Environmental Engineering Reference
In-Depth Information
2006; Smyth et al., 2009) have been implemented in GEM-MACH, including gas-
phase, aqueous-phase, and heterogeneous chemistry and many aerosol processes.
GEM-MACH15 is a limited-area forecast configuration of GEM-MACH. It
uses a continental-scale domain with 15-km horizontal grid spacing on a 348 ×
465 rotated latitude-longitude grid (Fig. 1b). The 58 vertical levels extend to 0.1
hPa on a hybrid sigma-pressure vertical coordinate. Time-dependent meteorological
lateral boundary conditions (LBCs) are provided hourly by GEM15, EC's global
variable-mesh regional forecast model. Monthly-average concentration vertical
profiles for different species are used to provide chemical LBCs. Version 1.3.0 of
GEM-MACH has been used in this study.
The SMOKE emissions processing system (v2.5) was used to produce input
anthropogenic emission files on the GEM-MACH15 rotated latitude-longitude grid
from the 2006 Canadian, 2005 U.S., and 1999 Mexican national emissions inventories.
Biogenic emissions are estimated on-line using the BEIS v3.09 algorithms.
2. Representation of PM Processes
Like CHRONOS, to reduce execution time GEM-MACH15 employs a simple 2-
bin representation of the PM size distribution (Bin 1 is 0-2.5 μm aerodynamic
diameter in size and Bin 2 is 2.5-10 μm), but PM chemical composition is treated
in more detail in GEM-MACH15 and additional processes affecting PM concent-
rations have been included. CHRONOS considers six chemical components: SO
4
;
NO
3
; NH
4
; secondary organic carbon (sOC); H
2
O; and “primary” PM. GEM-
MACH15 considers nine chemical components, as it separates the CHRONOS
“primary” component into elemental carbon (EC), primary organic carbon (pOC),
crustal material (CM), and sea salt (SS).
Both models represent inorganic gas-particle partitioning, PM sedimentation
and dry deposition, in-cloud scavenging, and secondary organic aerosol (SOA)
formation (CHRONOS uses the Pandis et al. (1992) SOA scheme whereas GEM-
MACH15 uses the Jiang (2004) scheme). In addition GEM-MACH15 considers
sea-salt emissions, aerosol nucleation, condensation, coagulation, and below-cloud
scavenging, and aerosol activation and aqueous-phase chemistry. To calculate
inter-bin condensational/evaporative transfers, the two bins are subdivided into
sub-bins to account for size dependence more accurately. The same approach is
used to calculate dry deposition velocities.
3. Results
GEM-MACH15 and CHRONOS predictions of surface O
3
and PM
2.5
concent-
rations have been compared for a 36-day period during summer 2008. Initial
chemical conditions were provided by running GEM-MACH15 in a series of 12-h
