nucleus

Towards a comprehensive understanding of the impact of natural aerosols on clouds

Cloud processes are of particular importance for the evolution of weather and climate, as they regulate the global distribution of precipitation further influencing the hydrological cycle, and affecting the Earth’s radiative budget. Atmospheric aerosol particles can serve as cloud condensation nuclei (CCN) and ice nuclei particles (INP) exerting a large influence in cloud properties. One of the foremost reasons for the large uncertainty associated to aerosol-cloud interactions is a lack of a broad knowledge of particle sources and how particles evolve to become effective CCN and INP. Therefore, the starting hypothesis of NUCLEUS is that natural aerosols can serve as seeds for cloud droplets and trigger ice crystals formation depending on their origin (chemical composition, size and morphology) and the transformation processes that these particles undergo in the atmosphere. To shed light in this scientifically sound topic, we will investigate the role of natural aerosols (pollen and dust) in cloud formation from an experimental point of view using in-situ techniques. We will use a 3-stage approach:

In -measurements of cloud residuals
Continuous measurements of CCN and INP concentration and their characterization during cloud-free conditions
Laboratory experiments to investigate aerosol-generated samples of atmospheric aerosols of natural origin.

To investigate ice-nucleating particles we will build an automatic aerosol sampler and a Droplet Freezing Assay (DFA). The measurements will be performed in an extremely sensitive ecosystem to climate change, Sierra Nevada National Park, and will led to a major breakthrough in our understanding of the impact of natural aerosols on clouds and climate.

objectives

The general objective of NUCLEUS is to determine the role of natural particles (dust and biogenic particles) on cloud formation

Objective 1

To determine the ability of natural aerosols to activate in real clouds

Objective 2

To investigate the impact of dust in the CCN activation process

Objective 3

To estimate the efficiency of primary biogenic and dust particles to act as IN

Objective 4

To improve INP parameterizations based on ancillary aerosol information

work packages

In-cloud characterization of cloud residuals

Most studies investigating cloud condensation nuclei and ice nuclei measure CCN and INP by artificially activating the aerosol particles. However, it is possible to study CCN and INP properties directly inside clouds by measuring the so-called cloud residuals that remain when cloud droplets and ice crystals are dried. This can be achieved with a counterflow virtual impactor (CVI) inlet which enables the separation of cloud particles from unactivated aerosol particles. Since the CVI can measure both water and ice particles, cloud residuals may correspond to either CCN, INP, or results from in-cloud processes (e.g. impaction scavenging or secondary ice). Such measurements are very rare and there are only a handful of GCVI deployed worldwide.

In this WP we will deploy a GCVI inlet in Sierra Nevada, characterize its performance and measure cloud residuals properties.

Build-up of an INP laboratory

Measurements of INP are scarce due to the complexity of the measurement technique and sampling methods. The standard offline method consists in sampling particles in a substrate, and subsequent offline analysis of INP activation. This latter process starts with detaching the particles from the filter and then exposing the droplets to a cooling ramp observing the number of frozen droplets as a function of temperature (INP spectra).

In this WP we will construct our own INP laboratory based on a Droplet Freezing Assay (DFA) which is the most common offline method for INP spectra determination.

Ice nucleating ability of natural aerosol

Atmospheric ice particles play a dominant role in determining the physical properties of clouds and the chemical composition of the troposphere by exerting enormous influence on physical processes such as radiative transfer, precipitation, and cloud electrification. In mixed-phase clouds the role of ice crystals is particularly important because ice can influence the supercooled liquid water content through the Wegener–Bergeron–Findeisen process. Most precipitation in clouds initiates via the ice phase especially over land, which significantly influences the hydrological cycle and determines cloud lifetime. To predict the impact of the above processes and constrain estimates of the cloud radiation budget, it is imperative to understand the initiation and evolution of ice formation in the atmosphere.

Within this WP we will use the DFA chamber to investigate the role of natural aerosols as ice nuclei particles in the atmosphere, based on ambient as well as on laboratory measurements.