Abstract

New particle formation (NPF), referring to the nucleation of molecular clusters and their subsequent growth into the cloud condensation nuclei (CCN) size range, is a globally significant and climate-relevant source of atmospheric aerosols. Classical NPF exhibiting continuous growth from a few nanometers to the Aitken mode around 60–70 nm is widely observed in the planetary boundary layer (PBL) around the world, but not in central Amazonia. Here, classical NPF events are rarely observed in the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50 nm (CN< 50) particles into the PBL and their subsequent growth. Central aspects of our understanding of these processes in the Amazon have remained enigmatic, however. Based on more than six years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO, Feb 2014 to Sep 2020), we analyzed the diurnal and seasonal patterns as well as meteorological conditions during 254 of such Amazonian growth events on 217 event days, which show a sudden occurrence of particles between 10 and 50 nm in the PBL, followed by their growth to CCN sizes. The occurrence of events was significantly higher during the wet season, with 88 % of all events from January to June, than during the dry season, with 12 % from July to December, probably due to differences in the condensation sink (CS), atmospheric aerosol load, and meteorological conditions. Across all events, a median growth rate (GR) of 5.2 nm h−1 and a median CS of 0.0011 s−1 were observed. The growth events were more frequent during the daytime (74 %) and showed higher GR (5.9 nm h−1) compared to nighttime events (4.0 nm h−1), emphasizing the role of photochemistry and PBL evolution in particle growth. About 70 % of the events showed a negative anomaly of the equivalent potential temperature (∆θ'e) – as a marker for downdrafts – and a low satellite brightness temperature (Tir) – as a marker for deep convective clouds – in good agreement with particle injection from the UT in the course of strong convective activity. About 30 % of the events, however, occurred in the absence of deep convection, partly under clear sky conditions, and with a positive ∆θ'e anomaly. Therefore, these events do not appear to be related to downdraft injection and suggest the existence of other currently unknown sources of the sub-50 nm particles.

Highlights

  • New particle formation (NPF) refers to the nucleation of nanometer-sized molecular clusters from gaseous precursors and their subsequent condensational growth (e.g., Kulmala et al, 2004; Dal Maso, 2005; Kirkby et al, 2011; Kulmala et al, 2012; Kerminen et al, 2018)

  • The newly formed particles grow through condensation of 10 semi- and low-volatile gases as well as coagulation into the cloud- and, climate-relevant size range with diameters, D, larger than ∼80 nm (Kerminen et al, 2018; Pöhlker et al, 2018)

  • This study focuses on particle number size distributions (PNSDs) obtained from a Scanning Mobility Particle Sizer (TSI Inc., 10 Shoreview, USA; classifiers: first model 3080, later 3082; DMA: 3081; condensation particle counter: CPC 3772) sampling from the 60 m inlet at the 80 m high so-called triangular mast (02° 08.602'S, 59° 00.033'W; 130 m a.s.l.) at the Amazon Tall Tower Observatory (ATTO) site

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Summary

Introduction

New particle formation (NPF) refers to the nucleation of nanometer-sized molecular clusters from gaseous precursors and their subsequent condensational growth (e.g., Kulmala et al, 2004; Dal Maso, 2005; Kirkby et al, 2011; Kulmala et al, 2012; Kerminen et al, 2018). The newly formed particles grow through condensation of 10 semi- and low-volatile gases as well as coagulation into the cloud- and, climate-relevant size range with diameters, D, larger than ∼80 nm (Kerminen et al, 2018; Pöhlker et al, 2018). Rizzo et al (2018) discussed the 20 occurrence of sub-50 nm particle growth events (‘Amazonian banana plots’), which resemble the behavior of ‘classical NPF’, but starting at much larger initial diameters (> 20 nm). These ‘Amazonian banana plots’ were observed in 3 % of the 749 days and were associated mainly with convective downdrafts. It is clear that in Amazonia these characteristic events are less frequent

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