Overview of MAM¶

The atmospheric aerosol is a population of particles having a wide range of sizes, composition, morphologies, and attachment states that are affected by a large number of source, sink, transformation, and transport processes.

The modal aerosol module (MAM) embedded in the U.S. Department of Energy's (DOE's) Energy Exascale Earth System Model (E3SM) provides a simplified but fairly complete treatment of the aerosol lifecycle that is suitable for global climate models, and is similar (in terms of complexity) to aerosol treatments in many other global atmospheric general circulation models (AGCMs). Two categories of aerosol processes are currently included in MAM:

clear-air, or all-air, processes: emissions, nucleation (new particle formation), condensation/evaporation of chemical species,
water uptake (condensation/evaporation of water), coagulation (particle collision), sedimentation (gravitational settling), and turbulent dry deposition;
cloud-related processes: aqueous chemistry in cloud droplets, activation and resuspension, wet removal, and subgrid vertical transport by convective clouds.

This section provides an overview of the basic physical concepts, mathematical methods, and simplifying assumptions used in MAM to provide a context for more detailed descriptions of the above-listed processes in the next sections.

Nomenclature and basic assumptions¶

As a trade-off between simplicity and accuracy, MAM mathematically describes the size distribution of aerosol particles within a grid cell of the host atmospheric model (in our case EAM) by the sum of several (e.g., 3, 4, or 7) log-normal functions of the particle size, referred to as "modes". The various modes can have different mean sizes and composition (see Figure 1 below and Table 1 in the section on the modal method).

Figure 1. Aerosol modes in MAM4

MAM4 modes

Figure: A schematic showing the log-normal modes in MAM4, their size range, and compositions. From Wang et al. (2019, DOI:10.1029/2019MS001851)

Some chemical compositions are soluble, for which the non-water part of a particle is referred to as the "dry" part and the entire particle (including the absorbed water) is referred to as the "wet" particle. Below we list the basic assumptions made in MAM that are shared by all subcomponent parameterizations that describe individual aerosol processes:

Aerosol particles are spherical; their volumes are additive.
A particle can contain one or more chemical species. Particles within a log-normal mode are uniform in composition (i.e., the different chemical species are internally mixed). The species considered in MAM are listed in the table below. The composition of individual log-normal modes is shown in the figure above.
Only two attachment states are explicitly treated: interstitial (i.e., suspended in the air) and cloud-borne (i.e., dissolved or suspended in cloud droplets or ice crystals). Interstitial and cloud-borne particles are treated separately in MAM, but using the same mode definitions.
The log-normal modes are expressed as continuous functions of particle diameter. The mean size of a mode can evolve with time, but the width (standard deviation) of the distribution is fixed, i.e, it takes a prescribed value with no spatial or temporal variation. Further details of those distribution functions are explained in the section on the modal method.
MAM also considers a few gas-phase species that can condense on or evaporate from aerosol particles. These gas species are listed in the table below.

Aerosol species in MAM4

"Mw host" is the molecular weight specified in the host model (unit: kg kmol\(^{−1}\)). "Mw microphysics" is the molecular weight used in the aerosol microphysics parameterizations (unit: kg kmol\(^{−1}\)). Density is given in the unit of kg m\(^{-3}\).

Species	Abbreviation	Mw host	Mw microphysics	Density
sulfate	SO4	115	115	1.77E3
primary organic aerosol	POA or POM	12	150	1.00E3
secondary organic aerosol	SOA	12	150	1.00E3
black carbon	BC	12	12	1.70E3
sea salt	NCL	58.5	58.5	1.90E3
dust	DST	135	135	2.60E3
marine organic aerosol	MOA or MOM	250092	250092	1.601E3