On the photochemical production of new particles in the coastal boundary layer

Concurrent measurements of ultra‐fine (r<5 nm) particle (UFP) formation, OH and SO 2 concentrations in the coastal environment are examined to further elucidate the processes leading to tidal‐related homogeneous heteromolecular nucleation. During almost daily nucleation events, UFP concentration approached ≈300,000 cm −3 under conditions of solar radiation and low tide. Simultaneous measurements of OH illustrate that, as well as occurring during low tide, these events occur during conditions of peak OH concentration, suggesting that at least one of the nucleating species is photochemically produced. Derived H 2 SO 4 production also exhibited remarkable coherence, although phase‐lagged, with UFP formation, thus suggesting its involvement, although binary nucleation of H 2 SO 4 and H 2 O can be ruled out as a plausible mechanism. Ternary nucleation involving NH 3 seems most likely as a trigger mechanism, however, at least a fourth condensable species, X, is required for growth to detectable sizes. Since UFP are only observed during low tide events, it is thought that species X, or it's parent, is emitted from the shore biota ‐ without which, no nucleation is detected. Species X remains to be identified. Model simulations indicate that, in order to reproduce the observations, a nucleation rate of 10 7 cm −3 s −1 , and a condensable vapour concentration of 5 × 10 7 cm −3 , are required.