Abstract: A conditional sampling technique for flux measurement involves measuring vertical velocity of the movement of air or gas, collecting samples of the air and its components in an "up" canister when the vertical velocity in the upward direction exceeds a threshold, and collecting samples in a "down" canister when the vertical velocity in the downward direction exceeds a threshold. Sensing intervals for measuring and collecting are repeated for a predetermined sampling time period. The passageways to the canisters are always either fully closed or fully opened. At the end of the sampling time period, the standard deviation of the vertical velocities is calculated, and the concentration of a particular component in the samples collected in the canisters is determined by chemical or other analysis.