Mar 23, 2015

Most AERMOD dispersion modeling is conducted using meteorological data from a nearby airport.  As long as the airport is reasonably representative of your source location, there is nothing wrong with that: airports are often a source of high quality data and located near major population centers and thus, major air pollution sources.  

But airport data isn't always the best option for dispersion modeling.  A great example is the Los Angeles, California area. The Los Angeles airport is located right on the Pacific Ocean and to make room for its four runways, has more flat, open space than the rest of Los Angeles.  If you are using AERMOD to model a facility that is only a few kilometers farther inland, and in the heart of urban Los Angeles, wind and temperature patterns at your site may be very different from those at the airport.  In other parts of the world, a nearby airport simply may not exist, or it may not record an important meteorological parameter needed by AERMOD, such as cloud cover.  Whatever the problem, “onsite” meteorological data can be the solution.

What is Onsite Meteorological Data and Where Can You Get It?

In the air dispersion modeling world, onsite meteorological data usually refers to any data collected outside of the mainline weather stations typically found at airports.  The most common example of onsite data is data that is truly “on site” - coming from a met station or air quality monitoring station located on the property.  Often these stations are set up to comply with regulatory requirements or internal environmental, safety, and health needs.  In some cases, a regulatory agency will decide on its own that none of the nearby airports are a suitable match, and will require that onsite meteorological data be collected for a year or more before the site can be modeled with AERMOD.  If you are in this situation and are looking to set up a new onsite meteorological station, Meteorological Solutions is a great company offering these services.

However, setting up a station yourself isn't the only way to get onsite data.  Any nearby air quality monitoring station that has meteorological data can likely be used as a data source.  Often, national, state, or local regulatory agencies make their meteorological data available online or may share their data at no charge to anyone who asks. Nearby industrial facilities that operate their own stations are also sometimes willing to share data. Even if these facilities want to share some of the costs of the station, avoiding a year-plus wait to collect your own data may be well worth the cost.

What Do You Need to Process Onsite Meteorological Data?

For an onsite station to be usable for AERMOD, it usually must record at least the following:

  1. Wind speed and direction
  2. Air temperature
  3. Either:
    • Cloud cover (%) or
    • Solar radiation AND temperature difference (e.g., difference between temperature measured at two heights) 
If only some of these parameters are available, it is possible to use onsite data supplemented by another station, such as a nearby airport.

In addition to these basic requirements, a regulatory agency may have strict requirements for data quality, instrument precision, and so forth.  For example, U .S. EPA publishes detailed guidance on recommended (and in some cases required) characteristics of an onsite meteorological station.

How To Use Onsite Meteorological Data?

Onsite meteorological data can be processed using the AERMET preprocessor. BREEZE AERMET is a great tool for this, as it helps with the special AERMET setup required to use onsite data.  AERMET can accept a wide variety of file formats as onsite data input but there are a number of formatting requirements, such as which units must be used for each parameter.  Additionally, the BREEZE meteorological data processing team regularly processes onsite data, and has even been selected by state regulatory agencies to process their data into an AERMOD-ready format.

Once onsite data is run through AERMET, a .SFC and .PFL file will be produced that can be used in AERMOD in exactly the same way as more traditional airport meteorological data.

Need additional help? Get answers to your questions by asking an expert in the BREEZE team. Contact us at

Meteorological Solutions is a subsidiary of Trinity Consultants, Inc.