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What is an $\ce{NO_x}$ level? I have read some online - but I'm confused as what is used to calculate $\ce{NO_x}$. Reported $\ce{NO_x}$ levels by the government (where I live - Canada) have an $\ce{NO_x}$ level with the attached "On an $\ce{NO2}$ mass basis" What does that mean? I said $\ce{NO_x}$ doesn't exist.

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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.

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  • $\begingroup$ If eighteen hundred tons of NOx was released - does that mean nine hundred was NO2 ? ( I understand they add the sum together ) do you "not" add all the other combination possible ( N2O etc etc ) ? Is the NO2 level the most important ? $\endgroup$ – Sam Feb 8 '17 at 1:39
  • $\begingroup$ NOx is the sum of the two mono-nitrogen oxides $\ce{NO}$ and $\ce{NO2}$ only, ie. not $\ce{N2O}$, etc. Regarding your first sentence, no, that's not quire right. If eighteen hundred tons of NOx were released, there could technically be any amount of $\ce{NO2}$ released up to eighteen hundred pounds, with NO making up the rest of it so that the total equals eighteen hundred pounds. Let me know if that doesn't make sense :) $\endgroup$ – airhuff Feb 8 '17 at 3:43
  • $\begingroup$ All of this is great btw / thank-you . Is NOx a science ? Or .... is it a generic term that represents ( what ever proportion may be ) levels . If this is true , do we know what the actual NO and NO2 levels are ? Is NOx only relevant in climate change ? NO2 is poison right ..... ? ( I'm being straight up - just a working class guy with a question ) Thank you so much $\endgroup$ – Sam Feb 9 '17 at 6:06
  • $\begingroup$ @Sam, great that this is making some sense. Atmospheric NOx is most important in it's roll in ground-level ozone production (that's bad, not like high in the stratosphere where we need an ozone layer). Our air is mostly made of nitrogen and oxygen, but under "normal" conditions they don't react to form NOx. Under the heat of an internal combustion engine, the do. The reason for lumping the 2 together is that they are part of a cycle; during daytime, NO2 is photochemically converted to NO producing ozone along the way. At night, the ozone converts the NO2 back to NO again. I hope that helps ;) $\endgroup$ – airhuff Feb 9 '17 at 6:22

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