The PSI rack contains two instruments: an Aethalometer and the WHOPS (White-light humidified optical particle spectrometer). 

     The Aethalometer measures the black carbon (BC) concentration using an optical technique. Ambient air is drawn through the inlet port (flow rates are typically around 4lpm) and collected on a quartz fiber filter tape, which is illuminated by LED’s of different wavelengths. The Aethalometer continuously analysis the blackening of the filter and at the same time also a reference signal of the unloaded filter is detected. In this way the attenuation signal, due to blackening of the filter, can be determined. Knowing the attenuation and optical components allows us to obtain the BC content of the aerosol deposit. When the filter spot gets too dark, or after a defined measurement time, the filter tape moves forward. In this way a continuous analysis is possible. 

     The WHOPS contains a PROMO 3000H with two Welas sensors (Palas GmbH). This is an optical particle counter that uses an intense source of white light (Osram XBO-75 Xenon short arc lamp) to illuminate a certain measuring volume. The sensing volume has a patented T-shape to minimize “border-zone” errors. The illuminated aerosols scatter light which is then detected by a photomultiplier (PMT) that is arranged at a 90° observation angle. Based on a calibration with PSL particles, the intensity of the scattered light can be converted to a certain particle diameter.  When measuring the velocity of the particles through the sampling volume, the flow can be calculated. Since each signal received by the PMT corresponds to one particle, the number of particles for a certain time range is known. As a result the particle concentration can be calculated. The first Welas sensor measures simply the total dry number size distribution (polydisperse measurement), whereas the second is situated in an isolated box. With this second sensor we want to see how particles grow when they are exposed to elevated relative humidities (RH) around 95%. To achieve this goal, the dried particles first go through a DMA (Differential Mobility Analyzer) where particles with a certain fixed diameter are selected. In the next step, these monodisperse particles pass through a custom build humidifier and finally they enter a growing chamber with a residence time of about 10 seconds). The 2nd Welas is then used to determine the hygroscopic growth factor of the monodisperse aerosol. A valve is situated just before the humidifier. If it is off all particles end up in the humidifier but when it’s on, the dry particles go through a bypass and are finally measured in the second Welas sensor. This bypass can be used to compare the diameter selection of the DMA with the measured diameter t from the WELAS. This can be used to retrieve optical properties of the aerosol (i.e. the effective refractive index).