How does wind affect the ambient radiation level ?

Air from the continent contains more natural radioactive material (which may be gaseous) released from rock and soil when compared to that from the ocean. When the wind is northerly, it brings the natural radioactive material from upwind land areas to Hong Kong, raising the ambient radiation level (see case 2 given here). On the contrary, when the wind is from the south, there is much less natural radioactive material over the ocean and the ambient radiation level is lower.