It is particularly noticeable in wintertime. The disagreeable stuff, which can hang over the cities like a dome, is caused by automobile exhausts. PAN affects people by irritating exposed skin and causing eyes to water. PAN measurements were first made in Calgary from a downtown monitoring station 10 metres above ground level. There is a one or two hour delay between the time traffic volume is at its peak and the greatest concentrations of PAN are measured because the photochemical reaction takes place over time.
Maximum concentrations occur around noon hour.
The highest concentrations measured in either city were seven parts per billion which may not seem like much, but some people are sensitive to PAN at concentrations of about 10 parts per billion. Nevertheless, Peake noted that most people don't notice any effect until concentrations reach 50 parts per billion. In the winter, we get temperature inversions that trap automobile emissions, causing pollutant concentrations to rise. But, because temperatures are low and we don't have much sunlight, PAN concentrations are not much different than in summer. The difference in summer is we seldom get inversions even though the temperatures are higher and there's more sunlight.