The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes
<p>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 th...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-11-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/12/5879/2019/amt-12-5879-2019.pdf |
| _version_ | 1818206263708745728 |
|---|---|
| author | S. Düsing B. Wehner T. Müller A. Stöcker A. Wiedensohler |
| author_facet | S. Düsing B. Wehner T. Müller A. Stöcker A. Wiedensohler |
| author_sort | S. Düsing |
| collection | DOAJ |
| description | <p>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.</p>
<p>A sophisticated set of laboratory experiments was therefore conducted to
investigate the effect of fast RH changes (dRH <span class="inline-formula">∕</span> d<span class="inline-formula"><i>t</i></span>) on the particle light-absorption coefficient (<span class="inline-formula"><i>σ</i><sub>abs</sub></span> in Mm<span class="inline-formula"><sup>−1</sup></span>) 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<sup>®</sup> 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
(<span class="inline-formula"><i>ρ</i><sup>*</sup></span>) ranged from 3.1 to 99.6 mg m<span class="inline-formula"><sup>−2</sup></span> in the case of the STAP and
ammonium sulfate and 1.2 to 37.6 mg m<span class="inline-formula"><sup>−2</sup></span> in the case the MA200.
Investigating BC-loaded cases, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ρ</mi><mtext>BC</mtext><mo>*</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d3ff6015cdd5bfb972ef121b5d2d2b8e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-5879-2019-ie00001.svg" width="20pt" height="15pt" src="amt-12-5879-2019-ie00001.png"/></svg:svg></span></span> was in the range of 2.9 to
43.0 and 1.1 to 16.3 mg m<span class="inline-formula"><sup>−2</sup></span> for the STAP and MA200, respectively.</p>
<p>Both instruments revealed opposing responses to relative humidity changes
(<span class="inline-formula">Δ</span>RH) 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 <span class="inline-formula"><i>σ</i><sub>abs</sub></span> per absolute change of RH (<span class="inline-formula">Δ<i>σ</i><sub>abs</sub>∕Δ</span>RH) of 0.14 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the clean case, 0.29 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the case of BC-loaded filters, and 0.21 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> 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 <span class="inline-formula">−0.4</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> 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<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> and,
in the case of ammonium sulfate loading and in the BC-loaded case, 0.17 to 0.62 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span>. In the
ammonium sulfate case, the minimum response shown by the MA200 was <span class="inline-formula">−0.42</span> with a maximum of <span class="inline-formula">−0.36</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> and a minimum of <span class="inline-formula">−0.42</span> and maximum <span class="inline-formula">−0.37</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the case of BC.</p>
<p>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<span class="inline-formula"><sup>−1</sup></span> s<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span>. 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<span id="page5880"/> 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.</p>
<p>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.</p> |
| first_indexed | 2024-12-12T04:10:15Z |
| format | Article |
| id | doaj.art-58d80b7098e4491fb3a29c155e391ca6 |
| institution | Directory Open Access Journal |
| issn | 1867-1381 1867-8548 |
| language | English |
| last_indexed | 2024-12-12T04:10:15Z |
| publishDate | 2019-11-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Measurement Techniques |
| spelling | doaj.art-58d80b7098e4491fb3a29c155e391ca62022-12-22T00:38:37ZengCopernicus PublicationsAtmospheric Measurement Techniques1867-13811867-85482019-11-01125879589510.5194/amt-12-5879-2019The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemesS. Düsing0B. Wehner1T. Müller2A. Stöcker3A. Wiedensohler4Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, GermanyLeibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, GermanyLeibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, GermanyDepartment of Statistics, Ludwig-Maximilians-Universität München, 80539 Munich, GermanyLeibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany<p>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.</p> <p>A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast RH changes (dRH <span class="inline-formula">∕</span> d<span class="inline-formula"><i>t</i></span>) on the particle light-absorption coefficient (<span class="inline-formula"><i>σ</i><sub>abs</sub></span> in Mm<span class="inline-formula"><sup>−1</sup></span>) 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<sup>®</sup> 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 (<span class="inline-formula"><i>ρ</i><sup>*</sup></span>) ranged from 3.1 to 99.6 mg m<span class="inline-formula"><sup>−2</sup></span> in the case of the STAP and ammonium sulfate and 1.2 to 37.6 mg m<span class="inline-formula"><sup>−2</sup></span> in the case the MA200. Investigating BC-loaded cases, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="italic">ρ</mi><mtext>BC</mtext><mo>*</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d3ff6015cdd5bfb972ef121b5d2d2b8e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-5879-2019-ie00001.svg" width="20pt" height="15pt" src="amt-12-5879-2019-ie00001.png"/></svg:svg></span></span> was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m<span class="inline-formula"><sup>−2</sup></span> for the STAP and MA200, respectively.</p> <p>Both instruments revealed opposing responses to relative humidity changes (<span class="inline-formula">Δ</span>RH) 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 <span class="inline-formula"><i>σ</i><sub>abs</sub></span> per absolute change of RH (<span class="inline-formula">Δ<i>σ</i><sub>abs</sub>∕Δ</span>RH) of 0.14 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the clean case, 0.29 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the case of BC-loaded filters, and 0.21 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> 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 <span class="inline-formula">−0.4</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> 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<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> and, in the case of ammonium sulfate loading and in the BC-loaded case, 0.17 to 0.62 Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span>. In the ammonium sulfate case, the minimum response shown by the MA200 was <span class="inline-formula">−0.42</span> with a maximum of <span class="inline-formula">−0.36</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> and a minimum of <span class="inline-formula">−0.42</span> and maximum <span class="inline-formula">−0.37</span> Mm<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span> in the case of BC.</p> <p>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<span class="inline-formula"><sup>−1</sup></span> s<span class="inline-formula"><sup>−1</sup></span> %<span class="inline-formula"><sup>−1</sup></span>. 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<span id="page5880"/> 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.</p> <p>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.</p>https://www.atmos-meas-tech.net/12/5879/2019/amt-12-5879-2019.pdf |
| spellingShingle | S. Düsing B. Wehner T. Müller A. Stöcker A. Wiedensohler The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes Atmospheric Measurement Techniques |
| title | The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes |
| title_full | The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes |
| title_fullStr | The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes |
| title_full_unstemmed | The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes |
| title_short | The effect of rapid relative humidity changes on fast filter-based aerosol-particle light-absorption measurements: uncertainties and correction schemes |
| title_sort | effect of rapid relative humidity changes on fast filter based aerosol particle light absorption measurements uncertainties and correction schemes |
| url | https://www.atmos-meas-tech.net/12/5879/2019/amt-12-5879-2019.pdf |
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