Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 1: Aerosol size distribution, hygroscopicity, and new model parametrizations for CCN prediction

Abstract

Size-resolved long-term measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations as well as hygroscopicity were conducted at the remote Amazon Tall Tower Observatory (ATTO) in the central Amazon Basin over a one-year period and full seasonal cycle (March 2014–February 2015). The presented measurements provide a climatology of CCN properties for a characteristic 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 observed 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), elevated 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, 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 modelling purposes, we compare different approaches of predicting CCN number concentration and present a novel parameterization, which allows accurate CCN predictions based on a small set of input data

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…Details about the measurement mode can be found in Andreae et al (2017). The error in supersaturation resulted from the calibration uncertainty, as described by Pöhlker et al (2016); it is estimated to be in the range of 10 %.…”

“…Several studies contributed to our current understanding of rainfall variability at different scales. Machado et al (2002) discussed the complex diurnal cycle interaction at a synoptic scale, while Laurent et al (2002) examined the mesoscale convective system initiation and propagation, and Rickenbach (2004) showed the importance of nocturnal clouds in rainfall in south-western Amazonas. Silva , Petersen and Rutledge (2001), and Cifelli et al (2002) have all published findings using TRMM-LBA data to describe the microphysical properties of the rainfall field, cloud processes, and biospheric interactions in this region.…”

What do we learn from long-term cloud condensation nuclei number concentration, particle number size distribution, and chemical composition measurements at regionally representative observatories?

“…3b) suggests that most particles are larger than 100 nm which will already activate at supersaturations lower than SS=0.5 %. Regarding the lower activation ratio at higher SS, Pöhlker et al (2016) link it to the influence of nearby biomass burning emissions and hence smaller less hygroscopic particles. Also, previous studies (e.g., Gunthe et al, 2009) confirm this finding by showing that particles with an electrical mobility diameter 25 < 90 nm are less hygroscopic than larger particles, owing to the difference in composition.…”

“…Table 1 lists instrumental and operational details. All information regarding each station's inlet system, instrument descriptions and sampling details is 5 given in the related data descriptor paper (Schmale et al, 2017), except for the rainforest station (ATTO), which is described in Pöhlker et al (2016). Since the focus is on long-term observations rather than short-term intensive field campaigns, the data used were chosen to cover at least 75 % of each season within twelve consecutive months.…”

“…The aerosol chemical 25 speciation monitor (ACSM), deployed at all other stations, has been introduced by Ng et al (2011) and the first official ACTRIS intercomparison is described in Crenn et al (2015). The intercomparison covers all quadrupole ACSMs, except the one deployed at ATTO, which is described in Pöhlker et al (2016). On Jungfraujoch, a time-of-flight ACSM was operated as described by .…”