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A study of night-time chemistry of HO x radicals in the Northern and Southern Hemisphere

This study investigates the night-time chemistry of HOx radicals under unpolluted conditions in the Northern and Southern Hemisphere. The research aims to understand the factors influencing radical concentrations and the main reactions driving the radical cycle. Two models based on the Master Chemical Mechanism are used to simulate night-time chemistry in these conditions. The results show the importance of organic compounds even in unpolluted environments. Perturbation analysis and eigenvalue/eigenvector decomposition provide insights into the dynamics of the kinetic system.

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A study of night-time chemistry of HO x radicals in the Northern and Southern Hemisphere

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  1. A study of night-time chemistry of HOx radicals in the Northern and Southern Hemisphere R. Sommariva1, L. Whitehouse1, M.J. Pilling1, W.J. Bloss1, Z. Fleming2, D.E. Heard1, J.D. Lee3, P.S. Monks2 1) University of Leeds, Leeds, UK 2) University of Leicester, Leicester, UK 3) University of York, York, UK

  2. Field Campaigns • SOAPEX-2 took place in January–February 1999 at Cape Grim, on the north-western tip of Tasmania (40°41’ S, 144°41’ E). • Cape Grim frequently experiences unpolluted air from the Southern Ocean (baseline sector: 190°-280°). • Late evening and early morning measurements of HO2 and night-time measurements of total peroxy radicals were taken on 15 – 16 February. • NAMBLEX took place in July–September 2002 at Mace Head, on the west coast of Ireland (53°19’ N, 9°54’ W). • Air masses arriving at Mace Head from the Atlantic Ocean are considered representative of the background conditions in the Northern Hemisphere (clean sector: 180°-300°). • During the night between the 31 August and 1 September measurements of HO2 by FAGE and total peroxy radicals by PERCA were taken.

  3. Measurements

  4. Objective The aim of this work was to study the night-time chemistry of HOx in the Northern and Southern Hemisphere under unpolluted conditions with no significant night-time radical sources. Questions • What determines the radical concentration at night? • What are the main reactions driving the radical cycle? • What determines the loss from the radical pool?

  5. Modelling • Two models based upon the Master Chemical Mechanism (http://mcm.leeds.ac.uk/MCM/) with different chemical complexity have been used to study night-time chemistry in unpolluted conditions. • The models were built using the procedure outlined in Carslaw et al., 1999. They were constrained to measured H2, O3, NOx, photolysis rates, HCHO, H2O and Temperature. Additionally they were constrained to: • Only the clean model was used for SOAPEX-2, while both models were used for NAMBLEX. • CO and CH4 (clean model). • CO, CH4, 23 NMHCs and 3 Oxygenates (fulloxy model).

  6. SOAPEX-2 15-16 February NAMBLEX 31 August-1 September NAMBLEX 31 August-1 September Model Results The agreement with the measurements is much better with the fulloxy model (NAMBLEX) than with the clean model showing the importance of organic compounds even under unpolluted conditions.

  7. SOAPEX-2, 15-16 February NAMBLEX, 31 August-1 September HO2 Rate of Production/Destruction Analysis On both campaigns main sources of HO2 were the reaction of OH with CO and CH4 and main sinks were the recycling reactions with O3 and NO. Radicals were lost via peroxy-peroxy reactions (mainly HO2+HO2) and aerosol uptake. Radical sources in these unpolluted conditions were negligible.

  8. where Perturbation Analysis • To get a better insight into the dynamics of the kinetic system a perturbation analysis was performed on a simplified version of the clean model. A perturbation Dc was applied to the set of ODEs representing the kinetic system and the propagation of the perturbation was studied. • The variation of the perturbation in time can be written as: • If Λ is the diagonal matrix of eigenvalues of the Jacobian matrix J and U the matrix of the eigenvectors, the solution of the system is:

  9. where Eigenvalue/ Eigenvector Decomposition • For species i the evolution of the perturbation in time is given by: • The perturbation propagates through the system and then relaxes on different timescales (t). Each eigenvalue (l) is related to a different timescale (li = -ti-1) and thus determines the decay of the perturbation on that timescale. • To understand the physical meaning of the eigenvectors a new set of variables (modes) Dz = U-1Dc are defined: • The matrix of the eigenvectorsU-1 can be considered as the linear transformation of the species into the modes showing how each species contributes to the mode associated with that eigenvalue or timescale.

  10. Results (I) • A version of the clean model including simplified inorganic, CO, CH4 chemistry was used for the perturbation analysis. • The system consists of three radicals (OH, HO2, CH3O2). The other species can be considered invariant on these timescales. • The system relaxes on three timescales described by three eigenvalues (l1, l2, l3).

  11. Results (II) • The fastest timescale is described by l1 (~ -1 s-1). This time-scale corresponds to the recycling of radicals through the system and is controlled mainly by OH+CO and OH+CH4 reactions. • The second timescale is described by l2 (~ -1×10-3s-1). In the associated eigenvector the CH3O2 contribution is of the same order of magnitude as the HO2 contribution indicating a strong influence of the OH+CH4 route to HO2 formation. The timescale described by l2 then corresponds to the slow conversion of CH3O2 to HO2 by reaction with NO. • The slowest time-scale is described byl3 (~ -5×105s-1). All three radicals contribute equally to the associated eigenvector. The process involves loss of the coupled radical pool via peroxy-peroxy reactions (mainly HO2+HO2). The long and increasing timescale is a result of the small and decreasing peroxy radical concentrations. • While l2 and l3 are similar in both campaigns, l1 is about 40% higher in NAMBLEX. l1 is approximately equal to the reciprocal of OH lifetime. The difference can be attributed to higher [CO] in the Northern Hemisphere. • A small radical source (O3+butenes) results in a steady radical concentration at night. This does not affect the perturbation analysis.

  12. Conclusions • The agreement between the modelled and measured HO2 and RO2 at night is reasonably good when the model is constrained to hydrocarbons and oxygenates. • Under unpolluted conditions and in the absence of significant radical sources, the radical concentrations remain non-zero at night because loss via second-order radical-radical reactions becomes increasingly small. • The system relaxes in three timescales corresponding to: • Propagation reactions of OH with CO and CH4 • Slow conversion of CH3O2 to HO2via reaction with NO • Removal from the radical pool via radical-radical reactions (HO2+HO2) AcknowledgementsSOAPEX-2 and NAMBLEX participants (Aberystwyth, Birmingham, Cambridge, East Anglia, Leeds, Leicester, UMIST, York Groups), Australian Bureau of Meteorology, Cape Grim and Mace Head staff, BADC, CSIRO-Melbourne, DIAC.

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