Occurrence of pristine aerosol environments on a polluted planet

Hamilton, Douglas S.; Lee, Lindsay; Pringle, K. J.; Reddington, Carly L.; Spracklen, Dominick V.; Carslaw, K. S.

doi:10.1073/pnas.1415440111

Cited by 165 publications

(197 citation statements)

References 54 publications

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“…Therefore, a direct anthropogenic influence on the marine aerosol upwind of and in the Caribbean, brought on by man-made fires, cannot be completely excluded. This is in line with results reported in Hamilton et al (2014), where it is stated that pristine aerosols (i.e., those free of anthropogenic influence) on Earth are largely only found in the Southern Hemisphere, while these pristine environments are only found temporarily and with a spatially patchy distribution in the Northern Hemisphere. Information on particle composition derived from our data suggests that the majority of particles in the CCN size is made up of sulfates, and these particles mostly originated from new particle formation.…”

Section: Discussionsupporting

confidence: 92%

“…Aerosols arriving on Barbados often have been considered clean and/or typical marine due to the incoming air masses being free of anthropogenic influences for thousands of kilometers prior to arrival. But we will show that even this aerosol is not free of continental, and hence possibly anthropogenic, influences, as already suggested by Hamilton et al (2014), who claim that pristine atmospheric aerosols today are found mainly only in the Southern Hemisphere and only very rarely in the Northern Hemisphere.…”

supporting

confidence: 77%

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Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

et al. 2016

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Abstract. The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between groundbased and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (N total ) ranged from as low as 100 up to 800 cm −3 , while number concentrations for cloud condensation nuclei (N CCN ) at a supersaturation of 0.26 % ranged from some 10 to 600 cm −3 . N total and N CCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both N total and N CCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for N CCN below 200 cm −3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive N CCN from measured number size distributions, showing that this is a valid approach to obtain N CCN . Although the total particulate mass sampled on filters was found to be dominated by Na + and Cl − , this was found to be contributed by a small number of large particles (> 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to N total and up to 15 % to N CCN . This is in accordance with finding no correlation between N total and wind speed.

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Section: Discussionsupporting

confidence: 92%

supporting

confidence: 77%

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

et al. 2016

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“…Many key portions of the globe remain undersampled, for example, the Southern Ocean, the maritime and continental tropics, much of Asia, and both polar regions. In an effort to replicate preindustrial aerosol−cloud relationships, it is helpful to study those regions of the present-day atmosphere that approximate preindustrial conditions (71). In such regions, clouds are particularly susceptible to small increases in CCN levels (72); the remote Southern Ocean is a prime example.…”

Section: Observations Of Aerosol−cloud Relationshipsmentioning

confidence: 99%

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

Seinfeld

Bretherton

Carslaw

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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610

474

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The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth's clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol−cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol−cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.climate | aerosol−cloud effects | general circulation models | radiative forcing | satellite observations Clouds play a key role in Earth's radiation budget, and aerosols serve as the seeds upon which cloud droplets form. Anthropogenic activity has led to an increase in aerosol particle concentrations globally and an increase in those particles that act as cloud condensation nuclei (CCN) and ice nucleating particles (INP). The effect of an increase in aerosols on cloud optical properties, and associated radiative forcing, is the most uncertain component of historical radiative forcing of Earth's climate caused by greenhouse gases (GHGs) and aerosols. The Intergovernmental Panel on Climate Change (IPCC) AR5 assessment of climate forcing factors (Fig. S1) ascribes "high" confidence to the estimate of direct aerosol radiative forcing (mean

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“…These episodes have been defined according to Moran-Zuloaga et al (2017). 1 The question to what extent the Amazonian wet season approximates a pristine and pre-industrial state, which is unaffected by human activities, has been actively debated over the last 30 years (Andreae, 2007;Martin et al, 2010b;Hamilton et al, 2014). Although the wet season is free from major pollution influence, there is evidently a persistent background pollution in varying states of dilution.…”

Section: Aerosol and Ccn Time Series For Representative Wet Season Comentioning

confidence: 99%

Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 2: Variability and characteristic differences under near-pristine, biomass burning, and long-range transport conditions

Pöhlker

Ditas

Saturno

et al. 2017

Preprint

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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 para-5 metrization 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: ), 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. Atmos. Chem. Phys. Discuss., https://doi

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Occurrence of pristine aerosol environments on a polluted planet

Cited by 165 publications

References 54 publications

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 2: Variability and characteristic differences under near-pristine, biomass burning, and long-range transport conditions

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