Complex sentence: Passenger volumes remained lower than what Dorval Airport could handle when renovated and when the decision was finally made to consolidate passenger traffic at one airport, Dorval was chosen, and Mirabel was relegated to the role of a cargo airport.
Split into simpler sentences: Passenger volumes remained lower than what Dorval Airport could handle when renovated. When the decision was finally made to consolidate passenger traffic at one airport, Dorval was chosen, and Mirabel was relegated to the role of a cargo airport.