I know for a fact- based off a teacher in school, the best time to go out, if you want clean air is 4am. It's cool, crisp, fresh, and it's a point where all the previous day's pollutants have had a chance over night to settle, and the day hasn't totally started for new one's to get kicked up. However... from your pictures, I want to wager a guess that you don't have nearly as much to worry about pollutant wise during the day as I would here in downtown San Diego.... hah.

Grats on biking again! I've kind of given up my late night video games for biking during the day, unconciously given then up, I still play but I pass out hours earlier, almost at a decent time some nights, just from being tired thanks to getting real exercise in during the day.