The role of deposition in limiting the hazard extent of dense-gas plumes

Abstract

Accidents that involve large (multi-ton) releases of toxic industrial chemicals and form dense-gas clouds often yield far fewer fatalities, casualties and environmental effects than standard assessment and emergency response models predict. This modeling study, which considers both dense-gas turbulence suppression and deposition to environmental objects (e.g. buildings), demonstrates that dry deposition to environmental objects may play a significant role in reducing the distance at which adverse impacts occur--particularly under low-wind, stable atmospheric conditions which are often considered to be the worst-case scenario for these types of releases. The degree to which the released chemical sticks to (or reacts with) environmental surfaces is likely a key parameter controlling hazard extents. In all modeled cases, the deposition to vertical surfaces of environmental objects (e.g. building walls) was more efficient in reducing atmospheric chemical concentrations than deposition to the earth's surface. This study suggests that (1) hazard extents may vary widely by release environment (e.g. grasslands vs. suburbia) and release conditions (e.g. sunlight or humidity may change the rate at which chemicals react with a surface) and (2) greenbelts (or similar structures) may dramatically reduce the impacts of large-scale releases. While these results are demonstrated to be qualitatively consistent with the downwind extent of vegetation damage in two chlorine releases, critical knowledge gaps exist and this study provides recommendations for additional experimental studies.

Cl2 deposition on soil matrices

“…Of this, 12 metric tons will react quickly ([A] 2 is the faster-reacting content). This is consistent with the conclusions from modeling studies for a stable atmosphere and low-wind conditions [11,20,21].…”

“…In addition, note that the effective rate of Cl − appearance is initially fast and slows as [Cl 2 ]t is increased. This demonstrates that a constant deposition velocity for Cl 2 cannot be universally applied to large-scale releases, though constant deposition velocities were used in prior modeling studies [11,20,21]. Second, the inset to Fig.…”

“…velocity-the density of Cl 2 is 2.5 times higher than air [10]-though this is disputed by the assumptions in a recent modeling study [11].…”

Gaps in toxic industrial chemical model systems: Improvements and changes over past 10 years

“…What has happened since 2008 and 2010, and are there still knowledge gaps in estimating removal processes? The Dillon 56 comprehensive review of deposition modeling approaches and evaluation histories for dense TIC plumes has been very useful. Limited deposition experiments were carried out as part of JR I and a few results of the analysis of these data and some laboratory data have been presented 57–59 .…”

Assessment of an irritant gas plume model for epidemiologic study

“…A multitude of sensors and prepared artificial deposition matrices surrounding the release site are utilized to analyze the dispersion, transport and interaction of Cl2 and other gases post-release. This area of research has also expanded to account for deposition beyond greenbelts, such as on structures and other manmade objects (Dillon, 2009; Jonsson et al, 2005). In regards to greenbelts (areas of vegetation in and around urban development), there has been some research performed in the area of phytotoxic injury as a result of chlorine exposure, including the use of phytotoxic indicators during post-accident investigation (Griffiths and Smith, 1990; Khan and Abbasi, 2001).…”