Ground-based observation of clusters and nucleation-mode particles in the Amazon

Daniela Wimmer,Tuukka Petäjä,Paulo Artaxo,Hanna Elina Manninen,Fernando Goncalves Morais,Radovan Krejci,Scot Turnbull Martin,John Backman,Veli-Matti Kerminen,Alessandro Franchin,Jian Wang,Joel Brito,Chongai Kuang,Juha Kangasluoma,Markku Kulmala,Stephany Buenrostro Mazon,Tuomo Nieminen

doi:10.5194/acp-18-13245-2018

Abstract

Abstract. We investigated atmospheric new particle formation (NPF) in the Amazon rainforest using direct measurement methods. To our knowledge this is the first direct observation of NPF events in the Amazon region. However, previous observations elsewhere in Brazil showed the occurrence of nucleation-mode particles. Our measurements covered two field sites and both the wet and dry season. We measured the variability of air ion concentrations (0.8–12 nm) with an ion spectrometer between September 2011 and January 2014 at a rainforest site (T0t). Between February and October 2014, the same measurements were performed at a grassland pasture site (T3) as part of the GoAmazon 2014/5 experiment, with two intensive operating periods (IOP1 and IOP2 during the wet and the dry season, respectively). The GoAmazon 2014/5 experiment was designed to study the influence of anthropogenic emissions on the changing climate in the Amazon region. The experiment included basic aerosol and trace gas measurements at the ground, remote sensing instrumentation, and two aircraft-based measurements. The results presented in this work are from measurements performed at ground level at both sites. The site inside the rainforest (T0t) is located 60 km NNW of Manaus and influenced by pollution about once per week. The pasture (T3) site is located 70 km downwind from Manaus and influenced by the Manaus pollution plume typically once per day or every second day, especially in the afternoon. No NPF events were observed inside the rainforest (site T0t) at ground level during the measurement period. However, rain-induced ion and particle bursts (hereafter, “rain events”) occurred frequently (643 of 1031 days) at both sites during the wet and dry season, being most frequent during the wet season. During the rain events, the ion concentrations in three size ranges (0.8–2, 2–4, and 4–12 nm) increased up to about 104–105 cm−3. This effect was most pronounced in the intermediate and large size ranges, for which the background ion concentrations were about 10–15 cm−3 compared with 700 cm−3 for the cluster ion background. We observed eight NPF events at the pasture site during the wet season. We calculated the growth rates and formation rates of neutral particles and ions for the size ranges 2–3 and 3–7 nm using the ion spectrometer data. The observed median growth rates were 0.8 and 1.6 nm h−1 for 2–3 nm sized ions and particles, respectively, with larger growth rates (13.3 and 7.9 nm h−1) in the 3–7 nm size range. The measured nucleation rates were of the order of 0.2 cm−3 s−1 for particles and 4–9×10-3 cm−3 s−1 for ions. There was no clear difference in the sulfuric acid concentrations between the NPF event days and nonevent days (∼9×105 cm−3). The two major differences between the NPF days and nonevent days were a factor of 1.8 lower condensation sink on NPF event days (1.8×10-3 s−1) compared to nonevents (3.2×10-3 s−1) and different air mass origins. To our knowledge, this is the first time that results from ground-based sub-3 nm aerosol particle measurements have been obtained from the Amazon rainforest.

Highlights

Atmospheric new particle formation (NPF) and growth has been estimated to account for a major, if not dominant, fraction of cloud condensation nuclei (Merikanto et al., 2009; Wang and Penner, 2009; Yu and Luo, 2009; Dunne et al, 2016; Kulmala et al, 2016)
The results presented in this work are from measurements performed at ground level at both sites
When comparing the two measurement sites, the most apparent differences include (i) the almost factor of 3 lower intermediate negative ion concentrations at the T3 site compared to the T0t site (6 and 5 cm−3 during the wet and dry season, respectively, at T3; 17 cm−3 during both the wet and dry season at T0t)

Summary

Introduction

Atmospheric new particle formation (NPF) and growth has been estimated to account for a major, if not dominant, fraction of cloud condensation nuclei (Merikanto et al., 2009; Wang and Penner, 2009; Yu and Luo, 2009; Dunne et al, 2016; Kulmala et al, 2016). The formation of atmospheric nanoparticles is a multi-stage process, in which stable clusters form from gas-phase precursors, followed by the activation of these clusters and their further growth (Kulmala et al, 2014). Atmospheric NPF is occurring frequently in many environments (e.g., Kulmala et al, 2004; Manninen et al, 2010), the Amazon basin is one of the locations where the initial steps in the formation of nanoparticles have not been previously observed from ground-based measurements (Martin et al, 2010a). In the Amazon basin, emissions and oxidation of volatile organic compounds (e.g., Lelieveld et al, 2008), aerosol activation to cloud droplets, and eventually rain formation are tightly connected with synoptic processes, such as deep convection (e.g., Lelieveld et al, 2008; Wang et al, 2016).

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Conclusion