AERMOD or SCREEN3 Air Dispersion Concentration Calculator

AERMOD is a regulatory-grade, Gaussian plume model endorsed by the U.S. EPA (Appendix W to 40 CFR Part 51) for detailed, site-specific air dispersion modeling. It incorporates boundary-layer meteorology, terrain effects, and building wake algorithms (PRIME). SCREEN3, also EPA-approved but simplified, uses worst-case meteorological conditions (e.g., Pasquill-Gifford stability class F, 2 m/s wind) and omits terrain/building effects—making it suitable for screening-level assessments only. AERMOD requires hourly meteorological data (surface + upper-air), while SCREEN3 relies on annual averages or default values. Per EPA guidance, SCREEN3 may overpredict concentrations by 2–10× compared to AERMOD under complex conditions; thus, SCREEN3 results typically trigger follow-up AERMOD analysis if exceedances occur.

Exit velocity and temperature critically influence plume rise via buoyancy and momentum forces—directly impacting effective stack height and dilution. AERMOD calculates physical and buoyant plume rise using Briggs’ equations (EPA AP-42 Section 12.3), where higher exit velocity increases momentum-driven rise, and greater temperature differential (ΔT = T_exit − T_ambient) enhances buoyant rise. Underestimating either parameter can artificially lower effective stack height, leading to non-conservative concentration predictions—especially within 1–3 km downwind. EPA Appendix W mandates using measured or manufacturer-specified exit parameters; default assumptions (e.g., 0 m/s velocity or ambient temperature) violate modeling best practices and may invalidate regulatory submissions.

No—SCREEN3 is explicitly prohibited for formal NSR or Prevention of Significant Deterioration (PSD) permit demonstrations per EPA’s Guideline on Air Quality Models (Appendix W, 2023 update). SCREEN3 is designated solely for preliminary screening: identifying whether more rigorous modeling (e.g., AERMOD) is warranted. For NSR/PSD, EPA requires AERMOD (or approved alternatives like CALPUFF) with site-specific meteorological data, terrain, and building data where applicable. Using SCREEN3 for final compliance could result in permit rejection or enforcement action. Always verify state implementation plans (SIPs); some states (e.g., Texas TCEQ) explicitly ban SCREEN3 for permitting. Document all assumptions and justify model selection per 40 CFR §51.100–§51.110.

Receptor height strongly influences predicted concentrations due to vertical concentration gradients near ground level. AERMOD assumes uniform mixing within the surface layer, but concentrations peak near the surface for neutral/stable conditions. For residential exposure, EPA and WHO recommend a receptor height of 1.5 m (breathing zone) per AP-42 Chapter 1 and EPA’s Risk Assessment Guidance for Superfund (RAGS). Using 0 m overpredicts deposition-related exposure; using >2 m underpredicts inhalation risk. In urban settings with buildings, consider additional receptors at rooftop levels if evaluating indoor infiltration or secondary exposure pathways—but primary health-based assessments must include 1.5 m. Always pair with appropriate averaging times (e.g., 1-hr max for acute effects, annual average for chronic risk).

EPA Appendix W mandates minimum 1-year of concurrent surface (wind speed/direction, temperature, cloud cover, precipitation) and upper-air (temperature, wind speed/direction at ≥3 levels) data—preferably from an airport or certified monitoring station within 50 km. Data must be processed into AERMET-compatible format (e.g., .bin files) with quality assurance checks for gaps, outliers, and instrument drift. Shorter datasets (≥3 months) are conditionally acceptable only with statistical justification (e.g., representative seasonal coverage) and EPA pre-approval. Use of default or interpolated data invalidates regulatory submittals. Additionally, terrain and land-use data (e.g., USGS 1:24k elevation, NLCD 2019) must be included. All inputs must be documented per EPA’s Model Documentation System (MDS) requirements in 40 CFR §51.112.

Zero concentrations typically arise from AERMOD’s internal numerical thresholds or physical constraints—not model error. First, concentrations below 1×10⁻¹² μg/m³ are truncated to zero for computational efficiency. Second, under highly stable atmospheric conditions (e.g., Class F, low wind), plume rise may be minimal and lateral dispersion extremely narrow, causing predicted concentrations beyond ~50 m to fall below detection thresholds. Third, incorrect coordinate projection (e.g., mismatched UTM zones) or receptor grid misalignment can place receptors outside the modeled domain. Verify receptor placement relative to stack coordinates, check MET input stability classification, and review AERMOD’s .out file for warnings about ‘no valid receptors’ or ‘plume not intersecting grid’. Always validate with diagnostic plots (e.g., contour maps) before concluding absence of impact.

Use AERMOD’s PRIME (Plume Rise Model Enhancements) algorithm to simulate building downwash effects—required by EPA Appendix W for stacks shorter than 2.5× building height or within 5× building width of any structure. Input precise 3D building geometry (height, length, width, location) via BPIPPRM.DAT or GIS-derived files. PRIME adjusts effective stack height and dispersion coefficients based on wake-induced turbulence. Omitting buildings for stacks near structures can underestimate concentrations by 2–5× at pedestrian level. EPA recommends building data from LiDAR or municipal GIS (e.g., NYC DOF, USGS 3DEP), validated against field surveys. Note: SCREEN3 lacks building algorithms entirely—never use it for sites with significant nearby structures per EPA’s Building Downwash Guidance (2021).

No—EPA Appendix W prohibits default terrain in regulatory AERMOD runs, regardless of topography. Even gentle slopes (>1%) alter wind flow and dispersion patterns measurably. AERMOD’s terrain processor (AERMAP) requires high-resolution digital elevation data (≤30 m resolution, e.g., USGS 1/3-arcsecond DEM) to compute terrain-following grids and adjust mixing heights. Using coarse or default terrain may underestimate concentrations by up to 40% in valleys or overestimate in ridges due to erroneous plume trajectory. EPA’s Technical Support Document (TSD) for AERMOD v23310 explicitly states that ‘terrain omission or simplification invalidates model results for permitting.’ Always generate custom terrain files and validate elevations against surveyed control points—especially within 5 km of the stack, where terrain effects dominate.