Modeling for sources emitting criteria pollutants may be required to demonstrate compliance with National Ambient Air Quality Standards (NAAQS).  If a demonstration is needed,  guidlines specify that ambient backgrounds be added to predicted concentrations.  Ambient backgrounds are calculated via data made available from various air monitoring stations located across the region.  The appropriate selection of a particular air monitoring station depends on several factors including, but not limited to, the following:

• proximity of the monitor to the modeled source
• location of the monitor with respect to the modeled source (e.g. upwind or downwind based on prevailing winds)
• available years of data
• sighting objective of the monitor
• completeness of data
• availability of data from a potentially more representative monitor

When calculating a background it is desirable that data from the last three years be used.  Following is a description of how to calculate backgrounds for the various pollutants / averaging periods:

• For pollutants with annual averaging periods, the highest of the three highest annual concentrations should be used. 
• For pollutants with a 24-hour, 8-hour, 3-hour, or 1-hour averaging period (with the exception of PM-10), the highest of the high second high values should be used. 
• For the 24-hour averaging period for PM-10, the fourth highest 24-hour concentration observed over the three year period should be used. 
• For the quarterly averaging period for Pb, the highest quarterly concentration observed over the three year period should be used.

Data from the last three years will not always be available from an otherwise representative monitor.  In these cases, the AQD can be consulted as to which alternatives can be considered.

The following EPA website can be used to access air monitoring data (link).