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Aerosol Life Cycle
Aerosols are short-lived climate forcers. They originate from a wide range of sources, evolve quickly through mixing and processes that are dependent on chemistry and meteorology, and are regularly removed from the atmosphere by precipitation and settling. This creates a highly heterogeneous distribution of aerosol in space and time, leading to difficulty in understanding aerosol properties and radiative impacts on climate. Depending on their properties and the environmental conditions in which they reside, aerosols can cool the surface or warm the atmosphere, thereby driving changes in circulation patterns, and can also modify the microphysical properties of clouds which further affects circulations. Aerosols have historically been the largest source of uncertainty in model predictions of future climate both in their clear-sky radiative effects and their impacts on cloud development and radiative properties.
The primary objective of ASR's Aerosol Life Cycle Working Group research is to understand and quantify the processes associated with the aerosol life cycle, the direct impact of aerosols on the Earth’s radiative balance, and the nature and distribution of cloud condensation nuclei (CCN) with the goal of improving their representations and thereby reducing the uncertainty in global and regional climate simulations and projections.
To this end, the working group addresses integrated chemical, physical and radiative processes from emissions, nucleation, transport, and aging to removal. We seek an understanding of the impact of these processes on the spatial and temporal distributions of global aerosol, the natural versus anthropogenic attribution of aerosol, and the relationship among physicochemical, cloud activating, and optical properties of aerosol. To understand and efficiently represent these processes at all pertinent scales, the ALWG will employ in situ and remote sensing observations from surface-based, airborne and satellite platforms from the process-level to the global scale, together with laboratory studies and modeling efforts.
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.
- Number publications have originated from researchers associated with the ALWG since its inception in 2009.
- ALWG members have led several field campaigns focused on different aspects of aerosol properties and processes including most recently the Biomass Burning Observation Project (BBOP), an airborne campaign that sampled near-source forest and agricultural fire plumes in the US west and southeast, and the Green Ocean Amazon (GoAmazon) campaign which is providing information on how aerosols, along with changes to heat and energy at the surface, influence cloud cycles under clean conditions in Amazon basin and under pollutant outflow from a tropical megacity.
- ASR scientists have proposed two conceptually new approaches that tackle the problem of modeling particle formation by looking at the process as a series of acid-base chemical reactions that now include interactions with amines and ammonia. By identifying key steps in the series, these approaches can quantitatively predict formation rates and concentrations of newly formed particles, while keeping the computational cost low enough to be suitable for inclusion in large-scale atmospheric models.
- ASR funded projects provided the first molecular elucidation and quantification of a direct and ubiquitous source of extremely low volatility organic compounds (ELVOCs) produced in the gas phase from oxidation of monoterpenes and other VOCs. This source is capable of explaining nanoparticle growth in boreal regions, and a significant fraction of low-volatility SOAs that are currently missing, or poorly described, in atmospheric models. These new insights help to more accurately quantify the effects of biogenic VOC emissions on new particle formation, abundance of cloud condensation nuclei, and potential related climate feedbacks.
- ASR researchers have developed new aerosol modules and models, such as the MOdel for Simulating Aerosol Interactions and Chemistry (MOSAIC) that better represent the aerosol lifecycle when evaluated using laboratory and ARM measurements. MOSAIC is now part of several community models, including the Weather Research and Forecasting (WRF) model and is currently being implemented in the DOE Accelerated Climate Model for Energy (ACME).
- ASR funding supported development of a quantitative mixing state metric needed to bring single particle measurements and modeling together into a common framework to better understand the importance of mixing state on aerosol optical and nucleating properties.