# Indirectly Measuring Diffusion of NO2 into Water

I am working on a data science project with the central hypothesis is that air-born pollutants can become water-born through diffusion. The example I am using as a proof of concept is $$\ce{NO2}$$. The problem I am having is that I do not understand the chemistry of how it interacts with $$\ce{H2O}$$ outside a laboratory setting that well.

$$\ce{NO2}$$ reacts with $$\ce{H2O}$$ to form $$\ce{HNO3}$$, $$\ce{HNO2}$$, $$\ce{H+}$$, and $$\ce{NO3-}$$. In addition, $$\ce{HNO2}$$ also reacts with water to form $$\ce{NO}$$, $$\ce{H+}$$, and $$\ce{NO3-}$$.

Do these reaction happen so fast that measuring dissolved $$\ce{NO2}$$ or $$\ce{HNO2}$$ in a sample an unreliable measurement?

If so, what is the best measurement I can use? $$\ce{H+}$$ and $$\ce{NO3-}$$ are both part of a lot of other very complicated biological pathways and have even more possible sources, so there is no way they can be relied upon. That leaves $$\ce{HNO3}$$, $$\ce{HNO2}$$, or $$\ce{NO}$$ as possibilities. Which of those would be the best to use as an indirect measurement?

Thanks for any help you can offer in advance. I am decent enough at chemistry in a lab, but application to real world systems is way beyond my abilities.

NO2 is a free stable radical, better express as $$\ce{.NO2}$$, and being a radical, does not directly react with H2O, or H+ or OH-.

The reaction mechanism likely proceed via a self-reaction creating nitrogen tetroxide:

$$\ce{.NO2 + .NO2 <=> N2O4 }$$

In a diluting medium of air, depending on the concentration of nitrogen dioxide, this reaction is kinetically expected to be slow.

When it does occur, it can be followed by:

$$\ce{N2O4 + H2O -> HNO2 + HNO3 }$$

Source: See Reactions (9) and (10) on Page 28 of this paper.

Now, one can ignore the intermediate reaction mechanics, for simplicity, however, this does present a problem for some, as yourselves, as clearly the rate determining step is not even cited.

Also, if you review the paper, there are other reaction equilibriums that you may have to take into consideration. As a sidebar, what the paper does not mention is that aqueous nitrite is a photo-sensitive producing products, so you may wish to control the amount of lab light exposure as well.