Aerosol–cloud interactions (ACI) constitute the single largest uncertainty in anthropogenic radiative forcing. To reduce the uncertainties and gain more confidence in the simulation of ACI, models need to be evaluated against observations, in particular against measurements of cloud condensation nuclei (CCN). Here we present a data set – ready to be used for model validation – of long-term observations of CCN number concentrations, particle number size distributions and chemical composition from 12 sites on 3 continents. Studied environments include coastal background, rural background, alpine sites, remote forests and an urban surrounding. Expectedly, CCN characteristics are highly variable across site categories. However, they also vary within them, most strongly in the coastal background group, where CCN number concentrations can vary by up to a factor of 30 within one season. In terms of particle activation behaviour, most continental stations exhibit very similar activation ratios (relative to particles > 20 nm) across the range of 0.1 to 1.0 % supersaturation. At the coastal sites the transition from particles being CCN inactive to becoming CCN active occurs over a wider range of the supersaturation spectrum. Several stations show strong seasonal cycles of CCN number concentrations and particle number size distributions, e.g. at Barrow (Arctic haze in spring), at the alpine stations (stronger influence of polluted boundary layer air masses in summer), the rain forest (wet and dry season) or Finokalia (wildfire influence in autumn). The rural background and urban sites exhibit relatively little variability throughout the year, while short-term variability can be high especially at the urban site. The average hygroscopicity parameter, κ , calculated from the chemical composition of submicron particles was highest at the coastal site of Mace Head (0.6) and lowest at the rain forest station ATTO (0.2–0.3). We performed closure studies based on κ –Köhler theory to predict CCN number concentrations. The ratio of predicted to measured CCN concentrations is between 0.87 and 1.4 for five different types of κ . The temporal variability is also well captured, with Pearson correlation coefficients exceeding 0.87. Information on CCN number concentrations at many locations is important to better characterise ACI and their radiative forcing. But long-term comprehensive aerosol particle characterisations are labour intensive and costly. Hence, we recommend operating “migrating-CCNCs” to conduct collocated CCN number concentration and particle number size distribution measurements at individual locations throughout one year at least to derive a seasonally resolved hygroscopicity parameter. This way, CCN number concentrations can only be calculated based on continued particle number size distribution information and greater spatial coverage of long-term measurements can be achieved.

Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions: Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with D_Ait ≈ 70 nm and N_Ait ≈ 160 cm⁻³, weak accumulation mode with D_acc ≈ 160 nm and N_acc ≈ 90 cm⁻³), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κ_Ait ≈ 0.12, κ_acc ≈ 0.18). Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (D_Ait ≈ 80 nm, N_Ait ≈ 120 cm⁻³ vs. D_acc ≈ 180 nm, N_acc ≈ 310 cm⁻³), an increased abundance of dust and salt, and relatively high hygroscopicity (κ_Ait ≈ 0.18, κ_acc ≈ 0.35). The coarse mode is also significantly enhanced during these events. Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (D_Ait ≈ 70 nm, N_Ait ≈ 140 cm⁻³ vs. D_acc ≈ 170 nm, N_acc ≈ 3400 cm⁻³), very high organic mass fractions ( ∼ 90 %), and correspondingly low hygroscopicity (κ_Ait ≈ 0.14, κ_acc ≈ 0.17). Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130 nm, N_CN, 10 ≈ 1300 cm⁻³), with high sulfate mass fractions (∼ 20 %) from volcanic sources and correspondingly high hygroscopicity (κ_{<  100 nm} ≈ 0.14, κ_{>  100 nm} ≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110 nm, N_CN, 10 ≈ 2800 cm⁻³) with an organic-dominated composition and sharply decreased hygroscopicity (κ_{<  150 nm} ≈ 0.10, κ_{>  150 nm} ≈ 0.20). Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions). The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

The Amazon rain forest experiences the combined pressures from human-made deforestation and progressing climate change, causing severe and potentially disruptive perturbations of the ecosystem's integrity and stability. To intensify research on critical aspects of Amazonian biosphere–atmosphere exchange, the Amazon Tall Tower Observatory (ATTO) has been established in the central Amazon Basin. Here we present a multi-year analysis of backward trajectories to derive an effective footprint region of the observatory, which spans large parts of the particularly vulnerable eastern basin. Further, we characterize geospatial properties of the footprint regions, such as climatic conditions, distribution of ecoregions, land cover categories, deforestation dynamics, agricultural expansion, fire regimes, infrastructural development, protected areas, and future deforestation scenarios. This study is meant to be a resource and reference work, helping to embed the ATTO observations into the larger context of human-caused transformations of Amazonia. We conclude that the chances to observe an unperturbed rain forest–atmosphere exchange at the ATTO site will likely decrease in the future, whereas the atmospheric signals from human-made and climate-change-related forest perturbations will increase in frequency and intensity.

The Amazon rainforest is a sensitive ecosystem experiencing the combined pressures of progressing deforestation and climate change. Its atmospheric conditions oscillate between biogenic and biomass burning (BB) dominated states. The Amazon further represents one of the few remaining continental places where the atmosphere approaches pristine conditions during occasional wet season episodes. The Amazon Tall Tower Observatory (ATTO) has been established in central Amazonia to investigate the complex interactions between the rainforest ecosystem and the atmosphere. Physical and chemical aerosol properties have been analyzed continuously since 2012. This paper provides an in-depth analysis of the aerosol's optical properties at ATTO based on data from 2012 to 2017. The following key results have been obtained. The aerosol scattering and absorption coefficients at 637 nm, σ_sp,637 and σ_ap,637, show a pronounced seasonality with lowest values in the clean wet season (mean ± SD: σ_sp,637 = 7.5±9.3 M m⁻¹; σ_ap,637 = 0.68±0.91 M m⁻¹) and highest values in the BB-polluted dry season (σ_sp,637 = 33±25 M m⁻¹; σ_ap,637 = 4.0±2.2 M m⁻¹). The single scattering albedo at 637 nm, ω₀, is lowest during the dry season (ω₀ = 0.87±0.03) and highest during the wet season (ω₀ = 0.93±0.04). The retrieved BC mass absorption cross sections, α_abs, are substantially higher than values widely used in the literature (i.e., 6.6 m² g⁻¹ at 637 nm wavelength), likely related to thick organic or inorganic coatings on the BC cores. Wet season values of α_abs = 11.4±1.2 m² g⁻¹ (637 nm) and dry season values of α_abs = 12.3±1.3 m² g⁻¹ (637 nm) were obtained. The BB aerosol during the dry season is a mixture of rather fresh smoke from local fires, somewhat aged smoke from regional fires, and strongly aged smoke from African fires. The African influence appears to be substantial, with its maximum from August to October. The interplay of African vs. South American BB emissions determines the aerosol optical properties (e.g., the fractions of black vs. brown carbon, BC vs. BrC). By analyzing the diel cycles, it was found that particles from elevated aerosol-rich layers are mixed down to the canopy level in the early morning and particle number concentrations decrease towards the end of the day. Brown carbon absorption at 370 nm, σ_ap,BrC,370, was found to decrease earlier in the day, likely due to photo-oxidative processes. BC-to-CO enhancement ratios, ER_BC, reflect the variability of burnt fuels, combustion phases, and atmospheric removal processes. A wide range of ER_BC between 4 and 15 ng m⁻³ ppb⁻¹ was observed with higher values during the dry season, corresponding to the lowest ω₀ levels (0.86–0.93). The influence of the 2009/2010 and 2015/2016 El Niño periods and the associated increased fire activity on aerosol optical properties was analyzed by means of 9-year σ_sp and σ_ap time series (combination of ATTO and ZF2 data). Significant El Niño-related enhancements were observed: in the dry season, σ_sp,637 increased from 24±18 to 48±33 M m⁻¹ and σ_{ap, 637} from 3.8±2.8 to 5.3±2.5 M m⁻¹. The absorption Ångström exponent, å_abs, representing the aerosol absorption wavelength dependence, was mostly <1.0 with episodic increases upon smoke advection. A parameterization of å_abs as a function of the BC-to-OA mass ratio for Amazonian aerosol ambient measurements is presented. The brown carbon (BrC) contribution to σ_ap at 370 nm was obtained by calculating the theoretical BC å_abs, resulting in BrC contributions of 17 %–29 % (25th and 75th percentiles) to σ_ap 370 for the entire measurement period. The BrC contribution increased to 27 %–47 % during fire events under El Niño-related drought conditions from September to November 2015. The results presented here may serve as a basis to understand Amazonian atmospheric aerosols in terms of their interactions with solar radiation and the physical and chemical-aging processes that they undergo during transport. Additionally, the analyzed aerosol properties during the last two El Niño periods in 2009/2010 and 2015/2016 offer insights that could help to assess the climate change-related potential for forest-dieback feedbacks under warmer and drier conditions.

In the Amazonian atmosphere, the aerosol coarse mode comprises a complex, diverse, and variable mixture of bioaerosols emitted from the rain forest ecosystem, long-range transported Saharan dust (we use Sahara as shorthand for the dust source regions in Africa north of the Equator), marine aerosols from the Atlantic Ocean, and coarse smoke particles from deforestation fires. For the rain forest, the coarse mode particles are of significance with respect to biogeochemical and hydrological cycling, as well as ecology and biogeography. However, knowledge on the physicochemical and biological properties as well as the ecological role of the Amazonian coarse mode is still sparse. This study presents results from multi-year coarse mode measurements at the remote Amazon Tall Tower Observatory (ATTO) site. It combines online aerosol observations, selected remote sensing and modeling results, as well as dedicated coarse mode sampling and analysis. The focal points of this study are a systematic characterization of aerosol coarse mode abundance and properties in the Amazonian atmosphere as well as a detailed analysis of the frequent, pulse-wise intrusion of African long-range transport (LRT) aerosols (comprising Saharan dust and African biomass burning smoke) into the Amazon Basin. We find that, on a multi-year time scale, the Amazonian coarse mode maintains remarkably constant concentration levels (with 0.4 cm⁻³ and 4.0 µg m⁻³ in the wet vs. 1.2 cm⁻³ and 6.5 µg m⁻³ in the dry season) with rather weak seasonality (in terms of abundance and size spectrum), which is in stark contrast to the pronounced biomass burning-driven seasonality of the submicron aerosol population and related parameters. For most of the time, bioaerosol particles from the forest biome account for a major fraction of the coarse mode background population. However, from December to April there are episodic intrusions of African LRT aerosols, comprising Saharan dust, sea salt particles from the transatlantic passage, and African biomass burning smoke. Remarkably, during the core period of this LRT season (i.e., February–March), the presence of LRT influence, occurring as a sequence of pulse-like plumes, appears to be the norm rather than an exception. The LRT pulses increase the coarse mode concentrations drastically (up to 100 µg m⁻³) and alter the coarse mode composition as well as its size spectrum. Efficient transport of the LRT plumes into the Amazon Basin takes place in response to specific mesoscale circulation patterns in combination with the episodic absence of rain-related aerosol scavenging en route. Based on a modeling study, we estimated a dust deposition flux of 5–10 kg ha⁻¹ a⁻¹ in the region of the ATTO site. Furthermore, a chemical analysis quantified the substantial increase of crustal and sea salt elements under LRT conditions in comparison to the background coarse mode composition. With these results, we estimated the deposition fluxes of various elements that are considered as nutrients for the rain forest ecosystem. These estimates range from few g ha⁻¹ a⁻¹ up to several hundreds of g ha⁻¹ a⁻¹ in the ATTO region. The long-term data presented here provide a statistically solid basis for future studies of the manifold aspects of the dynamic coarse mode aerosol cycling in the Amazon. Thus, it may help to understand its biogeochemical relevance in this ecosystem as well as to evaluate to what extent anthropogenic influences have altered the coarse mode cycling already.

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 ($\mathrm{\Delta }{\mathit{\theta }}_{\text{e}}^{\prime }$ ) – 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 $\mathrm{\Delta }{\mathit{\theta }}_{\text{e}}^{\prime }$ 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.