Comparisons of CALPUFF and AERMOD for Vermont Applications Examining differing model performance for a 76 meter and 12 meter (stub) stack with emission characteristics for wood combustion. With Building Downwash and Examining Near Field Impacts.

The Modeled Scenario For both models using a 2 km square domain centered over Burlington High School. Building Downwash occurs from the High School Building Structure. The ‘Stub’ modeled at 1 meter above the building structure with emission characteristics of an outdoor wood boiler. The Tall Stack (76 Meters), with an 18 meter/second exit Velocity, characteristics of wood combustion for electrical generation. Same Emission Rate for Both Stacks.

A Necessary Initial Step – Comparison of AERMOD Impacts using Current and Outdated AERSURFACE Geophysical Processing for AERMET. Where version 1 was directionally independent, out to 3 km. radial extent, ASOS locations =/- 0.5 km. accuracy (Prior EPA Guidance). Version 2 (Current EPA Guidance) – smaller radial extent (1 km.), requires better ASOS siting - +/- 100 meters after contacting ASOS sites for assistance. Surface roughness calculations were performed on a directionally specific basis which will improve estimates of the turbulence parameters. The turbulence estimated for a given wind direction is very sensitive to land use and terrain variations. Monthly Variation Specified to represent longer winters.

Normalized Comparison of Impacts for Old and New AERSURFACE Processing

Significantly Lower Impacts Predicted by the Tall Stack (76 Meters), for 1 km. Radial Extent of Landuse. (The airport area constitutes a larger percent of the land use, so the Friction Velocity averages lower, so Plume Does not impact the surface as much).

for the 76 meter and 12 meter (stub) stack with emission characteristics for wood combustion Associating the maximum hourly impacts with meteorological measures – paired in time and space. Examining CALPUFF model performance for the subset of CALM and variable wind direction (WDIR), hours that AERMOD discards.

Meteorological Field Production CALPUFF - Albany, NY Upper Air Combined with Burlington, VT Sfc. Meteorology. AERMOD – Burlington, VT Sfc. Meteorology.

Dispersion Model Options In this Comparison it was assumed that AERMOD was the better tool, and efforts were made to achieve similarity in predicted impacts between the 2 models by altering CALPUFF options. Comparisons were drawn for different downwash algorithms, dispersion estimate methods, many other aspects of CALPUFF Simulation. A Failing – The CTDM-Like dispersion calculation option for stable and neutral conditions was not possible to evaluate because on-site Met data was not available.

Method used to compute dispersion coefficients (MDISP) Default: 3 ! MDISP = 3 ! 1 = dispersion coefficients computed from measured values of turbulence, sigma v, sigma w 2 = dispersion coefficients from internally calculated sigma v, sigma w using micrometeorological variables (u*, w*, L, etc.) 3 = PG dispersion coefficients for RURAL areas (computed using the ISCST multi-segment approximation) and MP coefficients in urban areas 4 = same as 3 except PG coefficients computed using the MESOPUFF II eqns. 5 = CTDM sigmas used for stable and neutral conditions. For unstable conditions, sigmas are computed as in MDISP = 3, described above. MDISP = 5 assumes that measured values are read

CALPUFF Options Terrain Adjustment Method. Transitional plume rise. Downwash - ISC method, PRIME method. Partial plume penetration of elevated inversion. Dispersion Coefficients. For CALPUFF Maximum hourly impacts, these variables did not alter predictions more than 20%.

Averaged Wind and Mixing Height values for CALPUFF Maximum Impact Hours

Averaged Wind and Mixing Height values for AERMOD Maximum Impact Hours

AERMOD Maximum Impacts for ‘Stub’ Stack – Mixing Heights Appear too high. CALPUFF is simulating maximum impacts from the Tall (76 meter), stack under calm conditions throughout lowest 200 meters of the atmosphere with significant stagnation / puff buildup.

Associating Maximum Hourly Predictions from CALPUFF and AERMOD with Meteorological Measures – (August 4, 1998).

Examining Meteorology for August 4, 1998, 7 a.m. (Hour of Maximum impacts for Tall Stack with CALPUFF).

UTC versus Local (EST) Time? From Raw ASOS DATA - EST - Matches with CALPUFF- 08,04,1998,06, 61, 59, 0, 0.0,,-999, 29.85,0.5,1000, 0.00, 08,04,1998,07,-999,-999,-999,-999,,-999,-999,-999,-999,-999, 08,04,1998,08, 68, 61, 0, 0.0,,-999, 29.86,0.0,1000, 0.00, From AERMET.pfl File – AERMET is Specified in UTC?

Examining CALM hours For AERMOD, with the stub stack – 3 hours were discarded out of the top 5 hours of maximum predicted impacts, 18 hours for top 50 (the CALM Hours). For the tall stack no hours were discarded.

Examination of Calpuff Predictions for set of CALM and variable wind direction Hours in ASOS Data excluded by AERMOD

Examination of Calpuff Predictions for set of CALM and variable wind direction Hours in ASOS Data excluded by AERMOD for Maximum 50 Hourly Impacts.

Future Work Examine CALPUFF for CALM,Variable Wind Direction Hours for VT ASOS Sites in sheltered locations (Springfield, Rutland). For these locations calm hours are greater than 4000 hours per year. Current Intercomparison may not be paired in time. This will be reexamined. Continue to compare CALPUFF and AERMOD handling of specific hours of meteorology after rerunning CALPUFF with CTDM-like Dispersion.