Abstract
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 within 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 6 years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO; February 2014 to September 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 1.1 × 10−3 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 (Δθe′) – 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 Δθe′ anomaly. Therefore, these events do not appear to be related to downdraft transport and suggest the existence of other currently unknown sources of 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)
This study focuses on particle number size distributions (PNSDs) obtained from a scanning mobility particle sizer (SMPS) with an inlet located at 60 m above ground
This study reports the statistical characterization of aerosol particle growth events in the sub-50 nm size range (10– 50 nm), based on continuous measurements (February 2014 to September 2020) of PNSDs at a remote site in central Amazonia
Summary
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 semi-volatile and low-volatility gases as well as coagulation into the cloud- and, climate-relevant size range with diameters, D, larger than ∼ 80 nm (see definitions in Kulmala et al, 2012; Kerminen et al, 2018). While the smallest diameters in, for instance, boreal forest are typically in the range of a few nanometers, the “Amazonian bananas” rather start between about 20 and 40 nm (Kulmala et al, 2012; Kerminen et al, 2018). Varanda Rizzo et al (2018) discussed the occurrence of such sub-50 nm particle growth events in the Amazon and found them only in 3 % of the 749 d examined, associated mainly with convective downdrafts. The Amazonian bananas start at larger diameters and are comparatively rare relative to the classical events, e.g., in boreal forests (Nieminen et al, 2018; Dada et al, 2018)
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