Study Design. This study focused on a single location (freeway) in one city. Additional locations will be needed to fully assess air pollutant concentration gradients from different roadway types; different traffic patterns; geographic locations; meteorological conditions, or other characteristics.
Methods. The analytical methods implemented during this study followed EPA standard methods and Federal Reference Methods for performing ambient air measurements. Refinements to methods can and do occur over the course of time due to improved technologies and measurement techniques, however the most current technologies and techniques were implemented for this study.
Data. Uncertainties in the data may be considered in two parts: overall data integrity, individual measurements. Electronic data streaming was utilized whenever possible to lessen the chance of human error (i.e., transcription error) and ensure overall data integrity. When hardcopy data sheets, notes, chain-of-custody forms were utilized; an EPA staff member reviewed the hardcopy and verified the data. Quality assurance of the data (i.e., individual measurements) is an on-going process and often occurs during more specific data analysis. Given that this project generated gigabytes of data, thorough quality assurance of the data is an on-going activity.