Why does NO have a residence time of 4 days?

If $\ce{NO}$ is unstable, why does it have an atmospheric lifetime of 4 days? I know that it has a lone pair and one unpaired electron making it highly reactive. Therefore, shouldn't it react immediately in the atmosphere?

• Before you ask "why", ask "if". – Ivan Neretin Jun 6 '17 at 19:27
• Four days seems long, and there are other variables involved, mainly ozone concentration as NO + O3 is a major sink process for NO. Could you give a source for your stated value? Thx. – airhuff Jun 6 '17 at 21:13

It could be possible that you're thinking of the overall lifetime of $\ce{NO_x}$ ($\ce{NO2 + NO}$), which indeed does have a longer atmospheric lifetime than $\ce{NO}$ individually and which even, at around 5 km altitude, has a lifetime of ~4 days. The two interconvert during the day due to photo-chemistry, and the ratio of the two is dependent on the photolysis rate of $\ce{NO2}$ ($j_{\ce{NO2}}$) and on the rate coefficient of the reaction between $\ce{NO}$ and $\ce{O3}$.
As $j_{\ce{NO2}}$ is constant with altitude, the ratio between $\ce{NO}$ and $\ce{NO2}$ is dependent upon the reaction rate coefficient between $\ce{NO}$ and $\ce{O3}$. This, in turn, is governed by the temperature, and under lower temperatures, the reaction rate is slower. Combined with a decrease in the number concentration of ozone (due to the decrease in the number concentration of air with increasing altitude, if we assume a constant mixing ratio of ozone throughout the troposphere, of course), the ratio favours $\ce{NO}$ over $\ce{NO2}$.
At higher altitudes, then, you do see a situation where there is more $\ce{NO}$ than $\ce{NO2}$, and where the overall lifetime of $\ce{NO_x}$ can be higher than the 1 to 2 days found at the surface (reaching, sometimes, almost 12 days in the upper troposphere).