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ALEX CHADWICK, host:

Back now with Day to Day. In a moment, how does he go so fast in the water? Another gold for Michael Phelps. First, the air in Beijing, never mind the water.

Ken Rahn is professor emeritus of atmosphere chemistry at the University of Rhode Island, and he's also a senior research scholar at Tsinghua University in Beijing. He's online monitoring the data from Chinese air-quality stations all over Beijing and reporting results to us over the last week or so. Ken, welcome back to the program.

Dr. KENNETH RAHN (Atmospheric Chemistry Expert): Hi Alex, nice to be with you again.

CHADWICK: So the last we talked before the opening ceremonies, the air pollution index was in the mid-80s, not healthy by our standards but OK by Chinese standards. What's the number today? How do things look?

Dr. RAHN: Well, it plunged a couple days after the opening ceremony, and we've had - we're now in the low 30s in Beijing, which means everybody can relax and breathe good air. That's because of some rain and clouds and wind that has shifted away from the south into the north and northeast and so on.

CHADWICK: I saw, watching the events over the last couple of days, torrential rains, huge rains just pouring down.

Dr. RAHN: Yeah, they had some, and they're going to stay cloudy for another couple of days, with on and off rain, and then the system is going to start to change Thursday or Friday or so. It'll be sunny and bright, and that will bring back the south winds, and the air pollution will start to climb, probably, over the weekend. But for this week, everybody in Beijing can relax and have a good time.`

CHADWICK: One thing that you told me, it really matters what time of day you're competing.

Dr. RAHN: Oh yeah. Air pollution goes through very distinct cycles during the day, and each pollutant has its own cycle. Ozone, for example, peaks in the early to mid-afternoon, and particulate matter peaks briefly at morning rush hour and then goes through a minimum and has a bigger peak in the late afternoon and early evening.

CHADWICK: So if you want the best performance, the best air, when should you be competing?

Dr. RAHN: That's hard to say because you fix it for one, and you unfix it for the other, so to speak. Probably in the morning, in the relatively early morning. By that, I mean like nine,10,11,12 is overall the best time, I would say. Ozone hasn't had a chance to build up, and the particulate matter is declining from its rush hour mini-maximum. Around two to four in the afternoon, particulate matter is the lowest, but ozone is starting to climb. So there's that little window just around noon or late, late morning.

CHADWICK: Ken Rahn, professor emeritus of atmospheric chemistry at the University of Rhode Island and a senior research scholar at Tsinghua University in Beijing. Ken, thanks again.

Dr. RAHN: Always a pleasure, Alex.

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