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. Atmospheric brown carbon (BrC) makes a substantialcontribution to aerosol light absorption and thus global radiativeforcing. Although BrC may change the lifetime of the clouds and ultimatelyaffect precipitation, little is known regarding the optical properties andformation of BrC in the clouds. In the present study, the light-absorptionproperties of cloud droplet residual (cloud RES) were measured by coupled aground-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 ofwater-soluble organic carbon (WSOC) in the collected cloud water andPM2.5 samples were also obtained, associated with the concentration ofwater-soluble ions. The mean light-absorption coefficient (Abs370) ofthe 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. TheAbs365 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 absorptionefficiency (MAE365) was 0.17 ± 0.07 and 0.31 ± 0.21 m2 g−1, respectively. A comparison of thelight-absorption coefficient between BrC in cloud RES and cloud INT particles, and WSOCin cloud water and PM2.5 indicates a considerable contribution (48 %–75 %)of water-insoluble BrC to total BrC light absorption. Secondary BrCestimated by minimum R squared (MRS) method dominated the total BrC in cloudRES (67 %–85 %), rather than in the cloud-free (11 %–16 %) and cloud INT(9 %–23 %) particles. It may indicate the formation of secondary BrC duringcloud processing. Supporting evidence includes the enhanced WSOC anddominant contribution of the secondary formation and biomass burning factor(&gt;80 %) to Abs365 in cloud water provided by positivematrix factorization (PMF) analysis. In addition, we showed that thelight absorption of BrC in cloud water was closely related to humic-likesubstances and tyrosine-like and/or protein-like substances (r&gt;0.63, p&lt;0.01), whereas only humic-like substances for PM2.5, as identified byexcitation-emission matrix fluorescence spectroscopy.

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