Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

Abstract

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 organicdominated 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 activa...

Urban influence on the concentration and composition of submicron particulate matter in central Amazonia

“…The air masses of Bkgd-2 may have experienced less wet deposition and may represent more extensive atmospheric oxidation than those of Bkgd-1. They may also have carried PM contributions from out-of-basin sources, which would be consistent with the higher sulfate and black carbon concentrations of Bkgd-2 compared to those of Bkgd-1 (Chen et al, 2009;Pöhlker et al, 2018).…”

Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

“…Atlantic, and the Amazon Basin have found pollution layers in the free troposphere with similar characteristics (e.g., Andreae et al, 1988;Diab et al, 1996;Thompson et al, 1996;Bozem et al, 2014;Marenco et al, 2016). Recent studies in the central basin, at the Amazon Tall Tower Observatory (ATTO), and in northeastern edge of the Amazon discovered indications for a significant abundance of African smoke during the Amazonian dry season (Barkley et al, 2019;Pöhlker et al, 2019Pöhlker et al, , 2018Saturno et al, 2018b;135 Wang et al, 2016). However, robust quantitative data from observations and/or models (e.g., African BC and CCN fractions in the basin) have remained sparse.…”

“…The CCN concentrations at a supersaturation of 0.5 %, NCCN (S = 0.5 %), were calculated using long-term scanning mobility particle sizer (SMPS) data and the κ-Köhler parametrization as described in Pöhlker et al (2016). For more details about the aerosol optical properties characterization and CCN observations we refer to Saturno et al (2018b) and Pöhlker et al (2016Pöhlker et al ( , 2018, respectively. Further details on CO measurements conducted at ATTO site can be found in Winderlich et al 250 (2010) and Andreae et al (2015).…”

“…The large-scale trade wind circulation over the tropical Atlantic region is defined by the position of the 470 intertropical convergence zone (ITCZ), which oscillates north-southwards over the year. The ITCZ movement is a main factor of the pronounced aerosol seasonality in the central Amazon Martin et al, 2010;Moran-Zuloaga et al, 2018;Pöhlker et al, 2019Pöhlker et al, , 2018Saturno et al, 2018b). Based on BTs, which reflect the large-scale trade wind circulation patterns, we investigated the origin and age of the UPL up to 10 days prior to its observation.…”

“…During the wet season (February to May), trace gas and aerosol emissions from the regional biosphere predominantly regu-100 late atmospheric cycling, precipitation patterns, and regional climate (Pöhlker et al, 2012;Pöschl et al, 2010). Average wet season black carbon (BC) mass concentrations, MBC, are ~0.07 µg m -3 and MBC approaches zero during pristine episodes (Andreae and Gelencsér, 2006;Pöhlker et al, 2018). In contrast, the dry season (August to November) is characterized by intense and persistent BB emissions, changing substantially the atmospheric composition and cycling (Artaxo et al, 2013;Rizzo et al, 2013).…”