Reacting NaOH in solution with O3 I use common air to produce ozone. However the gas after the ozone generator does contain $\ce{NO2}$ gas, which I have to remove for the subsequent steps. I tried to remove the $\ce{NO2}$ with a stainless steel gas wash bottle filled with distilled water. This works fine, however the distilled water will subsequently get charged with $\ce{NO2}$ and turn to dilute nitric acid.

After a few days of operation, I have changed the water and tested its acidity. The $\mathrm{pH}$ was around 0.4 - it was possible to etch a copper pipe with it. So the formation of this acid is not very good for the stainless steel gas wash bottle. I have decided to add 2 more stainless steel gas wash bottles and fill the first one with $\ce{H2O + NaOH}$, in order to neutralize the $\ce{HNO3}$. The other two bottles are for capturing the remaining traces of $\ce{NO2}$.

Bottle 1: 5 liter $\ce{H2O}$ + $\pu{15g}$ $\ce{NaOH}$
Bottle 2: 5 liter $\ce{H2O}$
Bottle 3: 5 liter $\ce{H2O}$

Now I have done some ozone measurements with and without these gas wash bottles. The result was as follows:


It seemed as if $\ce{O3}$ would pass through pure $\ce{H2O}$ without any reaction once the solution is saturated and the saturation seems to happen almost immediately.
However the solution with the $\ce{NaOH}$ seems to react the first 2-3 minutes and then suddenly stop reacting, which is actually good, because it does not use up ozone, however I have no clue what is happening in detail.




Here are some charts from the measurement. Upper chart: test of the $\ce{H2O + NaOH}$ solution. Lower chart: test of the $\ce{H2O}$. Note: the lower the value in the chart, the higher the ozone concentration.

The duration of each test was about 5 minutes. I did measure the $\mathrm{pH}$ of the $\ce{NaOH}$ containing solution and it basically did not change due to the ozone treatment. (It was around 12.35 before and after ozone bubbling...)



Any idea what is happening there and why?
You could oxidize some hydroxide ions, to wit:

$\ce{OH^- + O3 -&gt; HO2^- + O2}$

Such an oxygen transfer is favored by strongly basic solutions because $\ce{HO2^-}$ is a weaker base ($\mathrm{p}K_\mathrm{a}$ hydrogen peroxide = 11.75) than $\ce{OH^-}$, as well as ozone being a more powerful oxidizer than diatomic oxygen.