To meet these objectives, the ALWG has established concentrated efforts in the following areas. The foundation for each group is the data provided by the ARM User Facility used in close association with laboratory experiments and process- to global-scale modeling efforts.
New Particle Formation
Newly formed particles in the atmosphere can drive concentrations of CCN and cloud properties. The goal of this focus group is to develop models that accurately predict particle nucleation and growth rates and the composition of potential CCN, and to incorporate these into regional and global atmospheric models. Nucleation rates determine how many newly formed particles could potentially serve as CCN. Growth rates determine whether or not nucleated particles will actually reach CCN sizes before they are lost by coagulation with pre-existing particles. The chemical species that contribute to growth determine the composition, and thus character, of the CCN. Previous research has shown that both nucleation and growth rates are much higher that were predicted by models.
Aerosol Aging and Mixing State
Aerosol mixing state plays a role in determining the climate impact of aerosol particles through optical and cloud nucleating effects, however the significance of this role is currently an open question. This focus group studies the sensitivity of climate-relevant properties of particles to the aerosol mixing state and the level of complexity required to represent these properties in models. These goals are achieved by bridging what is learned from aerosol property measurements, process-centered focus groups, and the modeling community in order to improve the representation of aerosol mixing state in regional to global scale models.
Secondary Organic Aerosol Formation
Secondary organic aerosol (SOA) comprises a large fraction of the total submicron aerosol mass concentration of the Earth’s atmosphere. Models, however, have difficulty reproducing the observed SOA mass and number concentrations, size distributions, and other physical and chemical properties relevant to climate. A number of recent field, laboratory, and modeling studies have provided evidence that SOA production and properties from biogenic species may be strongly coupled to anthropogenic activities. The goal of this focus area is to bring together laboratory, field, and model understanding and research of SOA to improve large-scale model implementations of important anthropogenic-biogenic interactions, to improve representation of intrinsic properties such as refractive index and hygroscopicity that are relevant to the simulation of radiative balance and climate of the atmosphere.
Aerosol Direct Radiative Forcing
Each of the above processes results in an aerosol assemblage at any one place and time that has a distinct set of optical properties and vertical distribution. The radiative effect of the aerosol is dependent on these properties as well as the local environmental conditions such as relative humidity and surface reflectivity. The working group as a whole aims to characterize aerosol direct radiative forcing as a function of space and time by considering the unique geographical distributions of sources, processes, and environmental conditions across the globe.