How Plume Plotter (AERMOD version) works

Plume Plotter uses AERMOD ([1]) to calculate the concentration of pollution at each location, at ground level. AERMOD uses AERMET to preprocess meteorological data. Below we use "AERMOD" to refer to the AERMOD/AERMET combination.

Plume Plotter is used in two ways: a real-time mode and a historical mode. Differences between these are noted below.

AERMOD is provided with the usual parameters of the emissions source:

We currently use a "pollutant ID" of "other", meaning that AERMOD will not perform any chemical simulations (e.g., converting NO to NO2). This allows the concentration of all other pollutants to be derived simply from the oxides of nitrogen concentrations by multiplying by the relative emission rates.

AERMOD also requires a few parameters of the area near the emissions source:

AERMOD is designed to be run over long periods, but the real-time Plume Plotter runs it for a single point in time. This is done by pretending that the latest weather observation is for the next hour, since AERMOD only understands whole hours. For example, if the latest weather observation is timed 17:15, we feed it to AERMOD as the observation for 18:00 and run AERMOD for the period including only the hour 18:00. This could introduce some error around sunrise or sunset; e.g., if sunset occurs between 17:15 and 18:00, the 17:15 reading might be wrongly treated as a nighttime one.

Even if run for a 1-hour period, AERMOD expects all previous observations on the same day, since midnight, as well as the upper air readings from the previous midnight. Therefore, these are all supplied.

Weather observations used are:

These are obtained from the nearest reliable weather station on Weather Underground, with a secondary weather station used for solar radiation (because few record this). In the real-time plume plotter, we assume the cloud cover is 100%, because there is no real-time source of cloud cover data (except by asking the user, which rarely works), and cloud cover data is used by AERMOD only at night anyway. In historical runs of Plume Plotter, the actual cloud cover is obtained from historical datasets.

Although AERMOD has ways of dealing with calm winds, these do not work when we use an "onsite" file for weather observations, as we do. AERMOD erroneously treats observations with calm winds as missing observations. We therefore replace these by very light (0.1m/s) winds from a random direction. Moreover, AERMOD then replaces these (and all wind speeds below 0.28m/s) by 0.28m/s.

Upper air observations provided are:

for various heights in the atmosphere (AERMOD only uses those up to 5000m). We use the midnight observations from the closest upper air station to the incinerator, obtained from uwyo.edu or meteocentre.com.

Note that weather observations are sometimes missing (some weather stations are unreliable) and even the upper air data is missing occasionally. AERMOD can tolerate some missing observations but sometimes they might cause AERMOD to fail. In historical runs of Plume Plotter, missing observations are filled in manually. The real-time Plume Plotter often fails after midnight, when the upper air data becomes out of date, until the new upper air data (measured at midnight) can be downloaded (about 1:45am).

Plume Plotter makes use of terrain data (OS Terrain 50) from Ordnance Survey. This is converted to DEM format and preprocessed by AERMAP (offline) to be used by AERMOD. This allows AERMOD to model dispersion correctly for the terrain.

Plume Plotter also handles building downwash. The report ([2]) states:

"The presence of adjacent buildings can significantly affect the dispersion of the atmospheric emissions in various ways. Wind blowing around a building distorts the flow and creates zones of turbulence. The increased turbulence can cause greater plume mixing. Also, the rise and trajectory of the plume may be depressed slightly by the flow distortion. This downwash leads to higher ground level concentrations closer to the stack than those which would be present without the building."

The building(s) near the incinerator stack were measured from the plans in the respective planning applications, and fed to the BPIP preprocessor (offline). BPIP generated information for AERMOD to correctly model building downwash.

Stack tip downwash is also modelled. This doesn't require any effort from Plume Plotter; it is just a default option in AERMOD.

AERMOD has many options, some of which might be more realistic. In particular there are several options that "address concerns regarding model performance under low wind speed conditions". However, we stick to the regulatory default options in order to avoid bias.

Weather data

All weather data (except cloud cover) is provided by Weather Underground. The real-time Plume Plotter downloads the latest weather observations and runs AERMOD approximately every 15 minutes. For historical runs, hourly historical weather observations are used.

Maps

Maps are provided by Google (© Google).

Background levels

Background levels of pollutants are listed on clicking "DETAILS".

Software versions

Results

Professional modellers employed by incinerator builders make it impossible to reproduce their results. They use proprietary software and do not reveal their data or parameters. In contrast, Plume Plotter's results are intended to be reproducible: the above software (downloadable from epa.gov) can be run on Plume Plotter's input files, which are available on request to ash@plumeplotter.com.

References

1. AERMOD Modeling System, EPA.

2. URBASER GLOUCESTERSHIRE AIR QUALITY ASSESSMENT, Fichtner, 2012.