinetics of the second-order decay of CH3O2 via its
self–reaction observed in the Highly Instrumented Reactor for Atmospheric
Chemistry (HIRAC). Good agreement was found; i.e. the calibration factors
obtained using the two methods had overlapping error limits at the
1σ level.
However, radicals are difficult to detect accurately and, particularly as
FAGE is not an absolute and direct method, may be subject to systematic
errors and hence require validation using complementary methods. Recently
we intercompared measurements of HO2 concentrations by the indirect
FAGE method and the direct and absolute CRDS method within HIRAC, and
we demonstrated good agreement, within 10 % and 16 % at 150 mbar and 1000, respectively (Onel et al., 2017b), which validates the FAGE method for
HO2. In this work, CH3O2 measurements by FAGE and CRDS within
HIRAC are intercompared at 80 mbar for a mixture of 3:1He/O2 and at
100 and 1000 mbar for air.
ExperimentalCH3O2 generation in HIRAC
The HIRAC chamber (Glowacki et al., 2007) is constructed from SAE 304
stainless steel and has an internal volume of ∼2.25 m3,
the contents of which are homogenised by four mixing fans. Eight 50 mm
diameter quartz tubes are mounted radially inside the chamber and extend
along its ∼2 m length. Each of the eight tubes house a UV
lamp that is used to initiate chemical reactions. The lamps can be changed
to different wavelength outputs depending on the chemical precursors to be
used. The FAGE instrument is connected to the HIRAC chamber through an
ISO-K160 flange with an O-ring compression fitting to allow the inlet
distance from the wall of the chamber to be varied. The 380 mm long inlet
allows the instrument to sample well away from the inner walls of the HIRAC
chamber and avoid chemical processes at the metal surface. Because the FAGE
system removes gas from the HIRAC chamber, a constant flow of synthetic air
is introduced into the chamber to maintain a constant pressure. The CRDS
set-up is described in Sect. 2.3.
The experiments were conducted inside the HIRAC chamber at 295 K using three
different pressure and gas mixtures. The first used 80 mbar total pressure of
helium (BOC, >99.99 %) and oxygen (BOC, >99.999 %) in the ratio of He/O2=3:1. The second and third
mixtures both used synthetic air obtained by mixing oxygen with nitrogen
(BOC, >99.998 %) in the ratio N2/O2=4:1 at 100 and 1000 mbar total pressure, respectively. CH3O2 was generated in
the chamber by photolysing one of two precursor gas mixtures. The first
CH3O2 precursor system was a mixture of Cl2 (Sigma Aldrich,
≥99.5 %) and CH4 (BOC, CP grade, 99.5 %), where the
Cl2 was photolysed at ∼365 nm (Phillips, TL-D36W/BLB,
λ=350–400 nm) to generate CH3O2 via the following reactions:
R6Cl2+hv(365 nm)→Cl+Cl,R7CH4+Cl→CH3+HCl,R1CH3+O2+M→CH3O2+M.
Typical reagent concentrations were [CH4]=1.2–2.5×1016 molecule cm-3 and [Cl2]=1.1–5.5×1015 molecule cm-3. The second method used the photolysis of
acetone (Sigma Aldrich, HPLC grade, ≥99.9 %) at 254 nm (GE G55T8/OH
7G lamps) to produce CH3O2 via Reactions (R8) and (R9) followed by Reaction (R1):
R8(CH3)2CO+hv(254 nm)→2CH3+COR9(CH3)2CO+hv(254 nm)→CH3+CH3CO
Typical initial concentrations were [(CH3)2CO]=8.8×1014 molecule cm-3. In the FAGE calibration experiments using
the kinetic decays [Cl2]0=1.1×1014 molecule cm-3 with CH4 at one of two
concentrations: 2.5×1016 and
2.5×1017 molecule cm-3. In the kinetic experiments
performed to determine the absorption cross section of CH3O2 at
7487.98 cm-1, [Cl2]0=1.1×1014 molecule cm-3 and [CH4]0=2.5×1016 molecule cm-3 at 80 mbar He/O2=3:1 and [Cl2]0=1.0×1015 molecule cm-3 and [CH4]0=2.4×1016 molecule cm-3 at 100 mbar N2/O2=4:1.
FAGE instrument and calibration for CH3O2
The FAGE instrument in HIRAC has been described in detail previously (Winiberg et al., 2015; Onel et al., 2017a, b). The
instrument has a ∼1 m long black anodised aluminium sampling
tube with an inner diameter of 50 mm. The interior of the tube is held at a
low pressure (3.3 mbar for a HIRAC pressure, pHIRAC of 1000 mbar of
synthetic air and 0.9 mbar for pHIRAC=100 mbar synthetic air and
pHIRAC=80 mbar mixture of He/O2=3:1) and draws sample gas
in through a 1 mm diameter pinhole mounted on one end of the tube at a rate
of ∼3 SLPM. Two fluorescence cells are integrated into the
tube, the centre of the first cell is ∼300 mm from the
pinhole, and the centre of the second cell is a further ∼300 mm downstream, followed by a line of tubing that is connected to a
rotary-backed roots blower pump system (Leybold Trivac D40B and Ruvac WAU
251). The first cell is used to detect OH radicals but is not relevant to
this work and is not discussed further, whereas the second cell is used for
the CH3O2 measurements detailed here. The CH3O2 radicals
sampled through the FAGE pinhole at 1000 mbar in HIRAC reached the detection
region in about 85 ms. High-purity NO (BOC, N2.5 nitric oxide) is injected
at 2.5 sccm using a mass flow controller (Brooks 5850S) into the centre of
the gas flow ∼25 mm prior to the second cell to convert
CH3O2 radicals into CH3O. Pulsed laser light at 297.79 nm is
directed through the cell, propagates perpendicular to the gas flow and
is used to excite the A2A1(ν3′=3)←X2E(ν3′′=0) transition of CH3O. The off-resonant red-shifted
fluorescence (320–430 nm) from CH3O (A) is subsequently detected by a
microchannel plate photomultiplier (Photek PMT325) using photon counting.
Measurements are made at an excitation wavelength of 297.79+2.5 nm in
order to determine the laser background, which is subtracted to leave only
signal due to CH3O fluorescence.
The FAGE technique is not absolute and therefore determination of the
calibration factor, CCH3O2
(counts cm3 molecule-1 s-1 mW-1), is required
to convert the measured signal,
SCH3O2 (counts s-1 mW-1), to the CH3O2 concentration:
[CH3O2]=SCH3O2CCH3O2.
Calibration at atmospheric pressure – H2O vapour photolysis in
the presence of excess CH4
This calibration procedure has been described in detail previously (Winiberg et al., 2015; Onel et al., 2017b), as such only important points
are presented here. CH3O2 radicals were generated by photolysing
water vapour in air (BOC, synthetic BTCA 178) at 184.9 nm to produce OH
radicals, which then reacted with methane (BOC, CP grade, 99.5 %) to
produce CH3O2:
The subsequent air/radical mixture was then sampled by the FAGE instrument.
The concentration of CH3O2 generated is given by
[CH3O2]=OH=H2O⋅σ⋅Φ⋅F⋅t,
where σ is the absorption cross section of water vapour at 184.9 nm,
(7.22±0.22)×10-20 cm2 molecule-1 (Cantrell et al., 1997; Creasey et al., 2000); Φ is the
photodissociation quantum yield of OH at 184.9 nm (unity); t is the residence
time of the gas in the photolysis field, which is ∼16.6 and
∼8.3 ms at 20 and 40 SLPM, respectively; and F is the lamp flux
at 184.9 nm. The product F⋅t is determined using chemical actinometry (Winiberg et al., 2015). The 184.9 nm
photon flux, F, is proportional to the electrical current supplied to the
photolysis lamp and is varied to produce a range of CH3O2 radical
concentrations. A typical calibration plot of the FAGE LIF signal versus
generated [CH3O2] calculated using Eq. (2) is shown in Fig. S2 in the Supplement. An average of four calibrations gave
CCH3O2=(8.03±1.37)×10-10 counts cm3 molecule-1s-1 mW-1 where the error represents the overall uncertainty
(17 %) calculated using the statistical error (7 %) and the systematic
error (16 %) at the 1σ level (Onel et al.,
2017b).
Calibration using kinetics of the CH3O2 temporal decay
The calibration described in the previous section is only valid when FAGE is
sampling at atmospheric pressure. However, when sampling from lower
pressures, as described in Sect. 2.1, the FAGE cell pressure decreases (0.9 mbar sampling from 100 mbar) and the calibration is no longer valid. An
alternative calibration procedure using the kinetics of the CH3O2
self-reaction inside the HIRAC chamber allowed the FAGE instrument to be
calibrated under the same conditions of pressure as the intercomparison
experiments, including at lower pressures. Table 1 shows the sensitivity
factors, CCH3O2, obtained
for each set of chamber conditions. Radicals were generated in the chamber
in the same manner as those described in Sect. 2.1. However, instead of
measuring steady-state radical concentrations, the lamps were switched off
and on at ∼120 s intervals to produce a series of
second-order decays, typically four per experiment, in which CH3O2
undergoes loss via self-reaction:
R5aCH3O2+CH3O2→CH3OH+CH2O+O2,R5bCH3O2+CH3O2→CH3O+CH3O+O2.
Assuming no wall loss for CH3O2, the kinetic decays can be
described by the integrated second-order rate equation:
1[CH3O2]t=1[CH3O2]0+2⋅kobs⋅Δt,
where [CH3O2]t is the radical concentration at time t of the
decay; [CH3O2]0 is the initial concentration at the time
t0, when the lamps are switched off; Δt=t-t0; and
kobs is the observed rate coefficient. The observed rate coefficient is
larger than the second-order rate coefficient of just the CH3O2
recombination Reaction (R5) as the methoxy radicals generated by Reaction (R5b) react rapidly with oxygen present in large excess to produce HO2
(Reaction R3), which in turn reacts with CH3O2 (Reaction R12).
R3CH3O+O2→CH2O+HO2,R12CH3O2+HO2→0.9CH3OOH+0.1CH2O+0.1H2O+O2.
As each HO2 radical consumes rapidly one CH3O2 species on the
timescale of Reaction (R5), the CH3O2 decay is described by
second-order kinetics (Sander and Watson,
1980, 1981; McAdam et al., 1987; Kurylo and
Wallington, 1987; Jenkin et al., 1988; Simon et al., 1990), with
kobs=k5(1+r5b), where r5b is the branching ratio for Reaction (R5b). By using the IUPAC recommendations (Atkinson et al., 2006):
k5=(3.5±1.0)×10-13 molecule-1 cm3 s-1 and r5b=0.37±0.06, a value of 4.8×10-13 molecule-1 cm3 s-1 is obtained for kobs.
Modelling of the decay process with a variety of CH3O2 and
HO2 concentrations after the lamps were switched off and following the
establishment of steady-state conditions showed that Eq. (3) was valid
within experimental error. With k5=3.5×10-13 molecule-1 cm3 s-1 (Atkinson et al., 2006), a faster
observed rate constant (defined by Eq. 3) was obtained from the model with
a value, 4.9×10-13 molecule-1 cm3 s-1,
consistent with that recommended by IUPAC, (4.8±0.6)×10-13 molecule-1 cm3 s-1 (1σ uncertainty;
Atkinson et al., 2006). Substituting Eq. (1) into Eq. (3) allows the
measured signal over the decay to be related to the instrument sensitivity
by
1(SCH3O2)t=1(SCH3O2)0+2⋅kobs⋅ΔtCCH3O2,
where (SCH3O2)t and
(SCH3O2)0 are the FAGE signal at time
t and t0, respectively. Taking the reciprocal of Eq. (4) gives
(SCH3O2)t=1(SCH3O2)0+2⋅kobs⋅ΔtCCH3O2-1,
which is then used to fit to the experimental data with kobs fixed to
the value recommended by IUPAC for 298 K, 4.8×10-13 molecule-1 cm3 s-1, using the Levenberg–Marquardt
algorithm. Figure 1 shows an example CH3O2 self-reaction decay
trace obtained at 1000 mbar, where the red line shows the result of the
fitting process.
An example of a second-order decay of the FAGE CH3O2
signal (normalised for laser power fluctuations) with 0.1 s time
resolution (black open circles) recorded at 295 K and a 1000 mbar air
mixture. CH3O2 was generated using [Cl2] of ∼1.1×1014 molecule cm-1 and
[CH4] of ∼2.5×1016 molecule cm-3.
At time zero (∼400 s) the photolysis lamps were turned off to
allow the radicals to decay. The data were fitted to Eq. (5) (excluding the
wall loss rate, kloss; red line) and Eq. (6) (including kloss; blue
dashed line) using the Levenberg–Marquardt algorithm. The obtained value for
the sensitivity factor was the same for both fits: CCH3O2=(1.17±0.04)×10-9 counts cm3 molecule-1 s-1 mW-1. The CCH3O2 errors given above represent statistical
uncertainties at the 1σ level.
However, as the HIRAC chamber is constructed from steel, the potential for a
loss of CH3O2 to the walls was investigated. As circulation fans
were used during all the experiments, the “movement” of CH3O2
radicals within the chamber is in part molecular diffusion and in part
convection. Therefore, the parameter kloss is controlled by both
convection and diffusion processes. By incorporating the wall loss as a
first-order process, Eq. (5) becomes
(SCH3O2)t=1(SCH3O2)0+2⋅kobskloss⋅CCH3O2×exp(kloss⋅Δt)-2⋅kobskloss⋅CCH3O2-1
Fitting Eqs. (5) and (6) to the experimental data is also shown in Fig. 1.
The extracted values for the sensitivity factor are the same for the fit
without and with wall loss included: CCH3O2=(1.17±0.04)×10-9 counts cm3 molecule-1 s-1 mW-1 (statistical
errors at the 1σ level). The close overlap of the fits without and with
wall loss included and the small values extracted for kloss (upper limit
of ∼1×10-5 s-1) fitting Eq. (6)
demonstrates that wall losses are very small and can be neglected. This is
evidenced further by the lack of an observable radical gradient across the
chamber diameter as shown in Fig. S5. In
addition, modelling the CH3O2 decays including a wall loss for
HO2 in the range of measured values, 0.03–0.09 s-1 (Onel et al., 2017a), showed a minor impact of the wall loss of HO2 on kobs, i.e.
kobs within 98 %–95 % agreement with the IUPAC preferred value of (4.8±0.6)×10-13 molecule-1 cm3 s-1 (1σ uncertainty; Atkinson et al., 2006).
Average sensitivity factors for the FAGE instrument using the
CH3O2 kinetic decay method under each chamber environment.
Examples of these decays can be found in Fig. 1 above and in Figs. S3 and S4, and the reported values are typically from an average of eight
decays. All the data were fitted using Eq. (5). The errors given in the
table are overall uncertainties (13 %) at the 1σ level.
Table 1 shows the average sensitivity factors obtained by fitting Eq. (5) to a
typical number of eight temporal decays of
SCH3O2 under each of the chamber conditions,
and example decay traces for the 80 and 100 mbar experiments can be found in
Figs. S3 and S4, respectively. These factors are used for their
respective experimental conditions. For the 1000 mbar intercomparison
experiments with CRDS, an average of the water vapour photolysis sensitivity
factor at 1000 mbar (CCH3O2,H2O=(8.03±1.37)×10-10 counts cm3 molecule-1 s-1 mW-1)
and the average sensitivity factor obtained from the kinetic decay
(CCH3O2,kinetic=(1.16±0.15)×10-9 counts cm3 molecule-1 s-1 mW-1) (Table 1) is used,
giving CCH3O2,avg.=(9.81±2.03)×10-10 counts cm3 molecule-1 s-1 mW-1. We make a
brief comment regarding the difference in the sensitivity factors
CCH3O2,H2O and CCH3O2,kinetic, for which the ratio is
∼0.7, showing a ∼30 % difference, although
the two calibration methods have overlapping error limits at the 2σ level. The kinetic method relies on the rate coefficient kobs for the
CH3O2 self-reaction as recommended by IUPAC (Atkinson et al.,
2006), which has a quoted 2σ uncertainty of 23 %. In a separate
paper we will present a detailed study of the kinetics of the
CH3O2 self-reaction, and its temperature dependence, and report a
revised rate coefficient for this reaction at 298 K.
As the pressure in the FAGE detection cell was 2–3 orders of magnitude lower
than the corresponding pressure in HIRAC (vide supra in Sect. 2.2), the concentrations
of the reagents (Cl2, methane and acetone) were also 2–3 orders of
magnitude lower in the fluorescence cells than the reagent concentrations in
HIRAC. However, a potential effect of the reagents (Cl2, methane and
acetone) on the FAGE sensitivity factor in the HIRAC experiments was
investigated. Two different concentrations of CH4 were used in the
kinetic method for FAGE calibration at 80 mbar of He+O2 in HIRAC to
find practically the same sensitivity factor: (3.80±0.50)×10-9 counts cm3 molecule-1 s-1 mW-1 for 2.5×1016 molecule cm-3CH4 (2.8×1014 molecule cm-3 in the fluorescence cell) and (3.86±0.50)×10-9 counts cm3 molecule-1 s-1 mW-1 for
2.5×1017 molecule cm-3CH4 (2.8×1015 molecule cm-3 in the fluorescence cell). As shown in Fig. S1 there is a good agreement between the laser excitation
scans of CH3O obtained from the CH3O2 generated in HIRAC
using the two methods: acetone photolysis in the presence of O2 and Cl2 photolysis in the
presence of CH4 and O2. In addition, a good agreement has been
previously found between the laser excitation spectra of CH3O generated
using the reaction of CH4 with OH (generated by the 184.9 nm photolysis
of water) in the presence of O2 and directly through the 184.9 nm photolysis of CH3OH (Onel et al., 2017b). Therefore, no effect of the used reagents on the
laser excitation spectrum of CH3O was found.
FAGE measurements of CH3O2 concentration gradient across the
HIRAC diameter
Measurement of radical gradients across the chamber diameter have been
performed previously for HO2 radicals (Onel et al., 2017a), where no
gradient was observed until measuring <10 cm from the chamber wall
where the signal began to decrease, ultimately by ∼16 % at
the point at which the FAGE sampling pinhole was level with the chamber
walls. To investigate any similar gradient effects for CH3O2, a
steady-state concentration of CH3O2 was generated in the chamber
at atmospheric pressure by photolysing O3 in the presence of air and
methane:
R13O3+hv(254nm)→O2+O(1D),R14O(1D)+CH4→CH3+OH,R1CH3+O2+M→CH3O2+M.
Ozone and methane were present in the chamber at ∼2.5×1013 and 2.5×1017 molecule cm-3, respectively. The FAGE inlet was translated
across the width of the chamber and the CH3O2 signal was observed
to show no decrease within the ∼10 % 1σ statistical
error of each measurement up until the point at which the pinhole was level
with the chamber walls. Moving the instrument further backwards positioned
the pinhole inside the ISO-K160 coupling flange and effectively
∼4 cm behind the chamber walls where there is likely to be
little air movement. This position is analogous to that of the CRDS mirrors,
which are recessed into the chamber walls as they mount to the outside of
the chamber (see Sect. 2.3). In this position a signal drop of
∼14 % was observed, within the statistical error margins
of the measurements. A plot of the radical gradient is shown in Fig. S5.
CRDS set-up
The optical path of the CRDS spectrometer within the HIRAC chamber is shown
in Fig. 2 and is the same spectrometer as used to probe HO2 across the
chamber's diameter, which has been described previously (Onel et al.,
2017a).
Longitudinal (horizontal) section of the HIRAC chamber.
The CRDS spectrometer probes the CH3O2 concentration as an average
across the chamber's diameter, while the FAGE instrument probes
CH3O2 in the chamber at a single point close to the centre.
The cavity is formed by two highly reflective 1 in. diameter mirrors
(99.999 %, Layertec, curvature radius = 1 m) housed in custom-built
mounts that allow the mirrors to be tilted slightly whilst maintaining a
gas-tight seal. The position of the mirror on the laser injection side is
modulated along the cavity axis by a few micrometres using a piezoelectric
transducer at ∼10 Hz, with the overall distance between the
two mirrors being ∼1.4 m. Laser light of ∼1.335µm is generated by a distributed feedback (DFB) fibre pig-tailed
diode laser (NTT Electronics, NLK1B5EAAA) held in a butterfly laser diode
mount (Thorlabs LM14S2). The electrical current that drives the laser diode
and thermoelectric cooler is generated by a Thorlabs ITC502 driver. The DFB
is connected to an in-line optical isolator (Thorlabs IO-H-1335APC), an
acousto-optic modulator (AOM, Gooch & Housego Fibre-Q M040-0.5C8H-3-F2S)
and a fibre collimator (Thorlabs CFC-8X-C). The laser light is then guided
into the cavity by two silver mirrors (Thorlabs PF10-03-P01). On the
detection side of the cavity, light leaking out of the mirror is directed
onto another silver mirror that guides the light through a f=30 mm focusing
lens (Thorlabs LA1805-C) onto an InGaAs photodiode (Thorlabs DET10C/M) that
is isolated from ambient light by a 1250 nm long-pass filter (Thorlabs
FELH1250). The photodiode signal is amplified (FEMTO DLPCA-200) and sent to
a data acquisition unit (DAQ, National Instruments USB-6361) and to a
custom-built comparator that acts as a trigger unit. The comparator compares
the amplified photodiode signal with a manually adjustable threshold
voltage, and upon reaching a preset threshold the AOM is switched off,
stopping the injection of light into the cavity within tens of nanoseconds
and initiating a ring-down event. The DAQ is simultaneously triggered and
acquires the signal by a ring-down method. The system resets after a set time (typically
5 ms) and is ready for the next event. The acquired data are processed using a
custom-made LabView program that fits the ring-down events with an
exponential function to extract the ring-down time, τ. Filters are
applied to process the ring-down events to exclude potential outliers caused
by dust particles passing through the beam and false positives (when the
acquisition is triggered by a transient noise spike), so that only
legitimate ring-down events are taken into account. The ring-down time can
then be converted into the absorption coefficient, α:
α=1c×(1/τ-1/τ0),
where τ and τ0 are the ring-down times with and without
CH3O2 radicals present, respectively, and c is the speed of light.
τ0 would be obtained in a typical experiment by recording
ring-down events for ∼1 min before switching on the
photolysis lamps in the chamber. As it is not possible to measure τ0 and τ simultaneously, the background was monitored regularly
during each experiment by switching off the photolysis lamps and allowing
the signal to return to the baseline.
The molecular chlorine delivery did not result in a change in the measured
ring-down time. However, delivery of the methane and acetone reagents led to
a decrease in the ring-down time indicating that, in the concentrations
delivered to the chamber, methane and acetone absorbed in the wavenumber
range used in the present work (∼7486–7491 cm-1). An
absorption coefficient of ∼8×10-9 cm-1
was measured for [acetone] ≈9×1014 molecule cm-3 at the typical measurement point of 7487.98 cm-1 (vide infra). An
absorption coefficient in the range (0.7–1.4) ×10-8 cm-1 was determined at 7487.98 cm-1 for CH4 in typical
concentrations in the FAGE–CRDS intercomparison experiments in the range
(1.2–2.5) ×1016 molecule cm-3. The background ring-down
time τ0 (Eq. 7) contained the contributions of the reagents,
methane or acetone, and was monitored regularly during the experiments by
turning off the chamber lamps (vide supra).
The CH3O2 absorption feature used in these measurements is a band associated with the A2A′←X2A′′ electronic transition
centred around 7488 cm-1, and it has been documented in previous work
(Faragó et al., 2013; Atkinson and Spillman, 2002; Pushkarsky et al.,
2000). There are interfering methane and water vapour lines in this region,
and these together with the change in [CH3O2] during longer (10 min) scanning times did not allow us to generate a continuous high-resolution scan across the CH3O2 transition. Instead, as shown in
Fig. 3, the absorption spectrum was mapped out as a series of point
measurements at fixed wavelengths, normalised by the absorption at the
optimum measurement point, 7487.98 cm-1, where the absorption feature
is sufficiently strong and furthest in wavelength from interfering methane
absorption lines and where the CH3O2 cross section was determined
(Sect. 3.2). The absorption coefficient of CH4 was about 7 times lower
at 7487.98 cm-1 than at 7489.16 cm-1, i.e. at the peak of the
CH3O2 spectral feature where Fittschen (2019) reported σCH3O2. Therefore, 7487.98 cm-1 (rounded to 7488 cm-1
henceforth) was chosen as the measurement point instead of the value of
7489.16 cm-1 used by Fittschen (2019). Each datum point in Fig. 3 was
obtained by measuring the absorption coefficient, α7488cm-1, and the baseline (lamps on, then off) at 7488 cm-1
followed by measuring αCH3O2 and baseline at another wavelength
on the absorption feature and then reverting to measuring at
7488 cm-1 again. This pattern was repeated multiple times for
different wavelengths to build up an absorption feature, with all data
points normalised to α7488cm-1 and then multiplied by
the CH3O2 cross section at 7488 cm-1 (Sect. 3.2) to obtain
the absorption spectrum shown in Fig. 3. The method was used to measure the
CH3O2 absorption spectrum under each of the three experimental
conditions detailed in Sect. 2.1: 80 mbar (He+O2) and 100 and
1000 mbar of synthetic air.
CH3O2 absorption spectrum at 295 K. The
measured absorption spectrum scaled to the absolute cross section determined
at 7488 cm-1 using the kinetics of the CH3O2 decay monitored
using CRDS (Sect. 3.2 below). The black line represents the CH3O2
spectrum measured by Faragó et al. (2013) at 67 mbar He/O2∼1:1 but with the absolute cross section scaled to reflect
the recent update reported by Fittschen (2019) giving σ7489cm-1=2.2×10-20 cm2 molecule-1.
ResultsCH3O2 absorption
spectrum and comparison with the literature
Figure 3 shows that the relatively broad absorption feature obtained in this
work in the range from ∼7486 to 7491 cm-1 is almost the
same at 80 mbar He/O2=3:1 and at 100 and 1000 mbar of synthetic
air. As shown in Fig. 3, the spectrum found in this work agrees well with
the general shape of the CH3O2 spectrum measured by Faragó et
al. (2013) at 67 mbar He/O2∼1:1 but scaled to reflect
the very recent update to the absolute absorption cross section reported by
Fittschen (2019), which gave σ7489cm-1=2.2×10-20 cm2 molecule-1. The peaks at the top of the
spectral feature reported by Faragó et al. (2013) are not reproduced in
this work owing to the method of generating the spectrum (Sect. 2.3).
Previously Pushkarsky et al. (2000) measured the CH3O2 absorption
spectrum over a larger wavenumber range (7300–7700 cm-1), where the
ν12 transition is located at 7488 cm-1 in agreement with this
work. In addition, if the CH3O2 spectrum at 27 mbar
N2/O2=4:1 reported by Atkinson and Spillman (2002) was
shifted by ∼2 cm-1 toward higher wavenumbers compared to
this work and the study of Faragó et al. (2013), the shape of the ν12 band from Atkinson and Spillman is in agreement with the results
shown in Fig. 3.
The similarity of the results at 80 mbar He/O2=3:1 and at 100 and
1000 mbar of air reported in this work and their agreement with the previous
measurements performed at relatively low pressures (Fittschen, 2019;
Faragó et al., 2013; Atkinson and Spillman, 2002; Pushkarsky et al.,
2000) can be explained by an overlap of several individual absorption lines
resulting in a spectral structure in the range from ∼7486 to
7491 cm-1 with practically no pressure dependence observed between
∼30–1000 mbar. Therefore, it can be assumed that the
absorption cross section at 7488 cm-1, σ (7488 cm-1), is
the same under the conditions used in this work, i.e. at 80 mbar of He and
O2 and at 100 and 1000 mbar of air.
Determination of the absorption cross section of CH3O2 at 7488
cm-1
The kinetics of the CH3O2 temporal decay by its self-reaction
(Reaction R5) were used to determine the absorption cross section of
CH3O2 at 7488 cm-1, i.e. σ (7488 cm-1). Note that the
cross section used is not the more standard integrated cross section used by
HITRAN and other spectral databases. CH3O2 radicals were generated
by using CH4/Cl2/synthetic-air mixtures (Sect. 2.1) with the
chamber UV lamps switched on to generate Cl atoms (Reaction R6). By extinguishing the
UV lamps, CH3O2 radicals were removed by self-reaction and wall
loss. Figure 4 shows an example of a kinetic decay obtained at 100 mbar
N2/O2=4:1 using CRDS. The experimental data were fitted by
using two functions described by Eqs. (8) and (9), which are closely related
to Eqs. (5) and (6) used in the analysis of the CH3O2 decays
measured using FAGE. Equation (8) assumes that the wall loss of
CH3O2 is negligible, and hence the removal of CH3O2 can
be described by the integrated second-order rate law equation, leading to
αt=1α0+2⋅kobs.Δtσ(7488cm-1)-1,
where αt is the CH3O2 absorption coefficient at 7488 cm-1 and at time t; α0 is the absorption coefficient at
time zero of the reaction when the lamps are switched off, i.e. t0; Δt=t-t0; and kobs is the observed rate coefficient of the
self-reaction at 298 K, i.e. kobs=(4.8±0.6)×10-13 cm3 molecule-1 s-1 (Atkinson et al., 2006).
Second-order decay of the CH3O2 absorption
coefficient at 7488 cm-1 monitored by CRDS. Experiment carried out at
295 K and 100 mbar of synthetic air; [CH4]0=2.4×1016 molecule cm-3 and [Cl2]0=1.0×1015 molecule cm-3. At time 2205 s the photolysis lamps were
turned off (time t0). Fitting Eq. (8) to the data (red line) gave
σ(7488 cm-1)=(1.47±0.07)×10-20 cm2 molecule-1. A fit including the wall loss rate, kloss (Eq. 9), is shown by the blue dashed line and resulted in σ(7488cm-1)=(1.50±0.07)×10-20 cm2 molecule-1. The error limits are statistical errors at the 1σ level.
For completeness, Eq. (9) includes the CH3O2 wall loss as a
first-order process, leading to
αt=1α0+2⋅kobs.kloss⋅σ(7488cm-1)×exp(klossΔt)-2⋅kobs.kloss⋅σ(7488cm-1)-1,
where kloss is the rate coefficient describing the CH3O2 wall
loss (Onel et al., 2017a).
Figure 4 shows that the fits given by Eqs. (8) and (9) to the data overlap
over all of the temporal CH3O2 decay and the values of σ (7488 cm-1) extracted by the two fits are in a very good agreement:
(1.47±0.07)×10-20 cm2 molecule-1 (Eq. 8)
and (1.50±0.07)×10-20 cm2 molecule-1 (Eq. 9), where the quoted errors are statistical uncertainties. The values
extracted for kloss by fitting Eq. (9) to the CRDS data were small and
similar to the values obtained by fitting Eq. (6) to the kinetic decays
monitored by FAGE. An upper limit of ∼1×10-5 s-1 was obtained for kloss in both FAGE and CRDS measurements,
showing that wall losses are negligible. From fitting Eq. (8) to the
temporal decays obtained at 100 mbar of synthetic air, an averaged value of
(1.51±0.19)×10-20 cm2 molecule-1 was
obtained, where the error represents 1σ overall uncertainty
(13 %). Fitting Eq. (8) to the data at 80 mbar He/O2=3:1 (Fig. S6) gave an average value of σ(7488 cm-1) =(1.46±0.17)×10-20 cm2 molecule-1 (1σ overall
uncertainty), in very good agreement with the value at 100 mbar of air. The
average of the results at 80 mbar He/O2=3:1 and 100 mbar of air,
1.49×10-20 cm2 molecule-1, is in excellent
agreement with the determination of Atkinson and Spillman (2002): σmax(ν12)=(1.5±0.8)×10-20 cm2 molecule-1 and consistent with the estimation of
∼1.0×10-20 cm2 molecule-1 for
σmax(ν12) obtained using the CH3O2 spectrum
reported by Pushkarsky et al. (2000). To enable a comparison at 7487.98 cm-1 with the very recent measurement of Fittschen (2019), who found
2.20×10-20 cm2 molecule-1 at 7489.16 cm-1,
σ(7487.98 cm-1) =1.49×10-20 cm2 molecule-1 obtained in this work was multiplied by the σ(7489.16 cm-1) :σ(7487.98 cm-1) ratio obtained by using
the high-resolution spectrum reported by Faragó et al. (2013) (Fig. 3).
The obtained value, σ(7489.16 cm-1) =(1.9±0.3)×10-20 cm2 molecule-1, is in reasonable agreement
with the result of Fittschen (2019), i.e. σ(7489.16 cm-1) =2.2×10-20 cm2 molecule-1.
Determination of the CRDS limit of detection
The CRDS limit of detection (LOD) has been computed using plots of the square
root of the Allan–Werle variance (Werle et al., 1993; Onel et al., 2017a)
obtained by continuously recording single ring-down events for 1–2 h after
delivering either acetone or methane in typical concentrations to the
chamber filled with the bath gas (He/O2=3:1 at 80 mbar and
synthetic air at 100 and 1000 mbar, respectively). The square root of the
Allan–Werle variance, here referred to as the Allan–Werle deviation, σA(n), gives an estimate of the error,
δα, between successively measured absorption coefficients
for a given averaging size n. For a signal-to-noise ratio (S/N) of 2, the limit
of detection for CH3O2 was determined as LODCH3O2=(2δαmin)/σCH3O2, where σCH3O2=1.49×10-20 cm2 molecule-1 is the
CH3O2 cross section at 7488 cm-1 and is shown in Table 2.
The optimum CRDS sensitivity under all conditions is achieved by averaging
∼500 ring-down events, requiring ∼77 s at an
acquisition rate of 6.5 Hz on average, with an example shown in Fig. 5.
An example of the Allan–Werle deviation plot (the plot of
the square root of the Allan–Werle variance) of the absorption coefficient
at 7488 cm-1 in the absence of CH3O2 and the presence of a
typical acetone concentration of 8.8×1014 molecule cm-3
at 1000 mbar against the number of ring-down events averaged, n. For S/N=2, the minimum detectable absorption coefficient for a single ring-down
measurement is 4.5×10-10 cm-1, which decreases to a
minimum of 2.89×10-11 cm-1 after n=500 (requiring 77 s at an acquisition rate of 6.5 Hz).
As the filter (FELH1250 Thorlabs, cut-off wavelength: 1250 nm) used to
cut off the laboratory visible light from the background of the CRDS
measurements allowed some of the 254 nm light generated by the HIRAC lamps
to be transmitted and then detected by the InGaAS photodiode detector, the
CRDS sensitivity was worse in the experiments using acetone/O2 and 254 nm lamps as a source of CH3O2 compared to the experiments using
Cl2/CH4/O2 and UV black lamps to generate CH3O2.
Therefore, separate Allan–Werle deviation plots were constructed using
measurements of single ring-down events after filling HIRAC with the bath
gas and turning the 254 nm lamps on. Then, the composite error, calculated
as the sum in quadrature of δα obtained in the presence of
acetone and the 254 nm lamps turned off, and δα, determined in the absence of acetone but
keeping the 254 nm lamps turned on, were used to determine the LOD of CRDS in
the acetone/O2, and 254 nm light experiments (Table 2). The composite LOD (with acetone/O2 and 254 nm lamps) was on average ∼55 % greater
than the LOD determined with the UV lamps off and in the absence of acetone; on average the LOD with Cl2/CH4/O2 and UV black lamps was ∼40 % higher than LOD with the bath gas.
CRDS detection limits for CH3O2 calculated at 80 mbar He/O2=3:1 and 100 and 1000 mbar of synthetic air for
single ring-down measurements (Δt=0.15 s), the optimum averaging
time obtained from Allan–Werle deviation plot (Fig. 5 shows an example),
Δtopt. (77 s under all experimental conditions) and Δt=60 s.
a Using the composite error calculated as the sum in quadrature of
δα obtained using a typical concentration of acetone, 8.8×1014 molecule cm-3, and δα determined
in the absence of acetone but keeping the 254 nm lamps turned on during all
measurements. b[CH4]=2.4×1016 molecule cm-3.
As the daytime concentrations of CH3O2 have been calculated using
an atmospheric box model to peak at ∼107–108 molecule cm-3 (Whalley et al., 2010, 2011, 2018), the current CRDS sensitivity is insufficient for the detection
of ambient [CH3O2]. The typical concentrations of CH4 and
acetone in ambient air are orders of magnitude lower than [CH4] and
[(CH3)2CO] used in the HIRAC experiments. However, water vapour,
which is present in the atmosphere in much larger concentrations (typically
∼1017 molecule cm-3) than in HIRAC for these
experiments (∼1013–1014 molecule cm-3)
will significantly absorb in this wavelength region and contribute towards
the background of the measurements. The limits of detection shown in Table 2
allow for HIRAC measurements of [CH3O2]≳¯1010 molecule cm-3 in steady state (where averaging times of ∼60 s can
be used) under all conditions used, and kinetic measurements of
[CH3O2]≳¯1011 molecule cm-3 with the present instrument
resolution time (0.15 s) at 80 mbar He/O2=3:1 and 100 mbar of air.
The relatively long ring-down times achieved here require the lasers to be
blocked for several milliseconds during which the full exponential ring-down time is
measured. This imposes an upper limit to the ring-down rate. The achieved
rate is significantly smaller (6.5 Hz on average) for the following reasons.
The width of the resonances of the optical cavity is of the order of 1 kHz,
much narrower than the laser linewidth. This makes the injection of light
into the cavity inefficient. Reducing the laser linewidth, e.g. with optical
feedback techniques, could significantly increase the injection efficiency
and the ring-down rate. Moreover, the resonance frequencies jitter and drift
due to the unavoidable vibrations associated with the operation of the HIRAC
chamber. The cavity length was actively modulated in order to repeatedly
force coincidence of laser and resonance frequency. Due to the poor
injection efficiency mentioned above, however, not every coincidence
resulted in a ring-down event. Furthermore, a significant fraction of the
ring-down events have to be discarded because of the passage of dust
particles, moved around by the fans within the chamber, through the cavity
axis. The use of an additional optical filter to cut off the 254 nm light
from the background of the CRDS measurements is expected to improve the CRDS
sensitivity if the 254 nm lamps are used in HIRAC. The CRDS sensitivity
could be further improved by mounting the cavity mirrors along the HIRAC
length, which would result in a cavity of about 2 m length containing
CH3O2 radicals, and hence above the current 1.4 m length.
Although the origin band centred at 7388 cm-1 is about 3 times
stronger than the methyl torsional band at 7488 cm-1 (Pushkarsky et
al., 2000; Chung et al., 2007), the latter was targeted because absorption
due to water vapour is between 1 and 3 orders of magnitude weaker
there (assuming 1 % v/v, atmospheric pressure) (Gordon et al., 2017).
Intercomparison of CRDS and FAGE CH3O2 measurements
All the intercomparison measurements have been performed at 7488 cm-1,
where the CH3O2 cross section was determined using CRDS (Sect. 3.2). For the measurements at 80 mbar He/O2 (3:1) and 100 mbar N2/O2 (4:1), CH3O2 was generated either from the
photolysis of acetone at 254 nm in the presence of O2 or from the
photolysis of Cl2 using UV black lamps in the presence of
CH4/O2. At 1000 mbar of synthetic air, the overlap of the methane
absorption lines due to the pressure broadening resulted in a significant
CH4 absorption over the range from 7486 to 7491 cm-1 in the
background of the CH3O2 measurements. Therefore, all the
measurements at 1000 mbar have been carried out using the photolysis of
acetone/O2 at 254 nm. The data recorded by CRDS using acetone/O2
were more scattered than the CRDS data recorded using
Cl2/CH4/O2 for the reasons discussed above (see Figs. 6a and
8a in comparison with Fig. 7a) and were the main contributors to the scatter
on [CH3O2]CRDS in the correlation plots (Figs. 6b, 7b and 8b
below). There was less signal noise present in the FAGE measurements, where
the most significant source of noise is the shot noise (Poisson noise),
which increases with the number of photons counted by the detector (Figs. 1,
S3 and S4) and results in a scatter on the FAGE data growing with
[CH3O2] in Figs. 6a, 7a and 8a.
(a) Comparison of CH3O2 measurement at 80 mbar He/O2 (3:1) where the lamps were turned on at t∼250 s
for ∼5 min to generate CH3O2 and then turned off
again. The measurement by FAGE is shown in red and the measurement by CRDS
is plotted in black. CH3O2 radicals were generated using the 254 nm photolysis of (CH3)2CO (8.8×1014 molecule cm-3). The 1σ statistical errors generated by the data
averaging are shown as grey (CRDS) and red (FAGE) shadows. (b) Correlation
plot at 80 mbar He/O2 (3:1) combining the data obtained using
acetone/O2 and 254 nm lamps with the data generated using
Cl2/CH4/O2 and UV black lamps. [CH3O2] measured by FAGE
is plotted against [CH3O2] measured by CRDS. The linear fit to the
data generates a gradient of 1.03±0.05 and an intercept of (-1.7±0.5)×1010 molecule cm-3. The linear fits
were generated using the orthogonal distance regression algorithm; fit
errors at the 2σ level. In both panels [CH3O2]FAGE was
determined using a calibration factor of 3.83×10-9 counts cm3 molecule-1 s-1 mW-1, and [CH3O2]CRDS
was calculated using a cross section of 1.49×10-20 cm2 molecule-1. Each point is a value averaged over 3 s.
Comparison of CH3O2 measurement (a) and the
correlation plot at 100 mbar of N2/O2 (4:1) (b). In both figures
[CH3O2]FAGE was computed using a calibration factor of 2.80×10-9 counts cm3 molecule-1 s-1 mW-1 and
[CH3O2]CRDS was determined using a cross section of 1.49×10-20 cm2 molecule-1. Each point is a value
averaged over 5 s. Panel (a) shows the measurement by FAGE (red) and the
measurement by CRDS (black) where the CH3O2 radicals were
generated by the photolysis of Cl2 (2.5×1015 molecule cm-3) in the presence of CH4 (2.4×1016 molecule cm-3) and O2. The UV black lamps were alternately switched on and
off: the lamps were turned off at t∼40 s and then turned on
at t∼250 s for ∼5 min before being switched
off again. The 1σ statistical errors generated by the data averaging
are shown as grey (CRDS) and red (FAGE) shadows. Panel (b) combines the
data obtained using acetone/O2 and 254 nm lamps with the data generated
using Cl2/CH4/O2 and UV black lamps. [CH3O2] measured
by FAGE is plotted versus [CH3O2] measured by CRDS. The linear fit
to the data is obtained using the orthogonal distance regression algorithm
and results in a gradient of 0.95±0.02 and an intercept of
(7.0±0.4)×109 molecule cm-3; fit errors
given at the 2σ level.
(a) Comparison of CH3O2 measurements at 1000 mbar of synthetic air where the lamps were turned off at t∼40 s and then on at t∼200 s for ∼2 min before
being switched off again. The measurement by FAGE is shown in red and the
measurement by CRDS is plotted in black. CH3O2 radicals were
generated using the 254 nm photolysis of (CH3)2CO (8.8×1014 molecule cm-3). The 1σ statistical errors generated
by the data averaging are shown as grey (CRDS) and red (FAGE) shadows. (b) Correlation plot of all the data generated at 1000 mbar of air.
[CH3O2] measured by FAGE is plotted against [CH3O2]
measured by CRDS. The linear fit to the data is generated using the
orthogonal distance regression algorithm and results in a gradient of
1.09±0.06 and an intercept of (1.1±0.3)×1010 molecule cm-3; fit errors given at the 2σ level. In
both panels [CH3O2]FAGE was determined using a calibration
factor of 9.81×10-10 counts cm3 molecule-1 s-1 mW-1 and [CH3O2]CRDS was calculated using a
cross section of 1.49×10-20 cm2 molecule-1. Each
point is a value averaged over 5 s.
As the acquisition rate of CRDS (6.5 Hz in average) differed compared to the
FAGE acquisition rate (in the range 1–10 Hz) the comparison data were
averaged to enable comparison of [CH3O2] by the two instruments at
the same moments of time. The averaging interval of time was chosen in the
range 3–5 s, depending on the comparison measurement, to average at least 10
ring-down events over each time interval. This was done because the CRDS data were filtered to
exclude outliers caused by dust particles passing through the light beam
trapped in the optical cavity and the number of encountered “dust events”
varied from one experiment to another.
CH3O2 was generated over a range of concentrations: 2–26×1010 molecule cm-3 at 80 mbar of He+O2 mixture, 2–60×1010 molecule cm-3 at 100 mbar of
synthetic air and 2–30×1010 molecule cm-3 at 1000 mbar of synthetic air. The comparison involved both periods with lamps on,
where the concentration of CH3O2 was changing slowly, and where
the lamps were turned off and the rapid decay of CH3O2 was
observed. Figures 6a, 7a and 8a show examples of time-resolved
CH3O2 concentrations where the lamps were turned on and off. CRDS
absorption coefficients were converted into concentrations using the
absorption cross section determined by studying the second-order
recombination kinetics, σ(7488 cm-1) =(1.49±0.19)×10-20 cm2 molecule-1 (Sect. 3.2). The FAGE signals
were converted into [CH3O2] using the sensitivity factor derived
from the analysis of the temporal decays of CH3O2 at 80 mbar of He+O2 mixture and 100 mbar of air, (3.83±0.50)×10-9 and (2.80±0.37)×10-9 counts cm3 molecule-1 s-1 mW-1, respectively. The data in the correlation plots of the
CH3O2 concentrations determined by FAGE (y axis) and CRDS
(x axis) (Figs. 6b, 7b and 8b) were fitted using an orthogonal distance
linear regression fit (Boggs et al., 1987), which accounts for
errors in both the y and x directions. The gradient of the correlation plot
of the CH3O2 concentrations determined by FAGE (y axis) and CRDS
(x axis) at 80 mbar of He+O2 (Fig. 6b) is 1.03±0.05,
showing an overall level of agreement within 3 %. The gradient of the
correlation plot of the CH3O2 concentrations determined by FAGE
(y axis) and CRDS (x axis) at 100 mbar of air (Fig. 7b) is 0.95±0.02,
showing an overall level of agreement within 5 %.
At 1000 mbar of air, the FAGE signal observed in HIRAC could be calibrated
in one of two ways: either via the photolysis of water vapour to generate OH
followed by reaction with CH4 to form CH3O2, or via the
kinetic analysis of second-order temporal decays of CH3O2. The
conversion of the FAGE signals into [CH3O2] at 1000 mbar air for
the intercomparison with CRDS shown in Fig. 8a and b was based on the
average of the results of the water vapour calibration method and the
kinetic decay calibration method, which gives C‾CH3O2=(9.81±2.03)×10-10 counts cm3 molecule-1 s-1 mW-1, Sect. 2.2.2). The gradient of the overall correlation plot (Fig. 8b) using C‾CH3O2 is
1.09±0.06, showing agreement to within 9 %. Figure S7 shows separately the two correlation plots
obtained using the sensitivities from the two methods of calibration for
FAGE: CCH3O2=(8.03±1.37)×10-10 counts cm3 molecule-1s-1mW-1 (water vapour calibration
method) and CCH3O2=(1.16±0.15)×10-9 counts cm3 molecule-1s-1 mW-1 (second-order kinetic decay method). The
gradients of the two linear fits are 1.35±0.07 (water vapour
calibration) and 0.92±0.05 (kinetic method of calibration).
Therefore, a significantly better agreement (within 8 %) was obtained by
using the kinetic method for the calibration of FAGE compared with using the
water vapour method for calibration of FAGE (35 % agreement). Better
agreement is expected when using the kinetic method to calibrate FAGE, as
this is the same method used to determine the absorption cross section and
hence calibrate the CRDS method, and the intercomparison is not affected by
any error in the rate coefficient, kobs, for the CH3O2
self-reaction. We consider that the main contribution to the discrepancy in
CCH3O2 values obtained by the two methods of calibration derives from an
overestimation of the reported value of the observed rate coefficient for
the CH3O2 self-reaction, kobs=(4.8±0.6)×10-13 molecule-1 cm3 s-1 (1σ error) at 298 K (Atkinson et al., 2006). In a subsequent paper we will report a revised
kobs, which will bring the two methods of calibration into agreement.
Conclusions
An intercomparison between the recently developed indirect method for the
measurement of the CH3O2 radicals using fluorescence assay by gas
expansion (FAGE) (Onel et al., 2017b) and the direct
cavity ring-down spectroscopy (CRDS) method has been performed within the
Leeds Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC). CRDS
detected CH3O2 by using the A←X (ν12) electronic transition at 7488 cm-1. The CH3O2 radical was
generated from the photolysis of mixtures of either
Cl2/CH4/O2 or acetone/O2 at room temperature and three
total pressures, 80 mbar of He/O2=3:1 and 100 and 1000 mbar of
N2/O2=4:1, and CH3O2 was measured simultaneously using the two
methods.
At all pressures FAGE was calibrated using the kinetics of the
CH3O2 second-order decay by self-reaction. At 1000 mbar the
conventional 185 nm photolysis of water vapour in the presence of excess
CH4 and O2 was used to calibrate FAGE in addition to the kinetic
method. The two calibration methods have overlapping error limits at
the 2σ level (34 % for the water vapour photolysis method and 26 %
for the kinetic method) as it has been found previously (Onel et al., 2017b). The difference between
CCH3O2 (water vapour method) and
CCH3O2 (kinetic method) has been discussed in detail previously (Onel
et al., 2017b). In the case of HO2, a very good agreement (difference
within 8 %) between CHO2 (water vapour method) and
CHO2 (kinetic method) was
obtained previously (Onel et al., 2017a; Winiberg et al., 2015), which
suggests that the production of OH and HO2 from the photolysis of water
vapour in air can be quantified robustly. We consider it unlikely that there
is a significant error in the fraction of OH which is converted to
CH3O2 upon the addition of methane. We consider instead that the
discrepancy between the two calibration methods is due to an overestimation
of the reported value of kobs for the CH3O2 self-reaction (Atkinson et al., 2006); the two methods of calibrations agree if
kobs is reduced by 25 %–30 %, which is close to the reported 2σ uncertainty in the rate coefficient (Atkinson et al., 2006). The average
value of the sensitivity factor obtained from the two calibration methods,
C‾CH3O2=(9.81±2.03)×10-10 counts cm3 molecule-1 s-1 mW-1, corresponds to a limit of detection (LOD) for CH3O2 of 1.18×108 molecule cm-3 for a S/N of 2 and 60 s averaging
period. The FAGE sensitivity factor increased by ∼3 times by
decreasing the pressure in the FAGE detection cell (from 3.3 mbar,
corresponding to a total HIRAC pressure of 1000, to 0.9 mbar, corresponding
to a total chamber pressure of 100 or 80 mbar).
The CH3O2 absorption cross section at 7488 cm-1 at 100 mbar
of air and 80 mbar of He/O2=3:1 was determined using the kinetics
of the CH3O2 second-order decays: σCH3O2=(1.49±0.19)×10-20 cm2 molecule-1. No change in
the shape of the CH3O2 spectrum with pressure was found from the
reduced pressures (100 mbar of air and 80 mbar of He/O2=3:1) to
1000 mbar of air, showing that σCH3O2 is almost independent
of pressure. For a time averaging of 60 s, the calculated CRDS LOD using the
Allan–Werle deviation plots and σCH3O2 is around 8×109 molecule cm-3 using acetone/O2 and 254 nm light at
all operating pressures and 6×109 molecule cm-3 using
CH4/Cl2 and black lamps at the reduced pressures.
The FAGE–CRDS intercomparison used measurements of CH3O2 under
steady-state conditions (photolysis lamps on) as well as rapid decays in
[CH3O2] (lamps switched off) to cover large concentration ranges:
2–26×1010 molecule cm-3 at 80 mbar of He+O2 mixture, 2–60×1010 molecule cm-3 at 100 mbar of air and 2–30×1010 molecule cm-3 at 1000 mbar of air. A good agreement between [CH3O2]FAGE and
[CH3O2]CRDS was obtained under all conditions as shown by the
gradient of the correlation plots: 1.03±0.05 at 80 mbar
He/O2, 0.95±0.02 at 100 mbar air and 1.09±0.06
at 1000 mbar air (using an average of the sensitivity factors for the two
FAGE calibration methods). The study provides a validation for the indirect
FAGE method for CH3O2 measurements, in agreement with the previous
FAGE validation for HO2 measurements (Onel et al., 2017a).
Data availability
Data presented in this study are available from the authors upon request
(chmlo@leeds.ac.uk and d.e.heard@leeds.ac.uk).
The supplement related to this article is available online at: https://doi.org/10.5194/amt-13-2441-2020-supplement.
Author contributions
LO and AB setup the CRDS spectrometer and performed experiments and data analysis, together with assistance from JH; MG and NN built the CRDS spectrometer; LO prepared the article, with assistance from AB and with contributions from GH, LW, PWS, GADR and DEH, who also commented on all results.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work has received funding from the Natural Environment Research Council
(NERC grant no. NE/M011208/1), the National Centre for Atmospheric
Science and the European Union's Horizon 2020 research and innovation
programme through the EUROCHAMP-2020 Infrastructure Activity under grant
agreement no. 730997. Alexander Brennan thanks NERC for a studentship awarded in the
framework of the SPHERES doctoral training programme (NE/L002574/1). The
authors thank Christa Fittschen for helpful discussions on the absorption
cross section of CH3O2.
Financial support
This research has been supported by the Natural Environment Research Council (grant no. NE/M011208/1), the European Union's Horizon 2020 through the EUROCHAMP-2020 Infrastructure Activity (grant no. 730997) and the SPHERES doctoral training programme (NE/L002574/1).
Review statement
This paper was edited by Jean-Francois Doussin and reviewed by three anonymous referees.
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