Search

The wet season atmosphere in the central Amazon resembles natural conditions with minimal anthropogenic influence, making it one of the rare pre-industrial-like continental areas worldwide. Previous long-term studies have analyzed the properties and sources of the natural Amazonian background aerosol. However, the vertical profile of the planetary boundary layer (PBL) has not been assessed systematically. Since 2017, such a profile assessment has been possible with the 325 m 5 high tower at the Amazon Tall Tower Observatory (ATTO), located in a largely untouched primary forest in central Amazonia. This study investigates the variability of submicrometer aerosol concentration, size distribution, and optical properties at 60 and 325 m height in the Amazon PBL. The results show significant differences in aerosol volumes and scattering coefficients in the vertical gradient. The aerosol population was well-mixed throughout the boundary layer during the daytime but became separated upon stratification during nighttime. We also found a significant difference in the spectral dependence of 10 the scattering coefficients between the two heights. The analysis of rainfall and related downdrafts revealed changes in the aerosol populations before and after rain events, with absorption and scattering coefficients decreasing as optically active particles are removed by wet deposition. The recovery of absorption and scattering coefficients is faster at 325 m than at 60 m. Convective events were concomitant with rapid increases in the concentration of sub-50 nm particles, likely associated with downdrafts. We found that the aerosol population near the canopy had a significantly higher mass scattering efficiency than at 15 325 m. It was also observed a clear spectral dependence, with values for λ = 450, 525 and 635 nm of 7.74±0.12, 5.49±0.11 and 4.15±0.11 m² g-1, respectively, at 60 m, while at 325 m, the values were 5.26±0.06, 3.76±0.05 and 2.46±0.04m² g-1, respectively. The equivalent aerosol refractive index results, which were obtained for the first time for the wet season in central Amazon, show a slightly higher scattering (real) component at 60 m compared to 325 m, of 1.33 and 1.27, respectively. In contrast, the refractive index's absorptive (imaginary) component was identical for both heights, at 0.006. This study shows that the aerosol physical properties at 60 and 325 m height are different, likely due to aging processes, and strongly depend on 5 the photochemistry, PBL dynamics, and aerosol sources. These findings provide valuable insights into the impact of aerosols on climate and radiative balance and can be used to improve the representation of aerosols in global climate models. [ABSTRACT FROM AUTHOR]

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 60-70nm is widely observed in the planetary boundary layer (PBL) around the world, but not in central Amazonia. Here, classical NPF events are rarely observed in the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50nm (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 six years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO, Feb 2014 to Sep 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 50nm 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 5.2nmh-1 and a median CS of 0.0011s-1 were observed. The growth events were more frequent during the daytime (74%) and showed higher GR (5.9nmh-1) compared to nighttime events (4.0nmh-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 (∆θ'e) - 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 ∆θ'e anomaly. Therefore, these events do not appear to be related to downdraft injection and suggest the existence of other currently unknown sources of the sub-50nm particles. [ABSTRACT FROM AUTHOR]

The Green Ocean Amazon experiment - GoAmazon2014/5 explored the interactions between natural biogenic forest emissions from Central Amazonia and urban air pollution from Manaus. Previous GoAmazon2014/5 studies showed that nitrogen oxides (NOx = NO + NO2) and sulfur oxides (SOx) emissions from Manaus strongly interact with biogenic volatile organic compounds (BVOCs), affecting secondary organic aerosol (SOA) formation. In previous studies, ground based and aircraft measurements provided evidence of SOA formation and strong changes in aerosol composition and properties. Aerosol optical properties also evolve, and their impacts on the Amazonian ecosystem can be significant. As particles age, some processes such as SOA production, black carbon (BC) deposition, particle growth, and the BC lensing effect change the aerosol optical properties, affecting the solar radiation flux at the surface. This study analyzes data and models SOA formation using the Weather Research and Forecasting with Chemistry (WRF-Chem) model to assess the spatial variability of aerosol optical properties as the Manaus plumes interact with the natural atmosphere. The following aerosol optical properties are investigated: single scattering albedo (SSA), asymmetry parameter (gaer), absorption Ångström exponent (AAE), and scattering Ångström exponent (SAE). These simulations were validated using ground based measurements at three experimental sites: Amazon Tall Tower Observatory - ATTO (T0a), downtown Manaus (T1), Tiwa Hotel (T2) and Manacapuru (T3), as well as the G1 aircraft flights. WRF-Chem simulations were performed over seven days during March 2014. Results show a mean biogenic SOA (BSOA) mass enrichment of 512 % at the T1 site, 450 % in regions downwind of Manaus such as the T3 site and 850 % in areas north of the T3 site in simulations with anthropogenic emissions. The SOA formation is rather fast, with about 80 % of the SOA mass produced in 3-4 hours. Comparing the plume from simulations with and without anthropogenic emissions, SSA shows a downwind reduction of approximately 10 %, 11 % and 6 % at the T1, T2 and T3 sites, respectively. Other regions, such as those further downwind of the T3 site, are also affected. Gaer values increased from 0.62 to 0.74 at the T1 site and from 0.67 to 0.72 at the T3 site when anthropogenic emissions are active. During the Manaus plume aging process, a plume tracking analysis shows an increase in SSA from 0.91 close to Manaus to 0.98 160 km downwind of Manaus as a result of SOA production and BC deposition. [ABSTRACT FROM AUTHOR]

In the period between July 9th and 11th, 2014 a Friagem event reached the central Amazon region causing significant changes in microclimate and atmospheric chemistry. On July 11th, the southwest flow related to the Friagem converged with the easterly winds in the central Amazon region. The interaction between these two distinct air masses formed a convection band, which intensified over the Manaus region and the Amazon Tall Tower Observatory (ATTO) site. The satellite images show the evolution of convective activity on July 11th, which lead to 21 mm of precipitation in the ATTO site. Moreover, the arrival of the Friagem caused a sudden drop in temperature and a predominance of southerly winds, which could be seen in Porto Velho between July 7th and 8th and in Manaus and ATTO site from July 9th to 11th. The results of ERA-Interim reanalysis and Brazilian developments on the Regional Atmospheric Modeling System (BRAMS) simulations show that this Friagem event coming from the southwest, carries a mass of air with higher O3 and NO2 mixing ratios and lower CO mixing ratio compared to the airmasses present at the central Amazon. At lake Balbina the Friagem intensifies the local circulations, such as the breeze phenomena. At the Manaus region and ATTO site, the main effects of the Friagem event are: a decrease in the incoming solar radiation (due to intense cloud formation), a large temperature drop and a distinct change in surface O3 and CO2 mixing ratios. As the cold air of the Friagem was just in the lower 500 m the most probable cause of this change is that a cold pool above the forest prevented vertical mixing causing accumulation of CO2 from respiration and very low O3 mixing ratio due to photochemistry reduction and limited mixing within the boundary layer. [ABSTRACT FROM AUTHOR]

Urbanization and deforestation have important impacts on atmospheric particulate matter (PM) over Amazonia. This study presents observations and analysis of submicron PM1 concentration, composition, and optical properties in central Amazonia during the dry season. The focus is on delineating the anthropogenic impact on the observed quantities, especially as related to the organic PM1. The primary study site was located 70km to the west of Manaus, a city of over two million people in Brazil. As part of the GoAmazon2014/5 experiment, datasets from a large suite of instrumentation were employed. A high-resolution time-of-flight aerosol mass spectrometer (AMS) provided data on PM1 composition, and aethalometer measurements were used to derive the absorption coefficient babs,BrC of brown carbon (BrC) at 370nm. The relationships of babs,BrC with AMS-measured quantities showed that the absorption was associated with less-oxidized, nitrogen-containing organic compounds. Atmospheric processing appeared to bleach the BrC components. The organic PM1 was separated into different classes by positive-matrix factorization (PMF). Estimates of the effective mass absorption efficiency associated with each PMF factor were obtained. Biomass burning and urban emissions appeared to contribute at least 80% of babs,BrC while accounting for 30 to 40% of the organic PM1 mass concentration. In addition, a comparison of organic PM1 composition between wet and dry seasons revealed that only a fraction of the nine-fold increase in mass concentration between the seasons was due to biomass burning. An eight-fold increase in biogenic secondary organic PM1 was observed. A combination of decreased wet deposition and increased emissions and oxidant concentrations, as well as a positive feedback on larger mass concentrations are thought to play a role in the observed increases. Fuzzy c-means clustering identified three clusters to represent different pollution influences during the dry season, including "baseline" (dry season background, which includes biomass burning), "event" (increased influence of biomass burning and long-range transport of African volcanic emissions), and "urban" (Manaus influence on top of the background). The baseline cluster was associated with a mean mass concentration of 9 ± 3μgm-3. This concentration increased on average by 3μgm-3 for both the urban and the event clusters. The event cluster was characterized by remarkably high sulfate concentrations. Differences in the organic PM1 composition for the urban cluster compared to the other two clusters suggested a shift in oxidation pathways as well as an accelerated oxidation cycle due to urban emissions, in agreement with findings for the wet season. [ABSTRACT FROM AUTHOR]

The Amazon rain forest is considered a very sensitive ecosystem that could be significantly affected by a changing climate. It is still one of the few places on Earth where the atmosphere in the continent approaches near-pristine conditions for some periods of the year. The Amazon Tall Tower Observatory (ATTO) has been built in central Amazonia to monitor the atmospheric and forest ecosystem conditions. The atmospheric conditions at the ATTO site oscillate between biogenic and biomass burning (BB) dominated states. By using a comprehensive ground-based aerosol measurement setup, we studied the physical and chemical properties of aerosol particles at the ATTO site. This paper presents results from 2012 to 2017, with special focus on light absorbing aerosol particles. The aerosol absorption wavelength dependence (expressed as the absorption Ångström exponent, åabs) was usually below 1.0 and increased during the presence of smoke transported from fires in the southern and eastern regions of the Amazon or advected from savanna fires in Africa. In this study, the brown carbon (BrC) contribution to light absorption at 370 nm was obtained by calculating the theoretical wavelength dependence of åabs (WDA). Our calculations resulted in BrC contributions of 17-29% (25th and 75th percentiles) to total light absorption at 370 nm (σap 370) during the measurement period (2012-2017). The BrC contribution increased up to 27-47% during fire events occurring under the influence of El Niño, between September and November 2015. An extended time series of ATTO and ZF2 (another Amazon rain forest sampling site) data showed enhanced light scattering and absorption coefficients during El Niño periods in 2009 and 2015. Long-range transport (LRT) aerosol particles that reached the central Amazon Basin from Africa or from southern Amazon exhibited a wide range of black carbon (BC) to carbon monoxide (CO) enhancement ratios (ERBC) (between 4 and 15 ng m-3 ppb-1) reflecting the variability of fuels, combustion phase, and removal processes in the atmosphere. Higher ERBC were measured during the dry season when we observed values up to 15 ng m-3 ppb-1, which were related to the lowest single scattering albedo (ω0) measured during the studied period, (0.86-0.93). A parameterization of åabs as a function of the BC to OA mass ratio was investigated and was found applicable to tropical forest emissions but further investigation is required, especially by segregating fuel types. Additionally, important enhancements of the BC mass absorption cross-section (αabs) were found over the measurement period. This enhancement could be linked to heavy coating of the BC aerosol particles. In the future, the BC mixing state should be systematically investigated by using different instrumental approaches. [ABSTRACT FROM AUTHOR]

Size-resolved 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 full seasonal cycle (Mar 2014-Feb 2015). In a companion part 1 paper, we presented an in-depth CCN characterization based on annually as well as seasonally averaged time intervals and discuss different parametrization strategies to represent the Amazonian CCN cycling in modelling studies (M. Pöhlker et al., 2016b). The present part 2 study analyzes the aerosol and CCN variability in original time resolution and, thus, resolves aerosol advection and transformation for the following case studies, which represent the most characteristic states of the Amazonian atmosphere: 1. Near-pristine (NP) conditions, defined as the absence of detectable black carbon (< 0.01 µg m-3), showed their highest occurrence (up to 30%) in the wet season (i.e., Mar-May). On average, the NP episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode: DAit = 70 nm, NAit = ~200 cm-3 vs. weaker accumulation mode: Dacc = 170 nm, Nacc = ~60 cm-3), a mostly organic particle composition, and relatively low hygroscopicity levels (κAit = 0.12 vs. κacc = 0.18). The NP CCN efficiency spectrum shows that the CCN population is sensitive to changes in supersaturation (S) over a wide S range. 2. Long-range transport (LRT) conditions frequently mix Saharan dust, African combustion smoke, and sea spray aerosols into the Amazonian wet season atmosphere. The LRT episodes (i.e., Feb-Apr) are characterized by an accumulation mode dominated size distribution (DAit = 80 nm, NAit = 120 cm-3 vs. Dacc = 180 nm, Nacc = 300 cm-3), a clearly increased abundance of dust and salt compounds, and relatively high hygroscopicity levels (κAit = 0.18, κacc = 0.34). The LRT CCN efficiency spectrum shows that the CCN population is highly sensitive to changes in S in the low S regime. 3. Biomass burning (BB) conditions dominate the Amazonian dry season. A selected characteristic BB episode shows a very strong accumulation mode (DAit = 70 nm, NAit = ~140 cm-3 vs. Dacc = 170 nm, Nacc = ~3400 cm-3), particles with very high organic fractions (> 90%), and correspondingly low hygroscopicity levels (κAit = 0.14, κacc = 0.17). The BB CCN efficiency spectrum shows that the CCN population is highly sensitive to changes in S in the low S regime. 4. Mixed pollution conditions show the superposition of African (i.e., volcanic) and Amazonian (i.e., biomass burning) aerosol emissions during the dry season. The African aerosols showed a broad monomodal distribution (D = 130 nm, N = ~1300 cm-3), with very high sulfate fractions (20%), and correspondingly high hygroscopicity (κAit = 0.14, κacc = 0.22). This was superimposed by fresh smoke from nearby fires with one strong mode (D = 113 nm, Nacc = ~2800 cm-3), an organic-dominated aerosol, and sharply decreased hygroscopicity (κAit = 0.10, κacc = 0.20). These conditions underline the rapidly changing pollution regimes with clear impacts on the aerosol and CCN properties. Overall, this study provides detailed insights into the CCN cycling in relation to aerosol-cloud interaction in the vulnerable and climate-relevant Amazon region. The detailed analysis of aerosol and CCN key properties and particularly the extracted CCN efficiency spectra with the associated fit parameters provide a basis for an in-depth analysis of aerosol-cloud interaction in the Amazon and beyond. [ABSTRACT FROM AUTHOR]

The intercontinental transport of aerosols from the Sahara is likely to play a significant role in nutrient cycles in the Amazon rainforest, since it carries many types of minerals to these otherwise low-fertility lands. Iron is one of the micronutrients essential for plant growth, and the Amazon rainforest is iron-limited. The main aim of this study was to assess the input and potential impact of iron bioavailability from Saharan dust, namely, the soluble fraction Fe(II)/Fe(III). Seven other soluble elements that are also essential for plants were measured. Dust particles entrained in the air were collected and analyzed, but not dust deposited in rainfall as atmospheric washout. The sampling campaign was carried out at the ATTO site (Amazon Tall Tower Observatory), from March to April 2015, and samplers were placed both above and below the canopy. Mineral dust aerosol at ATTO showed peak concentrations for Fe(III) (47.6 ng m−3), Fe(II) (16.2 ng m−3), Na (470 ng m−3), Ca (194 ng m−3), K (64.7 ng m−3), and Mg (88.8 ng m−3) during the presence of dust transported from the Sahara, as determined by remote ground-based and satellite sensing data and backward trajectories. Atmospheric transport of weathered Saharan dust, followed by surface deposition, results in substantial iron bioavailability across the rainforest canopy. The seasonal deposition of dust rich in soluble iron and other minerals is likely to affect both bacteria and fungi within the topsoil and on canopy surfaces, and especially benefit highly bioabsorbent epiphytes, such as lichens. In this scenario, Saharan dust can provide essential macronutrients and micronutrients to plant roots, and also directly to plant leaves. The influence on the ecology of the forest canopy and topsoil would likely be different from that of nutrients from the weathered Amazon bedrock, which provides the main source of soluble mineral nutrients. [ABSTRACT FROM AUTHOR]