The correct answer is A (1.5 m/s wind speed, Pasquill Class F stability) because this combination produces the least atmospheric dispersion , resulting in the highest downwind concentration of a toxic release.

CCPS explains that atmospheric dispersion depends heavily on wind speed and atmospheric stability class . Pasquill Class F represents very stable atmospheric conditions (typically nighttime with low wind), where vertical and lateral mixing is minimal. This causes released material to remain concentrated in a narrow plume rather than dispersing.

Low wind speed ( 1.5 m/s ) further reduces dilution because the plume moves slowly and experiences limited mixing with surrounding air. As a result, contaminants remain more concentrated over a longer distance .

In contrast, higher wind speeds ( 5 m/s ) and more neutral conditions ( Class D ) promote turbulence and mixing, which dilute the plume and reduce concentration. Even though option C also uses Class F stability, the slightly higher wind speed (3 m/s) results in more dispersion than in option A.

CCPS

emphasizes that worst-case toxic dispersion scenarios typically assume low wind speed and stable atmospheric conditions , as these produce the highest potential exposure levels and are critical for emergency planning and risk assessment.