Redox-active quinone and nonquinone moieties represent the electron exchange capacity (EEC) of natural organic matter (NOM), playing an important role in the electron transfer link of microbes and transformation of contaminants/metal minerals. However, the corresponding transformation of quinone/phenol and their respective influence on the EECs during reduction and reoxidation remain poorly characterized. Besides, it is still controversial whether nonquinones donate or accept electrons. Herein, we demonstrated that reoxidation of NOM after reduction can form new phenolic/quinone moieties, thus increasing the EEC. The assessment for the EEC, including the electron-donating capacity (EDC) and electron-accepting capacity (EAC), of nonquinones reflects the contribution of sulfur-containing moieties with considerable EDCs and EACs. In contrast, nitrogen-containing moieties donate negligible electrons even at E h = +0.73 V. The contributions of both thiol and amine moieties to the EEC are greatly affected by adjacent functional groups. Meanwhile, aldehydes/ketones did not display an EAC during the electron transfer process of NOM. Furthermore, substantially increased EDC at E h from +0.61 to +0.73 V could not be fully explained using thiol and phenolic moieties, suggesting the contribution of unknown moieties with high oxidation potential. The overall findings suggest that the roles of new quinones/phenol (derived from the addition of oxygen to condensed aromatic/lignin-like components) during redox dynamic cycling and thiol species should be considered in assessing the electron transfer processes of NOM.

Sulfate aerosols exert a net cooling effect on the earth-atmosphere system, yet their radiative forcing remains associated with largest of uncertainties in the assessment of climate change. One of the contributing factors is the poor understanding of the sulfate formation pathways, which are thought to be following mostly the mass-dependent fractionation model (i.e., Δ 33 S ~ 0). However, globally, urban sulfate aerosols exhibit significant non-zero Δ 33 S compositions (from -0.6‰ to +0.6‰), resulting in sulfur mass-independent fractionation (S-MIF) processes. The origin(s) of these S-MIF anomalies remain(s) unclear. Here, we conducted dual-isotope (Δ 33 S, δ 34 S) probing of sulfate aerosols from summertime megacity Delhi in South Asia. A shift towards concomitantly high Δ 33 S (from +0.2‰ to +0.5‰) and low δ 34 S (from +5‰ to +1‰) values was observed with the influx of mineral dust. The Fe-to-Al tracer showed significant correlations with sulfate loadings (R 2 =0.84) and Δ 33 S signatures (R 2 =0.77). As such, we postulate that the SO2 photo-oxidation on mineral dust generates S-MIF anomaly ~ +0.35±0.10‰, thereby also explaining the previously observed Δ 33 S values worldwide. Together, the findings help deconvolute S-isotope dynamics in urban regions wherein, contrary to prevailing paradigm, non-anthropogenic factor (i.e., mineral dust) is found to influence the aerosol sulfate-induced pollution affecting air quality/human health. SYNOPSISSulfate-related pollution in urban regions-affecting air quality/human health-could be linked to non-anthropogenic factor i.e., mineral dust.

Deposition of atmospheric mercury (Hg) is the most important Hg source on the high-altitude Himalayas and Tibetan Plateau. Herein, total gaseous Hg (TGM) at an urban and a forest site on the Tibetan Plateau was collected respectively from May 2017 to October 2018, and isotopic compositions were measured to clarify the influences of landforms and monsoons on the transboundary transport of atmospheric Hg to the Tibetan Plateau. The transboundary transported anthropogenic emissions mainly originated over Indo-Gangetic Plain and carried over the Himalayas by convective storms and mid-tropospheric circulation, contributing over 50% to the TGM at the Lhasa urban site, based on the binary mixing model of isotopes. In contrast, during the transport of TGM from South Asia with low altitude, the uptake by evergreen forest in Yarlung Zangbo Grand Canyon largely decreased the TGM level and shifted isotopic compositions in TGM at the Nyingchi forest site, which are located at the highaltitude end of the canyon. Our results provided direct evidence from Hg isotopes to reveal the distinct patterns of transboundary transport to the Tibetan Plateau shaped by landforms and climates, which is critical to fully understand the biogeochemical cycling of Hg in the high-altitude regions.