The influence of the baseline drift on the resulting extinction values of a CAPS PMex

Abstract. The effect of the baseline drift on the resulting extinction values of three CAPS PMex monitors 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 where observed using a baseline period of 5 min. These values can be even higher for longer baseline periods. A new method is shown to minimize this effect. Modified continuous baseline values are calculated in a post-processing step using cubic smoothing splines. With this approach the extinction artefacts are diminished and the effective scattering of the resulting extinction values is reduced by about 50 %.