Comparison of an open path differential optical absorption spectroscopy system and a conventional in situ monitoring system on the basis of long-term measurements of SO 2, NO 2, and O 3

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A field-based intercomparison study was conducted to evaluate the performance of a line-integrating monitoring technique (a commercial differential optical absorption spectroscopy, DOAS, system by Opsis AB, Sweden) in concert with a conventional in situ monitoring technique (MACSAM-2 (or MS2) system, Japan). In the course of our study, the mixing ratios of three trace gases including SO 2, NO 2, and O 3 were measured routinely from the Ban Po district of Seoul during a 13 month period (June 1999–August 2000). The data obtained from two different systems were used to evaluate various aspects of DOAS performance. The differences in the observed mixing ratios between two techniques, if assessed in terms of the percent difference (PD) values between different data sets, were in general rather compatible not only among different species but also as a function of varying time scale. The differences in the measured mixing ratio between the two systems were also examined statistically using linear regression analysis. Results of the regression analysis indicated the existence of significant correlations among all trace gases monitored, confirming the strong compatibility between the two systems. The effects of meteorological factors on the DOAS performance were also examined through investigation of the mixing ratio differences between two systems and the concurrently determined environmental parameters. According to our analysis, it is concluded that the level of agreement between the two systems can be affected by the variations in the spatial mixing conditions. Although some uncertainties remain to be resolved, our preliminary attempts to evaluate an open path monitoring technique clearly demonstrated that consideration of meteorological conditions may be required to properly assess the DOAS performance due to its capacity to cover spatial scale over the open path length.