Abstract
Abstract. 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 (g) 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 %; however the AERONET absorption optical depth (AAOD) was 10 %–20 % higher than the in situ measurements, and this positive discrepancy increased to 30 % 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 K h−1 in the planetary boundary layer (PBL), respectively, during the pollution period. The fraction of BrC absorption increased from 12 % to 40 % 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.
Highlights
The optical properties of aerosol, which cause aerosol to scatter or absorb solar radiation, have caused important radiative impacts on the earth system (Intergovernmental Panel on Climate Change 2013 – IPCC 2013)
Aircraft vertical profiles were performed on a daily basis as the flight time, which is indicated by the vertical bars (Fig. 1)
Each pollution event was classified as the pollution initialization, development and peak pollution period, corresponding to the following pollution levels: the clean period (CP; PM2.5, surface < 35 μg cm−3), transition period (TP; 35 μg cm−3 < PM2.5, surface < 200 μg cm−3) and heavy pollution period (HP; PM2.5, surface > 200 μg cm−3)
Summary
The optical properties of aerosol, which cause aerosol to scatter or absorb solar radiation, have caused important radiative impacts on the earth system (Intergovernmental Panel on Climate Change 2013 – IPCC 2013). The heating rate caused by light-absorbing aerosol was reported to vary as a function of the height and range at 0.3–2.1 K d−1 for the polluted PBL over Europe (Kedia et al, 2010; Ferrero et al, 2014, 2018) and 0.3–2.5 K d−1 for southern Asia (Tripathi et al, 2007; Ramana et al, 2007; Ramachandran and Kedia, 2010; Chakrabarty et al, 2012). The results here paint a full picture of vertical profiles of aerosol optical properties over the Beijing region and investigate the radiative forcing effect of aerosol during the heavy pollution events
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