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. This work presents airborne observations of sub-3 nm particles in the lowertroposphere and investigates new particle formation (NPF) within an evolvingboundary layer (BL). We studied particle concentrations together withsupporting gas and meteorological data inside the planetary BL over a borealforest site in Hyytiälä, southern Finland. The analysed data werecollected during three flight measurement campaigns: May–June 2015,August 2015 and April–May 2017, including 27 morning and 26 afternoonvertical profiles. As a platform for the instrumentation, we used a Cessna172 aircraft. The analysed flight data were collected horizontally within a30 km distance from SMEAR II in Hyytiälä and vertically from 100 mabove ground level up to 2700 m. The number concentration of 1.5–3 nmparticles was observed to be, on average, the highest near the forest canopytop and to decrease with increasing altitude during the mornings of NPF eventdays. This indicates that the precursor vapours emitted by the forest play akey role in NPF in Hyytiälä. During daytime, newly formed particleswere observed to grow in size and the particle population became morehomogenous within the well-mixed BL in the afternoon. During undefined dayswith respect to NPF, we also detected an increase in concentration of1.5–3 nm particles in the morning but not their growth in size, whichindicates an interrupted NPF process during these undefined days. Verticalmixing was typically stronger during the NPF event days than during theundefined or non-event days. The results shed light on the connection betweenboundary layer dynamics and NPF.

Vertical profiles of sub-3 nm particles over the boreal forest