What is an NOx level
The atmospheric $\ce{NO_x}$ (usually pronounced as "nox" or "knocks") concentration is the sum of the $\ce{NO}$ and $\ce{NO2}$ species.  These get combined into a single term as they each play a roll in photochemical ozone formation. $\ce{NO_x}$ can form as a byproduct of the combustion of fuels containing nitrogen, like coal, or from atmospheric nitrogen in the high-temperature environment of an internal combustion engine.

Because photochemical ozone production requires $\ce{NO_x}$, volatile organic carbon (VOC) and sunlight, the $\ce{NO_x}$ concentration is really most relevant for environments already rich in VOC that get plenty sunlight.  Modern automobiles have emission systems that have significantly reduced VOC emissions.   

Depending on just where you live in Canada, $\ce{NO_x}$ levels are likely very important for photochemical ozone production during the summer, when you have plenty of natural VOC's available from local vegetation (be it forest, tundra, or any other of your rich and diverse ecosystems).  

During the wintertime in Canada, or summer in places like the desert Southwest US, the atmospheric VOC concentrations are small.  In these cases it is the VOC concentration rather than that of $\ce{NO_x}$ that limits ozone production.  Of course the deserts also get plenty of the other ingredient, sunlight. 

Depending on your latitude, there may be enough sunlight to produce some ozone, and due to temperature inversions the lower rate of ozone production can still lead to the buildup of ozone; so long as you have some $\ce{NO_x}$ and VOC's present.