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Haze particles as a key air pollutant contain high level of toxins, which were hypothesized to inhibit phytoplankton growth when deposited to the ocean, and thus indirectly affect the climate. However, field observations have yet to provide conclusive evidence to confirm this hypothesis. Onboard microcosm experiments in the Northwest Pacific Ocean (NWPO) show that haze particles collected at the East Asia continent had an inhibition impact on phytoplankton growth only when at very high particle loading (2 mg/L). In contrast, haze particles at low and medium loadings (0.03-0.6 mg/L) stimulated phytoplankton growth and shifted phytoplankton size structure toward larger cells, primarily due to the supply of inorganic nitrogen nutrients from the particles. Model simulations showed that haze particle loading in NWPO surface seawater was usually more than an order of magnitude lower than 2 mg/L. This indicates that haze particles are unlikely to cause harm but to stimulate phytoplankton growth in the nitrogen-limited NWPO. Ocean biogeochemical modeling further shows that deposited nitrogen significantly enhanced surface ocean chlorophyll a concentration in the winter and spring of 2014. Overall, these results demonstrate that haze particles stimulate rather than inhibit primary production in the NWPO.

Plain Language Summary Increasing anthropogenic emissions of air pollutants from fast-developing East Asia leads to increasing deposition of aerosol particles to the Northwest Pacific Ocean (NWPO). Such particles contain both nutrients such as nitrogen and phosphorus and toxins such as copper and organic pollutants to the marine phytoplankton, which have contrasting effects on marine ecosystems and carbon uptake. However, the actual impact remains unknown. Detailed onboard incubation experiments confirm that Chinese haze particles have an overall stimulation effects on phytoplankton under representative ocean conditions in NWPO. Toxicity to phytoplankton is only observed when the added haze particle concentration is unrealistically high. Simulations suggest that nitrogen in the atmospheric deposition contributes significantly to the surface chlorophyll a concentration in NWPO and enhances carbon fixation, which indirectly offsets global warming.