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

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 0.8Mm−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 5 approximately 4Mm−1 where observed using a baseline period of 5min. 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%. 10

and pressure condition is subtracted from the respective raw signals, called total loss (loss). The resulting values are averaged over the period of baseline duration. This value is called last baseline (lastbaseline), which only depends on device parameters, in particular the degree of contamination of the cavity mirrors, but also on the concentration of absorbing gases.
The resulting values for particle extinction σ ep for the following normal measuring period is calculated as: where g is the geometry factor, considering the effect of the purge air on the effective optical path length. The crucial point is that the measurement is calculated using the baseline values, which are assumed to be constant for a certain period and lag behind in time.
A detailed description of the Instrument is given by Massoli et al. (2010). CAPS PMex has already been compared and characterized in combination with other instruments (Petzold et al., 2013)  For this purpose, exemplary measurements at an urban background station are analysed. In addition, a possible approach in post-processing is proposed to reduce the influence of the baseline drift.

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2 Experimental set-up In order to analyse the influence of the baseline drift on the resulting extinction values, measurements of ambient air were carried out at the Leibniz Institute for Tropospheric Research (Leipzig, Germany) over a period of two weeks. The measurement site, classified as an urban background station, is influenced by two main roads and rail traffic, as well as a small gas power plant.

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The measurements were performed with three different CAPS PMex monitors of different wavelengths: CAPS-blue (450 nm), CAPS-green (530 nm) and CAPS-red (630 nm). The sampling rate for all CAPS PMex was set to 1 Hz. The baseline period was set to 5 min with 60 s duration and 30 s flushing time.
In addition, the concentration of equivalent black carbon (eBC) was measured with a multi-angle absorption photometer (MAAP) at the same inlet system with a time resolution of 1 min. Furthermore, the NOx concentration was measured with an 25 APNA-370 Ambient NOx Monitor at an separate inlet at the roof top with 3 min resolution.
To analyse the influence of the variability of the gas concentration of the carrier gas and to rule out the influence of aerosols on the resulting extinction values, an additional filter was installed upstream of the three CAPS PMex. According to a zero filter test, values are expected to be around zero for the whole period. Deviations from this indicate a systematic error.

Variability of background signal
Time series for the loss signals of all three CAPS PMex, as well as the eBC and NO 2 concentrations are shown in Fig. 1.

Artefacts from internal baseline correction
As previously mentioned, variations in the baseline by changes in the gas composition may occure with values up to 4 Mm −1 min −1 .
Hence, the assumption of a constant baseline value for internal data processing may cause uncertainties.

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Any changes of the baseline during a normal measuring period due to changes in gas composition are immediately misinterpreted as aerosol extinction. Furthermore, due to the forward extrapolation, the internal lastbaseline value is phase shifted to the supposedly correct value. It is possible to reduce these artefacts by using a new method for calculating the baseline. For the post-processing the loss values for the baseline period were extracted, subtracted by the corresponding Rayleigh value and used as predictor variables for interpolation with cubic smoothing splines. The cubic smoothing spline function (smooth.spline) provided by R (R Core Team, 2013) was used for this purpose. A free smoothing parameter must be chosen, which depends on many factors, e.g.
baseline period and duration but also on device noise etc.. Therefore, a suitable parameter must be found for each individual device and application, e.g. by minimizing the artefacts with particle free ambient air according to the analysis shown here.
This approach results in a continuous time series of current baseline values, without phase-shift relative to the loss signal (see Fig. 2). The resulting extinction values improve significantly. In Fig. 3 the resulting histograms and statistical parameters for particle extinction for all instruments and the entire time series are shown. As expected, the mean value remains almost  absorption is maximized, and to a lesser extent at 530 nm, without correction, measurement precision is completely limited by the intervals between baseline measurements at a level far above short term noise levels. However, with the correction scheme, the data can be integrated for long periods of time in order to improve precision. For instance, at 450 nm, the precision is improved by a factor of approximately 4.

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The effect of the baseline drift on the measurement values of three different CAPS PMex for an urban background station was analysed. The drift can be up to 0.8 Mm −1 min −1 . For internal data processing, it is assumed that the baseline does not change for the following measurement period. In combination with a fast variable background signal or baseline drift, this can lead to measurement artefacts. The effect of baseline drift is additive, therefore, the relative error is higher for low particle extinction.
The use of cubic smoothing splines to calculate the current baseline values is a more adequate method for a variable back-30 ground and leads to a significant improvement in the particle extinction values. Artefacts for particle extinction almost disappear and variability decreases. Any other approach that provides a continuous time series for the baseline without phase shift seems just as useful. This approach is generally valid for all devices that are affected by a drift, but due to the measuring principle of the CAPS PMex this fact is especially important.