Christopher Pöhlker

Biogenic Potassium Salt Particles as Seeds for Secondary Organic Aerosol in the Amazon

Salty Origins of Fresh Water Cloud droplets above the Amazonian rain forest form mostly around organic aerosols, but the source of the aerosols has been a mystery. Pöhlker et al. (p. 1075) report that particles rich in potassium salts emitted by Amazonian vegetation can act as the seeds for the growth of organic aerosol particles that function as condensation nuclei for water droplets. These specks of biogenic salts provide a surface for the condensation of low- or semi-volatile organic compounds formed by the atmospheric oxidation of isoprene and terpenes, molecules produced in great abundance by many kinds of Amazonian plants. Potassium salt particles account for the previously mysterious initiation sites of aerosol growth above the Amazonian rainforest. The fine particles serving as cloud condensation nuclei in pristine Amazonian rainforest air consist mostly of secondary organic aerosol. Their origin is enigmatic, however, because new particle formation in the atmosphere is not observed. Here, we show that the growth of organic aerosol particles can be initiated by potassium-salt–rich particles emitted by biota in the rainforest. These particles act as seeds for the condensation of low- or semi-volatile organic compounds from the atmospheric gas phase or multiphase oxidation of isoprene and terpenes. Our findings suggest that the primary emission of biogenic salt particles directly influences the number concentration of cloud condensation nuclei and affects the microphysics of cloud formation and precipitation over the rainforest.

ACRIDICON–CHUVA Campaign: Studying Tropical Deep Convective Clouds and Precipitation over Amazonia Using the New German Research Aircraft HALO

AbstractBetween 1 September and 4 October 2014, a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rain forest. The new German research aircraft, High Altitude and Long Range Research Aircraft (HALO), a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German–Brazilian Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON– CHUVA) venture to quantify aerosol–cloud–precipitation interactions and their thermodynamic, dynamic, and radiative effects by in situ and remote sensing measurements over Amazonia. The ACRIDICON–CHUVA field observations were carried out in cooperation with the second intensive operating period...

The Green Ocean Amazon Experiment (GoAmazon2014/5) Observes Pollution Affecting Gases, Aerosols, Clouds, and Rainfall over the Rain Forest

AbstractThe Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed t...

Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall

The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.

Satellite retrieval of cloud condensation nuclei concentrations by using clouds as CCN chambers

Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft velocities (Wb). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb and the satellite-retrieved cloud base drop concentrations (Ndb), which is the same as CCN(S). Validation against ground-based CCN instruments at Oklahoma, at Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25° restricts the satellite coverage to ∼25% of the world area in a single day.

Substantial convection and precipitation enhancements by ultrafine aerosol particles

Up with ultrafine aerosol particles Ultrafine aerosol particles (smaller than 50 nanometers in diameter) have been thought to be too small to affect cloud formation. Fan et al. show that this is not the case. They studied the effect of urban pollution transported into the otherwise nearly pristine atmosphere of the Amazon. Condensational growth of water droplets around the tiny particles releases latent heat, thereby intensifying atmospheric convection. Thus, anthropogenic ultrafine aerosol particles may exert a more important influence on cloud formation processes than previously believed. Science, this issue p. 411 Water droplet condensation by ultrafine aerosol particles fuels more intense atmospheric convection. Aerosol-cloud interactions remain the largest uncertainty in climate projections. Ultrafine aerosol particles smaller than 50 nanometers (UAP<50) can be abundant in the troposphere but are conventionally considered too small to affect cloud formation. Observational evidence and numerical simulations of deep convective clouds (DCCs) over the Amazon show that DCCs forming in a low-aerosol environment can develop very large vapor supersaturation because fast droplet coalescence reduces integrated droplet surface area and subsequent condensation. UAP<50 from pollution plumes that are ingested into such clouds can be activated to form additional cloud droplets on which excess supersaturation condenses and forms additional cloud water and latent heating, thus intensifying convective strength. This mechanism suggests a strong anthropogenic invigoration of DCCs in previously pristine regions of the world.

Efflorescence upon humidification? X‐ray microspectroscopic in situ observation of changes in aerosol microstructure and phase state upon hydration

The phase and mixing state of atmospheric aerosols is a central determinant of their properties and thus their role in atmospheric cycling and climate. Particularly, the hygroscopic response of aerosol particles to relative humidity (RH) variation is a key aspect of their atmospheric life cycle and impacts. Here we applied X-ray microspectroscopy under variable RH conditions to internally mixed aerosol particles from the Amazonian rain forest collected during periods with anthropogenic pollution. Upon hydration, we observed substantial and reproducible changes in particle microstructure, which appear as mainly driven by efflorescence and recrystallization of sulfate salts. Multiple solid and liquid phases were found to coexist, especially in intermediate humidity regimes. We show that X-ray microspectroscopy under variable RH is a valuable technique to analyze the hygroscopic response of individual ambient aerosol particles. Our initial results underline that RH changes can trigger strong particle restructuring, in agreement with previous studies on artificial aerosols.

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

The Amazon rain forest 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 rain forest ecosystem and the atmosphere. Physical and chemical aerosol properties have been analyzed continuously since 2012. This paper provides an

Imaging Molecular Reaction and Diffusion in Organic Aerosol Particles

1. Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland. 2. Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland 3. Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, United Kingdom 4. Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany 5. Aix Marseille Université, CNRS, LCE UMR 7376, 13331 Marseille, France 6. Department of Physics, University of Helsinki, 00014 Helsinki, Finland 7. Université Lyon 1, CNRS, UMR 5256, IRCELYON, 69626 Villeurbanne, France 8. School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States 9. Department of Chemistry, University of California, Irvine, CA 92617, United States

Biological ice nuclei and the impact of rain on ice nuclei populations

With 18% of total US landmass devoted to croplands, farmland is a potentially major source of biogenic particles to the atmosphere. We investigated two farms as potential sources of biological ice nuclei (IN). We found that each of these farms contained abundant INA bacteria on the vegetation; however, airborne ina gene concentrations were typically below detectable limits, demonstrating a disconnect between local vegetative sources and the air above them. The question remains, then, as to how biological IN are released into the atmosphere. In a second study, we investigated how precipitation impacted the concentration and composition of IN. Results from these measurements show that ground level IN concentrations were enhanced during rain events, and that some portion of these IN were biological. In this paper, we present results from both of these studies, and discuss modified measurement techniques aimed at characterizing the often very low number concentrations of biological IN.