Research and development activities of the late 1980s, and the subsequent adoption of the 1990 Clean Air Act Amendments (CAAA), signify that the U.S. Environmental Protection Agency (EPA) clearly anticipated that optical remote sensing (ORS) techniques would play an important role in measuring airborne pollutants, especially those among the 189 hazardous air pollutants listed in the CAAA.1 A growing interest in developing ORS techniques to measure gaseous pollutants spurred intensive research and development activities, which were highlighted in a series of specialty conferences organized by the Air & Waste Management Association from 1992 through 1996. In fact, during the late 1980s and early 1990s, EPA’s National Environmental Response Team performed several emissions measurement studies using ORS techniques to support hazardous waste site remediation under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).2 These studies led to the development of an EPA guidance document for an open-path Fourier Transform Infrared (FTIR) technique3 and, consequently, the approval of EPA’s Compendium Method TO-16 in 1999.4 In addition, several equivalency tests for a Differential Optical Absorption Spectroscopy (DOAS) technique were approved for ozone, nitrogen dioxide, and sulfur dioxide. Despite these efforts, few practical applications for ORS techniques were put into practice in the United States and interest in the methods was beginning to fade by the end of the century. The myriad reasons for this waning interest are beyond the scope of this article; suffice it to say that many of the research programs that once thrived were scaled back considerably. Although ORS research activities were on the decline by the mid-1990s, development of these technologies did not cease entirely. In 1996, researchers from EPA National Risk Management Research Laboratory’s Air Pollution Prevention and Control Division (NRMRL/APPCD) were tasked with investigating and characterizing ammonia emissions from swine farm waste lagoons. Realizing the difficulties and expense associated with conducting a large number of point measurements using conventional monitoring methodologies, the researchers recognized the potential inherent in using open-path FTIR technologies. Based on their previous experience in development of emissions measurement methods using open-path FTIR,5,6 the NRMRL/APPCD researchers decided that ORS technologies were best suited for measuring large area emissions sources. Back in 1996, the researchers simply recognized that ORS techniques provided them with reasonable concentration measurements over a large spatial area. However, they still faced the problem of obtaining emissions flux estimates from area sources based on ORS concentration data.