The optical properties and in-situ observational evidence for the formation of brown carbon in clouds

• Published in: Atmospheric chemistry and physics Vol. 22; no. 7; pp. 4827 - 4839
• Main Authors: Guo, Ziyong, Yang, Yuxiang, Hu, Xiaodong, Peng, Xiaocong, Fu, Yuzhen, Sun, Wei, Zhang, Guohua, Chen, Duohong, Bi, Xinhui, Wang, Xinming, Peng, Ping'an
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 12-04-2022; Copernicus Publications
• Subjects: Absorption; Absorption coefficient; Absorptivity; Aerosol absorption; Aerosol light absorption; Aerosols; Analysis; Analytical methods; Biomass burning; Burning; Carbon; Cloud droplets; Cloud formation; Cloud water; Clouds; Counterflow; Digestive absorption; Efficiency; Electromagnetic absorption; Excitation spectra; Fluorescence; Fluorescence spectroscopy; Inlets (waterways); Light; Light absorption; Mountains; Observational studies; Optical properties; Organic carbon; Particulate matter; Polycyclic aromatic hydrocarbons; Radiative forcing; Spectroscopy; Tyrosine; Water; Water chemistry; United States > US; China
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-22-4827-2022

Atmospheric brown carbon (BrC) makes a substantial
contribution to aerosol light absorption and thus global radiative
forcing. Although BrC may change the lifetime of the clouds and ultimately
affect precipitation, little is known regarding the optical properties and
formation of BrC in the clouds. In the present study, the light-absorption
properties of cloud droplet residual (cloud RES) were measured by coupled a
ground-based counterflow virtual impactor (GCVI) and an aethalometer
(AE-33), in addition to the cloud interstitial (cloud INT) and ambient
(cloud-free) particles by PM2.5 inlet-AE-33 at Mt. Tianjing (1690 m a.s.l.), a remote mountain site in southern China, from November to December 2020. Meanwhile, the light-absorption and fluorescence properties of
water-soluble organic carbon (WSOC) in the collected cloud water and
PM2.5 samples were also obtained, associated with the concentration of
water-soluble ions. The mean light-absorption coefficient (Abs370) of
the cloud RES, cloud INT, and cloud-free particles were 0.25 ± 0.15,
1.16 ± 1.14, and 1.47 ± 1.23 Mm−1, respectively. The
Abs365 of WSOC was 0.11 ± 0.08 Mm−1 in cloud water and 0.40 ± 0.31 Mm−1 in PM2.5, and the corresponding mass absorption
efficiency (MAE365) was 0.17 ± 0.07 and 0.31 ± 0.21 m2 g−1, respectively. A comparison of the
light-absorption coefficient between BrC in cloud RES and cloud INT particles, and WSOC
in cloud water and PM2.5 indicates a considerable contribution (48 %–75 %)
of water-insoluble BrC to total BrC light absorption. Secondary BrC
estimated by minimum R squared (MRS) method dominated the total BrC in cloud
RES (67 %–85 %), rather than in the cloud-free (11 %–16 %) and cloud INT
(9 %–23 %) particles. It may indicate the formation of secondary BrC during
cloud processing. Supporting evidence includes the enhanced WSOC and
dominant contribution of the secondary formation and biomass burning factor
(>80 %) to Abs365 in cloud water provided by positive
matrix factorization (PMF) analysis. In addition, we showed that the
light absorption of BrC in cloud water was closely related to humic-like
substances and tyrosine-like and/or protein-like substances (r>0.63, p<0.01), whereas only humic-like substances for PM2.5, as identified by
excitation-emission matrix fluorescence spectroscopy.