The optical properties and in-situ observational evidence for the formation of brown carbon in clouds
oleh: Z. Guo, Z. Guo, Z. Guo, Y. Yang, Y. Yang, Y. Yang, X. Hu, X. Hu, X. Hu, X. Peng, X. Peng, X. Peng, Y. Fu, Y. Fu, Y. Fu, W. Sun, W. Sun, W. Sun, G. Zhang, G. Zhang, G. Zhang, D. Chen, X. Bi, X. Bi, X. Bi, X. Wang, X. Wang, X. Wang, P. Peng, P. Peng, P. Peng
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2022-04-01 |
Deskripsi
<p>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 PM<span class="inline-formula"><sub>2.5</sub></span> 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 PM<span class="inline-formula"><sub>2.5</sub></span> samples were also obtained, associated with the concentration of water-soluble ions. The mean light-absorption coefficient (Abs<span class="inline-formula"><sub>370</sub></span>) of the cloud RES, cloud INT, and cloud-free particles were 0.25 <span class="inline-formula">±</span> 0.15, 1.16 <span class="inline-formula">±</span> 1.14, and 1.47 <span class="inline-formula">±</span> 1.23 Mm<span class="inline-formula"><sup>−1</sup></span>, respectively. The Abs<span class="inline-formula"><sub>365</sub></span> of WSOC was 0.11 <span class="inline-formula">±</span> 0.08 Mm<span class="inline-formula"><sup>−1</sup></span> in cloud water and 0.40 <span class="inline-formula">±</span> 0.31 Mm<span class="inline-formula"><sup>−1</sup></span> in PM<span class="inline-formula"><sub>2.5</sub></span>, and the corresponding mass absorption efficiency (MAE<span class="inline-formula"><sub>365</sub></span>) was 0.17 <span class="inline-formula">±</span> 0.07 and 0.31 <span class="inline-formula">±</span> 0.21 m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span>, respectively. A comparison of the light-absorption coefficient between BrC in cloud RES and cloud INT particles, and WSOC in cloud water and PM<span class="inline-formula"><sub>2.5</sub></span> indicates a considerable contribution (48 %–75 %) of water-insoluble BrC to total BrC light absorption. Secondary BrC estimated by minimum <span class="inline-formula"><i>R</i></span> 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 (<span class="inline-formula"><i>></i>80</span> %) to Abs<span class="inline-formula"><sub>365</sub></span> 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 (<span class="inline-formula"><i>r</i><i>></i>0.63</span>, <span class="inline-formula"><i>p</i><i><</i>0.01</span>), whereas only humic-like substances for PM<span class="inline-formula"><sub>2.5</sub></span>, as identified by excitation-emission matrix fluorescence spectroscopy.</p>