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 a 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 because 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 (for a site in the UK or Ireland). Therefore, these are all supplied.
Weather observations used are:
In most cases, these are obtained from the nearest suitable weather station, with a backup weather station used for times when the main one is closed.
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 (0.1 m/s) winds from a random direction. Moreover, AERMOD then replaces these (and all wind speeds below 0.28 m/s) by 0.28 m/s.
Upper air observations provided are:
for various heights in the atmosphere (AERMOD only uses those up to 5000 metres). For the UK and Ireland, we use the midnight (UTC) observations from the closest upper air station to the incinerator. In general (in other countries), the upper air observations used are those recommended by the AERMOD guidelines.
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 plume plotter runs, missing observations are filled in automatically or manually. The real-time plume plotter usually 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: Ordnance Survey Terrain 50 (for GB), a similar dataset (for Northern Ireland), or SRTM (Shuttle Radar Topography Mission) data (for other countries). 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 are measured from the plans in the respective planning application, and fed to the BPIP preprocessor (offline). BPIP generates 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, to avoid bias.
Most weather data are provided by a nearby synoptic weather station or an airport (in METAR format). METAR reports are usually issued every 30 or 60 minutes, whereas synoptic weather station reports are usually hourly. A few plume plotters use specific local weather stations instead. The real-time plume plotter downloads the latest weather observations and runs AERMOD at intervals (ranging from 15 minutes to 1 hour or longer) that depend on the popularity of the plume plotter. For historical runs, hourly historical weather observations are used.
Maps are provided by OpenStreetMap (© OpenStreetMap contributors).
Background levels of pollutants are listed on clicking "DETAILS".
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.
1. AERMOD Modeling System, EPA.
2. URBASER GLOUCESTERSHIRE AIR QUALITY ASSESSMENT, Fichtner, 2012.