Review of Particulate Dry Deposition Models Currently Implemented in Atmospheric Dispersion Models

Atmospheric dispersion models are widely used obtaining deposition estimates for planning, emergency preparedness, real-time response, and post-event evaluations. Deposited materials pose potential hazards from both exposures to hazardous chemicals as well as radioactive materials. Such releases have similar potential inhalation and ingestion pathways. Radioactive materials have the additional hazard of radioactive shine from the deposited material. A reliable characterization of these potential deposition exposures is critical for management and mitigation of these hazards.

A review of the current status of dry deposition formulations for particulate matter used in currently deployed atmospheric dispersion models is presented. The review considers atmospheric models used in national/regional emergency response systems (such as the Interagency Modeling and Atmospheric Assessment Center (IMAAC)), and standalone emergency response models. The review provides conclusions and recommendations relative the adequacy of the current model formulations.

The review concludes that dry deposition formulations need to consider the full range of particle sizes including: 1) the accumulation mode range (0.1 to 1 micron diameter) and its minimum in deposition velocity, 2) smaller particles (less than .01 micron diameter) deposited mainly by molecular diffusion, 3) 10 to 50 micron diameter particles deposited mainly by impaction and gravitational settling, and 4) larger particles (greater than 100 micron diameter) deposited mainly by gravitational settling. The effects of the local turbulence intensity, particle characteristics, and surface element properties must also be addressed in the formulations.

Specific areas identified for improvements in the dry deposition formulations are 1) capability of simulating near-field dry deposition patterns, 2) capability of addressing the full range of potential particle properties, 3) incorporation of particle surface retention/rebound processes, and 4) development of dry deposition formulations applicable to urban areas. To improve dry deposition modeling capabilities, atmospheric dispersion models in which the dry deposition formulations are imbedded need better source-term plume initialization, improved in-plume treatment of particle growth processes, and better characterization of the properties of the receptor surfaces,

A major area where dry deposition modeling improvement is needed for national response applications is for the urban environment. Particulate dry deposition formulations used in current models were developed for much-less complex surface environments – that are largely inapplicable to the complex urban environment. An improved capability is urgently needed to provide surface-specific information to assess local exposure hazard levels in both urban and non-urban areas on roads, buildings, crops, rivers, etc.

A particulate dry deposition model improvement plan is described with a near-term and far-term component. Despite some conceptual limitations, it is concluded that the current formulations for particle deposition based on a resistance approach have proven to provide reasonable dry deposition simulations. For many models with inadequate dry deposition formulations, adding or improving a resistance approach will be the desirable near-term update. Resistance models however are inapplicable aerodynamically very rough surfaces such as urban areas. However it is also concluded that in the longer term an improved parameterization of dry deposition needs to be developed that will be applicable to all surfaces, and in particular urban surfaces.