Terminal velocity ($v_t$) is the velocity at which the gravitational downward force equals the aerodynamic drag force on a free-falling particle in still air. For vertical pneumatic conveying, the upward conveying air velocity ($v_{\text{air}}$) must be strictly greater than the particle's terminal velocity ($v_{\text{air}} \gt v_t$) to generate net positive aerodynamic lift. Thus, the terminal velocity of the material is lower than the air velocity.

Option analysis:

• Lower: Correct, particle terminal velocity must be lower than air velocity to allow upward transport.
• Higher: If terminal velocity exceeded air velocity, particles would settle and choke the pipe.
• Equal: The particles would remain stationary/suspended in mid-air (choking threshold).
• Zero: Physical particles have non-zero terminal velocity.