Figure 2.

Concentration of INPs active between −5 and −15°C in three samples of the Amazon River plume (black), two samples of the Tocantins River plume (red) and three samples of Atlantic surface water (blue). The Atlantic samples, all containing more than 35 g salt l⁻¹, were adjusted for freezing point depression. We analysed the same sample material three times: first, without any treatment (circles); second, after heating to 60°C (cross symbols); and third, after heating to 95°C (dash symbols). In addition, we analysed Amazon River plume samples in a 10-fold dilution with pure water (smaller symbols) to determine INP concentration at lower temperatures than was possible with undiluted samples (data in electronic supplementary material).