Transport of methyl bromide in Salinas Valley, California and its implications to human health risk assessment

The agricultural fumigant Methyl Bromide (MeBr) is an ozone depleting substance, and hence, it is banned from use. It is also a neurotoxicant, and a suspected developmental and reproductive toxicant.; After optimizing the MeBr analytical method (LOD = 48 ng.m−3 ) air sampling was conducted at 11 sites located on the adjacent mountains, and valley floors in Salinas Valley, California from September 1–4, 1995. The average concentrations ranged up to 8.98 μg m−3 .; Literature survey provided emission strengths required for ISCST3 (Industrial Source Complex Short Term 3) and CALMET/CALPUFF modeling. CALPUFF was driven by 3D meteorology from CALMET while ISCST3 assumed uniform weather over the modeling domain.; ISCST3 model successfully reproduced the diuranality in concentration patterns, transport of MeBr along Gabilan ranges and along the valley floor during day and nighttime, respectively. ISCST3 was unsuccessful in predicting the concentration trends at the receptors. ISCST3 mostly underpredicted, but within an order of magnitude of the measured concentrations. Sensitivity studies point at inadequate representation of valley meteorology and poor estimation of source strengths.; CALMET meteorological model resolved the vertical atmospheric structure, in addition to reproducing the sea breeze, terrain blocking and channeling effect, the up and down slope flows, and drainage flows. Predicted wind speeds and mixing heights were higher than the observed.; CALPUFF reproduced some main features of the measured concentration patterns, but mostly underpredicted the concentrations to within an order of magnitude. CALPUFF was unable to reproduce the concentrations trends at individual receptors. Sensitivity test indicated low source strengths. In comparison with ISCST3, CALPUFF's performance was slightly better.; The estimated exposure risk to the inhabitants of Salinas Valley was acceptable (MOE = 190). Based on the dispersion models-predicted worst case concentration, the MOE was between 95 and 119 indicating a need for increased attention to the risk of MeBr exposure.