## 3 Experimental
### Theoretical Filter Lifetime Capacity  
The proportion of active CaCO\({}_{3}\) (GCC) can be calculated from the total filter sample weight, which is the sum of the substrate plus coating. The substrate weighs 80 g\(\cdot\)m\({}^{-2}\). The coating (10 g\(\cdot\)m\({}^{-2}\) one-sided) makes up 11 w/w\(\%\) of the total sample weight of which carbonate (100/110) is 90 w/w\(\%\) of the coating and the sole carrier of the nitrogen species. Thus, 11 \(\times\) 90/100 = 9.9 w/w\(\%\) (approximately 10 w/w\(\%\)) of the sample is active CaCO\({}_{3}\) in the process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Samples** & **Mean \(\pm\) SD** &
\begin{tabular}{c} **Median** \\ **(Min–Max)** \\ \end{tabular} & **Skewness** & **Kurtosis** \\ \hline Filter box with H\({}_{2}\)O\({}_{2}\) & 11.6 \(\pm\) 4.1 & 10.7 & 1.710 & 5.249 \\ pre-oxidised samples & & (5.3–29.8) & & \\ \hline Filter box with non-sterilised & & & \\ samples displaying NO\({}_{2}\)\(\rightarrow\) NO conversion & 17.1 \(\pm\) 4.2 & 16.2 & 2.079 & 7.148 \\ \hline Ambient outdoor NO\({}_{x}\)\({}^{*}\) & 36.3 \(\pm\) 22.6 & 32.6 & 1.733 & 4.021 \\ \hline \hline \end{tabular} \({}^{*}\) Concentrations before and after filtering. SD: standard deviation.  
\end{table} 
Table 12: Descriptive statistics of NO\({}_{x}\) concentration (\(\mu\)g\(\cdot\)m\({}^{-3}\)) for filtered and unfiltered ambient outdoor air. Figure 27: Filter efficiency comparison between H\({}_{2}\)O\({}_{2}\) pre-oxidised (green line) and non-sterilised (NO\({}_{2}\)\(\rightarrow\) NO conversion, red line) samples mounted in the smaller filter box. Using the flow rate of 0.607 dm\({}^{3}\cdot\)min\({}^{-1}\) ambient air (Figure 21) with a time average NO\({}_{2}\) concentration taken over 6 days from 8 to 13 October 2019 at the Faculty of Technology and Metallurgy, University of Belgrade, (FTM) sampling point (Figure 18) equal to the annually averaged concentration for sampling point Mostar (SEPA) [30] of approximately 44.1 ug\(\cdot\)m\({}^{-3}\). This translates to a gas flow rate of 0.044 ug\(\cdot\)dm\({}^{-3}\times\) 0.607 dm\({}^{3}\cdot\)min\({}^{-1}\) = 0.027 ug\(\cdot\)min\({}^{-1}\) of NO\({}_{2}\). The box filter specification described has an active CaCO\({}_{3}\) content of approximately 10 w/w% of the filter elements (from above) and typically contains approximately 174.3 \(\times\) 0.1 g and approximately 17.4 g of CaCO\({}_{3}\). According to the stoichiometric reaction in Equation (1), 1 mol CaCO\({}_{3}\) + 2 mol NO\({}_{2}\) reacts to 1 mol CaCO(NO\({}_{3}\))\({}_{2}\). Thus, 17.4 g/100.0869 g\(\cdot\)mol\({}^{-1}\) (\(M_{\rm w}\) CaCO\({}_{3}\)), approximately 0.174 mol CaCO\({}_{3}\), can sorb 0.348 mol NO\({}_{2}\), which, at a flow rate of \(0.027\times 10^{-6}\) g\(\cdot\)min\({}^{-1}\)/46.0055 g\(\cdot\)mol\({}^{-1}\) (\(M_{\rm w}\) of NO\({}_{2}\)), approximately 0.587 \(\times\) 10\({}^{-9}\) mol\(\cdot\)min\({}^{-1}\), means the filter will last an extraordinarily long time under these conditions, i.e., (0.348/(0.587 \(\times\) 10\({}^{-9}\)) = 593 \(\times\) 10\({}^{6}\) min = 9.88 \(\times\) 10\({}^{6}\) h = 9.88 \(\times\) 10\({}^{6}\)/(8.76 \(\times\) 10\({}^{3}\)), which equates to approximately 1,130 years). Since the concentration of NO\({}_{2}\) in the outlet was effectively zero, this shows that there was enormous capacity at this flow rate and low concentration. Per unit weight of total filter, the consumption rate would be 1,130 years/17.4 g, which is \(>\) 64.94 years\(\cdot\)g\({}^{-1}\) CaCO\({}_{3}\). Realistically, the need to remove NO\({}_{2}\) from a larger physical street side volume would bring these figures into practical perspective. Filtering a 10 m length of a large street would require an air exchange of over 10 m \(\times\) 10 m \(\times\) 10 m \(\times\) 10 m = 1 000 m\({}^{3}\) at an estimated exchange rate of 100 m\({}^{3}\cdot\)min\({}^{-1}\). Therefore, the road-size filter lifetime, \(LT_{\rm filter\ road}\), would be approximately related to the experimental filter lifetime, \(LT_{\rm experimental}\), by Equation (6). \[