Gas-Phase Sulfur Intercomparison Experiment 2: Analysis and conclusions

A diffusion denuder, total sulfur chemiluminescence detector instrument for the measurement of SO2 was tested as the second part of the Gas-Phase Sulf ur Intercomparison Experiment (GASIE 2). The SO2 at mixing ratios between 2 7 and 182 parts per trillion by volume (pptv) was Provided by a dynamic dil ution apparatus. The data were kept blind from the other party and analyzed by two independent referees. The following was concluded: (1) The independ ent calibrations of each system are within a few percent. (2) The precision on any one day is better than day-to-day variability. (3) Runs in dry air show a small but significant nonzero intercept in correlation plots. (4) No effects of adding NO2 + O-3 or CO2 + CH4 + CO + dimethylsulfide are distin guishable. (5) A small but significant effect due to added H2O is evident i n both slope and intercept, but the source could not be discerned. (6) On a ny one day the systems can distinguish among 0, 20, and 40 pptv, but becaus e of day-to-day variability, they can only distinguish among 0, 30, and 60 pptv on different days.