A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

Telg, Hagen; Murphy, D. M.; Bates, T. S.; Johnson, James E.; Quinn, Patricia K.; Giardi, Fabio; Gao, R. S.

doi:10.1080/02786826.2017.1296103

Cited by 22 publications

(27 citation statements)

References 29 publications

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“…These photometers measure the aerosol-particle light-absorption coefficient (σ abs ) by detecting the change of attenuation of light due to deposited aerosol-particle mass on sample filter. They have been installed on airship platforms (Rosati et al, 2016), tethered balloon platforms (Ran et al, 2016;Ferrero et al, 2014Ferrero et al, , 2016, or unmanned aircraft systems (UASs; Markowicz et al, 2017;Telg et al, 2017;Bärfuss et al, 2018) to address the vertical BC distribution. To investigate human exposure to health-harming BC-containing aerosol particles from combustion sources, they have been used for mobile measurements (Cepeda et al, 2017, and references therein;Alas et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…To investigate human exposure to health-harming BC-containing aerosol particles from combustion sources, they have been used for mobile measurements (Cepeda et al, 2017, and references therein;Alas et al, 2018). Subramanian et al (2007), Vecchi et al (2013), and Lack et al (2008) have shown that liquid-like brown carbon can significantly bias filter-based absorption measurements, since this organic carbon wraps around filter fibers and alters their structural properties. Aerosol samples contain water vapor represented by its relative humidity (RH).…”

Section: Introductionmentioning

confidence: 99%

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The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes

et al. 2019

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Abstract. Measuring vertical profiles of the particle light-absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity (RH). These absorption photometers determine the particle light-absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (RH in %), thus influencing the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast RH changes (dRH ∕ dt) on the particle light-absorption coefficient (σabs in Mm−1) derived with two absorption photometers. The RH dependence was examined based on different filter types and filter loadings with respect to loading material and areal loading density. The Single Channel Tricolor Absorption Photometer (STAP) relies on quartz-fiber filter, and the microAeth® MA200 is based on a polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filters, filters loaded with black carbon (BC), and filters loaded with ammonium sulfate. The filter areal loading densities (ρ*) ranged from 3.1 to 99.6 mg m−2 in the case of the STAP and ammonium sulfate and 1.2 to 37.6 mg m−2 in the case the MA200. Investigating BC-loaded cases, ρBC* was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m−2 for the STAP and MA200, respectively. Both instruments revealed opposing responses to relative humidity changes (ΔRH) with different magnitudes. The STAP shows a linear dependence on relative humidity changes. The MA200 is characterized by a distinct exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 s running average output, the STAP reveals an absolute change in σabs per absolute change of RH (Δσabs∕ΔRH) of 0.14 Mm−1 %−1 in the clean case, 0.29 Mm−1 %−1 in the case of BC-loaded filters, and 0.21 Mm−1 %−1 in the case filters loaded with ammonium sulfate. The 60 s running average of the particle light-absorption coefficient at 625 nm measured with the MA200 revealed a response of around −0.4 Mm−1 %−1 for all three cases. Whereas the response of the STAP varies over the different loading materials, in contrast, the MA200 was quite stable. The response was, for the STAP, in the range of 0.17 to 0.24 Mm−1 %−1 and, in the case of ammonium sulfate loading and in the BC-loaded case, 0.17 to 0.62 Mm−1 %−1. In the ammonium sulfate case, the minimum response shown by the MA200 was −0.42 with a maximum of −0.36 Mm−1 %−1 and a minimum of −0.42 and maximum −0.37 Mm−1 %−1 in the case of BC. A linear correction function for the STAP was developed here. It is provided by correlating 1 Hz resolved recalculated particle light-absorption coefficients and RH change rates. The linear response is estimated at 10.08 Mm−1 s−1 %−1. A correction approach for the MA200 is also provided; however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a RH change could be reproduced. However, the given correction approach has to be estimated with other RH sensors as well, since each sensor has a different response time. And, for the given correction approaches, the uncertainties could not be estimated, which was mainly due to the response time of the RH sensor. Therefore, we do not recommend using the given approaches. But they point in the right direction, and despite the imperfections, they are useful for at least estimating the measurement uncertainties due to relative humidity changes. Due to our findings, we recommend using an aerosol dryer upstream of absorption photometers to reduce the RH effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large relative humidity gradients has to be low to minimize the observed RH effect. But this is simply not possible in some scenarios, especially in unmixed layers or clouds. Additionally, recording the RH of the sample stream allows correcting for the bias during post-processing of the data. This data correction leads to reasonable results, according to the given example in this study.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes

et al. 2019

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“…These photometers measure the aerosol particle light absorption coefficient (σabs) by detecting the change of attenuation of light due to deposited aerosol particle mass on sample filter. They have been installed on airship platforms (Rosati et al, 2016), tethered balloon platforms (Ran et al, 2016;Ferrero et al, 2014, Ferrero et al, 2016 or unmanned aircraft systems (UAS;Markowitz et al, 2017, Telg et al, 2017, Bärfuss et al, 2018 to address the 55 vertical BC distribution. To investigate human exposure to health-harming BC-containing aerosol particles from combustion sources, they have been used for mobile measurements (Capeda et al, 2017 and references therein;Alas et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

“…However, to address vertical profiling of BC with fast rh changes, particle light absorption measurements require a high temporal resolution of about seconds. Telg et al (2017) presented a study using an unmanned aircraft system (UAS) for vertical profiling of aerosol physical 75 properties including the aerosol particle light absorption coefficient measured by an absorption photometer. In their study, a significant decrease of σabs at around 1000 m altitude is visible.…”

Section: Introductionmentioning

confidence: 99%

“…In the WMO/GAW report 227 (2016), it is recommended to conduct aerosol sampling below 40% relative humidity to prevent measurements artefacts due to high relative humidity. Although the measurements of Telg et al (2017) have been conducted following these recommendations, this is a published example for the 80 bias in σabs measurements due to fast rh changes even for a rh below the 40% threshold.…”

Section: Introductionmentioning

confidence: 99%

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The effect of rapid relative humidity changes on fast filter-based aerosol particle light absorption measurements: uncertainties and correction schemes

Düsing¹,

Wehner²,

Müller³

et al. 2019

Preprint

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Abstract. Measuring vertical profiles of the particle light absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity. These absorption photometers determine the particle light absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (rh in %), influencing thus the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast rh changes (drh/dt) on the particle light absorption coefficient (σabs in Mm−1) derived with two absorption photometers. The rh dependency was examined based on different filter types and filter loadings with respect to loading material and loading areal density. Different filter material was used in the two examined instruments. The Single Channel Tri-Color Absorption Photometer (STAP; Brechtel Manufacturing Inc, 1789 Addison Way, Hayward, CA 94544, USA) relies on quartz-fiber filter (PALL LifeScience, Pallflex Membrane Filters Type E70-2075W) and the microAeth® MA200 (AethLabs, 1640 Valencia St, Suite 2C, San Francisco, CA 94110, USA) is based on a Polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filter, filter loaded with black carbon (BC) and filter loaded with ammonium sulfate. The filter loading areal densities (ρ*) ranged from 3.1 to 99.6 mg m−2 in the case of the STAP and ammonium sulfate, 1.2 to 37.6 mg m−2 considering the MA200. Investigating BC loaded cases, ρ*BC was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m−2 for the STAP and MA200, respectively. In addition, the effect of a silica-bead based diffusion on the rh effect was investigated. Both instruments revealed opposing responses to relative humidity changes (Δrh) with different amplitudes. Whereas the STAP shows a linear dependence to relative humidity changes, the MA200 is characterized by an exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 second average output, the STAP reveals an absolute change in σabs per absolute change of rh (Δσabs/Δrh) of 0.14 Mm−1 %−1 in the clean case, 0.29 Mm−1 %−1 in the case of BC loaded filters, and 0.21 Mm−1 %−1 considering filters loaded with ammonium sulfate. The 60-second running average of the particle light absorption coefficient at 625 nm measured with the MA200 revealed response of around −0.4 Mm−1 %−1 for all three cases. Whereas the response of the STAP varies over the different loading materials in contrast the MA200 was quite stable. The minimum and maximum response was for the STAP 0.17 Mm−1 %−1 and 0.24 Mm−1 %−1 considering ammonium sulfate loading and in the BC loaded case 0.17 Mm−1 %−1 and 0.62 Mm−1 %−1, respectively. The minimum response shown by the MA200 was −0.42 Mm−1 %−1 and −0.36 Mm−1 %−1 at maximum for ammonium sulfate and −0.42 Mm−1 %−1 and −0.37 Mm−1 %−1 in case of BC loading, respectively. Using the aerosol dryer upstream, the STAP did not change the behavior, but the amplitude of the observed effect was reduced by a factor of up to three. A linear correction function for the STAP was developed here. It is provided by correlating recalculated particle light absorption coefficients at 1 Hz time resolution against the change rate of rh. The linear response is estimated with 10.08 Mm−1 s−1 %−1 and can be used to correct for bias induced to rh changes at this time resolution. A correction approach for the MA200 is also provided, however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a rh change could be reproduced. Due to our findings, we recommend to use an aerosol dryer upstream of absorption photometers to reduce the rh effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large rh gradients has to be low to minimize the observed rh effect. Additionally, recording the rh of the sample stream allows correcting for the bias during post processing of the data. This data correction leads to reasonable results, according the given example in this study.

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Religious burning as a potential major source of atmospheric fine aerosols in summertime Lhasa on the Tibetan Plateau

Cui

Liu

Bai

et al. 2018

Atmospheric Environment

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A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

Cited by 22 publications

References 29 publications

The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes

The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes

The effect of rapid relative humidity changes on fast filter-based aerosol particle light absorption measurements: uncertainties and correction schemes

Religious burning as a potential major source of atmospheric fine aerosols in summertime Lhasa on the Tibetan Plateau

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