Tomas Rosicky's double earned 10-man Arsenal its sixth win in the past seven Premier League games -- 2-1 at West Brom -- and increased the pressure on Chelsea and Tottenham in the battle for a Champions League spot.
Rosicky broke the deadlock at the Hawthorns with his first league goal of the campaign and then added a second early in the second period.
Victory enabled Arsene Wenger's side to climb above Chelsea into fourth place, and Arsenal is now only a point behind Spurs with its two main rivals both in action Sunday.
"I am very pleased, it is three big points," Wenger said. "We have 22 points out of the last 27 but we know our target is to continue to do that until the end of the season. We know we have one target to win our games, then we don't have to look at Tottenham or Chelsea. If we lose our games, then you have to look at the results of the others. Our attitude is simple, we have to win the games.
"Can we finish fourth? I believe we will do it. We are conscious as well we have to give 100 percent commitment in every game and it will be difficult and edgy to the end. We are ready for the fight and we always tell the players 'don't look at everyone else, just focus on ourselves.' "
Arsenal was a worthy winner despite the continued absence of injured duo Jack Wilshere and Theo Walcott and have the momentum -- and no European distractions, unlike its main competitors -- entering the final phase of the season.
Wenger confirmed after Saturday's game that Wilshere and Walcott will be available for next weekend's clash with Norwich.
Albion only came to life after the introduction of substitute striker Romelu Lukaku and Mertesacker's red card, but the Baggies are comfortably placed in mid-table.
Arsenal needed a goal-line clearance from Rosicky to prevent Albion from taking the lead after 12 minutes. Chris Brunt's corner picked out Claudio Yacob's run to the near post, and his glancing header beat Lukasz Fabianski only for Rosicky to head away.
The Gunners did not look convincing at the back and Long created an opening from himself after cutting inside, but he failed to hit the target.
But after 20 minutes, Arsenal broke the deadlock through Rosicky's first Premier League goal of the campaign. Arteta's long pass was seized upon by Gervinho, who turned past Gareth McAuley before crossing for Rosicky, who headed home at close range.
West Brom tried to retaliate and Fabianski went down at the foot of a post to smother a Chris Brunt free kick.
But Arsenal was dominating the game and Gervinho extended Foster with a low shot on the turn.
Ramsey should have doubled Arsenal's lead two minutes before halftime but he poked the ball wide of the far post after good play by Giroud and Cazorla.
The second half continued in the same vein and Foster blocked a shot from the overlapping Bacary Sagna after being teed up by Gervinho.
After 51 minutes, Rosicky doubled Arsenal's lead with his second goal of the game.
Ramsey's pass was volleyed by Rosicky with plenty of power and Foster was unable to hold the shot. Rosicky was the first to react to the rebound and made no mistake from eight yards out.
"Tomas was fantastic. He is an important player for us. Unfortunately he was out injured for a long time," Wenger said. "But every year he gives us a push at the end of the season. I hope he finishes his career with us. I rate him that highly."
Koscielny was booked for bringing down Long, but the Gunners were in control of the game and Lukaku's weak free kick straight at Fabianski summed up Albion's performance.
There was a rare moment of danger for the Gunners when Fabianski blocked Brunt's near-post effort.
Albion was given a lifeline after 71 minutes when last defender Mertesacker was sent off for bringing down Long in the box, and Morrison converted the resulting penalty.
Wenger took off Gervinho and brought on Thomas Vermaelen to replace Mertesacker.
Albion came close to leveling was when McAuley headed just wide from a Brunt corner. Lukaku curled a shot just wide and Long fired over the bar, but Arsenal held on.
Information from Press Association was used in this report.