The air was clear all day the photo on the left was taken (Nov 2009). The air was increasingly hazy on the day the haze-filled air was photographed (also Nov 2009). Since strenuous hiking is an aerobic-like activity that induces deep breathing, I have curtailed hiking and other outdoor activities when airborne particulates are abundant. On warm days, the particulates can participate in chemical reactions that generate ozone. Similar photos present smog in Rocky (1; 2004) and more recently in Minnesota (2; 2009).

1. Rocky Mountain high — if you like smog
http://www.msnbc.msn.com/id/4398123/

2. Clearing the haze bit by bit [in Minnesota]
http://tinyurl.com/yfrgez7 [Star Tribune]