2. Experimental
2.1. Global chemistry transport model studies
The global chemistry transport model, STOCHEM used in this
study, is a 3-D model that has widely been used in studies of
tropospheric chemistry and subsequently enables advanced spatial
resolution of tropospheric pollution. The model transports amyriad
of chemical species that are subject to chemical reactions and
physical processes. It takes a Langrangian approach with respect to
transport as all species are advected in unison. The chemistry and
transport processes are subsequently uncoupled enabling local
determination of the timestep. Lagrangian cells are advected according
to winds taken from the Meteorological Office Unified
Model archive which stores 6-hourly wind, temperature, specific
humidity, tropopause height and boundary layer depth data on a
1.25 longitude 0.83 latitude grid. The STOCHEM model has an
associated advection timestep of 5 min which is sufficient to
monitor the chemical evolution of each of the 50,000 Lagrangian air
parcels but short enough to maintain numerical stability. An initial
description of the model was given by Collins et al. (1997) and an
updated reportwas detailed by Utembe et al. (2009). This particular
version of the model is described by Derwent et al. (2008) with an
organic aerosol module and a more extensive chemical scheme
introduced by Utembe et al. (2010, 2011). The chemical mechanism
used in STOCHEM, is the common representative intermediates
mechanism version 2 and reduction 5 (CRI v2-R5), referred to as
‘STOCHEM-CRI’. The detail of the CRI v2-R5 mechanism is given by
Jenkin et al. (2008), Watson et al. (2008), and Utembe et al. (2009)
with updates highlighted in Utembe et al. (2010). The mechanism
describes the oxidation of 27 emitted VOCs and consists of 229
chemical species taking part in 627 chemical and photochemical
2. Experimental2.1. Global chemistry transport model studiesThe global chemistry transport model, STOCHEM used in thisstudy, is a 3-D model that has widely been used in studies oftropospheric chemistry and subsequently enables advanced spatialresolution of tropospheric pollution. The model transports amyriadof chemical species that are subject to chemical reactions andphysical processes. It takes a Langrangian approach with respect totransport as all species are advected in unison. The chemistry andtransport processes are subsequently uncoupled enabling localdetermination of the timestep. Lagrangian cells are advected accordingto winds taken from the Meteorological Office UnifiedModel archive which stores 6-hourly wind, temperature, specifichumidity, tropopause height and boundary layer depth data on a1.25 longitude 0.83 latitude grid. The STOCHEM model has anassociated advection timestep of 5 min which is sufficient tomonitor the chemical evolution of each of the 50,000 Lagrangian airparcels but short enough to maintain numerical stability. An initialdescription of the model was given by Collins et al. (1997) and anupdated reportwas detailed by Utembe et al. (2009). This particularversion of the model is described by Derwent et al. (2008) with anorganic aerosol module and a more extensive chemical schemeintroduced by Utembe et al. (2010, 2011). The chemical mechanismused in STOCHEM, is the common representative intermediatesmechanism version 2 and reduction 5 (CRI v2-R5), referred to as‘STOCHEM-CRI’. The detail of the CRI v2-R5 mechanism is given byJenkin et al. (2008), Watson et al. (2008), and Utembe et al. (2009)with updates highlighted in Utembe et al. (2010). The mechanismdescribes the oxidation of 27 emitted VOCs and consists of 229chemical species taking part in 627 chemical and photochemical
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