# Assessment of Daytime HONO Emission Source from Asphal Surface to Urban Air
## 3 Results
### VOCs Dependency  
We measured the HONO production rate changes with varying toluene and hexane concentrations to test VOCs' role in the HONO production (Figure 4). Without toluene and hexane addition, no detectable amount of HONO production on the asphalt surface was observed in the chamber reaction with humidified air of 100 ppbv NO\({}_{2}\) under typical daylight conditions. If the HONO production mechanism on the asphalt surface is same as that on humic acid as reaction (R4), HONO can be produced only by NO\({}_{2}\) with light. Adding 100 ppbv of VOCs under the same NO\({}_{2}\) and light conditions, HONO production became clearly efficient at a rate of 5.2 \(\times\) 10\({}^{8}\) molecules cm\({}^{-2}\) s\({}^{-1}\). HONO production rapidly increased with toluene and hexane concentrations up to 500 ppbv and then became saturated at higher concentrations. Heterogeneous HONO production on photocatalytic mineral surfaces such as TiO\({}_{2}\) and humic acid is driven by a hydrogen source from water or humic acid itself. Thus, the observed toluene and hexane dependence of the HONO production rate in this study requires a different reaction path than reaction (R4) alone. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**\begin{tabular}{c} **Emission Rate** \\ **(Molecules** \\ **cm\({}^{-2}\) s\({}^{-1}\))** \\ \end{tabular} } & \multirow{2}{*}{**NO\({}_{2}\) (ppb)**} & \multirow{2}{*}{**RH (\%)**} & \multirow{2}{*}{**\
\begin{tabular}{c} **Irradiance** \\ **(W m\({}^{-2}\))** \\ \end{tabular} } & \multirow{2}{*}{**Wavelength** (nm)**} & \multirow{2}{*}{**RF**} \\  & & & & & & & \\ \hline \multirow{2}{*}{Paint} & White wall paint & \(1.3\times 10^{10}\) & 50 & 50 & 10.6 & 300–400 & [35] \\  & Photocatalytic paint & & & & & & \\  & (3.5\% TiO\({}_{2}\) & \(2.1\times 10^{10}\) & 40 & 40 & 8.5 & 340–400 & [36] \\  & nanoparticles) & & & & & & \\ Glass & Clean & \(1.3\times 10^{9}\) & 50 & 50 & 10.6 & 300–400 & [35] \\  & Urban gprime & \(1.5\times 10^{10}\) & 46 & 90 & 8.0 & 300–400 & [37] \\ Aerosol & & \(2.5\times 10^{9}\) & & 60 & & 400 \(<\lambda\) & [27] \\ Soil & & \(2.2\times 10^{11}\) & & & & & [37] \\  & & \(1.3\times 10^{8}\) & & & & & [38] \\  & & \(1.5\times 10^{9}\) & & & & & [39] \\ Forest & & \(2.4\times 10^{9}\) & & & & & [39] \\ canopy & & \(6.2\times 10^{9}\) & & & & & [40] \\  & & \(6.0\times 10^{8}\) & & & & & [18] \\  & & \(6.7\times 10^{9}\) & & & & & [26] \\  & & \(3.3\times 10^{9}\) & & & & & [20] \\ Asphalt & & 0.5–3.1\(\times 10^{9}\) & 50–300 & 50 & 11.3 & 295–385 &
\begin{tabular}{c} This study \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of HONO emission rate on surfaces of various materials. The asphalt surfaces used in this experiment are highly complex mixtures of organics and inorganics. In particular, the organic composition, mainly from petroleum in the sedimentary source, is not well characterized owing to many different geographical origins and refining processes. As asphalt from crude oil is a derivative of humic acids, asphalt has structural similarities with humic acids having numerous aromatic rings. In contrast to universal polycyclic structures, humic acids contain higher contents of oxidized and polar branches, such as alcohols and carbonates compared to asphalt. For this reason, humic acids are extremely hydrophilic, but the asphalt surface is very hydrophobic. The hydrophobic nature of the asphalt surface is likely to abate the water uptake to its surface, which may cause a much lower rate of HONO formation compared to the humic acid surface under humidified air conditions. However, VOCs and its oxidized derivatives can be easily absorbed to the asphalt surface and they can be an effective hydrogen source on the asphalt surface. In addition, it was reported that NO\({}_{2}\) conversion on humic acid surfaces was not proportional to the high light intensities [28] and even saturated in the light of early morning hours [39].