In aerosol and air-filtration science, the collection efficiency due to the interception mechanism increases with which operating or particle parameter(s)? Provide the most appropriate factor based on physical reasoning.

Introduction:
Interception is one of the three principal capture mechanisms in fibrous filters (along with inertial impaction and Brownian diffusion). It occurs when a particle closely follows the streamline but still contacts the fiber because its finite radius causes it to ‘‘touch’’ the collector. Understanding what increases interception efficiency helps in designing air filters, respirators, and bioprocess air-handling systems where particle removal is critical.

Given Data / Assumptions:

• Lamina-like flow near fibers is assumed (low local Reynolds number).
• Particles are non-evaporating, rigid spheres for conceptual clarity.
• Filter structure (fiber diameter, packing) remains unchanged during comparison.

Concept / Approach:
Interception efficiency depends strongly on the interception parameter R = d_p / d_f, where d_p is particle diameter and d_f is fiber diameter. Larger particles (higher d_p) at the same streamline proximity have a higher chance of contacting the fiber surface. Air velocity mainly influences inertial impaction (through Stokes number) rather than pure interception; for interception, velocity plays a weaker, secondary role when flow remains in the creeping regime around fibers.

Step-by-Step Solution:

Verification / Alternative check:
Classic fibrous filter models (e.g., Lee–Liu, Hinds) show interception terms scale with R; experiments confirm higher removal for larger particles via interception when diffusion and impaction are controlled.

Why Other Options Are Wrong:

• Increasing air flow velocity: Primarily enhances impaction, not pure interception.
• Both (a) and (b): Overstates the role of velocity for the interception mechanism.
• Increasing particle size (option d): This duplicates option (a); the correct idea is captured by (a).

Common Pitfalls:
Confusing impaction (inertia-dominated, velocity dependent) with interception (geometry dominated). Also, overlooking fiber diameter—the same particle collected more efficiently by interception with thinner fibers due to higher R.

Final Answer:
increasing particle diameter