In a cross-flow filtration process, the pressure drop $\left ( \Delta P \right )$ driving the fluid flow is $2$ atm, inlet feed pressure $\left ( P_{i} \right )$ is $3$ atm and filtrate pressure $\left ( P_{f} \right )$ is equal to atmospheric pressure. The average transmembrane pressure drop $\left ( \Delta P_{m} \right )$ is ___________ atm.