Stephanie King

Rainforest aerosols as biogenic nuclei of clouds and precipitation in the Amazon.

Clean or Dirty Aerosols strongly affect atmospheric properties and processes—including visibility, cloud formation, and radiative behavior. Knowing their effects in both clean and polluted air is necessary in order to understand their influence (see the Perspective by Baltensperger). Clarke and Kapustin (p. 1488) examine vertical atmospheric profiles collected above the Pacific Ocean, where air quality is affected by the transport of polluted air from the west, and find significant regional enhancements in light scattering, aerosol mass, and aerosol number associated with combustion. Aerosol particle concentrations in this region can exceed values in clean, unperturbed regions by over an order of magnitude. Thus combustion affects hemispheric aerosol optical depth and the distribution of cloud condensation nuclei. Pöschl et al. (p. 1513) discuss the composition of aerosols above the Amazon Basin, in the pristine conditions of the rainy season. The aerosols in this region are derived mostly from gaseous biogenic precursors, plants, and microorganisms, and particle concentration is orders of magnitude lower than in polluted continental regions. The majority of cloud condensation nuclei in the Amazon during the wet season are derived from biogenic precursors. The Amazon is one of the few continental regions where atmospheric aerosol particles and their effects on climate are not dominated by anthropogenic sources. During the wet season, the ambient conditions approach those of the pristine pre-industrial era. We show that the fine submicrometer particles accounting for most cloud condensation nuclei are predominantly composed of secondary organic material formed by oxidation of gaseous biogenic precursors. Supermicrometer particles, which are relevant as ice nuclei, consist mostly of primary biological material directly released from rainforest biota. The Amazon Basin appears to be a biogeochemical reactor, in which the biosphere and atmospheric photochemistry produce nuclei for clouds and precipitation sustaining the hydrological cycle. The prevailing regime of aerosol-cloud interactions in this natural environment is distinctly different from polluted regions.

Cloud condensation nucleus activity of secondary organic aerosol particles mixed with sulfate

from 23 to 37 m gm � 3 . CCN analysis was performed for 80to 150-nm particles having variable organic-sulfate volume fractions, which were estimated from the diameter of the organic-sulfate particle relative to that of the seed as well as independently from mass spectra. Critical supersaturation, which increased for greater SOA volume fraction and �� ;

Satellite images suggest a new Sargassum source region in 2011

In the summer of 2011, a major ‘Sargassum event’ brought large amounts of seaweed onto the beaches of the islands of the eastern Caribbean with significant effects on local tourism. We present satellite observations showing that the event had its origin north of the mouth of the Amazon in an area not previously associated with Sargassum growth. A significant concentration of Sargassum was detected in April, when it was centred at about 7° N latitude and 45° W longitude. By July it had spread to the coast of Africa in the east and to the Lesser Antilles and the Caribbean in the west. We have previously used images from MERIS (Medium Resolution Imaging Spectrometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) to show the value of satellite observations in tracking patterns of Sargassum. For the years 2003–2010, we were able to determine the seasonal distribution over the range of 20°–40° N latitude and 100°–40° W longitude covering the ‘Sargasso Sea’ region of the North Atlantic and the Gulf of Mexico. In 2011, satellite data showed a large shift in the distribution, whose cause is unclear.

Investigating primary marine aerosol properties: CCN activity of sea salt and mixed inorganic-organic particles.

Sea spray particles ejected as a result of bubbles bursting from artificial seawater containing salt and organic matter in a stainless steel tank were sampled for size distribution, morphology, and cloud condensation nucleus (CCN) activity. Bubbles were generated either by aeration through a diffuser or by water jet impingement on the seawater surface. Three objectives were addressed in this study. First, CCN activities of NaCl and two types of artificial sea salt containing only inorganic components were measured to establish a baseline for further measurements of mixed organic–inorganic particles. Second, the effect of varying bubble residence time in the bulk seawater solution on particle size and CCN activity was investigated and was found to be insignificant for the organic compounds studied. Finally, CCN activities of particles produced from jet impingement were compared with those produced from diffuser aeration. Analyses indicate a considerable amount of organic enrichment in the jet-produced particles relative to the bulk seawater composition when sodium laurate, an organic surfactant, is present in the seawater. In this case, the production of a thick foam layer during impingement may explain the difference in activation and supports hypotheses that particle production from the two methods of generating bubbles is not equal.

The Dynamic Shape Factor of Sodium Chloride Nanoparticles as Regulated by Drying Rate

The influence of drying rate on the dynamic shape factor χ of NaCl particles was investigated. The drying rate at the efflorescence relative humidity (ERH) of 45% was controlled in a laminar flow tube and varied from 5.5 ± 0.9 to 101 ± 3 RH s–1 at ERH, where RH represents one percent unit of relative humidity. Dry particles having mobility diameters of 23–84 nm were studied, corresponding to aqueous particles of 37–129 nm at the RH (57%) prior to drying. At each mobility diameter and drying rate, the critical supersaturation of cloud-condensation activation was also measured. The mobility diameter and the critical supersaturation were combined in an analysis to determine the value of χ. The measured values varied from 1.02 to 1.26. For fixed particle diameter the χ value decreased with increasing drying rate. For fixed drying rate, a maximum occurred in χ between 35- and 40-nm dry mobility diameter, with a lower χ for both smaller and larger particles. The results of this study, in conjunction with the introduced apparatus for obtaining quantified drying rates, can allow the continued development of a more detailed understanding of the morphology of submicron salt particles, with the potential for the follow-on development of quantitative modeling of evaporation and crystal growth at these dimensions.

Global remote sensing of Trichodesmium

The maximum chlorophyll index (MCI) from the Medium Resolution Imaging Spectrometer (MERIS) satellite imager gives a robust indicator of the presence of a variety of floating slicks, near-surface vegetation, and intense surface plankton blooms. The index responds to the presence of surface slicks of Trichodesmium due to the ‘red edge’ increase in radiance with increasing wavelength near 700 nm. Global composite images of this index can be used to map the distribution of surface slicks of Trichodesmium, showing seasonal and long-term variations. The MCI also responds to the ‘red edge’ in the spectral signature of Sargassum. The two species share some areas of common occurrence and it is important to distinguish between them. We have developed spectral techniques of distinguishing between these two, and for discriminating a variety of other confusing targets that occur in different areas. We feel that MERIS MCI can be a useful tool in monitoring global Trichodesmium spatial distribution, and its short- and long-term variation.

Satellite observations of seeding of the spring bloom in the Strait of Georgia, BC, Canada

ABSTRACT We extend our previous observations of the spring bloom in the Strait of Georgia, Canada using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images, to include both a longer time period (2000–2016) and a wider variety of ‘seeding’ bloom events. We have reported on blooms in inlets to the north of the Strait, which appear to seed the main spring bloom in a characteristic ‘Malaspina Dragon’ pattern, leading to significantly earlier spring bloom dates in some years. Here, we double the time period of those observations. We find no clear repetitions of the earlier dragon pattern, but several instances of seeding from other inlets, with varying impacts on spring bloom timing. In situ monitoring has increased in coverage in recent years in the main body of the Strait, and we have deployed a fluorometer to record blooms at one inlet location, but observations of blooms in inlets rely mostly on satellites. We discuss the spatial and temporal resolution requirements imposed by the spatial scale of inlets and the temporal scale of blooms and cloud cover. In these waters, we find chlorophyll fluorescence images more useful than the standard chlorophyll product, restricting the value of NASA’s Visible Infrared Imaging Radiometer Suite (VIIRS) sensor and suggesting increased value from ESA’s Ocean and Land Colour Imager (OLCI) in the future, especially when two instruments are in orbit.