Behavior of ultrafine particles in continental and marine air masses at a rural site in the United Kingdom

Particle size distribution measurements were made at a coastal site in the United Kingdom. These are presented, and the behavior of recently formed ul trafine particles is discussed. No ultrafine particles were observed in mar itime air masses; however, 3 to 7 nm particles were frequently observed at enhanced concentrations when the wind direction was from the land. Their fo rmation was favored at lower temperatures, when I ppbv or more of SO2 was p resent and in air masses that had not been aged extensively. On days when e nhanced ultrafine particle concentrations were observed, 3 nm particles inc reased sharply in the morning, approximately 30 to 90 min after the UV sola r flux first increased. By early afternoon the ultrafine particle concentra tion had returned to background levels. Rapid measurements of 5 nm particle s showed no correlation with turbulence parameters, although the boundary l ayer mixing scales were similar to growth times of freshly nucleated partic les to 5 nm diameter. However, ultrafine particle concentrations do correla te with the availability of sulphuric acid vapor. A delay of approximately an hour between the increase of H2SO4 in the morning and a large increase i n ultrafine particle concentrations is due to the growth of particles to ob servable sizes, not the nucleation process itself. An analysis of the times cales for growth showed that coagulation may be important immediately after the particles have nucleated but its effectiveness reduces as number conce ntration falls. Conversely, growth by condensation is initially slow due to the Kelvin effect but increases in importance as the particles reach obser vable sizes.