Collection and chemical analysis of atmospheric aerosols and riverine water

Atmospheric aerosols were collected at the Atmospheric Environment Monitoring Station in the Laoshan campus of the Ocean University of China, Qingdao, China (36.16° N, 120.50° E), about seven kilometers off the coast of the Yellow Sea (Fig. 1). The sampling was conducted under haze weather (visibility <10 km, relative humidity <90%) in January 2018 (Aerosol 1), January 2019 (Aerosol 2), and June 2019 (Aerosol 3) (Table ^S1). The aerosols were collected onto an acid washed filter (Whatman, 41#) through a high-volume sampler (KC-1000, Laoshan electronic instrument company of Qingdao) operated at a flow rate of 1.0 m³·min⁻¹ for about 20 hours uninterruptedly, and a blank was simultaneously collected at a flow rate of 0 m³·min⁻¹ under the same condition⁷⁴.

The aerosol sample was divided up into several aliquots for chemical analyses and the on-board microcosm experiments. For chemical analyses, two parts of the filter were extracted ultrasonically with deionized water (18.2 MΩ cm) in an ice-water bath for 40 minutes and the extracts was filtered instantly through a PES membrane (aperture 0.45 μm). The first extract was used for nutrient (NO₃^- + NO₂^-, NH₄⁺, DIP, and Si (OH)₄) analysis using an QuAAtro continuous-flow analyser (SEAL Analytical)⁴⁷. The second extract was digested using 15.5 mol·L⁻¹ HNO₃ and then evaporated. The solution was used for dissolved trace metal analysis using an Agilent 7500c ICP-MS⁷⁵. The detection limit was 0.4 μg·L⁻¹ for Fe and was 0.09 μg·L⁻¹ for other metals. The third part of the filter was used for aerosol amendments in the microcosm experiments. It was extracted before the microcosm experiments took place following the same procedure as described above for the nutrient analysis^76,77.

The riverine water was collected using a surface-water collector in the Xiaoqing River in 2019. The riverine water was filtered through acid-washed acetate fiber membranes (0.45 μm pore size) and stored in HDPE bottles at -20 °C. It was defrosted in the dark at room temperature (18–20 °C) before measuring the chemical composition. Nutrient concentrations (NO₃^- + NO₂^-, NH₄⁺, DIP, and Si (OH)₄) of riverine water were then measured using the QuAAtro continuous-flow analyser (SEAL Analytical)⁴⁷.