## 3 Experimental
### Filter Development, Test Methods, and Results
#### 3.1.4 Effect of Moisture  
To test the function of the coated substrate under forced through-flow conditions, small cut samples were placed in a clip-together design plastic cassette (Model CU37045-37 mm, Environmental Express, Mt. Pleasant, SC, USA) usually used for sampling dusts, fumes, and mists. However, the membrane was exchanged for an experimental active NO\({}_{x}\) absorbing filter. Such filter systems come with piping kits, and the set-up used to deliver synthetic NO\({}_{2}\) from a gas bottle under pressure-controlled flow rate is shown in Figure 16. Using this method, it was possible to investigate the gross impact of moisture, and, thus, test whether the humectant action of MNFC alone was sufficient to promote the salt conversion in the coating, or whether higher moisture levels were required. To do this, one sample was wetted using an aerosol spray to a level of approximately 50 w/w% deionised water. After exposure for 2 h at a volume flow rate of \(v\) = 200 cm\({}^{3}\cdot\)min\({}^{-1}\), the samples underwent extraction and analysis by ion chromatography.  
Ideally, and a subject for future work, would be to use a moisture-controlled chamber environment to pre-condition the inlet gas. Additionally, a full knowledge of moisture held in the coating would be of value in designing coating formulations and the humectant level. A rough estimate of moisture  
Figure 16: Filter samples mounted in cassettes to enable comparison of moisture content. Flow conditions and gas concentration data are also shown.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **NO\({}_{2}\)** \\ **(mg\(\cdot\)dm\({}^{-3}\))** \\ \end{tabular} & \begin{tabular}{c} **NO\({}_{3}\)** \\ **(mg\(\cdot\)dm\({}^{-3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **NO\({}_{2}\)** \\ **(mg\(\cdot\)kg\({}^{-1}\))** \\ **Dry Material** \\ \end{tabular} &
\begin{tabular}{c} **NO\({}_{3}\)** \\ **(mg\(\cdot\)kg\({}^{-1}\))** \\ **Dry Material** \\ \end{tabular} \\ \hline CaCO\({}_{3}\) exposed & 1.2 & 2.3 & 6.9 & 13.9 \\ \hline HAP (on CaCO\({}_{3}\)) exposed & 1.6 & 2.3 & 9.7 & 13.8 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Sorbed NO\({}_{x}\) gas results with respect to nitrite and nitrate species for the standard GCC CaCO\({}_{3}\) sample and the hydroxyapatite surface converted CaCO\({}_{3}\)content in the coating layer held within the MNFC humectant binder can be considered from prior art. For example, the review of [PERSON] et al. [28] offers a value of \(\leq\) 9 w/w% H\({}_{2}\)O based on nanofibrillar cellulose under the conditions of 60% RH at 20 \({}^{\circ}\)C. In this work, however, the important factor is that wet versus either ambient air moisture or moisture retention in the MNFC component under dry gas exposure results in a different balance of nitrite to nitrate salts.  
In a perfectly balanced system, an inlet flow rate of 200 cm\({}^{3}\cdot\)min\({}^{-1}\) would be expected to split equally between the two experimental samples. However, this is unlikely in practice. Because the flow resistance in each separate cassette is unknown, it is impossible to calculate the relative gas volume rate exposures. Nonetheless, the experiment provided a noteworthy result (Table 9) in that high-water content promoted the formation of nitrite with no extra nitrate formed when compared to the dry starting blank material, which was itself exposed since production to ambient storage air. This finding confirms that the use of humectant was superior to a separate moisture supply and suggests that analysis of nitrite could have been detecting dissolved NO\({}_{2}\) rather than the conversion of salt to nitrite.