Elisabet Weingartner

The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO₂. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO₂. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO₂. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO₂. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO2. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

Carbonaceous aerosols (CA) represent a chemically and physically diverse class of airborne particles. Their relevance for climate and health is significant, but poorly understood. Conventional metrics, such as particulate matter (PM10 or PM2.5), are insufficient to capture their impact. More specific indicators, such as elemental and organic carbon (EC and OC) and equivalent black carbon (eBC), often suffer from methodological artefacts, limited compatibility, and insufficient time resolution. In this work, we present an overview of the techniques and measurement systems developed by members of our group at FHNW for improved in situ quantification and characterization of CA. Photoemission-based sensors provide a real-time signal that serves as a proxy for particle-bound polycyclic aromatic hydrocarbons (PAHs), which include well known carcinogenic substances. Photothermal methods, such as photothermal interferometry (PTI) and photoacoustic spectroscopy (PAS), are used for direct observation of light absorption — an optical property directly linked to climate effects — without artefacts inherent to filter-based measurements. The fast thermal carbon totalizator (FATCAT) provides robust, unattended measurement of total carbon with high time resolution and generates fast-thermograms that reveal volatility and refractivity. Collectively, these methods address key limitations of current CA monitoring and support the long-term goal of integrating relevant metrics into air quality monitoring and climate observation networks.

Black carbon (BC) aerosol particles are emitted by the incomplete combustion of carbonaceous fuels. These particles absorb solar radiation and BC-dominated aerosol mixtures with low single scattering albedo have a positive radiative forcing, thus heating the atmosphere. Radiative transfer models make use of the BC mass absorption cross section (MACBC) to derive the radiative forcing of BC given a certain particle mass concentration. Freshly emitted BC has a MAC value of 8 ± 1 m2/g at 550 nm (Bond et al., 2013). However, MAC can increase as aerosols age in the atmosphere due to increase in particle coating. This is the so-called lensing effect, which leads to MACBC observations of up to 15 m2/g at 550 nm (Li et al., 2022; Savadkoohi et al., 2024). The effect of coatings and the evolution of MACBC with ageing have been and still are a matter of intense scientific discussions.The determination of MACBC is carried out in the lab and in the field using various methods for light absorption and BC mass measurement. The most common techniques for absorption measurement include filter-based attenuation measurements, whereas the most common technique for mass measurement is thermo-optical analysis, which quantifies elemental carbon mass (EC; EN 16909:2017). The development of more accurate techniques with operational and scientific advantages for both light absorption and BC mass quantification has led to more reliable MACBC field measurements, allowing researchers to have a clearer picture of how atmospheric ageing and regional conditions affect the optical properties of BC.In this study, we have reviewed 63 publications that provide atmospheric MACBC values and present the results in terms of aerosol type, measurement technique, regional variability, and how interpretation of results using these factors can help the community to use the appropriate MAC in models. We provide guidance and perspectives for future studies and how the literature on MACBC can be exploited and interpreted in order to improve radiative models that include BC.References Bond, T. C., Doherty, S. J., Fahey, D. W., Forster, P. M., Berntsen, T., DeAngelo, B. J., Flanner, M. G., Ghan, S., Kärcher, B., Koch, D., Kinne, S., Kondo, Y., Quinn, P. K., Sarofim, M. C., Schultz, M. G., Schulz, M., Venkataraman, C., Zhang, H., Zhang, S., … Zender, C. S. (2013). Bounding the role of black carbon in the climate system: A scientific assessment. Journal of Geophysical Research: Atmospheres, 118(11), 5380–5552. https://doi.org/10.1002/jgrd.50171Hanyang Li & Andrew A. May (2022) Estimating mass-absorption cross-section of ambient black carbon aerosols: Theoretical, empirical, and machine learning models, Aerosol Science and Technology, 56:11, 980-997, https://doi.org/10.1080/02786826.2022.2114311Savadkoohi, Marjan, Marco Pandolfi, Cristina Reche, Jarkko V. Niemi, Dennis Mooibroek, Gloria Titos, David C. Green, et al. (2023) The Variability of Mass Concentrations and Source Apportionment Analysis of Equivalent Black Carbon across Urban Europe. Environment International 178: 108081. https://doi.org/10.1016/j.envint.2023.108081.

The risk of Mycobacterium tuberculosis (Mtb) transmission can be high in crowded clinics. We developed a spatiotemporal model of airborne Mtb transmission based on the Wells-Riley equation. We collected environmental, clinical and person-tracking data in a South African clinic during COVID-19, when community or surgical masks were compulsory and ventilation was increased. We matched person movements with clinical records to identify the spatiotemporal location of infectious TB patients. We modeled the concentration of infectious doses (quanta) and estimated the individual risk of infection. Over five days, video sensors tracked 1,438 clinic attendees. CO² levels were low (median 431 ppm, IQR 406 ppm–458 ppm); the quanta concentration was higher in the morning than in the afternoon, and highest in the waiting room. The estimated risk of infection per clinic attendee was 0.05% (80%-credible interval (CrI) 0.01%–0.06%). It increased with the number of close contacts with infectious patients and the time spent in the clinic, and was 1.3-fold (95%-CrI 1.2–1.4) higher in scenarios without mask use and 2.1-fold (95%-CrI 0.9–5.0) higher with pre-pandemic ventilation rates, emphasizing the importance of ventilation. Spatiotemporal modeling can identify high-risk areas and evaluate the impact of infection control measures in clinics.

Carbonaceous aerosol is a relevant constituent of the atmosphere in terms of climate and health impacts. Nevertheless, measuring this component poses many challenges. There is currently no simple and sensitive commercial technique that can reliably capture its totality in an unattended manner, with minimal user intervention, for extended periods of time. To address this issue we have developed the fast thermal carbon totalizator (FATCAT). Our system captures an aerosol sample on a rigid metallic filter and subsequently analyses it by rapidly heating the filter directly, through induction, to a temperature around 800 ^∘C. The carbon in the filter is oxidized and quantified as CO₂ in order to establish the total carbon (TC) content of the sample. The metallic filter is robust, which solves filter displacement or leakage problems, and does not require a frequent replacement like other measurement techniques. The limit of detection of our system using the 3σ criterion is TC =0.19 µg-C (micrograms of carbon). This translates to an average ambient concentration of TC =0.32 µg-C m⁻³ and TC =0.16 µg-C m⁻³ for sampling interval of 1 or 2 h respectively using a sampling flow rate of 10 L min⁻¹. We present a series of measurements using a controlled, well-defined propane flame aerosol as well as wood-burning emissions using two different wood-burning stoves. Furthermore, we complement these measurements by coating the particles with secondary organic matter by means of an oxidation flow reactor. Our device shows a good correlation (correlation coefficient, R²>0.99) with well-established techniques, like mass measurements by means of a tapered element oscillating microbalance and TC measurements by means of thermal–optical transmittance analysis. Furthermore, the homogeneous fast-heating of the filter produces fast thermograms. This is a new feature that, to our knowledge, is exclusive of our system. The fast thermograms contain information regarding the volatility and refractoriness of the sample without imposing an artificial fraction separation like other measurement methods. Different aerosol components, like wood-burning emissions, soot from the propane flame and secondary organic matter, create diverse identifiable patterns.