Shona C. Smith, Tamsin L. Malkin, L. N. Farrugia, Fred Winiberg, Stephanie Orr, Andrew Goddard, Dwayne E. Heard and Paul W. SeakinsSchool of Chemistry, University of Leeds, Leeds, LS2 9JT, UK

������������

�� �� � � � � � �� �� � �� � �� � � � � � � � � �� �� � � �� � �� � �� �� � � �� � � � � �� � � � � � � � � � � � �� � � � �� � � � � � � � � � � � � � � � � �� � � �

� � � �� � � �� � � � � � � � � �� � �� � �� �� � � � � � � � � � � � � �� � � � � � �� �� ��! "� � � � �

is a NERC funded simulation chamber which utilises a range of instrumentation to study atmospherically relevant gas phase chemical reactions.2

HIRAC Design• 2.25 m3 stainless steel drum (Fig. 2).

• 4 mixing fans give mixing times of �70 s.• 8 rows of photolysis lamps (Fig. 3).

• Uniquely pressure and temperature variable (225 – 325 K).

• Equipped with FAGE (OH and HO2), CRDS (NO3), FTIR, GC-FID (VOCs) and commercial analysers (O3, NOx, H2O).

����������������� � ����� � � ��������� �������������������

���� �������� ������������

#$ %&' () * + (, -. ' ( + ($ * -#$ %&' () * + (, -. ' ( + ($ * -#$ %&' () * + (, -. ' ( + ($ * -#$ %&' () * + (, -. ' ( + ($ * -

/ 0 � 1 �� � � � � � � � �� � � � � �� �� � � � � � � � � � � � � � �� � � � �2 �� � � � �3 �� � �� � � � � � �4 � �655 55

• O3 + isoprene reaction carried out in 200 - 500 fold excess of cyclohexane and[isoprene], [cyclohexanone] and [cyclohexane] measured by GC-FID.

• YOH calculated from plots of type shown in Fig. 4 using equation 1:

��������������� � ���������

!���������

• O3+alkene reactions are recognised as an important source of OH radicals.

• Measurements of OH yields from O3+isoprene in literature range from 0.19 – 0.68.

• IUPAC recommend YOH = 0.25 and Master Chemical Mechanism (MCM v3.1) recommend YOH = 0.27.

• Currently no measurements of YHO2.

• MCM based isoprene ozonolysis mechanism assumes 50% ethene Criegee (CH2OOE) and 50% MVK / MACR Criegees (MVKOOA / MACROOA) formed (Fig. 1).1

• Branching ratios in Fig. 1 designed to give total YOH = 0.27 and YHO2 = 0.26 with no pressure dependence.

‘Biogenic isoprene is highly reactive and is emitted into the troposphere in greater quantities than any other non-methane hydrocarbon’ (Guenther et al.,1995)

789: ;<789: ;<789: ;<789: ;< =>? @AB CD EF GHI F JHI KL 789: M<789: M<789: M<789: M< N > H JH E KO PO EF QR O L

"��!�����

/ YOH from isoprene ozonolysis displays positive pressure dependence (Fig. 6).

• Excellent agreement between YOH measured using both direct and indirect techniques.

• At atmospheric pressure, YOH = 0.26 ± 0.02 in good agreement with previous determinations of Aschmann (0.27), Neeb and Moorgat (0.26) and Paulson (0.26).

Pressure / mbar YOH

1000 (ref 3)

0.26 ± 0.01 �

0.26 ± 0.02�

0.25 ± 0.01 �

850 0.25 ± 0.03�

750 0.23 ± 0.03 �

0.24 ± 0.03�

500 0.22 ± 0.02 �

0.22 ± 0.04�

250 0.19 ± 0.03 �

100 0.17 ± 0.03 �

• First measurements of YHO2 from isoprene ozonolysis measured directly using FAGE.

• YHO2 is independent of pressure.

• We recommend a value YHO2 = 0.26 ± 0.05 taken as the average of our measurements between 300 and 1000 mbar.

YOH = 0.26 ± 0.02for p = 1000 mbar

YHO2 = 0.26 ± 0.05for p = 300 - 1000 mbar

Atmospheric pressure YOH and YHO2 in agreement with MCM recommendations 3

Fig. 6: Pressure dependent OH and HO2 yields from the isoprene ozonolysis measured inside HIRAC (see Table 1).

Table 1: Pressure dependent YOHfrom isoprene ozonolysis. Symbols represent methods indicated in Fig. 6.

Fig. 1: MCM mechanism for isoprene ozonolysis. CH2OOE = ethene Criegee. MVKOOA/MACROOA = MVK/MACR Criegee.1

STUV U WX Y TZ [\]^SX \ TUV U WX Y TZ [\]

S_` X ab T\ T]^SX \ TUV U WX Y TZ [\]cedf

∆∆=

g&' () * + (, -. ' ( + ($ * -g&' () * + (, -. ' ( + ($ * -g&' () * + (, -. ' ( + ($ * -g&' () * + (, -. ' ( + ($ * -

• Temporal [OH] and [HO2] profiles measured by a FAGE instrument calibrated in HIRAC at relevant pressures using 2 new techniques.

• OH and HO2 temporal profiles fitted with a box model based on the MCM (201 species and 655 reactions) (Fig. 5).

• YOH and YHO2 were treated as variable fitting parameters.

(Eqn 1)

Fig. 4: [Cyclohexanone] as a function of change [isoprene] at different chamber pressures.

Fig. 5: Measured [OH] and [HO2] with MCM calculations (YOH and YHO2 variable parameters).

� � � ����� ���������#$� !% & �'� ��(������� ���) ��������#

1) S. M. Saunders et al., Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): troposphericdegradation of non-aromatic volatile organic compounds, Atmos. Chem. Phys., 3, pp. 161-180, 2003.2) D. R. Glowacki et al., Design and initial results from a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC), Atmos. Chem. Phys., 7, pp. 5371-5390, 2007. 3) T. L. Malkin et al., Measurements of OH and HO2 yields from the gas phase ozonolysis of isoprene, Atmos. Chem Phys., 10, pp.1441-1459, 2010.

• Kroll et al. (2001) observed no pressure dependence in prompt YOH from ethene ozonolysis.

• If CH2OOE channel (Fig 1. a) of isoprene ozonolysis is pressure independent, then positive pressure dependence of YOH

potentially indicates a change in distribution between CH2OOE (a) and MVKOOA / MACROOA (b) channels.

• Lack of pressure dependence of YHO2 would therefore indicate a redistribution of the branching ratios within the MACROOA / MVKOOA decomposition.

• Confirm hypothesis using a product study in which change in yields of additional products i.e. propene, MVK, MACR and HCHO are measured as a function of pressure.

a

b

0.0

0.5

1.0

1.5

2.0

2.5

300 550 800 1050 1300 1550 1800 2050 2300 2550

Reaction time / s

[Pro

duct

] / 1

013 m

olec

ule

cm-3 Measured isoprene Measured MACR

Measured MVK Measured propene

Modelled isoprene Modelled MVK

Modelled MACR Modelled propene

• Product studies have been carried out in HIRAC at 1012 mbar air (Fig. 7).

• These data were modelled using the isoprene ozonolysis mechanism extracted from the MCM v3.1 with AtChem.

• [MACR] and [propene] were underpredicted (by up to 50% and 32% respectively) whilst [MVK] was overpredicted (by up to 36%).

• Further product studies at reduced chamber pressures are underway along with analysis of simultaneously measured FTIR spectra.

Fig. 7: Measured and modelled concentrations of isoprene and several reaction products from the ozonolysis of isoprene in 1012 mbar of air.

������������