A dual-wavelength photothermal aerosol absorption monitor: design, calibration and performance

Abstract

Abstract. There exists a lack of aerosol absorption measurement techniques with low uncertainties and without artefacts. We have developed the two-wavelength Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, calibration and mode of operation and evaluate its applicability, limits and uncertainties. The 532 nm channel was calibrated with ∼ 1 µmol mol−1 NO2, whereas the 1064 nm channel was calibrated using measured size distribution spectra of nigrosin particles and a Mie calculation. Since the aerosolized nigrosin used for calibration was dry, we determined the imaginary part of the refractive index of nigrosin from the absorbance measurements on solid thin film samples. The obtained refractive index differed considerably from the one determined using aqueous nigrosin solution. PTAAM-2λ has no scattering artefact and features very low uncertainties: 4 % and 6 % for the absorption coefficient at 532 and 1064 nm, respectively, and 9 % for the absorption Ångström exponent. The artefact-free nature of the measurement method allowed us to investigate the artefacts of filter photometers. Both the Aethalometer AE33 and CLAP suffer from cross-sensitivity to scattering – this scattering artefact is most pronounced for particles smaller than 70 nm. We observed a strong dependence of the filter multiple scattering parameter on the particle size in the 100–500 nm range. The results from the winter ambient campaign in Ljubljana showed similar multiple scattering parameter values for ambient aerosols and laboratory experiments. The spectral dependence of this parameter resulted in AE33 reporting the absorption Ångström exponent for different soot samples with values biased 0.23–0.35 higher than the PTAAM-2λ measurement. Photothermal interferometry is a promising method for reference aerosol absorption measurements.

Comment on egusphere-2023-2641

“…Throughout the study, the instruments operated at a flow rate of 100 mlmin −1 in dual-spot sampling mode to compensate for filter loading effects (Drinovec et al, 2015). Although several papers in the literature have emphasised the need for a multi-instrument approach to determine the multi-scattering correction factor (C ref ) for aethalometer filters Yus-Díez et al, 2021;Ferrero et al, 2021), with photothermal interferometry being recently identified as a suitable technique to provide a reference measurement of this parameter (Drinovec et al, 2022), this study lacked multi-instrument co-location. As a result, a constant C ref of 1.3 was chosen to account for multiple light scattering within the filter fibres and between deposited particles and the filter, as suggested by the manufacturer (Aethlabs, 2024).…”

“…The deployment of photo-thermal and photo-acoustic instruments in field settings has been limited due to operational challenges, which has hindered the widespread adoption of these techniques. However, there has been a recent resurgence of interest in these methods with the development of a new generation of photometers and interferometers (Drinovec et al, 2022;Visser et al, 2020Visser et al, , 2023.…”

Insights into secondary organic aerosol formation from the day- and nighttime oxidation of PAHs and furans in an oxidation flow reactor

“…where σair is the air mass absorption efficiency (Table S5). Comparison between AE33 and photothermal interferometer PTAAM-2λ showed that a higher value of multiple scattering parameter C should be used (Drinovec et al, 2022) than the one provided by the manufacturer. A new C value of 3.75 was selected for the measured particle sizes.…”

Aerosol absorption by in-situ filter-based photometer and ground-based sun-photometer in an urban atmosphere

“…For example, the photo-acoustic technique is very sensitive to atmospheric conditions such as relative humidity, temperature and pressure (Langridge et al, 2013), while photothermal interferometry is sensitive to mechanical vibration, although it has recently gained new attention (e.g. Drinovec et al, 2022;Visser et al, 2020). Filter-based measurements are very simple to operate, but have the main disadvantages of filter-related artifacts, such as the filter loading and the multiple scattering effects within filter fibres and between the collected particles and the filter fibres, possibly leading to systematic errors in the measurements.…”