In situ vertical characteristics of optical properties and heating rates of aerosol over Beijing

• Main Authors: P. Tian, D. Liu, D. Zhao, C. Yu, Q. Liu, M. Huang, Z. Deng, L. Ran, Y. Wu, S. Ding, K. Hu, G. Zhao, C. Zhao, D. Ding
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/2603/2020/acp-20-2603-2020.pdf
• ISSN: 1680-7316; 1680-7324

<p>Characterizing vertical profiles of aerosol optical properties is important because relying on only the surface or column-integrated measurements cannot unambiguously constrain the radiative impacts of aerosol. This study presents series of vertical profiles of in situ measured multi-wavelength optical properties of aerosols during three pollution events from November to December 2016 over the Beijing region. For all pollution events, the clean periods (CPs) before pollution initialization showed a higher scattering Ångström exponent (SAE) and a smaller asymmetry parameter (<span class="inline-formula"><i>g</i></span>) with relatively uniform vertical structures. The heavy pollution periods (HPs) showed an increased particle size, causing these parameters to vary in the opposite way. During the transition periods (TPs), regional transport of aged aerosols at higher altitudes was found. The Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) matched the in situ measurements within 10&thinsp;%; however the AERONET absorption optical depth (AAOD) was 10&thinsp;%–20&thinsp;% higher than the in situ measurements, and this positive discrepancy increased to 30&thinsp;% at shorter wavelengths. The absorption of brown carbon (BrC) was identified by the increased-absorption Ångström exponent (AAE), and the heating rate of black carbon (BC) and BrC was estimated by computing the spectral absorption coefficient and actinic flux calculated by a radiative transfer model. BC and BrC had a heating rate of up to 0.18 and 0.05&thinsp;<span class="inline-formula">K h⁻¹</span> in the planetary boundary layer (PBL), respectively, during the pollution period. The fraction of BrC absorption increased from 12&thinsp;% to 40&thinsp;% in the PBL from the CP to the HP. Notably, a higher contribution of BrC heating was found above the PBL under polluted conditions. This study paints a full picture of shortwave heating impacts of carbonaceous aerosols during different stages of pollution events and highlights the increased contribution of BrC absorption especially at higher altitudes during pollution.</p>