Deposition & depletion: when the plume touches down
Everything in this guide so far treats the plume as permanent — every gram emitted stays airborne until it leaves the domain. For gases that's usually fine. For particles, and for gases that stick to surfaces, it isn't: mass leaves the plume on the way. AERMOD models that with deposition (mass arriving on the ground) and depletion (the plume getting lighter because of it). Both change concentrations — not always in the direction you'd guess.
Two ways out of the air
- Dry deposition — particles settle under gravity and both particles and gases are taken up by the surface the plume brushes against.
- Wet deposition — precipitation washes material out of the plume. This is why deposition runs need weather the concentration-only run doesn't: "precipitation code, precipitation rate, relative humidity, surface pressure, and cloud cover" (AERMOD User's Guide §3.2.2.12).
Each process produces two things you can ask the model for: the effect on
concentrations, and the deposition flux itself — how much mass
lands per square meter (the DEPOS/DDEP/WDEP output types). The flux
outputs are what multipathway risk assessment consumes: deposited mass is
the starting point for soil and food-chain exposure.
Depletion: the honest ledger
If mass is landing on the ground, the plume downwind must contain less of it. That bookkeeping is depletion, and AERMOD applies it automatically:
Beginning with version 04300, the dry and wet removal (depletion) mechanisms … will automatically be included in the calculated concentrations or deposition flux values if the dry and/or wet deposition processes are considered, unless the user specifies the NODRYDPLT and/or NOWETDPLT options.
Source · AERMOD User's Guide (EPA-454/B-26-001) §3.2.2.6.
Turning depletion off (NODRYDPLT/NOWETDPLT) "will result in a more
conservative estimate" (§3.2.2.6) — the model then deposits mass and
keeps it in the plume. That's a defensible simplification for a screen,
and a refinement to remove when a particulate demonstration is close.
The settling surprise
Intuition says deposition always lowers concentrations. The User's Guide says otherwise:
However, the inclusion of particle deposition effects may increase ground-level concentrations for some sources compared to the same source modeled as a gaseous emission due to gravitational settling on the particulate plume. The magnitude of this effect will depend on the source characteristics (elevated or low-level) and particle size distribution.
Source · AERMOD UG (EPA-454/B-26-001) §3.2.2.6.
The mechanism: settling tilts an elevated plume's centerline toward the ground, so heavy particles arrive at near-field receptors that the buoyant gas plume would have sailed over. Deposition is physics, not a discount — a coarse-particle stack plume can produce higher maxima close in and lower ones far out. For a ground-level dust source, by contrast, depletion mostly just drains the plume and far-field concentrations drop.
The three methods — and which are regulatory
AERMOD offers three deposition treatments, with sharply different regulatory standing (§3.2.2.12; Addendum A to Appendix W, AERMOD entry, "Physical Removal"):
| Treatment | Inputs (per source) | Regulatory status |
|---|---|---|
| Method 1 — particles, known size distribution | particle diameter categories, mass fractions, particle density (PARTDIAM/MASSFRAX/PARTDENS) | Regulatory — usable with DFAULT |
| Method 2 — particles, size distribution unknown | fine-mass fraction (< 2.5 µm) + representative mass mean diameter | ALPHA — non-regulatory |
| Gas deposition | seasonal categories by month, direction-specific land-use categories, pollutant-specific properties (GDSEASON/GDLANUSE/GASDEPOS) | ALPHA — non-regulatory |
The Guideline on Air Quality Models (Appendix W) is blunt about the boundary in its AERMOD model summary: "Currently, Method 1 particle deposition is available for regulatory applications. Method 2 particle deposition and gas deposition are currently alpha options and not available for regulatory applications" (40 CFR part 51, Appendix W, Addendum A, A.1, "Physical Removal"). And since version 19191, AERMOD "will issue a fatal error message and abort" if Method 2 or gas deposition is requested without the ALPHA keyword (§3.2.2.12).
Two practical footnotes: deposition is not implemented for the RLINE/RLINEXT, BUOYLINE, or SWPOINT source types (§3.2.2.6), and Method 1 lives or dies on the quality of its particle size distribution.
A worked example
A crushing plant needs a PM10 demonstration. Modeled as a gas-like pollutant (no deposition), the design value at the fence line is fine but a receptor cluster 800 meters downwind fails by 8%. The operation's stack test includes a particle size distribution: 40% of mass above 10 µm aerodynamic diameter, most of the rest between 2.5 and 10 µm. Entering that distribution as Method 1 inputs (diameters, mass fractions, density) lets the model settle and deplete the coarse fraction where physics says it lands — near the plant — and the 800-meter receptors clear the standard. The near-field values rise slightly (the settling surprise), so the fence-line margin is re-checked in the same run. The refinement is legitimate precisely because the size distribution is measured and documented, and because Method 1 is a DFAULT-compatible regulatory option.
In PlumeSmart
Sources carry optional Method 1 particle inputs (diameter categories, mass fractions, density), the ALPHA-status treatments are labeled as non-regulatory where offered, and deposition-flux outputs feed the risk assessment module — the deposition step of the wizard is where all of this is entered.