Measurement report: The effect of aerosol chemical composition on light scattering due to the hygroscopic swelling effect

• Main Authors: R. Ren, Z. Li, P. Yan, Y. Wang, H. Wu, M. Cribb, W. Wang, X. Jin, Y. Li, D. Zhang
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/9977/2021/acp-21-9977-2021.pdf

<p>Liquid water in aerosol particles has a significant effect on their optical properties, especially on light scattering, whose dependence on chemical composition is investigated here using measurements made in southern Beijing in 2019. The effect is measured by the particle light scattering enhancement <span class="inline-formula"><i>f</i></span>(RH), where RH denotes the relative humidity, which is found to be positively and negatively impacted by the proportions of inorganic and organic matter, respectively. Black carbon is also negatively correlated. The positive impact is more robust when the inorganic matter mass fraction was smaller than 40 % (<span class="inline-formula"><i>R</i>=0.93</span>, <span class="inline-formula"><i>R</i></span>: the Pearson's correlation coefficient), becoming weaker as the inorganic matter mass fraction gets larger (<span class="inline-formula"><i>R</i>=0.48</span>). A similar pattern was also found for the negative impact of the organic matter mass fraction. Nitrate played a more significant role in aerosol hygroscopicity than sulfate in Beijing. However, the deliquescence point of ambient aerosols was at about RH <span class="inline-formula">=</span> 80 % when the ratio of the sulfate mass concentration to the nitrate mass concentration of the aerosol was high (mostly higher than <span class="inline-formula">∼</span> 4). Two schemes to parameterize <span class="inline-formula"><i>f</i></span>(RH) were developed to account for the deliquescent and non-deliquescent effects. Using only one <span class="inline-formula"><i>f</i></span>(RH) parameterization scheme to fit all <span class="inline-formula"><i>f</i></span>(RH) processes incurs large errors. A piecewise parameterization scheme is proposed, which can better describe deliquescence and reduces uncertainties in simulating aerosol hygroscopicity.</p>