Ultrafine sea spray aerosol over the southeastern Pacific: Open-ocean contributions to marine boundary layer CCN

In this study, we measured sea spray aerosol (SSA) concentrations with diameters larger than 0.040 μm produced by open-ocean breaking waves over the SEP (southeastern Pacific). Robust statistics were established through repeated airborne flights over 1000 km along 20° S from the coastline of Chile to 85° W during VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land-Study Regional Experiment).

Non-volatile SSA number concentrations were inferred using a thermally resolved technique constrained for clean conditions with an à ngström exponent below 0.5, black carbon mass concentration at values lower than 15 ng m⁻³ and organic aerosol concentration less than 0.02 μg m⁻³. We found that number concentrations of SSAs active as cloud condensation nuclei (CCN) for a supersaturation of 0.25% varied between 17 and 36 cm⁻³, but these did not increase with the increasing mean wind speed typically observed further offshore along 20° S. Concurrent increases in mean offshore precipitation rate in excess of about 1 mm d⁻¹ indicate that scavenging of SSAs by precipitation exceeds increases in production at wind speeds above about 8 m s⁻¹.