Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer

<p>The ultra-high-sensitivity aerosol spectrometer (UHSAS) differs from most other optical particle spectrometers by using a high-power infrared (IR) laser to detect small particles and reduce the sizing ambiguity due to the non-monotonicity of scattering with particle size.</p> &...

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Autores principales:	S. G. Howell, S. Freitag, A. Dobracki, N. Smirnow, A. J. Sedlacek III
Formato:	article
Lenguaje:	EN
Publicado:	Copernicus Publications 2021
Materias:	Environmental engineering TA170-171 Earthwork. Foundations TA715-787
Acceso en línea:	https://doaj.org/article/1454b371c65d43b6aa5a2a3926ae7a5e
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spelling	oai:doaj.org-article:1454b371c65d43b6aa5a2a3926ae7a5e2021-11-26T15:41:12ZUndersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer10.5194/amt-14-7381-20211867-13811867-8548https://doaj.org/article/1454b371c65d43b6aa5a2a3926ae7a5e2021-11-01T00:00:00Zhttps://amt.copernicus.org/articles/14/7381/2021/amt-14-7381-2021.pdfhttps://doaj.org/toc/1867-1381https://doaj.org/toc/1867-8548<p>The ultra-high-sensitivity aerosol spectrometer (UHSAS) differs from most other optical particle spectrometers by using a high-power infrared (IR) laser to detect small particles and reduce the sizing ambiguity due to the non-monotonicity of scattering with particle size.</p> <p>During the NASA ORACLES project (ObseRvations of Aerosols above CLouds and their intEractionS) over the southeast Atlantic Ocean, the UHSAS clearly undersized particles in the biomass burning plume extending from southern Africa. Since the horizontal and vertical extent of the plume was vast, the NASA P-3B research aircraft often flew through a fairly uniform biomass burning plume for periods exceeding 30 min, sufficient time to explore the details of the UHSAS response by selecting single particle sizes with a differential mobility analyzer (DMA) and passing them to the UHSAS. This was essentially an in-flight calibration of the UHSAS using the particles of interest. Two modes of responses appeared. Most particles were undersized by moderate amounts, ranging from not at all for 70 <span class="inline-formula">nm</span> aerosols to 15 % for 280 <span class="inline-formula">nm</span> particles. Mie scattering calculations show that composition-dependent refractive index of the particles cannot explain the pattern. Heating of brown carbon or tarballs in the beam causing evaporation and shrinking of the particles is the most plausible explanation, though mis-sizing due to non-sphericity cannot be ruled out. A small fraction (10 %–30 %) of the particles were undersized by 25 % to 35 %. Those were apparently the particles containing refractory black carbon. Laboratory calibrations confirm that black carbon is drastically undersized by the UHSAS, because particles heat to their vaporization point and shrink.</p> <p>A simple empirical correction equation was implemented that dramatically improves agreement with DMA distributions between 100 and 500 <span class="inline-formula">nm</span>. It raised the median particle diameter by 18 <span class="inline-formula">nm</span>, from 163 to 181 <span class="inline-formula">nm</span>, during the August 2017 deployment and by smaller amounts during deployments with less intense pollution. Calculated scattering from UHSAS size distributions increased by about 130 %, dramatically improving agreement with scattering measured by nephelometers. The correction is only valid in polluted instances; clean marine boundary layer and free troposphere aerosols behaved more like the calibration spheres. We were unable to directly test the correction between 500 and 1000 <span class="inline-formula">nm</span>, though aerodynamic particle sizer (APS) data appear to show that the correction is poor at the largest diameters, which is no surprise as the composition of those particles is likely to be quite different than that of the accumulation mode. This adds to the evidence that UHSAS data must be treated cautiously whenever the aerosol may absorb infrared light. Similar corrections may be required whenever brown carbon aerosol is present.</p>S. G. HowellS. FreitagA. DobrackiA. DobrackiN. SmirnowN. SmirnowA. J. Sedlacek IIICopernicus PublicationsarticleEnvironmental engineeringTA170-171Earthwork. FoundationsTA715-787ENAtmospheric Measurement Techniques, Vol 14, Pp 7381-7404 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Environmental engineering TA170-171 Earthwork. Foundations TA715-787
spellingShingle	Environmental engineering TA170-171 Earthwork. Foundations TA715-787 S. G. Howell S. Freitag A. Dobracki A. Dobracki N. Smirnow N. Smirnow A. J. Sedlacek III Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
description	<p>The ultra-high-sensitivity aerosol spectrometer (UHSAS) differs from most other optical particle spectrometers by using a high-power infrared (IR) laser to detect small particles and reduce the sizing ambiguity due to the non-monotonicity of scattering with particle size.</p> <p>During the NASA ORACLES project (ObseRvations of Aerosols above CLouds and their intEractionS) over the southeast Atlantic Ocean, the UHSAS clearly undersized particles in the biomass burning plume extending from southern Africa. Since the horizontal and vertical extent of the plume was vast, the NASA P-3B research aircraft often flew through a fairly uniform biomass burning plume for periods exceeding 30 min, sufficient time to explore the details of the UHSAS response by selecting single particle sizes with a differential mobility analyzer (DMA) and passing them to the UHSAS. This was essentially an in-flight calibration of the UHSAS using the particles of interest. Two modes of responses appeared. Most particles were undersized by moderate amounts, ranging from not at all for 70 <span class="inline-formula">nm</span> aerosols to 15 % for 280 <span class="inline-formula">nm</span> particles. Mie scattering calculations show that composition-dependent refractive index of the particles cannot explain the pattern. Heating of brown carbon or tarballs in the beam causing evaporation and shrinking of the particles is the most plausible explanation, though mis-sizing due to non-sphericity cannot be ruled out. A small fraction (10 %–30 %) of the particles were undersized by 25 % to 35 %. Those were apparently the particles containing refractory black carbon. Laboratory calibrations confirm that black carbon is drastically undersized by the UHSAS, because particles heat to their vaporization point and shrink.</p> <p>A simple empirical correction equation was implemented that dramatically improves agreement with DMA distributions between 100 and 500 <span class="inline-formula">nm</span>. It raised the median particle diameter by 18 <span class="inline-formula">nm</span>, from 163 to 181 <span class="inline-formula">nm</span>, during the August 2017 deployment and by smaller amounts during deployments with less intense pollution. Calculated scattering from UHSAS size distributions increased by about 130 %, dramatically improving agreement with scattering measured by nephelometers. The correction is only valid in polluted instances; clean marine boundary layer and free troposphere aerosols behaved more like the calibration spheres. We were unable to directly test the correction between 500 and 1000 <span class="inline-formula">nm</span>, though aerodynamic particle sizer (APS) data appear to show that the correction is poor at the largest diameters, which is no surprise as the composition of those particles is likely to be quite different than that of the accumulation mode. This adds to the evidence that UHSAS data must be treated cautiously whenever the aerosol may absorb infrared light. Similar corrections may be required whenever brown carbon aerosol is present.</p>
format	article
author	S. G. Howell S. Freitag A. Dobracki A. Dobracki N. Smirnow N. Smirnow A. J. Sedlacek III
author_facet	S. G. Howell S. Freitag A. Dobracki A. Dobracki N. Smirnow N. Smirnow A. J. Sedlacek III
author_sort	S. G. Howell
title	Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
title_short	Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
title_full	Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
title_fullStr	Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
title_full_unstemmed	Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
title_sort	undersizing of aged african biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer
publisher	Copernicus Publications
publishDate	2021
url	https://doaj.org/article/1454b371c65d43b6aa5a2a3926ae7a5e
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Undersizing of aged African biomass burning aerosol by an ultra-high-sensitivity aerosol spectrometer

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