What month typically has the most days with Air Quality in the orange and red levels, and how many days would the average for those levels be?
Posted July 21, 2008
MIKE MOSS SAYS: Kevin, Using the verification statistics for Code Orange and Red air quality days in the Triangle provided by the NC Division of Air Quality, we find that July has the highest average combined number with 5. It's worth noting, though, that the averages for June and August are 4.4 and 3.8, respectively, so that the incidence of orange and red days is spread fairly evenly through the summer months, with notably lower averages for the cooler months. A couple of additional things to consider are that the number of Code Red days averages less than one in any given month, and also that in looking at yearly records, there is a great deal of variability in the timing and numbers of these days. For example, in 2006 there were only two days that even reached code orange, in 2004 only one, and in 1987 not a single exceedence day. On the other hand, in 1999 and 2002, there were 29 days, and in 1988 there were 37, including 3 Code Purples!
Our contact at NC DAQ also points out that there was a recent (beginning 2008) change in the National Ambient Air Quality Standards for 8-hour ozone concentrations that reduced the threshold concentration for code orange exceedences, and this could result in a near-doubling, on average, of the number of yearly exceedences due to ozone. Of course, this assumes that precursor emissions remain about the same, so the numbers may not double for long as the effects of new emission reduction/control requirements and replacement of older vehicles with newer, cleaner ones results in lower average ozone concentrations. In the meantime, it might be reasonable to expect the average number of combined orange and red days in mid-summer to run closer to 10 for few years as opposed to the previous value of 5 mentioned above.