PlumeSmartEPA Regulatory Air Dispersion Modeling
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Air Modeling GuideCovers AERMOD 24142 & 26135 — every excerpt version-stamped · current NAAQS

BPIPPRM: turning buildings into model inputs

AERMOD's downwash algorithm needs to know what each building looks like from the wind's point of view — and that changes with every wind direction. The Building Profile Input Program (BPIP), in its PRIME-input version BPIPPRM, is the U.S. Environmental Protection Agency (EPA) preprocessor that does the geometry. Its User's Guide states the job:

The Building Profile Input Program (BPIP) was designed to incorporate the concepts and procedures expressed in the GEP technical support document…, the Building Downwash guidance…, and other related documents … into a program that correctly calculates building heights (BH's) and projected building widths (PBW's).

Source · BPIP User's Guide (EPA-454/R-93-038, revised 2004) — quoted word-for-word from the source document.

One building, thirty-six widths

buildingwindprojected widthfor THIS directionwhat BPIP producesper stack, per 10° sector:building height + maximumprojected width (and, for thePRIME inputs, projected lengthand stack-offset values)only structures "nearby" the stackcount — within 5L, capped at 0.8 km(the GEP page's §51.100(jj) rule)the same run also reports eachstack's GEP formula heightAERMOD reads these as direction-arrays on the source pathway — 36 values per building input, per stack
The same building presents a different width to every wind direction — BPIP computes 36 direction-specific values (one per 10° sector)

A rectangular building side-on to the wind presents its full width; the same building corner-on presents its diagonal. BPIP resolves this by computing direction-specific values — "36 pairs of BH and PBW values … for each stack" (BPIP UG §3), one pair per 10-degree wind-direction sector. For each direction, BPIPPRM "uses the maximum projected length and maximum projected width" of the structure (AERMOD User's Guide, EPA-454/B-24-007, §3.3) — the footprint the wake actually sees. These arrays enter AERMOD on the source pathway as the building-input keywords (heights, widths, lengths, and stack-position offsets, 36 values each).

Two jobs in one program

BPIP is divided into two parts. The first part is based solely on the GEP technical support document and is designed to determine whether or not a stack is being subjected to wake effects from a structure or structures. Several values are determined such as the GEP stack height, GEP related BH's and PBW's. Flags are set to indicate which stacks are being affected by which structure wake effects.

Source · BPIP UG (EPA-454/R-93-038) §2.

So the same building geometry answers both questions from the Regulatory review part: what is this stack's Good Engineering Practice (GEP) formula height (H + 1.5L, from the controlling nearby structure), and is this stack wake-affected at all. The second part then computes the downwash building dimensions — "only if a stack is being influenced by structure wake effects" (§2).

Which structures participate is not a judgment call: the "nearby" rule from the GEP regulation applies — a structure counts within 5L of the stack, capped at 0.8 km (40 CFR §51.100(jj); see GEP stack height). A tall building beyond that radius contributes neither GEP credit nor modeled downwash.

Version discipline

Downwash results depend on the processor version, and the AERMOD User's Guide is unusually blunt about the experimental branch: the alternative equivalent-building method in draft BPIPPRM 19191_DRFT "is a research grade version that has been provided to the modeling community for testing, evaluation, and feedback and may not be used in a regulatory context" (EPA-454/B-24-007 §3.3). Regulatory demonstrations use the released BPIPPRM with its maximum-projected-dimension method.

Practical notes that save review cycles

  • Tiered buildings are entered as tiers. BPIP works from each tier's corner coordinates and height — a stepped structure is several tiers, not one bounding box.
  • Base elevations matter. Building heights and stack heights are compared through their bases; on sloping ground, wrong base elevations shift the GEP formula height and the wake geometry.
  • Every stack is processed against every nearby structure — the controlling building can differ stack by stack, and even direction by direction for the same stack.

In PlumeSmart

Buildings drawn on the facility map (or imported from an existing downwash file) run through BPIPPRM automatically — tier corners, heights, and base elevations from the map, direction-specific outputs written into the run's input file, and the processor's full output — including each stack's GEP calculations — kept with the run files, so the geometry a reviewer checks is exactly the geometry the model used.