in the atmosphere, in simple or complex terrain. PLUVUEII – A model that estimates atmospheric visibility degradation and atmospheric discoloration caused by plumes resulting from the emissions of particles, nitrogen oxides, and sulfur oxides. The model predicts the transport, dispersion, chemical reactions, optical effects and surface deposition of such emissions from a single point or area source. SCIPUFF – A puff dispersion model that uses a collection of Gaussian puffs to predict three-dimensional, time-dependent pollutant concentrations. In addition to the average concentration value, SCIPUFF predicts the statistical variance in the concentrations resulting from the random fluctuations of the wind. SDM – Shoreline dispersion model (SDM) is a Gaussian dispersion model used to determine ground-level concentrations from tall stationary point source emissions near a shoreline. SLAB – A model for denser-than-air gaseous plume releases that utilizes the one-dimensional equations of momentum, conservation of mass and energy, and the equation of state. SLAB handles point source ground-level releases, elevated jet releases, releases from volume sources and releases from the evaporation of volatile liquid spill pools. Screening models These are models that are often used before applying a refined air quality model to determine if refined modelling is needed. AERSCREEN – The screening version of AERMOD. It produces estimates of concentrations, without the need for meteorological data, that are equal to or greater than the estimates produced by AERMOD with a full set of meteorological data. The U.S. EPA released version 11060 of AERSCREEN on 11 March 2010 with a subsequent update, version 11076, on 17 March 2010. The U.S. EPA published the "Clarification memorandum on AERSCREEN as the recommended screening model" on 11 April 2011. CTSCREEN – The screening version of CTDMPLUS. SCREEN3 – The screening version of ISC3. TSCREEN – Toxics screening model (TSCREEN) is a Gaussian model for screening toxic air pollutant emissions and their subsequent dispersion from possible releases at superfund sites. It contains 3 modules: SCREEN3, PUFF, and RVD (Relief Valve Discharge). VALLEY – A screening, complex terrain, Gaussian dispersion model for estimating 24-hour or annual concentrations resulting from up to 50 point and area emission sources. COMPLEX1 – A multiple point source screening model with terrain adjustment that uses the plume impaction algorithm of the VALLEY model. RTDM3.2 – Rough terrain diffusion model (RTDM3.2) is a Gaussian model for estimating ground-level concentrations of one or more co-located point sources in rough (or flat) terrain. VISCREEN – A