Abstract
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-1min-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 post-processing 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 %.
Highlights
Aerosol particles affect the global albedo or radiation balance of the earth by interacting with solar and thermal radiation through absorption and scattering processes
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
Summary
Aerosol particles affect the global albedo or radiation balance of the earth by interacting with solar and thermal radiation through absorption and scattering processes. The aerosol scattering σsp, absorption σap and extinction σep coefficients, from which the single scattering albedo ω = σsp/σep is derived, are important parameters. A very similar measurement method is the cavity attenuated phase shift (CAPS) technique. The phase shift of the distorted signal caused by the effective optical path is measured by a vacuum photo diode on the opposite side. This is a robust, state-of-the-art and commercially available measurement method, which is used as a gas monitor to measure ambient NO2 concentration (Kebabian et al, 2005, 2008)
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