Fittschen, Int. J. Chem. Kinet. 7 263 K - hirac.leeds.ac.uk
1
Stephanie Orr , 1 Fred Winiberg, 1 Charlotte Brumby, 1 Iustinian Bejan, 1 Terry Dillon 2 and Paul Seakins 1 Direct detection of OH yields from a temperature dependent study of HOCH 2 O 2 + HO 2 in HIRAC 1 University of Leeds, UK; 2 University of York, UK Corresponding author: [email protected] Results • Direct measurements of OH using FAGE, confirming channel (2c). • Modelling performed in Kintecus using a chemical mechanism based on that described in Jenkin et al. (2007) [5] shows good agreement with measurements at all temperatures (example dataset at T = 273 K shown in Figure 3). • Branching ratios for R2 included in the model are: Y 2a = 0.5; Y 2b = 0.3; Y 2c = 0.2, as reported by Jenkin et al. (2007) [5]. • No temperature dependence of the OH channel observed, Y 2c = 0.2 gave a good model fits to the data at all measured temperatures (Figure 7). • Discrepancy between model/measurement Experimental • Experiments performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) (Figure 2). • Chamber has 8 rows of internal photolysis lamps (λ ~ 360 nm) and 4 mixing fans (total mixing time ~ 70 s). • Experiments conducted at T = 263, 273, 283 and 293 K and p = 1000 mbar and additional experiments to quantify wall loss rates. • HO 2 and HCHO generated by reacting methanol with chlorine atoms: References [1] J. Orlando and G. Tyndall, Chem. Soc. Rev. 41, 6294-6317 (2012); [2] J. Lelieveld et al., Nature, 452, 737-740 (2008); [3] F. A. F. Winiberg, Ph.D Thesis, University of Leeds (2014); [4] P. Morajkar, C. Schoemaecker, M. Okumura and C. Fittschen, Int. J. Chem. Kinet. 46, 245-259 (2014); [5] M. E. Jenkin, M. D. Hurley and T. J. Wallington, PCCP 9, 3149-3162 (2007); [6] T. L. Nguyen, L. Vereecken and J. Peeters, Z. Phys. Chem. 224, 1081-1093 (2010). Introduction • Recent studies have shown the reactions between organic peroxy radicals (RO 2 ) and the hydroperoxyl radical (HO 2 ) to be radical propagating [1]. • RO 2 + HO 2 reactions in pristine (low NOx) environments are the major tropospheric sink of RO 2 and key components of the OH-initiated oxidation of isoprene. • It is thought that they may proceed via OH-recycling mechanisms and hence account for model/measurement Conclusions and Future Work • Successful experiments of the reaction of HO 2 + HOCH 2 O 2 at T = 263 - 293 K and p = 1000 mbar. • Reactant HO 2 and products OH, HCHO and HCOOH were observed directly. • Only study to measure both radicals and stable products directly. • ROP and ROD analyses validate measurements of species; the target reaction is well understood. • Good agreement between measurements and literature (Y 2a = 0.5; Y 2b = 0.3; Y 2c = 0.2 gave good model fits) [5]. • No temperature dependence has been observed between T = 263 – 293 K, good model fits to data where Y 2C = 0.2 at all studied temperatures (Figure 7). • HO 2 measurements need to be scrutinised in more detail. • Constrain the system to measurements, including OH, and use the model to calculate Y 2C at all temperatures. • Future plans to look at HO 2 + CH 3 CHO equilibrium which is also of importance to the CH 3 C(O)O 2 + HO 2 reaction. Figure 1: The different product channels of the hydroxymethyl peroxy + HO 2 reaction. Figure 2: (a) The HIRAC chamber. (b) 8 rows of photolysis lamps inside the chamber. Instrument Species measured FTIR Methanol, formic acid, formaldehyde GC-FID Methanol Fluorescence Assay by Gas Expansion (FAGE) OH, HO 2 Commercial analysers NO, NO 2 , O 3 (all were below LODs throughout experiments) Cl 2 + hn → 2Cl Cl + CH 3 OH → CH 2 OH + HCl CH 2 OH + O 2 → HO 2 + HCHO • Reactants and products were monitored with a time resolution of ~60 s (Table 1). Table 1: Instruments used and species measured throughout experiments. (b) Understanding the mechanism • It is important to look at the contribution of individual reactions to overall product formation, to ensure that the overall process is well understood. • For our system, rate of production (ROP) and rate of destruction (ROD) analyses were carried out for the experimental data for all measurable species. • Figure 5 shows that the target reaction (HO 2 + HOCH 2 O 2 ) dominates OH formation chemistry through to t = 600 s. • Up to t = 600 s, OH loss reactions are well characterised (Figure 6). • Because ROP = ROD, which is well characterised, and the target reaction dominates OH production, we understand our target reaction well. • This is also the case for HO 2 , HCHO and for HCOOH (>99% produced from R2b&c, >97% removed by reactor walls). Therefore, experiments are sensitive to Y 2b and Y 2c . (a) discrepancies of HO x (OH + HO 2 ) [2]. • Reaction with formaldehyde (HCHO) is an important influence on HO 2 in the troposphere, especially at low temperatures, and is of importance to laboratory experiments where [HCHO] is high. The reaction is a key area of uncertainty in the CH 3 C(O)O 2 + HO 2 reaction, which has recently been studied in this laboratory [3]. • The HO 2 + HCHO equilibrium constant (R1) is well established in the literature and the rate constants, k 1 and k -1, have recently been measured directly [4]. However, the fate of HOCH 2 O 2 , which predominantly reacts with HO 2 (R2, Figure 1) , is less well characterised. • OH has been measured indirectly in a chamber study [5], which proposed that the reaction proceeds via three channels (R2a-c, Figure 1) - the only determination to date of these branching ratios. • Theoretical calculations have provided further evidence of an OH channel [6]. • This work aims to measure the branching ratio of the OH-producing channel of the hydroxymethyl peroxy + HO 2 reaction (channel (2c), Figure 1) and its variation with temperature. 0 200 400 600 0 1x10 7 2x10 7 3x10 7 4x10 7 5x10 7 6x10 7 7x10 7 0 200 400 600 0.0 2.0x10 10 4.0x10 10 6.0x10 10 8.0x10 10 1.0x10 11 1.2x10 11 1.4x10 11 0 200 400 600 0.0 5.0x10 12 1.0x10 13 1.5x10 13 2.0x10 13 0 200 400 600 2.0x10 14 2.5x10 14 3.0x10 14 3.5x10 14 0 200 400 600 0 1x10 13 2x10 13 3x10 13 4x10 13 5x10 13 [OH] / molecule cm -3 Time / s [HO 2 ] / molecule cm -3 Time / s [HCOOH] / molecule cm -3 Time / s [MeOH] / molecule cm -3 Time / s [HCHO] / molecule cm -3 Time / s Figure 3: Measured and modelled reactants and products using data obtained from an experiment at T = 273 K. Model includes IUPAC recommended rate constants. 0 200 400 600 0 1x10 6 2x10 6 3x10 6 4x10 6 5x10 6 6x10 6 7x10 6 8x10 6 [OH] / molecule cm -3 Time / s Cl + HO2 ==> ClO + OH HOCH2O2 + HO2 ==> HCOOH + HO2 + OH + O2 H + HO2 ==> OH + OH OH + HO2 ==> H2O OH + H2O2 ==> HO2 + H2O OH + Cl2 ==> Cl + HOCl OH + HCl ==> Cl + H2O OH + CH3OH ==> HCHO + HO2 + H2O OH + HCHO ==> CO + HO2 + H2O of HO 2 at all temperatures (30-50% over-prediction). • Measured OH yields at T = 293 K , where [OH] is approaching the limit of detection (LOD) of the FAGE instrument (~1.5 × 10 6 ), are in good agreement with the model (Figure 4). Figure 5: Relative contribution of OH production reactions at different reaction times. Figure 6: Relative contribution of OH destruction reactions at different reaction times. 60 s 600 s 600 s 60 s Figure 4: Measured and modelled OH for experiment at T = 293 K. 0 200 400 600 0.0 2.0x10 7 4.0x10 7 6.0x10 7 8.0x10 7 1.0x10 8 293 K 283 K 273 K 263 K [OH] / molcule cm -3 Reaction time / s Figure 7: Model/measurement comparisons of [OH] for T = 263, 273, 283 & 293 K.
Transcript of Fittschen, Int. J. Chem. Kinet. 7 263 K - hirac.leeds.ac.uk
Stephanie Orr,1 Fred Winiberg,1 Charlotte Brumby,1 Iustinian Bejan,1 Terry Dillon2 and Paul Seakins1
Direct detection of OH yields from a temperature dependent study of HOCH2O2 + HO2 in HIRAC
1University of Leeds, UK; 2University of York, UK Corresponding author: [email protected]
Results • Direct measurements of OH using FAGE, confirming
channel (2c).
• Modelling performed in Kintecus using a chemical
mechanism based on that described in Jenkin et al.
(2007) [5] shows good agreement with
measurements at all temperatures (example dataset
at T = 273 K shown in Figure 3).
• Branching ratios for R2 included in the model are: Y2a
= 0.5; Y2b = 0.3; Y2c = 0.2, as reported by Jenkin et al.
(2007) [5].
• No temperature dependence of the OH channel
observed, Y2c = 0.2 gave a good model fits to the
data at all measured temperatures (Figure 7).
• Discrepancy between model/measurement
Experimental
• Experiments performed in the Highly Instrumented Reactor for Atmospheric
Chemistry (HIRAC) (Figure 2).
• Chamber has 8 rows of internal photolysis lamps (λ ~ 360 nm) and 4 mixing fans
(total mixing time ~ 70 s).
• Experiments conducted at T = 263, 273, 283 and 293 K and p = 1000 mbar and
additional experiments to quantify wall loss rates.
• HO2 and HCHO generated by reacting methanol with chlorine atoms:
References [1] J. Orlando and G. Tyndall, Chem. Soc. Rev. 41, 6294-6317 (2012); [2] J. Lelieveld et al., Nature, 452, 737-740 (2008); [3] F. A. F. Winiberg, Ph.D Thesis, University of Leeds (2014); [4] P. Morajkar, C. Schoemaecker, M. Okumura and C. Fittschen, Int. J. Chem. Kinet. 46, 245-259 (2014); [5] M. E. Jenkin, M. D. Hurley and T. J. Wallington, PCCP 9, 3149-3162 (2007); [6] T. L. Nguyen, L. Vereecken and J. Peeters, Z. Phys. Chem. 224, 1081-1093 (2010).
Introduction • Recent studies have shown the reactions
between organic peroxy radicals (RO2)
and the hydroperoxyl radical (HO2) to be
radical propagating [1].
• RO2 + HO2 reactions in pristine (low NOx)
environments are the major tropospheric
sink of RO2 and key components of the
OH-initiated oxidation of isoprene.
• It is thought that they may proceed via
OH-recycling mechanisms and hence
account for model/measurement
Conclusions and Future Work
• Successful experiments of the reaction of HO2 + HOCH2O2
at T = 263 - 293 K and p = 1000 mbar.
• Reactant HO2 and products OH, HCHO and HCOOH were
observed directly.
• Only study to measure both radicals and stable products
directly.
• ROP and ROD analyses validate measurements of
species; the target reaction is well understood.
• Good agreement between measurements and literature
(Y2a = 0.5; Y2b = 0.3; Y2c = 0.2 gave good model fits) [5].
• No temperature dependence has been observed
between T = 263 – 293 K, good model fits to data where
Y2C = 0.2 at all studied temperatures (Figure 7).
• HO2 measurements need to be scrutinised in more detail.
• Constrain the system to measurements, including OH,
and use the model to calculate Y2C at all temperatures.
• Future plans to look at HO2 + CH3CHO equilibrium which
is also of importance to the CH3C(O)O2 + HO2 reaction.
Figure 1: The different product channels of the hydroxymethyl peroxy
+ HO2 reaction.
Figure 2: (a) The HIRAC chamber. (b) 8 rows of photolysis lamps inside the chamber.
Instrument Species measured
FTIR Methanol, formic acid, formaldehyde
GC-FID Methanol
Fluorescence Assay by Gas Expansion (FAGE) OH, HO2
Commercial analysers NO, NO2, O3 (all were below LODs throughout experiments)
Cl2 + hn → 2Cl
Cl + CH3OH → CH2OH + HCl
CH2OH + O2 → HO2 + HCHO
• Reactants and products were monitored with a time resolution of ~60 s (Table 1).
Table 1: Instruments used and species measured throughout experiments.
(b)
Understanding the mechanism • It is important to look at the contribution of individual
reactions to overall product formation, to ensure that the
overall process is well understood.
• For our system, rate of production (ROP) and rate of
destruction (ROD) analyses were carried out for the
experimental data for all measurable species.
• Figure 5 shows that the target reaction (HO2 + HOCH2O2)
dominates OH formation chemistry through to t = 600 s.
• Up to t = 600 s, OH loss reactions are well characterised
(Figure 6).
• Because ROP = ROD, which is well characterised, and the
target reaction dominates OH production, we understand
our target reaction well.
• This is also the case for HO2, HCHO and for HCOOH (>99%
produced from R2b&c, >97% removed by reactor walls).
Therefore, experiments are sensitive to Y2b and Y2c.
(a)
discrepancies of HOx (OH + HO2) [2].
• Reaction with formaldehyde (HCHO) is an important influence on HO2 in the troposphere, especially at low
temperatures, and is of importance to laboratory experiments where [HCHO] is high. The reaction is a key
area of uncertainty in the CH3C(O)O2 + HO2 reaction, which has recently been studied in this laboratory [3].
• The HO2 + HCHO equilibrium constant (R1) is well established in the literature and the rate constants, k1 and
k-1, have recently been measured directly [4]. However, the fate of HOCH2O2, which predominantly reacts
with HO2 (R2, Figure 1), is less well characterised.
• OH has been measured indirectly in a chamber study [5], which proposed that the reaction proceeds via
three channels (R2a-c, Figure 1) - the only determination to date of these branching ratios.
• Theoretical calculations have provided further evidence of an OH channel [6].
• This work aims to measure the branching ratio of the OH-producing channel of the hydroxymethyl peroxy +
HO2 reaction (channel (2c), Figure 1) and its variation with temperature.
0 200 400 600
0
1x107
2x107
3x107
4x107
5x107
6x107
7x107
0 200 400 600
0.0
2.0x1010
4.0x1010
6.0x1010
8.0x1010
1.0x1011
1.2x1011
1.4x1011
0 200 400 600
0.0
5.0x1012
1.0x1013
1.5x1013
2.0x1013
0 200 400 6002.0x10
14
2.5x1014
3.0x1014
3.5x1014
0 200 400 600
0
1x1013
2x1013
3x1013
4x1013
5x1013
[OH
] /
mo
lec
ule
cm
-3
Time / s
[HO
2] /
mo
lec
ule
cm
-3
Time / s
[HC
OO
H]
/ m
ole
cu
le c
m-3
Time / s [Me
OH
] /
mo
lec
ule
cm
-3
Time / s
[HC
HO
] /
mo
lec
ule
cm
-3
Time / s
Figure 3: Measured and modelled
reactants and products using data
obtained from an experiment at T = 273
K. Model includes IUPAC recommended
rate constants.
0 200 400 600
0
1x106
2x106
3x106
4x106
5x106
6x106
7x106
8x106
[OH
] /
mole
cule
cm
-3
Time / s
OH Formation @ 60 s
Cl + HOCH2OOH ==> HCOOH + OH + HCl
Cl + HO2 ==> ClO + OH
HOCH2O2 + HO2 ==> HCOOH + HO2 + OH + O2
H + HO2 ==> OH + OH
OH Formation @ 600 s
Cl + HOCH2OOH ==> HCOOH + OH + HCl
Cl + HO2 ==> ClO + OH
HOCH2O2 + HO2 ==> HCOOH + HO2 + OH + O2
H + HO2 ==> OH + OH
OH Formation @ 10 s
Cl + HOCH2OOH ==> HCOOH + OH + HCl
Cl + HO2 ==> ClO + OH
HOCH2O2 + HO2 ==> HCOOH + HO2 + OH + O2
H + HO2 ==> OH + OH
OH Formation @ 10 s
Cl + HOCH2OOH ==> HCOOH + OH + HCl
Cl + HO2 ==> ClO + OH
HOCH2O2 + HO2 ==> HCOOH + HO2 + OH + O2
H + HO2 ==> OH + OH
OH Destruction @ 60 sOH + HO2 ==> H2O
OH + H2O2 ==> HO2 + H2O
OH + HCOOH ==> CO2 + H + H2O
OH + HOCH2OOH ==> HOCH2O2 + H2O
OH + HOCH2OH ==> HCOOH + H + H2O
OH + Cl2 ==> Cl + HOCl
OH Destruction @ 600 sOH + HO2 ==> H2O
OH + H2O2 ==> HO2 + H2O
OH + HCOOH ==> CO2 + H + H2O
OH + HOCH2OOH ==> HOCH2O2 + H2O
OH + HOCH2OH ==> HCOOH + H + H2O
OH + Cl2 ==> Cl + HOCl
-7%
0% -5%
0%
-84%
-4%
OH Destruction @ 10 sOH + HO2 ==> H2O
OH + H2O2 ==> HO2 + H2O
OH + Cl2 ==> Cl + HOCl
OH + HCl ==> Cl + H2O
OH + CH3OH ==> HCHO + HO2 + H2O
OH + HCHO ==> CO + HO2 + H2O
-7%
0% -5%
0%
-84%
-4%
OH Destruction @ 10 sOH + HO2 ==> H2O
OH + H2O2 ==> HO2 + H2O
OH + Cl2 ==> Cl + HOCl
OH + HCl ==> Cl + H2O
OH + CH3OH ==> HCHO + HO2 + H2O
OH + HCHO ==> CO + HO2 + H2O
-7%
0% -5%
0%
-84%
-4%
OH Destruction @ 10 sOH + HO2 ==> H2O
OH + H2O2 ==> HO2 + H2O
OH + Cl2 ==> Cl + HOCl
OH + HCl ==> Cl + H2O
OH + CH3OH ==> HCHO + HO2 + H2O
OH + HCHO ==> CO + HO2 + H2O
of HO2 at all temperatures (30-50%
over-prediction).
• Measured OH yields at T = 293 K ,
where [OH] is approaching the limit
of detection (LOD) of the FAGE
instrument (~1.5 × 106), are in good
agreement with the model (Figure 4). Figure 5: Relative contribution of OH
production reactions at different
reaction times.
Figure 6: Relative contribution of OH
destruction reactions at different
reaction times.
60 s
600 s
600 s
60 s
Figure 4: Measured and modelled OH for
experiment at T = 293 K.
0 200 400 600
0.0
2.0x107
4.0x107
6.0x107
8.0x107
1.0x108
293 K
283 K
273 K
263 K
[OH
] /
molc
ule
cm
-3
Reaction time / s
Figure 7: Model/measurement comparisons of [OH] for
T = 263, 273, 283 & 293 K.
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