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Mass accommodation and gas–particle partitioning in secondary organicaerosols: dependence on diffusivity, volatility, particle-phase reactions,and penetration depth

Mass accommodation is an essential process for gas–particle partitioning of organic compounds in secondary organic aerosols (SOA). The mass accommodation coefficient is commonly described as the probability of a gas molecule colliding with the surface to enter the particle phase. It is often applied, however, without specifying if and how deep a molecule has to penetrate beneath the surface to be regarded as being incorporated into the condensed phase (adsorption vs. absorption).

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and theirintEractionS) project: aerosol–cloud–radiation interactions in the southeastAtlantic basin

Southern Africa produces almost a third of the Earth’s biomass burning (BB) aerosol particles, yet the fate of these particles and their influence on regional and global climate is poorly understood. ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) is a 5-year NASA EVS-2 (Earth Venture Suborbital-2) investigation with three intensive observation periods designed to study key atmospheric processes that determine the climate impacts of these aerosols.

ORACLES (ObseRvations of Aerosols above CLouds and theirintEractionS) project: aerosol–cloud–radiation interactions in the southeastAtlantic basin">Read more

Opinion: Cloud-phase climate feedback and the importance of ice-nucleatingparticles

Shallow clouds covering vast areas of the world’s middle- and high-latitude oceans play a key role in dampening the global temperaturerise associated with CO2. These clouds, which contain both ice andsupercooled water, respond to a warming world by transitioning to a statewith more liquid water and a greater albedo, resulting in a negative“cloud-phase” climate feedback component. Here we argue that the magnitudeof the negative cloud-phase feedback component depends on the amount andnature of the small fraction of aerosol particles that can nucleate icecrystals. We propose that a concerted research effort is required to reducesubstantial uncertainties related to the poorly understoodsources, concentration, seasonal cycles and nature of these ice-nucleatingparticles (INPs) and their rudimentary treatment in climate models. Thetopic is important because many climate models may have overestimated themagnitude of the cloud-phase feedback, and those with better representationof shallow oceanic clouds predict a substantially larger climate warming. Wemake the case that understanding the present-day INP population in shallowclouds in the cold sector of cyclone systems is particularly critical fordefining present-day cloud phase and therefore how the clouds respond towarming. We also need to develop a predictive capability for future INPemissions and sinks in a warmer world with less ice and snow and potentiallystronger INP sources.

On the ice-nucleating potential of warm hydrometeors in mixed-phase clouds

The question as to whether or not the presence of warm hydrometeors in clouds mayplay a significant role in the nucleation of new ice particles has been debatedfor several decades. While the early works of and indicated that it might be irrelevant, the more recentstudy of suggested otherwise. In this work, weattempt to quantify the ice-nucleating potential using high-fidelity flowsimulation techniques around a single hydrometeor and use favorable considerationsto upscale the effects to a collective of ice particles in clouds. While we find that ice nucleationmay be significantly enhanced in the vicinity of a warm hydrometeorand that the affected volume of air is much larger than previously estimated, it isunlikely that this effect alone causes the rapidenhancement of ice nucleation observed in some types of clouds, mainly due to the lowinitial volumetric ice concentration. Furthermore, it is demonstrated that the excess nucleationrate does not primarily depend on the rate at which cloud volume is sampled by the meteors’wakes but is rather limited by the exposure time of ice-nucleating particles to the wake,which is estimated to be of the order of few microseconds. It is suggested to further investigatethis phenomenon by tracking the trajectories of ice-nucleating particles in order to obtaina parametrization which can be implemented into existing cloud models to investigate second-order effectssuch as ice enhancement after the onset of glaciation.

Stratospheric aerosol layer perturbation caused by the 2019 Raikoke and Ulawun eruptions and their radiative forcing

In June 2019 a stratospheric eruption occurred at Raikoke (48∘ N,153∘ E). Satellite observations show the injection of ash and SO2 into the lower stratosphere and an early entrainment of the plume into a cyclone. Following the Raikoke eruption, stratospheric aerosol optical depth (sAOD) values increased in the whole Northern Hemisphere and tropics and remained enhanced for more than 1 year, with peak values at 0.040 (short-wavelength, high northern latitudes) to 0.025 (short-wavelength, Northern Hemisphere average). Discrepancies between observations and global model simulations indicate that ash may have influenced the extent and evolution of the sAOD. Top of the atmosphere radiative forcings are estimated at values between -0.3 and -0.4Wm-2 (clear-sky) and of -0.1 to -0.2Wm-2 (all-sky), comparable to what was estimated for the Sarychev eruption in 2009. Almost simultaneously two significantly smaller stratospheric eruptions occurred at Ulawun (5∘ S, 151∘ E) in June and August. Aerosol enhancements from the Ulawun eruptions mainly had an impact on the tropics and Southern Hemisphere. The Ulawun plume circled the Earth within 1 month in the tropics. Peak shorter-wavelength sAOD values at 0.01 are found in the tropics following the Ulawun eruptions and a radiative forcing not exceeding -0.15 (clear-sky) and -0.05 (all-sky). Compared to the Canadian fires (2017), Ambae eruption (2018), Ulawun (2019) and the Australian fires (2019/2020), the highest sAOD and radiative forcing values are found for the Raikoke eruption.

Long-term deposition and condensation ice-nucleatingparticle measurements from four stations across the globe

Long-term ice-nucleating particle (INP) data are presented from four semi-pristine sites located in the Amazon, the Caribbean, Germany and the Arctic. Average INP concentrations did not differ by orders of magnitude between the sites. For all sites short-term variability dominated the time series, which lacked clear trends and seasonalities. Common drivers to explain the INP levels and their variations could not be identified, illustrating the complex nature of heterogeneous ice nucleation.

Tropical Pacific climate variability under solar geoengineering: impacts onENSO extremes

Solar geoengineering has been introduced to mitigate human-caused global warming by reflecting sunlight back into space. This research investigates the impact of solar geoengineering on the tropical Pacific climate. We find that solar geoengineering can compensate some of the greenhouse-induced changes in the tropical Pacific but not all. In particular, solar geoengineering will result in significant changes in rainfall, sea surface temperatures, and increased frequency of extreme ENSO events.

Optical characterization of pure pollen types using a multi-wavelength Raman polarization lidar

Measurements of the multi-wavelength Raman polarization lidar Polly ^XT have been combined with measurements of pollen type and concentration using a traditional pollen sampler at a rural forest site in Kuopio, Finland. The depolarization ratio was enhanced when there were pollen grains in the atmosphere, illustrating the potential of lidar to track pollen grains in the atmosphere. The depolarization ratio of pure pollen particles was assessed for birch and pine pollen using a novel algorithm.

The dual-field-of-view polarization lidar technique: a new concept in monitoring aerosol effects in liquid-water clouds – theoretical framework

A novel lidar method to study cloud microphysical properties (of liquid water clouds) and to study aerosol–cloud interaction (ACI) is developed and presented in this paper. In Part 1, the theoretical framework including an error analysis is given together with an overview of the aerosol information that the same lidar system can obtain. The ACI concept based on aerosol and cloud information is also explained. Applications of the proposed approach to lidar measurements are presented in Part 2.

Sensitivity of the Southern Hemisphere circumpolar jet response to Antarctic ozone depletion: prescribed versus interactive chemistry

Ozone depletion over Antarctica was shown to influence the tropospheric jet in the Southern Hemisphere. We investigate the atmospheric response to ozone depletion comparing climate model ensembles with interactive and prescribed ozone fields. We show that allowing feedbacks between ozone chemistry and model physics as well as including asymmetries in ozone leads to a strengthened ozone depletion signature in the stratosphere but does not significantly affect the tropospheric jet position.

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