A LONG PATH DIFFERENTIAL OPTICAL-ABSORPTION SPECTROMETER AND EPA-APPROVED FIXED-POINT METHODS INTERCOMPARISON

Differential optical absorption spectrometry (DOAS) has been used by a number of investigators over the past 10 years to measure a wide rang e of gaseous air pollutants. Recently OPSIS AB, Lund, Sweden has devel oped and made commercially available a DOAS instrument that has a numb er of features which make the unit attractive for field monitoring stu dies in both remote and urban areas. The DOAS is composed of a broad b and light source (emission between 200 and 1000 nm) and a receiver-spe ctrometer assembly. The spectral data from the spectrometer are proces sed in real time using a PC computer to calculate the concentrations o f the pollutants programmed to be monitored by the system. The distanc e between the light source and receiver can range from 100 to 2000 m, depending on the pollutant to be monitored and species concentrations. In September and October of 1989 an OPSIS AB DOAS was operated in the Research Triangle Park, NC on the roofs of the two main Environmental Protection Agency laboratories. The distance between the light source and receiver was 557 m and the pollutants monitored were SO2, NO2, O3 , HCHO and HNO2. Comparisons between the federal reference and equival ent fixed-point methods measuring SO2, O3 and NO2 and simultaneous dat a derived from the DOAS had correlations typically greater than 0.90 d uring the evaluation.