The influence of the baseline drift on the resulting extinction values of a cavity attenuated phase shift-based extinction monitor (CAPS PMex)

Abstract

The effect of the baseline drift on the resulting extinction values of three cavity attenuated phase shift-based extinction monitors (CAPS PMex) with different wavelengths and the respective correlation with NO2 was analysed for an urban background station. A drift of more than 0.8 Mm-1 min-1 was observed for ambient air, with high probability caused by traffic-emissions-driven changes in carrier gas composition. The baseline drift leads to characteristic measurement artefacts for particle extinction. Artificial particle extinction values of approximately 4 Mm-1 were observed using a baseline period of 5 min. These values can be even higher for longer baseline periods. Two methods are shown to minimize this effect. Modified continuous baseline values are calculated in a postprocessing step using simple linear interpolation and cubic smoothing splines. Both methods are useful to reduce artefacts, although the use of cubic smoothing splines gives slightly better results. The extinction artefacts are diminished and the effective scattering of the resulting extinction values is reduced by about 50 %.