U.S. Air Quality

Ozone Climatology in the Philadelphia Air Quality Forecast Area: the NOx SIP Controls are Making a Difference

My esteemed M.S. advisor, Bill Ryan, writes a medium-range air quality outlook for the Mid-Atlantic region every day during the summer ozone season. Today he included an interesting update on ozone climatology in the Philadelphia forecast area, which basically encompasses the region shown in the image below from Google Earth, overlaid with the current AQI values (it’s a Good air quality day today).

Bill writes that in the Philadelphia forecast area, the 8-hour O3 standard (75 ppb) has been exceeded only once so far this season: 76 ppb on April 25. This is very unusual – since the NOx SIP controls were phased in beginning in 2003, Philadelphia has had an average of 10 exceedence days (defined as Code Orange or Red ozone) by mid-June. Prior to 2003, the number of exceedences was higher, with an average of 13 days for the period 1993-2002. The bottom line is that the NOx SIP controls seem to be making a difference, which is great news for the residents of the Philadelphia area, although perhaps not as much fun for forecasters!