The Arctic Ocean has experienced rapid warming and sea ice loss in recent decades, becoming the first open-ocean basin to experience widespread aragonite undersaturation [saturation state of aragonite (Ω _arag ) < 1]. However, its trend toward long-term ocean acidification and the underlying mechanisms remain undocumented. Here, we report rapid acidification there, with rates three to four times higher than in other ocean basins, and attribute it to changing sea ice coverage on a decadal time scale. Sea ice melt exposes seawater to the atmosphere and promotes rapid uptake of atmospheric carbon dioxide, lowering its alkalinity and buffer capacity and thus leading to sharp declines in pH and Ω _arag . We predict a further decrease in pH, particularly at higher latitudes where sea ice retreat is active, whereas Arctic warming may counteract decreases in Ω _arag in the future.

A comprehensive investigation using the air quality network and meteorological data of China in 2015 showed that PM_2.5 driven by cold surges from the ground level could travel up to 2000 km from northern to southern China within two days. Air pollution is more severe and prominent during the winter in north China due to seasonal variations in energy usage, trade wind movements, and industrial emissions. In February 2015, two cold surges traveling from north China caused a temporary increase in the concentration of PM_2.5 in Shanghai. Subsequently, the concentration of PM_2.5 in Xiamen increased to a high of 80 µg/m³_, which is double the average PM_2.5 concentration in Xiamen during the winter. This finding is a new long-range transport mechanism comparing to the well-established mechanism, with long-range transport more likely to occur in the upper troposphere than at lower levels. These observations were validated by results from the back trajectory analysis and the RAMS- CMAQ model. While wind speed was found to be a major facilitator in transporting PM_2.5 from Beijing to Xiamen, more investigation is required to understand the complex relationship between wind speed and PM_2.5 and how it moderates air quality in Beijing, Shanghai, and Xiamen.

The Arctic Ocean has turned from a perennial ice‐covered ocean into a seasonally ice‐free ocean in recent decades. Such a shift in the air‐ice‐sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO₂sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air‐sea CO₂flux and the regional CO₂sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation‐based estimation and a data‐driven box model evaluation. ThepCO₂observations and model results showed that summer CO₂uptake significantly increased by about 1.4 ± 0.6 Tg C decade⁻¹in the Chukchi Sea, primarily due to a longer ice‐free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO₂sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO₂uptake, which was, however, counteracted by a rapidly decreased air‐seapCO₂gradient therein. Therefore, the current and future Arctic Ocean CO₂uptake trends cannot be sufficiently reflected by the air‐seapCO₂gradient alone because of the sea ice variations and other environmental factors.

To examine seasonal and regional variabilities in metabolic status and the coupling of net community production (NCP) and air‐sea CO₂ fluxes in the western Arctic Ocean, we collected underway measurements of surface O₂/Ar and partial pressure of CO₂ (pCO₂) in the summers of 2016 and 2018. With a box‐model, we demonstrate that accounting for local sea ice history (in addition to wind history) is important in estimating NCP from biological oxygen saturation (Δ(O₂/Ar)) in polar regions. Incorporating this sea ice history correction, we found that most of the western Arctic exhibited positive Δ(O₂/Ar) and negative pCO₂ saturation, Δ(pCO₂), indicative of net autotrophy but with the relationship between the two parameters varying regionally. In the heavy ice‐covered areas, where air‐sea gas exchange was suppressed, even minor NCP resulted in relatively high Δ(O₂/Ar) and low pCO₂ in water due to limited gas exchange. Within the marginal ice zone, NCP and CO₂ flux magnitudes were strongly inversely correlated, suggesting an air to sea CO₂ flux induced primarily by biological CO₂ removal from surface waters. Within ice‐free waters, the coupling of NCP and CO₂ flux varied according to nutrient supply. In the oligotrophic Canada Basin, NCP and CO₂ flux were both small, controlled mainly by air‐sea gas exchange. On the nutrient‐rich Chukchi Shelf, NCP was strong, resulting in great O₂ release and CO₂ uptake. This regional overview of NCP and CO₂ flux in the western Arctic Ocean, in its various stages of ice‐melt and nutrient status, provides useful insight into the possible biogeochemical evolution of rapidly changing polar oceans.

The acidification of coastal waters is distinguished from the open ocean because of much stronger synergistic effects between anthropogenic forcing and local biogeochemical processes. However, ocean acidification research is still rather limited in polar coastal oceans. Here, we present a 17‐year (2002–2019) observational data set in the Chukchi Sea to determine the long‐term changes in pH and aragonite saturation state (Ω_arag). We found that pH and Ω_arag declined in different water masses with average rates of −0.0047 ± 0.0026 years⁻¹ and −0.017 ± 0.009 years⁻¹, respectively, and are ∼2–3 times faster than those solely due to increasing atmospheric CO₂. We attributed the rapid acidification to the increased dissolved inorganic carbon owing to a combination of ice melt‐induced increased atmospheric CO₂ invasion and subsurface remineralization induced by a stronger surface biological production as a result of the increased inflow of the nutrient‐rich Pacific water.