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

Ping Tian, Dantong Liu, Delong Zhao, Chenjie Yu, Quan Liu, Mengyu Huang, Zhaoze Deng, Liang Ran, Yunfei Wu, Shuo Ding, Kang Hu, Gang Zhao (+2 others)
2019 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> Characterizing vertical profiles of aerosol optical properties is important because only replying on the surface or column-integrated measurements is unable to 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 in Nov. to Dec. 2016 over Beijing region. For all pollution events, clean periods (CP) before pollution
more » ... before pollution initialization showed higher scattering Ångström exponent and smaller asymmetry parameter (<i>g</i>), and relatively uniform vertical structures. The heavy pollution (HP) periods showed increased particle size, causing these parameters to vary in the opposite way. During the transition periods (TP), regional transport of aged aerosols at upper level was found. The AERONET aerosol optical depth (AOD) matched the in-situ measurements within 10&amp;thinsp;%, however the AERONET absorption optical depth (AAOD) was 10&amp;ndash;20&amp;thinsp;% higher than in-situ measurements, and this positive discrepancy increased to 30&amp;thinsp;% at shorter wavelength. The absorption of brown carbon (BrC) was identified by increased absorption Ångström exponent (AAE), and the heating rate of black carbon (BC) and BrC was calculated by computing the wavelength-dependent absorption coefficient and actinic flux by the radiative transfer model. BC and BrC had heating rate up to 0.18&amp;thinsp;K/h and 0.05&amp;thinsp;K/h in the planetary boundary layer (PBL) respectively during the pollution period. The fraction of BrC absorption increased from 12&amp;thinsp;% to 40&amp;thinsp;% in the PBL from CP to HP period. Notably, higher contribution of BrC heating was found above the PBL under polluted condition. This study gives a full picture of shortwave heating impacts of carbonaceous aerosols during different stages of pollution event, and highlights the increased contribution of BrC absorption especially at higher level during pollution.</p>
doi:10.5194/acp-2019-780 fatcat:iek2ykmqobevnam3vvlcmvp6vm