Fertilization of the Northwest Pacific Ocean by East Asia air pollutants

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. On-board 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-1). In contrast, haze particles at low and medium loadings (0.03-0.6 mg L-1) stimulated phytoplankton growth and shifted phytoplankton size structure towards 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-1. This indicates that haze particles are unlikely to cause harm but to stimulate phytoplankton growth in the nitrogen-limited NWPO. Ocean biogeochemical modelling further show 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.