Simulation experiment in SAPHIR
Experiments were performed in the outdoor atmospheric simulation chamber
SAPHIR. Details of the chamber can be found in previous publications
e.g.. SAPHIR has a cylindrical shape (length
18 m, diameter 5 m, volume 270 m3 and consists of a
double-wall FEP (fluorethylene-propylene) film. A small overpressure
(45 Pa) prevents ambient air entering the chamber. The replenishment
flow that is required to maintain this pressure leads to a dilution of all
trace gases by approximately 3 to 5 % per hour. A shutter system shades the
chamber before the photooxidation experiment is started. Natural sunlight is
used to irradiate the mixture. Small sources of nitrous acid (HONO)
and formaldehyde (HCHO) are present in the sunlit chamber (100 to
200 pptvh-1). The photolysis of HONO is typically the primary
source for OH radicals and nitrogen oxides.
In total, four experiments were conducted in this study, two of them at low
NO (23 June 2016: NO < 70 pptv and 23 May 2017:
NO < 40 pptv) and two of them at high NO conditions
(20 August 2014: 0.7 to 6 ppbv NO and 17 May 2017: approximately
0.1 to 0.4 ppbv NO). Results from experiments performed at
similar conditions gave consistent results. The discussion of results focuses
on the two experiments for which the number of trace gas measurements was
highest and results for the other experiments are shown in the Supplement.
The experiments started with cleaning the chamber air by flushing out
impurities from previous experiments until trace gas concentrations were
below the detection limit of the instruments. The chamber air was first
humidified by flushing water vapour from boiling water into the dark chamber
(relative humidity approximately 70 %). In the low NO experiments,
approximately 140 ppbv ozone produced from a silent discharge
ozoniser (O3onia) was injected in the dark in order to suppress NO
concentrations. In contrast, 6 to 10 ppbv of NO2 or NO
were injected from a gas mixture in case of the high NO experiments.
In one of the two low NO experiments (23 May 2017), 20 ppbv
MVK (Sigma-Aldrich, purity 99 %) in water was injected in the dark
chamber from a Liquid Calibration Unit (LCU, Ionicon). MVK
(1.5 ppbv) was reinjected after 3.5 h of photooxidation. In the
other experiments, MVK was injected several times during the
experiment after an initial phase of illumination of the chamber air without
additional OH reactants by injecting liquid MVK into a heated
inlet line that is flushed by synthetic air. This procedure was similar to
that applied in previous studies e.g.. The photooxidation of MVK was then observed for several
hours. No significant particle formation was observed in the experiments so
that only gas-phase chemistry needs to be considered in the evaluation.
Instrumentation
Trace gas concentrations were measured with a comprehensive set of
instruments. Nitric oxide (NO) was detected by chemiluminescence (Eco
Physics) and nitrogen dioxide (NO2) by the same instrument but with a
blue-light converter in the inlet. In one of the experiments (17 May 2017),
no NOx measurements were available. A cavity ring-down instrument
(Picarro) monitored water vapour and carbon monoxide and a UV photometer
(Ansyco) detected ozone.
The total OH reactivity (inverse lifetime of OH) was measured
by a pump-probe method , in which the decay of
OH radicals produced by laser flash photolysis of ozone is observed by
laser-induced fluorescence (LP-LIF). OH reactivity gives a measure of
all OH reactant concentrations, so that potential gaps in the
detection of e.g. organic compounds that are relevant for the radical
chemistry can be identified e.g.. Unfortunately, the
instrument failed in 2014, thus OH reactivity was only measured in
one of the two high NO experiments.
Organic compounds were measured by a proton-transfer time-of-flight mass
spectrometer (PTR-TOF-MS, Ionicon), which was calibrated to quantify
MVK. Methylglyoxal (CHOCOCH3, MGLYOX), and
glycolaldehyde (HOCH2CHO) were quantified in one low and one high
NO experiment. In the other two experiments, performed in different
years, the PTR-TOF-MS was calibrated for MVK, but not for
methylglyoxal and glycolaldehyde for all experiments so that these species
could not be quantified in all experiments. Acetic acid was detected on the
same mass as glycolaldehyde in the PTR-TOF-MS instrument. However, model
calculations suggest that the contribution of acetic acid was less than
10 % of the total signal. Therefore, measurements represent glycolaldehyde
concentrations reasonably well.
A second PTR-TOF-MS instrument (PTR-3, Ionicon) quantified MVK
concentrations in the experiment on 23 May 2017. Measurements of both
instruments agreed within 20 %. In addition to direct measurements of
MVK concentrations, measurements of the OH reactivity can be
used to calculate the MVK concentration that was injected in the
experiments because the increase in OH reactivity at that point in
time can be attributed to the MVK concentration increase. The
comparison with the increase in MVK measurements by the PTR-TOF-MS
instrument shows good agreement.
In the experiments in 2017, formaldehyde was measured by the same
differential optical absorption spectroscopy (DOAS) instrument that also
detects OH radicals in the chamber . In the other
years, HCHO was measured by a Hantzsch monitor. The
1σ-precision of the formaldehyde measurement of 230 pptv is
less than that of the Hantzsch monitor (20 pptv), but it is
sufficiently high for the detection of HCHO in the experiments here.
Simplified OH oxidation scheme for MVK. Names of compounds
are assigned similar to MCM. Modifications to the MCM mechanism (M2) applied
in model sensitivity runs M1 and M2 (Table ) are shown as
grey arrows. Reaction yields are calculated for conditions of the experiment
with low NO (high ozone concentrations) on 23 May 2017. Grey numbers
refer to model run M2.
OH was detected by DOAS in all experiments except
for the high NO experiment in 2014. In addition, OH,
HO2 and RO2 radicals were measured by laser-induced
fluorescence (LIF). The instrument has been described in detail elsewhere
. OH concentrations measured
by LIF in the SAPHIR chamber have been shown to agree with measurements by
DOAS in several comparison exercises e.g.. Good agreement was also observed in this work so that
significant potential artefacts in the LIF detection scheme as reported for
some instruments in the field can
be excluded.
HO2 and RO2 are chemically converted to OH by the
reaction with NO prior to OH detection by laser-induced
fluorescence in the LIF instrument. The conversion of RO2 requires at
least two subsequent reactions with NO. First, RO2 is
converted through adding NO to HOx in a flow reactor upstream of the
fluorescence cell. Added CO in the reactor ensures that the
HOx consists predominantly of HO2, which has a small wall
loss compared to OH. The reactor is operated at higher pressure
(25 hPa) compared to the low-pressure (4 hPa) LIF detection
cell . The HOx is sampled from the reactor into the
LIF detection cell where HO2 is converted by a large excess of added
NO to OH. Operational parameters of the RO2 system are
optimized for the efficient detection of RO2 radicals that have a
similar reaction rate with NO as methylperoxy radicals. As a
consequence, RO2 radicals are not efficiently detected, if their
reaction with NO does not directly and quantitatively result in the
production of HO2. This is for example the case for the peroxy
radical HMVKBO2 (as named in the MCM) that is formed from the
reaction of MVK with OH (see below for details). This has to be
taken into account, if measured RO2 radicals are compared with model
calculations.
The HO2 detection cell consists of a fluorescence cell, in which
HO2 reacts with excess NO that is injected behind the inlet
nozzle. As shown for several LIF instruments, the HO2 signal can also
contain contributions from RO2 radicals that rapidly form HO2
in the reaction with NO . This
applies for those RO2 radicals which form an alkoxy radical
(RO) in the reaction with NO that rapidly produces HO2
and other products. The interferences from RO2 can be minimised;
however, if the instrument is operated with an NO concentration, for
which the HO2 to OH conversion efficiency is only
approximately less than 10 %. In this case, the RO2 to OH
conversion efficiency becomes much smaller for all RO2 species,
because the two reactions with NO needed to produce OH limit
the overall conversion efficiency. In this study, the HO2 channel of
the LIF instrument was operated such that RO2 interferences can be
assumed to be negligible.
In one of the experiments (23 May 2017), HO2 was additionally
detected by a newly developed chemical ionisation mass spectrometry (CIMS)
instrument using Br- as ionisation reagent. HO2 is detected
as cluster ion similar to the approaches reported by using an I- and Br- CIMS, respectively. Details
of this new instrument will be presented in a separate publication.
HO2 measurements of the CIMS instrument agreed with [HO2]
detected by the LIF instrument within 15 %.
Solar radiation was measured outside the chamber using a spectroradiometer.
Photolysis frequencies are then calculated by applying a model to transfer
outside conditions to conditions inside the chamber . Latest recommendations for absorption spectra and photolysis
yields are used.
Changes of reactions and additional reactions applied to the MCM.
Reaction
Reaction rate constant
Reference
MCM*:
R1:
OH + MVK → HMVKAO2
0.24×2.6×10-12exp(610KT-1) cm3s-1
R2:
OH + MVK → HMVKBO2
0.76×2.6×10-12exp(610KT-1) cm3s-1
HOCH2CHO + OH → products
8×10-12 cm3s-1
MVK + O3 → OH + products
0.16×8.5×10-16exp(-1520KT-1)
,
M1 (includes MCM*):
R3:
HMVKBO2 + HO2 → HMVKBOOH
0.34×0.625 KRO2HO2a
R4:
HMVKBO2 + HO2 → HMVKBO + OH
0.48×0.625 KRO2HO2a
R5:
HMVKBO2 + HO2 → BIACETOH + OH + HO2
0.18×0.625 KRO2HO2a
M2 (includes M1 and MCM*):
R7:
HMVKAO2 → HO2 + BIACETOOH
0.003 s-1b
This work
R8:
HMVKBO2 (+X)c → HO2 + HOCH2CHO
(0.006±0.004) s-1
This work
+HCHO + CO
HOCH2CO3 + NO2 → PHAN
0
This work
a value from MCM: 0.625 KRO2HO2 =2.91×10-13exp(1300KT-1) cm3s-1
b from theoretical calculation (see Table ).
c A reaction partner could not be determined from these experiments.
Model calculations
Model calculations were performed using the Master Chemical Mechanism in its
latest version 3.3.1 . A simplified reaction scheme is shown in
Fig. . The MCM mechanism was modified (MCM*) to take results
reported in literature into account and findings in this work. Details are
listed in Table .
Chamber specific properties were added such as dilution of traces gases due
to the replenishment flow. Sources for HONO and HCHO production
from the chamber were parameterised as described in previous publications
e.g..
Model calculations were constrained to physical parameters (pressure,
temperature, photolysis frequencies and dilution rate of trace gases). A small,
constant background OH reactivity of unknown OH reactants that
was measured by the OH reactivity instrument after humidification of
the chamber air was modelled as an OH reactant that converts OH
to HO2. However, the magnitude of this background reactivity was
small (<1 s-1) compared to the OH reactivity from
MVK during the experiment (>15 s-1) so that it did not
affect the chemistry.
Injections of trace gases were modelled as sources during the time of
injection, but injected trace gases were not constrained to measured values
at later times. [NO], [NO2] and [O3] were only
constrained to measurements for the high NOx experiment, because
differences between modelled and measured values would have led to
significant differences in other observables. No modelling could be performed
for one of the high NO experiments (17 May 2017) due to the lack of
NOx measurements. No measurements for the reaction rate constants of
RO2 species from MVK exist. The sensitivity of model results
to a change of the RO2 reaction rate constants, however, is rather
small so that their uncertainties could not explain observed
model–measurements discrepancies.
H-migration and HO2 elimination in
hydroxy-MVK-peroxy radicals. Barrier height Eb, reaction energy
Ereact and the rate coefficient k at a temperature of 300 K
are listed. Arrhenius expressions for a temperature range between 200 and
400 K are available in the Supplement.
Reactant
Reaction class
Product
Eb
Ereact
k (300 K)
kcal mol-1
kcal mol-1
s-1
HMVKAO2
-OH 1,5-H-shift
CH3-C(=O)-CH(O•)-CH2OOH
21.6
20.4
5.0×10-4
α-OH 1,4-H-shift
CH3-C(=O)-C•(OH)-CH2OOH
24.7
-6.2
3.3×10-3
-CH3 1,6-H-shift
C•H2-C(=O)-CH(OH)-CH2OOH
23.1
10.2
5.9×10-4
HMVKBO2
-OH 1,5-H-shift
CH3-C(=O)-CH(OOH)-CH2O•
22.5
20.6
8.8×10-5 a
α-OH 1,4-H-shift
CH3-C(=O)-CH(OOH)-C•HOH
25.1
6.5
3.2×10-5
-CH3 1,6-H-shift
C•H2-C(=O)-CH(OOH)-CH2OH
27.4
10.0
3.8×10-5
HO2 elimination
CH3-C(=O)-CH=CHOH + HO2
30.0
-1.5
6.1×10-10
a estimated at 0.01 s-1 by .
Quantum-chemical calculations
A set of H-migration reactions for the main MVK-derived peroxy
radicals was investigated by quantum chemical and theoretical kinetic
methodologies. The reactions studied included migration of hydroxyl,
α-OH, and methyl H-atoms; direct HO2
elimination forming an enol was also investigated (Table ).
Several methodologies were applied, as detailed in the supporting
information. From these data, the M06-2X/cc-pVTZ rovibrational data
, with CCSD(T)/aug-schwartz4(DT) single point
energy calculations extrapolated to the basis set limit were selected. All quantum chemical calculations were performed
using the Gaussian-09 program suite . The high-pressure
rate coefficients for each of the elementary processes was then calculated
using multi-conformer canonical transition state theory, MC-CTST
based on a rigid rotor harmonic oscillator
paradigm, an exhaustive search of the reactants and TS conformers, and
asymmetric Eckart tunnelling and WKB zero-curvature (ZCT) tunnelling. For the
1,4- and 1,6-H-shift in HMVKAO2, a large difference between
Eckart and ZCT tunnelling was found; the geometric average is reported here
(see Supplement).
Time series of radicals, inorganic and organic species during the
MVK photooxidation for the high NO experiment (20 August 2014)
together with results from model calculations applying MCM. Dark shaded areas
indicate the time before opening the chamber roof and vertical dashed line
times when trace gases were injected into the chamber. OH reactivity was not
measured during this experiment. NO, NO2 and O3 are
constrained to measurements in the model. RO2 loss rates (most lowest
right panel) are calculated from modelled HO2, RO2 and
NO concentrations. However, contributions from the reactions with
RO2 and HO2 or RO2 are too small to be visible.
Modelled acetic acid concentrations are small compared to modelled
glycolaldehyde concentrations (measured together in the PTR-TOF-MS).
Time series of radicals, inorganic and organic species during the
MVK photooxidation at low NO (23 May 2017) together with results from
model calculations applying MCM. Dark shaded areas indicate the time before
opening the chamber roof and vertical dashed line times when trace gases were
injected into the chamber. RO2 loss rates (most lowest right panel)
are calculated from modelled HO2, RO2 and NO
concentrations.