Introduction
Methyl peroxy (CH3O2) radicals are critical intermediates in the
atmospheric oxidation (Orlando and Tyndall, 2012) and
combustion of hydrocarbons (Zador et al., 2011). In the
remote atmosphere, CH3O2 is mainly formed by the reaction of
methane with the OH radical via abstraction of an H atom (Reaction R1), followed by
the reaction of the produced CH3 radical with O2 (Reaction R2).
OH+CH4→CH3+H2OCH3+O2+M→CH3O2+M
Methyl peroxy radicals can also be formed
from more complex species, e.g. the reaction of acetyl peroxy radicals with
HO2 in low-NOx environments or the reaction of acetyl peroxy
radicals with NO in anthropogenically influenced environments.
CH3O2 is predicted to be the most abundant peroxy radical in the
atmosphere, yet there are no specific measurements of its concentration.
Daytime concentrations estimated using a box model utilising the MCM (Master
Chemical Mechanism) version 3.3.1 (Saunders et al., 2003; Jenkin et al.,
2015) are ∼ 6 × 108 molecule cm-3 in the tropical
Atlantic Ocean in summer (Whalley et al., 2010),
∼ 2 × 108 molecule cm-3 in a tropical rainforest
(Whalley et al., 2011) and lower in polluted environments, for example
∼ 5 × 107 molecule cm-3 in London in summertime
(Whalley et al., 2017).
The reaction of CH3O2 with NO (Reaction R3) usually dominates the chemistry
of CH3O2, particularly in environments influenced by anthropogenic
NOx emissions, resulting in NO2 production and hence ozone
production:
CH3O2+NO→CH3O+NO2.
The subsequent reaction
of CH3O with O2 (Reaction R4) produces HO2, which in turn
oxidises another NO to NO2 (Reaction R5) with further production of
O3 and propagation of the HOx radical chain:
CH3O+O2→CH2O+HO2,HO2+NO→OH+NO2.
However, under low NOx levels (e.g. remote forested environments and the
marine boundary layer), the self-reaction of CH3O2 (Reaction R6)
and the reactions of CH3O2 with HO2 and other organic peroxy
(RO2) species are important radical removal/termination reactions. The
CH3O2 self-reaction occurs through two channels, Reactions (R6a)
and (R6b) (Tyndall et al., 1998):
CH3O2+CH3O2→CH3OH+CH2O+O2,CH3O2+CH3O2→CH3O+CH3O+O2.
Despite the importance of Reaction (R6), there are uncertainties of about a
factor of 2 in the value of its rate coefficient at room temperature,
k6, which has a range of
(2.7–5.2) × 10-13 cm3 molecule-1 s-1
(Atkinson et al., 2006); the preferred IUPAC
value is k6=3.5 × 10-13 cm3 molecule-1 s-1.
The previous kinetic studies used time-resolved UV-absorption spectroscopy to
detect CH3O2 radical, typically at 250 nm, (Sander and Watson,
1980, 1981; McAdam et al., 1987; Kurylo and Wallington, 1987; Jenkin et al.,
1988; Simon et al., 1990; Lightfoot et al., 1990). UV-absorption spectroscopy
is a relatively insensitive technique and hence the detection limits of
CH3O2 were quite high, for example approximately
4 × 1012 molecule cm-3 (Sander and Watson, 1980, 1981).
In addition, due to the broad, featureless spectra of RO2 species, which
often overlap, UV absorption is a relatively unselective technique for the
study of the kinetics of individual RO2. Therefore, there is a clear
need for the determination of k6 using a more selective method, which
will be addressed in subsequent studies.
At present, CH3O2 is not specifically measured in the atmosphere
by any direct or indirect method. Time-resolved continuous-wave cavity
ring-down spectroscopy (CRDS), using the ν12 transition of the A←X band at ∼ 1.3 µm, has been used to detect
CH3O2 directly in a photoreactor (Farago et al.,
2013; Bossolasco et al., 2014). However, the detection limit is not
sufficiently sensitive to enable tropospheric detection. Typically, the sum
of HO2 and all organic RO2 has been measured in the atmosphere,
making no distinction between HO2 and different RO2 species,
although more recently the sum of RO2 has been quantified separately
from HO2. One of the methods uses chemical ionisation mass spectrometry to
determine the sum HO2+∑i[RO2,i] or separately [HO2], depending on the control
of the flows of the NO and SO2 reagents (Hanke et al., 2002; Edwards
et al., 2003). The sum HO2+∑i[RO2,i] has also been determined for
many years by the peroxy radical chemical amplifier (PERCA) method, which
uses NO and CO to generate NO2 amplified by a chain reaction, and
subsequently measured by a variety of methods, for example luminol
fluorescence, laser-induced fluorescence (LIF) or cavity absorption methods
(Cantrell and Stedman, 1982; Cantrell et al., 1984; Miyazaki et al.,
2010; Hernandez et al., 2001; Green et al., 2006; Chen et al., 2016). A
modification of PERCA, using a denuder to remove HO2 has been used to
estimate the sum of RO2 (Miyazaki et al., 2010). ROxLIF is a
more recent method, which uses OH LIF detection at low pressure, known as
FAGE (fluorescence assay by gas expansion) (Fuchs et al., 2008; Whalley et
al., 2013). The ROxLIF method measures either [HOx] = [OH] + [HO2], by converting HOx into HO2 through addition of CO, or
ROx=[HOx]+∑i([RO2,i]+[ROi]), by titrating ROx to HO2
by added NO and CO. After the conversion into HO2, HO2 is
converted into OH in the FAGE chamber and detected by LIF. The sum
∑i[RO2,i] and the concentration of the
initial HO2 can be determined from the separate measurements of
HOx, ROx and OH. The limit of detection (LOD) of the ROxLIF method
is ∼ 0.1 pptv (2.5 × 106 molecule cm-3)
(Fuchs et al., 2008; Whalley et al., 2013). Recently, the interference
from certain types of RO2 radicals in the FAGE detection of HO2
was deliberately exploited to enable a partial RO2 speciation
(Whalley et al., 2013). The method was used in the Clean Air for London
campaign (ClearfLo) to distinguish 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).
As methoxy (CH3O) radicals can be generated by techniques such as
pulsed laser photolysis and microwave discharge and detected with high
sensitivity by LIF (Shannon et al., 2013; Chai et al., 2014; Albaladejo et
al., 2002; Biggs et al., 1993, 1997), the method has been used
in kinetic studies of a range of CH3O reactions. These studies used the
electronic excitation of the methoxy radical from the ground state to the
first electronically excited state (A2A1←X2E). The A←X excitation spectrum covers the range
∼ 275–317 nm and leads to fluorescence from several vibronic
bands in the near UV, which has been reported in a series of experimental and
theoretical studies (Inoue et al., 1980; Kappert and Temps, 1989; Powers et
al., 1997; Nagesh et al., 2014).
This paper reports the development of a new method for the selective and
sensitive detection of CH3O2 radicals using FAGE by titrating
CH3O2 to CH3O by reaction with added NO (Reaction R3) and then
detecting the resultant CH3O by off-resonant LIF with laser excitation
at ca. 298 nm. The method is similar to the standard method used for the
detection of HO2 radicals by FAGE through conversion of HO2 to OH
by reaction with added NO followed by OH on-resonance LIF at about 308 nm
(Heard and Pilling, 2003). As LIF is not an absolute detection method,
FAGE instruments require calibration, with the 184.9 nm photolysis of water
vapour in air using a mercury (Hg) Pen-Ray lamp being a common method
employed for generating known concentrations of OH and HO2 (Heard
and Pilling, 2003):
H2O⟶184.9nmOH+H,H+O2+M→HO2+M,
where M = N2, O2 and the photodissociation quantum yield of OH
and H is unity. In this study the photolysis of water vapour is performed in
the presence of excess methane to produce CH3O2:
CH4+OH→CH3+H2O,(R1)CH3+O2+M→CH3O2+M.(R2)
An alternative CH3O2 calibration is also presented, consisting of
the analysis of the kinetics of the CH3O2 decay by self-reaction
monitored by FAGE and compared with the water photolysis method. The studies
are performed within HIRAC (Highly Instrumented Reactor for Atmospheric
Chemistry), which is a 2.25 m3, custom-built, stainless steel chamber
simulating the ambient conditions (Glowacki et
al., 2007). HIRAC has been used in alternative calibrations of FAGE for OH
and HO2 using the temporal evolution of appropriate species, in
validation and development of new atmospheric measurement techniques as well
as in kinetic and mechanistic studies of atmospheric relevant reactions
(Malkin et al., 2010; Winiberg et al., 2015, 2016).
Vertical cross section of the FAGE fluorescence cells. The first
(left) fluorescence cell was used to detect OH fluorescence through a 308.8±5.0 nm bandpass filter (transmission > 50 %) and the
second cell to detect CH3O2 after titration with added NO to form
CH3O using a bandpass filter between 320 and 430 nm with an average
transmission > 80 %.
Direct LIF detection of CH3O radicals, which is also a key intermediate
in the oxidation of methane and other volatile organic compounds (VOCs) in the troposphere and formed by
reactions such as Reactions (R3) and (R6b), is also reported here. However, in the
atmosphere, CH3O is exclusively consumed by reaction with O2 (Reaction R4)
generating formaldehyde and recycling HO2, resulting in a very short
lifetime and consequently very low concentration (∼ 102–103 molecule cm-3). For this reason no measurements in
the atmosphere have previously been attempted. The photolysis of CH3OH
at 184.9 nm is used to estimate the FAGE sensitivity for CH3O. The
dominant photolysis channel of methanol between 165 and 200 nm generates
CH3O radicals (Wen et al., 1994; Kassab et al., 1983; Marston et al.,
1993):
CH3OH⟶165–200 nmCH3O+H.
A photodissociation quantum yield of CH3O of 0.86 ± 0.10 has been
found at 193.3 nm (Satyapal et al., 1989) in qualitative agreement
with analysis of the end-products of the methanol photodissociation at 184.9 nm (Porter and Noyes, 1959; Buenker et al., 1984). Here we report the
first measurements of CH3O concentrations in an atmospheric simulation
chamber. Methoxy radicals are generated by the CH3O2
self-reaction carried out within HIRAC at 295 K and 1000 mbar of N2
containing O2 in trace amounts to reduce the rate of removal of
CH3O by reaction with O2. This work enhances the capability of
HIRAC to measure short-lived radical species by the addition of both
CH3O2 and CH3O detection, and we discuss the potential of the
method for detection of CH3O2 in the atmosphere itself.
Experimental
The FAGE instrument
Details on the HIRAC-based FAGE instrument for the detection of OH and
HO2 has been presented previously (Winiberg et al., 2015). Figure 1
shows a schematic cross section of the instrument inlet and the two
fluorescence detection cells. The gas was sampled with a flow rate of
3.2 slm through a 1 mm diameter pinhole and passed down a 50 mm diameter
flow tube of 280 mm length first into the OH detection axis and, after a
further 300 mm, into the CH3O2 detection axis. The pressure in the
detection cells was maintained at 2.65 ± 0.05 Torr by using a
high-capacity rotary-backed Roots blower pumping system (Leybold, Trivac D40B and
Ruvac WAU251). CH3O2 radicals were titrated to CH3O by adding
high purity NO (BOC, N2.5 nitric oxide) with a typical 2.5 sccm flow rate
(further details in Sect. 2.2) ∼ 25 mm before the second detection
axis into the centre of the flow. The resultant CH3O radicals were
measured by LIF.
Probe laser light was generated by an Nd:YAG (JDSU Q201-HD) pumped dye laser
(Sirah Credo-Dye-N) using a DCM dye (Sirah) in ethanol and operating at a
5 kHz pulse repetition frequency, with a pulse width at half
maximum height of 25 ns, typical pulse
energy of 120 µJ pulse-1 and a linewidth of 0.08 cm-1
at 595 nm. The frequency doubled light at either ∼ 308 nm (OH
detection) or ∼ 298 nm (CH3O detection) was focused into fibre
optic cables to be delivered to the two detection cells. OH and CH3O
radicals were separately detected by LIF spectroscopy by exciting at
307.99 nm using the Q1(2) rotational line of the A2∑+(ν′=0)←X2Πi(ν′′=0) OH transition in the first
detection axis to monitor on-resonant fluorescence (308.8 ± 5.0 nm)
and excitation at 297.79 nm in the A2A1(ν3′=3)←X2E(ν3′′=0) CH3O transition in the second detection axis to
monitor red-shifted off-resonant LIF (320–430 nm). Here ν3 refers to
the C–O stretching vibrational mode of CH3O which demonstrates a
progression in the LIF spectrum (Inoue et al., 1980; Kappert and Temps, 1989;
Powers et al., 1997; Nagesh et al., 2014). The fluorescence in the two cells
was collected orthogonal to the gas flow by two microchannel plate
photomultiplier tubes (MCP-PMT) (Photek PMT325/Q/BI/G)
equipped with a 50 ns gate unit (Photek GM10-50) for gated photon counting,
and the signal was amplified using a pre-amplifier (Photek PA200-10). Further
details on the OH detection and calibration in HIRAC have been reported
previously (Winiberg et al., 2015).
The laser and photon-counting timing for CH3O detection was controlled
by a delay pulse generator (9520 series, Quantum Composers). The relatively
broad bandpass filter used for the collection of the CH3O fluorescence
(average transmission > 80 % between 320 and 430 nm) allowed
some red-shifted scattered light (presumably from the walls of the chamber)
generated by the probe laser to be transmitted and hence detected by the
MCP-PMT. In order to ameliorate this and reduce the background signal, the
gate unit was opened 100 ns
after the laser pulse to detect fluorescence integrated over a gate width of
2 µs. The optimum gate width of 2 µs (values in the range
1–3 µs were compared) is consistent with the CH3O
fluorescence lifetimes, calculated to be in the range of
0.9–1.5 µs, using the reported radiative lifetimes for CH3O
of 1.5 µs (Inoue et al., 1979), 2.2 µs (Ebata et al.,
1982), and 4 ± 2 µs (Wendt and Hunziker, 1979) and using the
fluorescence quenching rate coefficients of N2 and O2 (Wantuck et
al., 1987) to calculate the rate of quenching at the pressure in the FAGE
detection cell (2.65 ± 0.05 Torr). As the fluorescence lifetime of
CH3O(A) in the detection cell was 0.9–1.5 µs, delaying the
counting of the fluorescence by 100 ns makes very little difference
(∼ 10 %) in the fraction of fluorescence collected.
All LIF signals reported here were normalized to the probe laser power as
measured with a laser power meter (Maestro, Gentec-EO) before the start of
each LIF measurement. Fluctuations in the relative laser power were monitored
via a photodiode (UDT-555UV, Laser Components) during the measurements and
were accounted for in the signal normalisation. The LIF spectrum was
corrected for the laser-scattered background by subtracting the normalized
offline signal recorded over 60 s at the end of each LIF measurement using
an offline wavelength λ(offline = 300.29 nm) =λ(online = 297.79 nm) + 2.5 nm, well away from any CH3O
absorption. The signals were large enough that during conditions where
CH3O2 concentrations were constant (e.g. in calibrations or during
HIRAC experiments where steady-state concentrations were generated) it was
established that the laser wavelength was stable over a long period once the
laser wavelength had been tuned to the CH3O transition. Hence, the
online wavelength position for CH3O fluorescence detection was found
without using a reference cell. We are in the process of developing a
reference cell for field measurements in the future, when
the atmospheric concentrations of
CH3O2 (and hence CH3O after conversion) will be both lower and
more variable over short timescales. Figure 2 shows the laser excitation
spectrum centred at ∼ 298 nm in the ν3 vibronic band recorded
using an increment of Δλ=10-3 nm. The spectrum agrees
well with previous work (Inoue et al., 1980; Kappert and Temps, 1989; Shannon
et al., 2013). Figure 3 shows typical laser excitation scans performed over a
narrower range of wavelengths in order to locate λ(online). The LIF
spectra were obtained by using the CH3O or CH3O2 radicals
generated in a flow tube described in Sect. 2.3.1, with the flow-tube output
impinged close to the FAGE sampling inlet. The radicals were generated using
the 184.9 nm light output of a Hg Pen-Ray lamp by either the photolysis of
methanol in nitrogen to generate CH3O or the photolysis of water vapour
in synthetic air (to generate OH) in the presence of methane to form
CH3O2. The CH3O radicals were directly detected, while the
CH3O2 radicals were first converted to CH3O species by added
NO prior to the fluorescence detection cell (Fig. 1). Similar laser scans to
the scans shown in Fig. 3 were recorded by using the CH3O2 radicals
produced in a steady-state concentration in HIRAC using photolytic mixtures
of Cl2–CH4–air as described in Sect. 2.3.2. There were no
unexpected features in the laser scans recorded when FAGE sampled
CH3O2 radicals from HIRAC, consistent with no interference being
anticipated in the FAGE measurements of CH3O as there were no other
species in HIRAC absorbing at 298 nm and fluorescing at the wavelengths
transmitted by the bandpass filter (average transmission
> 80 % over 320–430 nm).
Laser excitation spectrum of the A2A1 (ν3′=3) ←X2E (ν3′′=0) transition of the methoxy
radical. CH3O radicals were obtained by photolysis of methanol in
N2 at 184.9 nm. Fluorescence cell pressure = 2.65 ± 0.05 Torr;
wavelength increment Δλ= 10-3 nm, with each point
corresponding to 5000 laser shots. The red arrow indicates the wavelength
λ(online) ∼ 297.79 nm used for the time-resolved kinetic
studies of CH3O2.
Typical laser excitation scans of CH3O performed over a much
smaller range of wavelengths. Methoxy radicals were generated using
OH and CH4
(black line) to produce 5.5 × 1010 molecule cm-3
CH3O2, subsequently titrated to CH3O by adding NO, and the
photolysis of methanol (red line) to generate
4.9 × 1010 molecule cm-3 CH3O directly. See main
text for the description of the methods and calibration. The signal was
normalised for the laser power (10.3 ± 0.3 mW
in the methane method and
8.7 ± 0.2 mW in the methanol
method). Fluorescence cell pressure = 2.65 ± 0.05 Torr; wavelength
increment Δλ= 10-3 nm, with each point corresponding to
5000 laser shots. The red arrow indicates the wavelength λ(online)
∼ 297.79 nm used for the time-resolved kinetic studies of
CH3O2.
Optimisation of the NO concentration for methyl peroxy radical
detection
As NO was added ∼ 25 mm prior the methoxy detection axis (Fig. 1), some of the methoxy radicals formed by Reaction (R3) reacted further
with NO before the fluorescence detection:
CH3O+NO→CH2O+HNO,CH3O+NO+M→CH3ONO+M,
where M = N2, O2. In addition to the above reactions, CH3O
reacts with O2 by Reaction (R4). Figure 4 shows the dependence of the
LIF signal on the concentration of NO obtained experimentally and by
numerical simulations using Reactions (R3)–(R4) and (R10)–(R11) and
outlined in the Supplement. A maximum signal was obtained with added
[NO] = 6.7 × 1013 molecule cm-3 for a reaction time
of 3 ms, estimated from the linear flow velocity within the FAGE reactor.
Figure 4 shows that the functional dependence with added [NO] of the
experimental CH3O signal and the simulated
[CH3O] / [CH3O2]0 ratio display the same shape
(within overlapping error limits),
with the numerical simulations showing that
[CH3O] / [CH3O2]0 at the detection axis was
∼ 0.4 (i.e. 40 % conversion to CH3O).
FAGE calibrations
CH3O and CH3O2 calibrations were carried out using the
conventional radical source, employed
in fieldwork for OH and HO2
calibrations (Heard and Pilling, 2003), that produces radicals in a flow tube impinging just outside the
FAGE inlet pinhole (Winiberg et al., 2015) and is described in Sect. 2.3.1.
Two methods of calibration have been used for CH3O2: the flow-tube
method and the kinetics of the self-reaction of CH3O2 carried out
in HIRAC.
FAGE signal (left axis) and the ratio
[CH3O] / [CH3O2]0 (right axis) as a function of the
concentration of NO for a reaction time of 3 ms. Black squares are
experimental CH3O signals (errors are 1σ) and red circles are
the ratio [CH3O] / [CH3O2]0 generated by numerical
simulations (percentage uncertainties are 20 %) using the chemistry
system outlined in the main text and described in further detail in the
Supplement.
Calibration for methoxy radicals
In the CH3O calibration experiments, nitrogen (BOC,
> 99.998 %) was used as carrier gas. Part of the N2 flow
was passed through a methanol (Sigma Aldrich, ≥ 99.9 %) bubbler
while the other portion bypassed the bubbler. The gas containing methanol
vapour was then passed through a square-cross-section flow tube of dimensions
13 × 13 mm
(internal) and 300 mm length with a flow rate of
40 slm (ensuring turbulent flow conditions), controlled by an electronic
flow controller (Brooks, 0–100 slm air). The collimated light of a Hg
Pen-Ray lamp (LOT-Oriel, Hg(Ar)) was directed across the
flow tube (close to the downstream end) to photolyse methanol vapour. The
flow-tube output was impinged close to the FAGE inlet to sample CH3O
radicals at atmospheric pressure through a 1 mm diameter pinhole (Fig. 1).
The concentration of CH3O radicals was calculated using Eq. (1):
[CH3O]=[CH3OH]σCH3OH,184.9nmΦCH3O,184.9nmF184.9nmΔt,
where σCH3OH,184.9nm is the absorption cross section
of methanol at 184.9 nm,
(6.35 ± 0.28) × 10-19 cm2 molecule-1,
obtained by averaging reported values (Dillon et al., 2005; Jimenez et al.,
2003; Nee et al., 1985); F184.9nm is the photon flux of
184.9 nm light; and Δt is the irradiation time of the gas. Although
it is known, based on end-product analysis, that the scission of the O–H
bond is a major photolysis channel of methanol at 184.9 nm (Buenker et al.,
1984; Porter and Noyes, 1959), the photodissociation quantum yield of
CH3O at 184.9 nm, ΦCH3O,184.9nm, has not yet been
reported. Here it is assumed
that ΦCH3O,184.9nm is equal to the photodissociation
quantum yield at 193.3 nm, ΦCH3O,193.3nm= 0.86 ± 0.10, which has been reported (Satyapal et al., 1989). In
order to determine the methanol vapour concentration in the flow tube,
[CH3OH], separate experiments were carried out with the same calibration
system to bubble deionised water instead of methanol with the same flow rate.
The water vapour concentration, [H2O], was measured using a dew-point
hygrometer (CR4, Buck Research Instruments) prior to the flow tube. Then
[CH3OH] was calculated using the averaged [H2O] and the vapour
pressures pCH3OH and pH2O at the temperatures
measured for CH3OH (13 ∘C) and H2O (15 ∘C) in the
bubbler:
[CH3OH]=H2OpCH3OHpH2O.
Equation (2) assumes that there were no losses of water vapour and methanol
vapour by condensation in the tubing connecting the bubbler to the flow tube.
This is as expected based on the small difference in temperature between the
bubbler (vide supra) and the connecting tubing (typically held at
∼ 20 ∘C) and as the gas going through the bubbler was diluted
with the gas bypassing the bubbler.
N2O photolysis at 184.9 nm to generate NO (via reaction of the
photoproduct (O1D) with N2O giving a
known yield of NO), which was subsequently measured using a commercial
analyser, was used as a chemical actinometer to obtain the product
F184.9nm × Δt (Winiberg et al., 2015) and
hence calculate [CH3O] via Eq. (1). The photolysis time, Δt, was
estimated to be 8.3 ms, using the volumetric flow rate and the geometric
parameters of the flow tube (assuming plug flow), and was in turn used to
determine F184.9nm. Although it is the product
F184.9nm × Δt which is used to calculate
[CH3O], any change in the volumetric flow rate between the calibration
and actinometry experiments will change Δt, and hence the product was
corrected for any changes in volumetric flow rate. A range of [CH3O] at
constant [CH3OH] was produced by changing the electrical current through
the Hg lamp between 0 and 20 mA, and hence F184.9nm, to
generate the calibration plot presented in Fig. 5.
Calibration for methyl peroxy radicals
Flow-tube method
Methyl peroxy radicals were generated by water photolysis at 184.9 nm
(Reaction R7) to give OH, followed by
the reaction with excess methane in air (BOC, synthetic BTCA 178) –
Reactions (R1)–(R2) to give CH3O2. The calibrations were performed
using the set-up described above. Methane (BOC, CP grade, 99.5 %) was
flowed at 82.5 sccm to convert OH into CH3, which subsequently reacted
rapidly with O2 to form CH3O2. Figure S1 (Supplement) shows an
example of the OH signal with and without CH4. The signal in the
presence of CH4 was 0.04 ± 0.04 of the signal in the absence of
CH4 showing that 0.96 ± 0.04 of OH was converted into
CH3O2. The result is in agreement with the estimation of the
fraction of OH titrated to CH3O2, 0.97, using a rate coefficient of
6.4 × 10-15 cm3 molecule-1 s-1 for the OH +
CH4 reaction (Atkinson et al., 2006) and an average residence time of OH
in the calibration flow tube of 11 ms determined using the volumetric flow
rate and the geometric parameters of the flow tube and position of the Hg pen
lamp.
The concentration of CH3O2 was determined using Eq. (3):
[CH3O2]=0.96[OH]=0.96[H2O]σH2O,184.9nmΦH2O,184.9nmF184.9nmΔt,
where σH2O,184.9nm 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), and ΦH2O,184.9nm is the photodissociation quantum yield of OH, which is equal to
unity. The values of F184.9nm and Δt were determined as
described in the Sect. 2.3.1. No loss of CH3O2 by reaction with the
HO2 radicals generated by the reaction of H atoms with O2
(Reaction R8) was encountered over the residence time of the radicals in the
calibration flow tube (∼ 11 ms) as CH3O2 reacts with
HO2 on a 10 s timescale as determined using a reaction rate
coefficient of 5.2 × 10-12 cm3 molecule-1 s-1
(Atkinson et al., 2006) and the radical concentrations in the flow tube. The
CH3O2 radicals sampled through the FAGE pinhole expansion to a
pressure of 2.65 Torr reached the detection region in about 85 ms, while the
calculated CH3O2 + HO2 reaction half-life at this reduced
pressure in the FAGE inlet was thousands of seconds, and any change in the
CH3O2 concentration is expected to be negligible.
FAGE calibration for CH3O at atmospheric pressure and 293 K;
laser power P=12.9± 0.3 mW and pressure in the detection cell is
2.65 ± 0.05 Torr. The FAGE signal, including the measurement with the
Hg lamp turned off ([CH3O] = 0), was obtained after subtraction of
the offline signal, 12.3 ± 0.9 counts s-1 mW-1. Averaging
time per point = 120 s. The error limits in [CH3O] and the FAGE
signal for the x and y axes respectively are representative of the
1σ overall uncertainty, which contains the total systematic and
statistical errors (see text for details of these). The error limits shown in
the legend are the standard errors in the slope and intercept of the fit to
the experimental data.
Figure 6 shows results obtained from three separate calibration experiments.
In the first two experiments air was humidified by passing a fraction of the
air flow (40 slm total flow rate) through a deionised water bubbler. The
hygrometer measured 7.5 × 1016 molecule cm-3 of water
vapour prior to the calibration flow tube, and the concentration of methane
in the flow tube was 5 × 1016 molecule cm-3. In the
second experiment, a series of FAGE measurements were performed using a
photon flux of ∼ 1.6 × 1014 photon cm-2 s-1 to generate ∼ 4.5 × 109 molecule cm-3 CH3O2. In the third experiment
[CH4] = 1017 molecule cm-3 and all the air flow (now at 20 slm) was passed through the water
bubbler to obtain 3 × 1017 molecule cm-3 H2O vapour. The concentration of CH3O2
was varied by changing the photon flux in the range of 0.5–1.5 × 1014 photon cm-2 s-1 to
generate [CH3O2] = 1.5–4.5 × 1010 molecule cm-3.
FAGE calibration for CH3O2 at atmospheric pressure and
293 K. The data were obtained by three separate experiments: two of them
generating [CH3O2] ≅ 4.5 × 109 molecule cm-3 in the calibration flow tube
(blue circles); laser power P=9.5±0.3 mW and 11.6 ± 0.4 mW
respectively and one experiment using [CH3O2] in the range of
1.5–4.5 × 1010 molecule cm-3 (red circles); P= 9.2 ± 0.2 mW. The pressure in the FAGE detection cell was
maintained at 2.65 ± 0.05 Torr in all experiments. Averaging time per
point = 120 s. The error limits in [CH3O2] and the FAGE signal
for the x and y axes respectively are representative for the 1σ
overall uncertainty, which contains the total systematic and statistical
errors. The error limits shown in the legend are the standard errors in the
slope and intercept of the fit to the experimental data.
CH3O2 second-order decay method
The principle behind this calibration method is that the second-order decay
of CH3O2 is dependent upon its initial concentration, and hence its
quantification offers an alternative way to calibrate the signal. The
experiments were performed in the HIRAC chamber at 295 K and 1 bar of
synthetic air obtained by mixing high purity oxygen (BOC,
> 99.999 %) and nitrogen (BOC, > 99.998 %)
in the ratio of O2 : N2= 1 : 4. Methane (BOC, CP grade,
2–3 × 1017 molecule cm-3) and molecular chlorine (Sigma
Aldrich, ≥ 99.5 %,
0.3–2.1 × 1014 molecule cm-3) were delivered to the
chamber. Eight UV black lamps (Phillips, TL-D 36W/BLB, λ= 350–400 nm) housed in quartz tubes mounted radially inside the reactive
volume were used to photolyse Cl2 to generate Cl atoms and initiate the
chemistry:
CH4+Cl→CH3+HCl,(R12)CH3+O2+M→CH3O2+M.(R2)
Numerical simulations using the chemical system described in Table S3 in the
Supplement showed that [Cl]0= 1–6 × 106 molecule cm-3 (varied by changing the initial [Cl2]). The
high excess of methane (2–3 × 1017 molecule cm-3)
relative to [Cl]0 ensured that the reactions of the Cl atoms with the
self-reaction products formaldehyde and methanol were negligible. In each
HIRAC experiment the lamps were alternatively turned on for 2–3 min and
then off over 1–2 min to generate a series of typically 3–4
CH3O2 kinetic decays.
In order to detect CH3O2 the FAGE instrument was coupled to HIRAC
through a custom-made ISO-K160 flange to sample the gas with a flow rate of
∼ 3 slm. For most measurements, the 1 mm pinhole of the 280 mm long
FAGE inlet sampled ∼ 230 mm from the chamber wall as in the OH
measurements reported previously (Winiberg et al., 2015). Additional
investigations into any CH3O2 gradient across the ∼ 600 mm
radius of HIRAC were conducted using measurements of CH3O2 formed
by the CH4 reaction with O1D generated by the photolysis
of O3 at 254 nm followed by the reaction of the produced CH3
radical with O2 at 295 K and 1 bar of synthetic air. An extended FAGE
inlet (length 520 mm) was used to sample along 500 mm across the chamber
starting with the inlet pinhole flush at the wall. A constant concentration
of CH3O2 was found (within the 10 % overall error of the
measurement) for all the sampled distances of 0–500 mm from the wall (note
that 0 mm here refers to the FAGE inlet being at an equivalent position to
the wall away from the mounting flange). The absence of a CH3O2
gradient across the chamber provides evidence of the efficacy of the mixing
in HIRAC and shows that the wall loss of CH3O2 is negligible and
hence that a shorter inlet, and hence distance from inlet to CH3O2
detection axis, could be used in future CH3O2 FAGE measurements
within HIRAC, improving further the sensitivity.
Methoxy radical measurements within HIRAC
The experiment was carried out in HIRAC at 295 K and 1 bar of N2 (BOC,
> 99.998 %), but without any NO added to the FAGE cell (the
cell furthest from the pinhole as shown in Fig. 1) so that [CH3O] is
measured directly. Initial concentrations in HIRAC were [CH4]0 = 4.50 × 1017 molecule cm-3 and
[Cl2]0= 5.57 × 1015 molecule cm-3. After adding the reagents into
the chamber, the lamps (vide supra) were turned on to generate CH3O by Reaction (R6b).