Atmospheric new-particle formation (NPF) is an important source of climatically relevant aerosol particles. Observations show that local scale variation in the number concentration of freshly formed particles is common inside the boundary layer. This variation remains poorly understood. The aim of this study was for the first time ever to identify, characterize and explain the dominant local scale spatial variation in the number concentration of nucleation mode particles over a rural boreal forest region by analyzing airborne and sufrace-based measurements.
The airborne measurements utilized an instrumented Zeppelin NT airship and a light Cessna 172 airplane. The measurement flights took place around the SMEAR II field station in Hyytiälä, Finland between the years 2013–2015. Number-size distributions and number concentrations of aerosol particles were measured down to nucleation mode sizes and the meteorological conditions were surveyed.
The most important inhomogeneities, in terms of increase in the number concentration and commonness, were found to be long, less than 10 km wide, boundary layer deep regions of nucleation mode particles that were roughly aligned with the mean flow. These regions were named NPF streets. The number concentration within the NPF streets could increase between 2...10 fold compared to the surroundings. At least one NPF street was observed over the measurement area on at least 43 % of the NPF event days measured. On May 8, 2013 the particle growth rate in the NPF street and in the concurrent regional NPF event was similar (~2 nm/h). Particle growth inside the small measurement area indicates that the particles were forming simultaneously along the length of the NPF street.
The NPF streets were found to be linked to horizontal roll vortices due to roll-enhanced NPF. However, only some (adjacent) rolls were able to enhance NPF significantly. Because of the large horizontal scale of roll vortex systems and NPF events, coupled with the relatively large increase in number concentration and commonness, the NPF streets might be an important source of aerosol particles over the boreal forest.
