Abstract
Abstract. A substantial fraction of the atmospheric aerosols originates from secondary new particle formation (NPF), where atmospheric vapours are transformed into particles that subsequently grow to larger sizes, affecting human health and the climate. In this study, we investigate aerosol size distributions at two stations located close to each other (∼ 20 km) but at different altitudes: urban (UGR; 680 m a.s.l., metres above sea level) and high-altitude remote (SNS; 2500 m a.s.l.) sites, both in the area of Granada, Spain, and part of AGORA observatory (Andalusian Global ObseRvatory of the Atmosphere). The analysis shows a significant contribution of nucleation mode aerosol particles to the total aerosol number concentration at both sites, with a contribution of 47 % and 48 % at SNS and UGR, respectively. Due to the important contribution of NPF events to the total aerosol number concentrations and their high occurrence frequency (> 70 %) during the study period, a detailed analysis of NPF events is done in order to get insight into the possible mechanisms and processes involved in NPF events at these contrastive sites. At SNS, NPF is found to be associated with the transport of gaseous precursors from lower altitudes by orographic buoyant upward flows. NPF events at the SNS site are always observed from the smallest measured sizes of the aerosol size distribution (4 nm), implying that NPF takes place in or in the vicinity of the high-altitude SNS station rather than being transported from lower altitudes. Although NPF events at the mountain site seem to be connected with those occurring at the urban site, growth rates (GRs) at SNS are higher than those at the UGR site (GR7−25 of 6.9 and 4.5 nm h−1 and GR4−7 of 4.1 and 3.6 nm h−1 at SNS and UGR, respectively). This fact could have special importance for the production of cloud condensation nuclei (CCN) and therefore for cloud formations which may affect regional/global climate, since larger GRs at mountain sites could translate to a larger survival probability of NPF particles reaching CCN sizes, due to the shorter time period needed for the growth. The analysis of sulfuric acid (H2SO4) shows that the contribution of H2SO4 is able to explain a minimal fraction contribution to the observed GRs at both sites (< 1 % and < 10 % for the 7–25 and 4–7 nm size ranges, respectively), indicating that other condensing vapours are responsible for the majority of particle growth, as well as the differing growth rates between the two sites. Results also show that the condensation sink (CS) does not play a relevant role in NPF processes at both sites and points to the availability of volatile organic compounds (VOCs) as one of the main factors controlling the NPF events at both sites. Finally, a closer analysis of the NPF events that were observed at the SNS site during a Saharan dust episode that occurred during the field campaign was carried out, evidencing the role of TiO2 and F2O3 together with VOCs in promoting new particle formation during this dust intrusion event. Although further investigation is needed to improve our understanding in this topic, this result suggests that climate effects of mineral dust and NPF are not disconnected from each other as it was commonly thought. Therefore, since mineral dust contributes to a major fraction of the global aerosol mass load, dust–NPF interaction should be taken into account in global aerosol-climate modelling for better climate change prediction.
Highlights
The formation of new atmospheric aerosol particles, and their subsequent growth, commonly known as new particle formation (NPF) events, has a substantial contribution to aerosol particle number concentration and affects the climate via aerosol–cloud interactions (Kerminen et al, 2018). Kulmala et al (2014) described NPF events as a five-step process: (1) chemical reactions in the gas phase producing lowvolatility vapour(s), (2) clustering, (3) nucleation or barrierless nucleation, (4) activation of clusters with a second group of vapours, and (5) subsequent multi-component condensational growth of nucleated particles to larger sizes
The average aerosol concentration measured at the UGR urban station was in the range of those obtained in the summer season (4–24 × 103 cm−3) in other European urban sites (e.g., Birmili et al, 2016; Gómez-Moreno et al, 2011; Pérez et al, 2010; Pey et al, 2010)
The results show that GR7−25 and growth rates in the range 4–7 (GR4−7) mean values at the Sierra Nevada remote station (SNS) remote station are larger than those observed at the UGR station (Fig. 4a) and that GR7−25 is always larger at the SNS station (Fig. 4b)
Summary
The formation of new atmospheric aerosol particles, and their subsequent growth, commonly known as new particle formation (NPF) events, has a substantial contribution to aerosol particle number concentration and affects the climate via aerosol–cloud interactions (Kerminen et al, 2018). Kulmala et al (2014) described NPF events as a five-step process: (1) chemical reactions in the gas phase producing lowvolatility vapour(s), (2) clustering, (3) nucleation or barrierless nucleation, (4) activation of clusters with a second group of vapours, and (5) subsequent multi-component condensational growth of nucleated particles to larger sizes. The formation of new atmospheric aerosol particles, and their subsequent growth, commonly known as new particle formation (NPF) events, has a substantial contribution to aerosol particle number concentration and affects the climate via aerosol–cloud interactions (Kerminen et al, 2018). Kulmala et al (2014) described NPF events as a five-step process: (1) chemical reactions in the gas phase producing lowvolatility vapour(s), (2) clustering, (3) nucleation or barrierless nucleation, (4) activation of clusters with a second group of vapours, and (5) subsequent multi-component condensational growth of nucleated particles to larger sizes. Despite the advancement in theoretical knowledge of NPF steps, large discrepancies have been found between the expected and observed properties of NPF under atmospheric conditions (Chu et al, 2019; Kulmala et al, 2017; Nieminen et al, 2018). Earlier studies indicated that ammonia (NH3) enhances aerosol nucleation, but recent laboratory and theoretical studies suggest that amines and highly oxygenated molecules (HOMs) play vital roles in enhancing nucleation and promoting the initial growth of newly formed particles in the atmosphere (e.g., Kulmala et al, 2013; Schobesberger et al, 2013; Tröstl et al, 2016; Ehn et al, 2014)
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