Aerosol particle formation in the upper residual layer

Abstract

Abstract. According to current estimates, atmospheric new particle formation (NPF) produces a large fraction of aerosol particles and cloud condensation nuclei in the earth’s atmosphere, therefore having implications for health and climate. Despite recent advances, atmospheric NPF is still insufficiently understood in the upper parts of the boundary layer (BL). In addition, it is unclear how NPF in upper BL is related to the processes observed in the near-surface layer. The role of the topmost part of the residual layer (RL) in NPF is to a large extent unexplored. This paper presents new results from co-located airborne and ground-based measurements in a boreal forest environment, showing that many NPF events (∼42 %) appear to start in the upper RL. The freshly formed particles may be entrained into the growing mixed layer (ML) where they continue to grow in size, similar to the aerosol particles formed within the ML. The results suggest that in the boreal forest environment, NPF in the upper RL has an important contribution to the aerosol load in the BL.

Answer to Referee #1

“…"The monthly distribution of upper RL NPF events follows the distribution of ML NPF events, with a peak during spring (Mar-May). This is in line with previous studies that classified NPF events in Hyytiälä (Dal Maso et al, 2005;Nieminen et al, 2014). This makes sense since the conditions favoring ML NPF would also favor upper RL NPF.…”

“…"The mean growth rate of the appearing particle modes was 2.2 nm h-1 which is similar to 2.5 nm h-1 reported by Nieminen et al (2014) for 3-25 nm particles during NPF events in Hyytiälä. "…”

Zeppelin-led study on the onset of new particle formation in the planetary boundary layer

“…The 3-10 nm number concentration increased from less than 100 cm -3 to ~400 cm -3 . Lampilahti et al (2020a) showed evidence that NPF frequently takes place in the interface between the residual layer and the free troposphere, disconnected from the ML. Precursor gases may be transported to these altitudes and the mixing over the interface layer could initiate nucleation.…”

“…Other results come from a boreal forest environment in southern Finland. Lampilahti et al (2020a) showed evidence that NPF may occur in the interface between the RL and the FT. O' Dowd et al (2009) observed the first signs of NPF in the surface ML and Leino et al (2019) showed that sub-3 nm particles have higher concentration close to surface. Laakso et al (2007) performed hot-air balloon measurements and concluded that NPF either took place throughout the ML or in the lower part of the ML.…”

Answer to Referee #2

“…Answer: The days were sunny spring days without snow (and one sunny day in July). On such days the ML is expected to be well-mixed and the particles should reach the surface in less than an hour or so (Stull, 1988). We looked at the soundings released at ∼11:20 and ∼17:20 from Hyytiälä during these days.…”