Simonich's measurements confirm that the air, although better, is still heavily polluted by U.S. standards.
On average, she said, she's been measuring particulate levels about six times higher than those seen in the 1996 Atlanta Olympics.
A good day in Beijing, she said, is roughly comparable to America's most polluted cities.
The Chinese were hoping for more than the improvements seen in Simonich's tests, but their efforts have been undercut by the smog-trapping climate.
"I think they've done as much as they can," Simonich said. "But the role that meteorology plays is so important."
The same weather that traps smog can make Beijing extremely hot and humid.
Goucher and Yoder Begley prepared for their competitions by training for a week in Houston, along with fellow distance runner Galen Rupp.
"The workouts went great," the group's coach, Alberto Salazar, told the Portland Oregonian newspaper. "We ran in hotter conditions than we expect to see in Beijing."
But that doesn't mean the heat won't be a major factor for less-prepared competitors.
"The air is so thick," Yoder Begley wrote on her blog. "You sweat through your clothes in minutes and stay wet all day."
The weather took a toll last weekend in the men's bicycling road race, a hilly 152-mile (245-kilometer) event in which 53 of 143 competitors dropped out.
"I worked hard, but the heat and humidity were too much for me," Dutch cyclist Karsten Kroon told the Salt Lake Tribune. "You feel your head explode."
Mari Holden, a 2000 silver-medalist-turned-coach, sympathized.
"I always had a hard time with heat and humidity," she told National Geographic News. "I've had instances where I ended up in the hospital."
Whatever the conditions, however, athletes say they are still driven to do their best.
As distance runner Shalane Flanagan told the Associated Press: "Unless I can't walk and my lungs are falling out, or I'm coughing up a lung, I'll be running."