B757-200 flight crew in-flight smelled an acrid electrical burning smell; they declared an emergency and diverted to the nearest suitable airport.

Narrative

At FL400 I smelled an acrid; burning electrical smell. I looked at the captain to see if he smelled the same thing - he concurred immediately and we both donned O2 masks and goggles. We ran the QRC for Smoke and Fumes and established crew communications. We then determined where we were going to go (sort of a coin toss between Option 1 and 2). Option 1 was behind us over the right wing and Option 2 was right off the nose; so we opted for the second - which at 85NM would give us a smooth 3:1 glidepath to landing while we coordinated the divert and ran the continuation of the checklist. We completed Smoke and Fumes for 'no determination of origin' considerations; turning off the utility busses and left recirc fan. We then selected Alternate on the Equip Cooling switch. We notified the flight attendants of our change in destination and to prep the cabin for landing (not prep for evacuation). We also asked them to determine if there was any smoke or fumes in the cabin; to which they responded; 'negative'. The Captain removed his mask briefly to see if the fumes had dissipated but he told me they were less but still present. I kept my mask and goggles in place throughout the descent and approach; removing them only after landing with the window cracked. We coordinated the divert with ATC and dispatch (via ACARS messages) and declared an emergency. All checklists; including Approach/Descent and Final Descent were completed in a timely manner. We were cleared direct to the visual approach fix where the weather was better than 5000 ceiling and 10+NM visibility. We kept our speed at 320 KIAS until 10 miles from the fix and then slowed and configured normally. Normal visual approach and landing and rollout followed. We turned off at first available taxiway; where fire trucks were waiting and had the crews check us for any exterior signs of smoke or fire. We also asked them to look in the area under the cockpit; near the E&E bay. They reported no smoke or fire and negative on the IR scan for heat sources. We opted to taxi in to the gate where we unloaded all passengers and crewmembers. There was still a slight odor of electrical burning in the cockpit after landing; but less than we experienced at altitude. There was never any indication or smell of smoke or fumes after of the cockpit door (nothing detected by flight attendants or passengers). Aircraft was turned over to maintenance and subsequent flight was cancelled.