Direct measurements (specifically the front-to-back ratio) of alpha radioactivity from radon progeny deposited on the front and back of a 400-wire mesh screen were performed under realistic aerosol conditions outdoors with two scintillation alpha detectors facing both sides of the screen simultaneously. Two flow regimes were tested: one which kept the flow Reynolds number (Ref) for the wire below one (200 L min-1) and a second with the flow Reynolds number for wire > 1 (400 L min-1). To estimate the radioactivity loss in the screen, parallel measurements of open and backup filters were also performed. The average front-to-back ratio expressed as a ratio of the potential alpha energy concentration as measured on the screen side facing the airflow to the other side of the screen for 200 L min-1 and 400 L min-1 was 3.42 +/- 0.32 (standard error) and 2.30 +/- 0.14 (standard error), respectively. The loss of radioactivity in the screen expressed as a ratio between the potential alpha energy concentration measured on an open filter minus backup filter to the sum of the potential alpha energy concentration measured on the front and back of the screen was 1.29 +/- 0.13 and 1.27 +/- 0.22 for 200 and 400 L min-1, respectively.