Apparatus and method for detecting presence of an analyte in ambient air. The apparatus comprising a valve for admitting ambient air, a line for conveying the air to a sensing element, a housing for isolating the environment around the sensing element, an inlet port for flowing the air tho the sensing element in a turbulent flow in a direction substantially perpendicularly to the sensing element, an outlet port for conveying the air from the sensing element, and a pump in communication with the ...