3. Continuous Measurements During climb or descent of the aircraft the radon concentration is not constant and it is no longer possible to determine the radon concentration directiy rrom the filter activity. Our method allows determination of the concentration profile by assuming that radioactive equilibrium remains fixed in the air. The variation of the number of atoms of each daughter present on the filter is equal to: d N, (+) dt where q is the air flow, N is the number of atoms of each type and A is the decay constant for each daughter. We assume that during the time interval from vo to t that the quantity of each element reaching the filter is constant and that the equilibrium remains After integration: N5(%) = Ad No (to) + B, Qo(t) ( N2(t) = Az Nz (ty) + Bz No{tg) + C3 Qo(t) N,(%) = A, N, (to) C: ) te established. ~~ jar = Q;(t) QtAi_y] N;_, (+) AG N;(t) + By N3( tg) + C, No(to) + D, Q(t) where A, B, C, and D are the constant coefficients for the time interval determined from the initial equations. The alpha activity of the filter during the time interval from by to t iss RY (t) = Kap No(t) +A, NC) where K is a coefficient allowing for the difference in detection efficiency for polonium-218 and polonium214. The radon concentration may thus be written: : Q2(%) = EEF1 Ago Fal) = MRabaN a( ty) FAYAN, (to) FAGBANGCEa) tou + CyuNalto) Knowing that at the start of sampling ty = 0, No(0) = 0, N2(0) = 0, and N, 69) = 0, (@