APM aerosol microphysics

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This page describes the APM (Advanced Particle Microphysics) option in GEOS-Chem.


The Advanced Particle Microphysics (APM) package was developed for implementation into GEOS-Chem at State University of New York (SUNY) at Albany. In the present version of the APM module, size-resolved microphysics for secondary particles (i.e., those formed from gaseous species) and sea salt has been treated with 40 sectional bins to represent sulfate (or secondary) particles and 20 sectional bins to represent sea salt particles. The bin structure is chosen to have relatively high resolution for the size range important to the growth of nucleated particles (a few nanometers) to cloud condensation nuclei (CCN). The growth of nucleated particles through the condensation of sulfuric acid vapor and equilibrium uptake of nitrate, ammonium, and secondary organic aerosol is explicitly simulated, along with the scavenging of secondary particles by primary particles (dust, black carbon, organic carbon, and sea salt). The amount of secondary species coated on primary particles (through condensation, coagulation, equilibrium uptake, and aqueous chemistry) are tracked.

Authors and collaborators

  • Fangqun Yu (U. of Albany) -- Principal Investigator
  • Gan Luo (U. of Albany)

APM User Groups

User Group Personnel Projects
U. of Albany Fangqun Yu ...

Computational Information

The APM model contains a number of computationally efficient schemes: (1) Usage of pre-calculated lookup tables for nucleation rates and coagulation kernels; (2) Variable size ranges for particles of different types; (3) Variable bin resolution; (4) Variable and optimized time steps for the coagulation calculations; (4) The coating of primary particles by sulfate is tracked using one tracer (sulfate mass) for each type of primary particles; (5) Nitrate, ammonium, and SOAs asso-ciated with sulfate are calculated based on the equilibrium partition. The above schemes enable the APM model to capture the main properties of atmospheric particles important for their direct and indirect radiative forcing while keeping the computational costs quite low. The computational time of the coupled model (127 tracers) with full microphysics and chemistry is only about 2.2 times longer than that of the model with full chemistry only (54 tracers). In the other words, the application of efficient schemes keeps the increase in computational costs as low as 86% at 100% increase in number of tracers associated with particle size and composition.

In our study, all simulations are running on 8-CPU Linux workstations with the 2.2 Ghz Dual Quad-Core AMD Opteron Processor 2354. The model system is compiled using OpenMP (http://www.openmp.org) for running in parallel. The original GEOS-Chem code has 54 tracers, and it takes 24.23 hours for one year full-chemistry simulations at 4x5 horizontal resolutions and 47 layers (GEOS-5 data). The coupled GEOS-Chem-APM model has 127 tracers (73 additional tracers: 40 for sulfate, 20 for sea salt, one for H2SO4 gas, 4 tracers for BC/OC from fossil fuel, 4 tracers for BC/OC from biomass/bio-fuel, and 4 for sulfate attached to dust, BC, primary OC, and sea salt particles). With full size-resolved microphysics (nucleation, condensation, coagulation, deposition, and scavenging) and chemistry, it takes the coupled model (127 tracers) 52.35 hours for the same year simulations on the same machine. In other words, our efficient schemes allow the increase in the computing cost per 100% increase in number of tracers (associated with particle size information) to (52.35/24.23-1)/(127/54-1) = 86%. Such a relatively small increase in the computing cost associated with full size-resolved microphysics is desirable and makes the future coupling of APM model with global climate model feasible.

We also carried out some preliminary simulations using the nested version of GEOS-Chem (with APM included) over East Asia. It takes about three days to simulate one-month (full chemistry, full size-resolved microphysics, 127 tracers, GEOS5, 0.5x0.666, 47 layers) in our 8-cpu machine.

Implementation notes

We will begin to work with GEOS-Chem support team to put APM into the standard version of GEOS-Chem after v8-02-05 is official released.


The first application of GEOS-Chem + APM focuses on predicting the number concentrations of particles in the troposphere. Significant amount of efforts have been devoted to validate the simulated global spatial distributions of particle number abundance, using a large amount of land-, ship-, and aircraft- based measurements. See following publications for details.

Yu, F., and G. Luo, Simulation of particle size distribution with a global aerosol model: Contribution of nucleation to aerosol and CCN number concentrations, Atmos. Chem. Phys., 9, 7691-7710, 2009.

Yu, F., G. Luo , T. Bates , B. Anderson , A. Clarke , V. Kapustin , R. Yantosca , Y. Wang , S. Wu, Spatial distributions of particle number concentrations in the global troposphere: Simulations, observations, and implications for nucleation mechanisms, J. Geophys. Res., revised, 2010.


Yu, F., and G. Luo, Simulation of particle size distribution with a global aerosol model: Contribution of nucleation to aerosol and CCN number concentrations, Atmos. Chem. Phys., 9, 7691-7710, 2009.

Known issues