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Coupled Aerosol Microphysics and Multiphase Halogen Modeling: Insights and Implications

This study explores a comprehensive modeling scheme integrating aerosol microphysics with multiphase halogen chemistry, based on adaptations of Pechtl et al. (2006). We analyze how variations in aerosol size distributions can influence the gas- and condensed-phase chemical interactions, focusing on three methods for generating discrete size distributions. Results from remote marine test cases demonstrate the significant impact of aerosol initialization on chemical exchange rates and deposition schemes, emphasizing the necessity of high-resolution aerosol size characterization for accurate modeling of gas-phase chemistry.

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Coupled Aerosol Microphysics and Multiphase Halogen Modeling: Insights and Implications

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  1. Coupled aerosol microphysics and multiphase halogen modelling Doug Lowe, Dave Topping, Gordon McFiggans University of Manchester (g.mcfiggans@manchester.ac.uk)

  2. Multiphase Halogen Chemistry Chemistry scheme based on the scheme of Pechtl et al (ACP, 2006) – with adaptations to condensed-phase scheme to accommodate PD-FiTE thermodynamics core.

  3. Lower particle number: no chemistry effect Higher particle surface area: increased chemical- exchange rates between gas- and condensed-phases Lower particle volume: slower condensed-phase reactions and lower particulate chemical source term Remote marine testcases – aerosol initialisation 3 methods of generating discrete aerosol size distributions from continuous distributions: 1) Preserve particle number and volume (N/V) 2) Preserve particle number and surface area (N/S) 3) Preserve particle surface area and volume (S/V) S / V N / S N / V NUMBER SURFACE AREA VOLUME Quasi log-normal seasalt mode (after Toyota et al., GRL, 2001)

  4. Organisation of Modelling Studies • Analysis: • Determine influence of initialisation assumptions. For this we will use fixed deposition rates • Determine influence of deposition rate parameterisations.

  5. Size resolution dependence of aqueous species 1 bin per mode: Photochemistry leads to acidification of both modes. Bisulphate dissociation decreases in non-seasalt mode. Diurnal nitrate & sulphate in both modes. Diurnal chloride, bromide & iodide cycling in both modes. 16 bins per mode: Even greater complexity in size segregated composition for all species Further increased structure in composition 4 bins per mode: More complex size segregated composition for all halogen and non-halogen species. Higher resolution exposes increased structure in composition

  6. Influence of Aerosol Initialisation (3/5) Seasalt plus single-bin non-seasalt; fixed turnover rate

  7. Influence of Aerosol Initialisation (4/5) Seasalt plus single-bin non-seasalt; fixed turnover rate

  8. Influence of Deposition Scheme (1/3) Seasalt plus single-bin non-seasalt

  9. Influence of Deposition Scheme (3/3) Seasalt only; offline volumetric averaged turnover rate – 1-bin lifetimes of 0.56 days

  10. Loss rates for each cycle

  11. Conclusions Condensed-phase: A high aerosol size-resolution is necessary to capture variations in sea-salt composition. Gas-phase: Differences in gas-phase chemistry between different aerosol size-resolutions appear to be primarily due to microphysical changes, not changes in condensed-phase chemistry. Choice of initialisation scheme is important for low-resolution models – aerosol surface area and volume must be conserved to accurately capture aerosol influence on gas-phase chemistry. Accurate on-line calculation of size-resolved aerosol deposition and emission rates are important for studying gas-phase chemistry – this is not possible with a low particle size resolution.

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