Aerosol hygroscopicity over the southeast Atlantic Ocean during the biomass burning season – Part 1: From the perspective of scattering enhancement
<p>Aerosol hygroscopicity plays a vital role in aerosol radiative forcing. One key parameter describing hygroscopicity is the scattering enhancement factor, <span class="inline-formula"><i>f</i></span>(RH), defined as the ratio of the scattering coefficient at...
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| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/24/13849/2024/acp-24-13849-2024.pdf |
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| Summary: | <p>Aerosol hygroscopicity plays a vital role in aerosol radiative forcing. One key parameter describing hygroscopicity is the scattering enhancement factor, <span class="inline-formula"><i>f</i></span>(RH), defined as the ratio of the scattering coefficient at humidified relative humidity (RH) to its dry value. Here, we utilize the <span class="inline-formula"><i>f</i></span>(80 %) from ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) 2016 and 2018 airborne measurements to investigate the hygroscopicity of aerosols, its vertical distribution, its relationship with chemical composition, and its sensitivity to organic aerosol (OA) hygroscopicity over the southeast Atlantic (SEA) Ocean during the biomass burning (BB) season.</p>
<p>We found that aerosol hygroscopicity remains steady above 2 km, with a mean <span class="inline-formula"><i>f</i></span>(80 %) of 1.40 <span class="inline-formula">±</span> 0.17. Below 2 km, aerosol hygroscopicity increases with decreasing altitude, with a mean <span class="inline-formula"><i>f</i></span>(80 %) of 1.51 <span class="inline-formula">±</span> 0.22, consistent with higher values of BB aerosol hygroscopicity found in the literature. The hygroscopicity parameter of OA (<span class="inline-formula"><i>κ</i><sub>OA</sub></span>) is retrieved from the Mie model with a mean value of 0.11 <span class="inline-formula">±</span> 0.08, which is in the middle to upper range compared to the literature. Higher OA hygroscopicity is related to aerosols that are more aged, oxidized, and present at lower altitudes. The enhanced biomass burning aerosol (BBA) hygroscopicity at lower altitudes is mainly due to a lower OA fraction, increased sulfate fraction, and greater <span class="inline-formula"><i>κ</i><sub>OA</sub></span> at lower altitudes.</p>
<p><span id="page13850"/>We propose a parameterization that quantifies <span class="inline-formula"><i>f</i></span>(RH) with chemical composition and <span class="inline-formula"><i>κ</i><sub>OA</sub></span> based on Mie simulation of internally mixed OA–(NH<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>2</sub></span>SO<span class="inline-formula"><sub>4</sub></span>–BC mixtures. The good agreement between the predictions and the ORACLES measurements implies that the aerosols in the SEA during the BB season can be largely represented by the OA–(NH<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>2</sub></span>SO<span class="inline-formula"><sub>4</sub></span>–BC internal mixture with respect to the <span class="inline-formula"><i>f</i></span>(RH) prediction. The sensitivity of <span class="inline-formula"><i>f</i></span>(RH) to <span class="inline-formula"><i>κ</i><sub>OA</sub></span> indicates that applying a constant <span class="inline-formula"><i>κ</i><sub>OA</sub></span> is only suitable when the OA fraction is low and <span class="inline-formula"><i>κ</i><sub>OA</sub></span> shows limited variation. However, in situations deviating these two criteria, <span class="inline-formula"><i>κ</i><sub>OA</sub></span> can notably impact scattering coefficients and aerosol radiative effect; therefore, accounting for <span class="inline-formula"><i>κ</i><sub>OA</sub></span> variability is recommended.</p> |
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| ISSN: | 1680-7316 1680-7324 |