Abstract
Abstract. Size-resolved long-term measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted at the remote Amazon Tall Tower Observatory (ATTO) in the central Amazon Basin over a 1-year period and full seasonal cycle (March 2014–February 2015). The measurements provide a climatology of CCN properties characteristic of a remote central Amazonian rain forest site.The CCN measurements were continuously cycled through 10 levels of supersaturation (S = 0.11 to 1.10 %) and span the aerosol particle size range from 20 to 245 nm. The mean critical diameters of CCN activation range from 43 nm at S = 1.10 % to 172 nm at S = 0.11 %. The particle hygroscopicity exhibits a pronounced size dependence with lower values for the Aitken mode (κAit = 0.14 ± 0.03), higher values for the accumulation mode (κAcc = 0.22 ± 0.05), and an overall mean value of κmean = 0.17 ± 0.06, consistent with high fractions of organic aerosol.The hygroscopicity parameter, κ, exhibits remarkably little temporal variability: no pronounced diurnal cycles, only weak seasonal trends, and few short-term variations during long-range transport events. In contrast, the CCN number concentrations exhibit a pronounced seasonal cycle, tracking the pollution-related seasonality in total aerosol concentration. We find that the variability in the CCN concentrations in the central Amazon is mostly driven by aerosol particle number concentration and size distribution, while variations in aerosol hygroscopicity and chemical composition matter only during a few episodes.For modeling purposes, we compare different approaches of predicting CCN number concentration and present a novel parametrization, which allows accurate CCN predictions based on a small set of input data.
Highlights
Introduction1.1 Atmospheric aerosols and cloudsIn our current understanding of the Earth’s climate system and its man-made perturbation, the multiscale and feedbackrich life cycles of clouds represent one of the largest uncertainties (Boucher et al, 2013; Stevens et al, 2016)
1.1 Atmospheric aerosols and cloudsIn our current understanding of the Earth’s climate system and its man-made perturbation, the multiscale and feedbackrich life cycles of clouds represent one of the largest uncertainties (Boucher et al, 2013; Stevens et al, 2016)
Size-resolved cloud condensation nuclei (CCN) measurements have been conducted at the remote Amazon Tall Tower Observatory (ATTO) site in the central Amazon, spanning a full seasonal cycle from March 2014 to February 2015
Summary
1.1 Atmospheric aerosols and cloudsIn our current understanding of the Earth’s climate system and its man-made perturbation, the multiscale and feedbackrich life cycles of clouds represent one of the largest uncertainties (Boucher et al, 2013; Stevens et al, 2016). Aerosol particles can originate from various natural and anthropogenic sources and span wide ranges of concentration, particle size, composition, as well as chemical and physical properties (Pöschl, 2005). Their activation into cloud droplets depends on their size, composition, and mixing state as well as the water vapor supersaturation (e.g., Köhler, 1936; Dusek et al, 2006; McFiggans et al, 2006; Andreae and Rosenfeld, 2008; Su et al, 2010). The knowledge obtained from the growing body of field data has been translated into different parametrization strategies that represent the cloud–aerosol microphysical processes in modeling studies (e.g., Nenes and Seinfeld, 2003; Petters and Kreidenweis, 2007; Su et al, 2010; Deng et al, 2013; Mikhailov et al, 2013)
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