We measure the surface tension of picoliter-volume droplets containing strong ionic surfactants and cosolutes and compare this to surface tension predictions using two independent surfactant partitioning models. Under high-water-activity conditions, experimental measurements and model predictions show no change when NaCl cosolute is replaced with sea salt. Model predictions show that total surfactant concentrations in the range of tens to hundreds of millimolar are required to lower the surface tension of accumulation-mode aerosol.