The dispersion of small aerosols in a concert hall is experimentally studied for estimating the risk of infection with SARS-CoV-2 during a concert. A mannequin was modified to emit an air stream containing aerosols and CO₂. The aerosols have a size distribution with a peak diameter $\left(\delta \right)$ close to 0.3 µm and a horizontal initial particle velocity ($v_{p,x}$) of 2.4 m/s. The CO₂-concentration (c) emitted simultaneously is 7500 ppm. It is investigated, if the spatial dissipation of aerosols and CO₂ can be correlated. This would allow the use of technically easier CO₂ measurements to monitor compliance with aerosol concentration limits. Both aerosol and CO₂ concentrations are mapped by different sensors placed around the mannequin. As a result, no significant enrichment of aerosols and CO₂ was obtained outside a radius of 1.5 m when the fresh air ventilation in the concert hall has a steady vertical flow with a velocity of $v_{g,z}=0.05 \mathrm{m}/\mathrm{s}$ and the installed ventilation system was operating at an air change rate per hour (ACH) of 3, corresponding to an air exchange rate of 51,000 m³/h. A Pearson correlation coefficient of 0.77 was obtained for CO₂ and aerosol concentrations measured simultaneously at different positions within the concert hall.

A new sensor for methane and carbon dioxide concentration measurements in biogas plants is presented. LEDs in the mid infrared spectral region are implemented as low cost light source. The combination of quartz-enhanced photoacoustic spectroscopy with an absorption path leads to a sensor setup suitable for the harsh application environment. The sensor system contains an electronics unit and the two gas sensors; it was designed to work as standalone device and was tested in a biogas plant for several weeks. Gas concentration dependent measurements show a precision better than 1% in a range between 40% and 60% target gas concentration for both sensors. Concentration dependent measurements with different background gases show a considerable decrease in cross sensitivity against the major components of biogas in direct comparison to common absorption based sensors.

Molecular oxygen is detected at low concentrations using photoacoustic spectroscopy despite its unfavorable photoacoustic properties. The system consists of a seed laser diode, a tapered amplifier and a quartz tuning fork based spectrophone, thus employing quartz enhanced photoacoustic spectroscopy (QEPAS). With this system a detection limit of 13 ppm is reached with a compact and long term stable setup. Further improvement of the detection limit is possible by adding suitable gases to the sample gas that promote the radiationless de-excitation of the oxygen molecules.