Observation of new particle formation over a mid-latitude forest facing the North Pacific

The number size distributions of aerosol particles were measured using a scanning mobility particle sizer (SMPS) at a mid-latitude forest in Japan during 20–30 August 2010. Four days during the observation period experienced new particle formation (NPF) under the conditions of low condensation and coagulation sinks. The formation rate of new particles was calculated to be in the range between 0.2 and 1.0 cm−3 s−1 for the NPF events. The growth rates of the newly formed mode of aerosol particles ranged between 5.0 and 15.7 nm h−1. The backward air mass trajectories revealed that the NPF occurred in clean maritime air masses originated from the North Pacific, which is characterized with the low mass concentrations of aerosol components. The results from the classification analysis of backward air mass trajectories indicate that the maritime air mass conditions are frequent at this forest site and are primarily in summer season. Large increases in the number concentrations of accumulation mode particles (above 90 nm) were followed by the increased precipitation rates in the afternoon hours on the NPF event days. Therefore, the newly-formed particles would be involved in the convective cloud formation and precipitation over the studied region.

► New aerosol particle formation was observed over a mid-latitude forest in Japan.
► The property of the aerosol during new particle formation is characterized.
► The significance of new particle formation is discussed.