Distribution of Aerosol Particles from Diesel Engines Behind Cruising Ships in the Baltic Sea

Ship diesel engines are a significant source of aerosol particles and gaseous pollutants for the marine atmospheric boundary layer (MABL). To assess the environmental impact of maritime transport on the MABL, it is important to understand how aerosol particles are transported, dispersed and transformed in ship exhaust plumes. The dynamics of aerosol particle size distributions, ranging from 5 nm to 4 µm at a height of 1.5 m above sea level, were measured on board a boat behind cruising ships in German waters in the Baltic Sea. Exhaust plumes were measured at ages of 2–60 s at distances of 20–600 m from cruising ships; the number concentration of 5–340 nm particles increased about a hundred times in comparison with background levels, while the mass of supermicron particles increased only by 1–6% compared to background levels. Changes in aerosol particle number size distributions were mainly driven by plume expansion. However, in plumes aged 6–16 s at distances of 50–150 m, a sudden drop in number concentration by 30% for 20 nm particles was observed. Their scavenging followed by fast sedimentation with sea spray microdroplets generated by ship movement is a possible explanation for this. A significant reduction in the residence time of nanoparticles may imply extensive environmental consequences.