Thanks to Gabriel Martinelli's absence with a foot injury, Trossard reprised his role on the left-hand side of the Arsenal attack, joining Kai Havertz and Bukayo Saka in an untouched front three.
Three weeks on from a 1-0 first-leg loss, where they failed to have a single shot on target, Arsenal once again struggled to unlock a compact Porto defence, marshalled by 41-year-old Pepe.
Joao Mario and Wendell were also earning their keep out wide — the latter did a magnificent job of neutralising Saka — but with 41 minutes gone, Trossard got the Gunners back on terms.
A neat passing sequence with Martin Odegaard saw the Gunners captain execute a perfect through ball to the Belgian, who kept his composure to pick out the far corner in front of the away end.
Trossard's effort at the Emirates Stadium saw the 29-year-old score for the third Champions League home game running, following previous strikes against PSV Eindhoven and Sevilla in the group stage.
Trossard was an unused substitute during the Gunners' 6-0 thrashing of Lens in November, meaning that he is now just the third player in Arsenal's history to score in each of their first three Champions League home games for the club.
Current teammate Saka was the second player to achieve the feat for the Gunners by netting against all of PSV, Lens and Sevilla in the group phase, eight years after Alexis Sanchez became the first Arsenal man to do so.
The Chilean attacker — now with Inter Milan at the age of 35 — netted against Galatasaray, Anderlecht and Borussia Dortmund in the 2014-15 group stage, before
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