oaete a peene ee ee te tereee we ee ee re ge ee eee te eeee te ee ENVIRONMENT | han a constant-velocity wind the measurementstations, and dot beginning of spill 1. The crosses in- The ATMAScode models atmospheric transport by solving the advection-diffusion equation, which simply states that the con- tions. Figure 2 shows a contour plot of the concentration isopleths 1 m above the ground 60s after the this experiment, the wind carried model. plots of marker-particle distribu- centration of pollutants at a given 400 point is determined by two I dicate the measurementstations.In muchof the plume to theleft of the detector array. | | processes: diffusion (the simple spreading that takes place without wind and the effects of gusts and eddies) and advection (blowing away in the average wind). ATMAS uses the particle-in-cell method,° which represents the mass of the emitted pollutant by marker particles whose trajectories through space are calculated according to _an equation of motion derived from the advection-diffusion equation. The array of marker particles gives us a graphic threedimensional representation of the vapor plume. To use the ATMAScode, we must specify boundary conditions, the characteristics of the LNG vapor source, and measuredvalues of wind speed and direction as a function of time. Forthe first three experiments we assumed a constant emission rate from a round LNG pool of constant size for a fixed period of time. In the fourth 0.585 g/m® 300 + | ~ E i 8 200 - — a 2 — 100 }- 29.2 g/m? test, because of significant changes 87.7 g/m" in wind velocity, we found it necessary to simulate the way in which the LNG pool spreads over 175 g/m? 0 the water surface. We did this by turning on a series of vapor sources with successively larger areas from time to time during the calculation. The various 5-m* tests are described in Table 1. The ATMAScode displays the x 0 i . 50 D | 100 Distance — m | 150 ; 200 Concentration isopleths calculated by the Oi, ATMAScode for the vapor plume 60 s after the start of thefirst 5-m’ spill experiment. The lower flammability limit (33 g/m? at 300 K) eventually extended to about 380 m. results of its calculations in contour plots of gas concentration, graphs of concentration vs time for each of 31