Roll vortices induce new particle formation bursts in the planetary boundary layer

Abstract

Abstract. Recent studies have shown the importance of new particle formation (NPF) to global cloud condensation nuclei (CCN) production, as well as to air pollution in megacities. In addition to the necessary presence of low-volatility vapors that can form new aerosol particles, both numerical and observational studies have shown that the dynamics of the planetary boundary layer (BL) plays an important role in NPF. Evidence from field observations suggests that roll vortices might be favorable for inducing NPF in a convective BL. However, direct observations and estimates of the potential importance of this phenomenon to the production of new aerosol particles are lacking. Here we show that rolls frequently induce NPF bursts along the horizontal circulations and that the small clusters and particles originating from these localized bursts grow in size similar to particles typically ascribed to atmospheric NPF that occur almost homogeneously at a regional scale. We outline a method to identify roll-induced NPF from measurements and, based on the collected data, estimate the impact of roll vortices on the overall aerosol particle production due to NPF at a boreal forest site (83 % ± 34 % and 26 % ± 8 % overall enhancement in particle formation for 3 and 10 nm particles, respectively). We conclude that the formation of roll vortices should be taken into account when estimating particle number budgets in the atmospheric BL.

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

“…During the PEGASOS northern mission in spring 2013, a Cessna 172 airplane carrying scientific instrumentation was deployed to measure aerosol particles, trace gases and meteorological variables in the lower troposphere alongside the Zeppelin. The measurement setup and instrumentation on board have been described in previous studies (Lampilahti et al, 2020b;Schobesberger et al, 2013;Leino et al, 2019;Väänänen et al, 2016) Basic meteorological variables (temperature, pressure, relative humidity) were measured on board. Particle number size distribution was measured using a SMPS (10-400 nm size range, 2 min time resolution) and the number concentration of >3 nm particles was measured using an ultrafine condensation particle counter (UF-CPC, TSI model 3776) at 1 s time resolution.…”

“…The 3-10 nm number concentration increased from less than 100 cm −3 to ∼ 400 cm −3 . Lampilahti et al (2021a) Figure 7. Vertical profile of 3-10 nm particle number concentration (black dots), temperature (blue line) and water vapor concentration (red line) measured on board the Cessna between 07:00-10:00 on 8 May 2013 in HTL.…”

“…Other results come from a boreal forest environment in southern Finland. Lampilahti et al (2021a) 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 a higher concentration close to the 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.…”

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

“…In addition a separate layer of sub-10 nm particles was observed above the ML in HTL. Lampilahti et al (2020b) showed that such layers in HTL are likely the result of NPF in the topmost part of the RL. Furthermore it was estimated that around 42% of the NPF events observed in HTL at the surface are entrained from such elevated layers.…”

Answer to Referee #2

“…The patchiness of nucleation mode particles can have other explanations such as vari-C5 able cloud cover (Wehner et al, 2007), land features (O'Dowd et al, 2009), and organized convection like roll vortices (Lampilahti et al, 2020).…”