Overestimation of aerosol soluble phosphorus using water extraction in comparison with seawater extraction

Aerosol phosphorus (P) dissolvable in seawater is a crucial nutrient in aerosol particles to fertilize marine ecosystem. However, the contents of dissolvable P in aerosol particles are usually quantified with water as the leaching medium. In this study, we collected 211 sample pairs of PM2.5 and total suspended particles (TSP) at Qingdao-a coastal city of China, and quantified and compared the contents of soluble P in PM2.5 and particles larger than 2.5 mu m (PM> 2.5 = TSP-PM2.5) extracted with Milli-Q water (DPwater) and prepared seawater (DPseawater). Overall, DPwater and DPseawater linearly correlated in both TSP and PM2.5 samples with the confidence of 99% (r = 0.82-0.86), and the aerosol P solubility quantified with water extraction was approximately 20% more than that with seawater extraction. The correlation between the dissolved inorganic P in water (DIPwater) and in seawater (DIPseawater) was more significant than that between the dissolved organic P in water (DOPwater) and seawater (DOPseawater). Moreover, DIPseawater was approximately 91% of DIPwater in PM2.5, and 74% in PM> 2.5, whereas DOPseawater was approximately 45% of DOPwater in both PM2.5 and PM> 2.5. The fluxes of the DIP and DOP to sea surface estimated based on the results of water extraction were approximately 10% and 50% larger than those of seawater extraction, respectively. Our results provide effective coefficients for converting aerosol inorganic and organic soluble P quantified with water extraction to that soluble in seawater, and enable better assessments of aerosol P fertilization to the marine ecosystem with data from water extraction.

Automated summary

This study collected 211 sample pairs of PM2.5 and total suspended particles (TSP) in Qingdao, China, and compared the soluble phosphorus (P) contents in different size fractions using water and seawater extraction. The results showed that the solubility of aerosol P quantified with water extraction was approximately 20% higher than that with seawater extraction.