Abstract
Abstract. We have developed a novel single-beam photothermal interferometer and present here its application for the measurement of aerosol light absorption. The use of only a single laser beam allows for a compact optical set-up and significantly easier alignment compared to standard dual-beam photothermal interferometers, making it ideal for field measurements. Due to a unique configuration of the reference interferometer arm, light absorption by aerosols can be determined directly – even in the presence of light-absorbing gases. The instrument can be calibrated directly with light-absorbing gases, such as NO2, and can be used to calibrate other light absorption instruments. The detection limits (1σ) for absorption for 10 and 60 s averaging times were determined to be 14.6 and 7.4 Mm−1, respectively, which for a mass absorption cross section of 10 m2 g−1 leads to equivalent black carbon concentration detection limits of 1460 and 740 ng m−3, respectively. The detection limit could be reduced further by improvements to the isolation of the instrument and the signal detection and processing schemes employed.
Highlights
According to estimates from the World Health Organization (WHO), particulate air pollution contributes to about 7 million premature deaths each year, making it one of the leading causes of early mortality worldwide (WHO, 2014)
Studies of short-term health effects suggest that black carbon (BC) particles, a component of carbonaceous aerosols, are a better indicator of the effect of harmful particulate substances from combustion sources exert on human health than any other metric (Janssen et al, 2011, 2012), and it is acknowledged that BC poses tremendous harm to public health
Noise from the pump in the AE33 is coupled through the sampling reservoir and adds to the baseline noise of the Photothermal interferometry (PTI) measurement. This can be seen at 17:30 local time (LT) in the measurement data when an absolute filter was inserted into the sampling line to determine the background signal level, better isolating the modulated single-beam PTI (MSPTI) from noise transported from the sampling volume through the sampling line
Summary
According to estimates from the World Health Organization (WHO), particulate air pollution contributes to about 7 million premature deaths each year, making it one of the leading causes of early mortality worldwide (WHO, 2014). In situ absorption methods have a further advantage over traditional measurements, namely the ability to traceably calibrate the instrument using an absorbing gaseous species such as NO2 (Arnott et al, 2000; Nakayama et al, 2015) or O3 (Lack et al, 2006, 2012; Davies et al, 2018) Such internal primary calibration standards are unavailable for filterbased instrumentation, which rely on comparative measurements with reference instruments and reference aerosols; are such calibration processes prone to biases, they cannot be performed in the field, requiring the instrument to be shipped to the calibration facility. In order to address the aforementioned difficulties associated with the PTI technique, a new PTI instrument employing a single laser and unique beam configuration has been developed This instrument greatly simplifies the alignment of the interferometer, maximises the sensitivity of the measurement and enables artefact-free measurement of aerosol absorption in the presence of absorbing gaseous species. Future improvement of the sensitivity and durability of the MSPTI is planned, enabling its use as a field-monitoring instrument
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