# Assessment of Daytime HONO Emission Source from Asphal Surface to Urban Air
## 4 Conclusions  
Photolytic HONO formations on an asphalt surface were tested along with the factors influencing the formation by using a continuous flow chamber system. The contributions of these factors to ambient daytime HONO concentrations observed during the summers of 2016 and 2019 in Seoul were assessed. The daytime means of observed HONO were 0.60 and 0.45 ppb at two different locations in Seoul. These values are comparable to other urban conditions and strongly suggest that some common daytime HONO sources exceed the rapid daytime HONO sinks. HONO production was evaluated using three factors on the asphalt surface within an experiment chamber: UV intensity, NO\({}_{2}\) concentration, and VOCs (toluene and hexane) concentration. HONO was not generated on the asphalt surface if any of these parameters were low, which confirms that these are all critical parameters in asphalt surface reactions. Although no HONO surface formation was found in the no-light condition, significant HONO formation of 6 \(\times\) 10\({}^{8}\) molecules cm\({}^{-2}\) s\({}^{-1}\) was observed even under very low radiation of UV (0.7 W m\({}^{-2}\)). This shows that the asphalt surface may be an important source of HONO in the early morning and late afternoon when other photoreactive sources are not readily available. We found that HONO formation on the asphalt surface was linearly dependent on NO\({}_{2}\). Moreover, we calculated the first-order conversion rate of NO\({}_{2}\) to HONO for 1.2 \(\times\) 10\({}^{-4}\) s\({}^{-1}\), which is two orders smaller than those of a TiO\({}_{2}\)-doped surface. No HONO formation on the asphalt surface was observed with humidified air and high-NO\({}_{2}\) conditions, unlike for other photoreactive minerals such as TiO\({}_{2}\). Saturated HONO production at high hydrocarbon concentrations and high light intensities was another distinct characteristic of the asphalt surface reaction. These results suggested that the HONO production mechanism on the asphalt surface was similar with that on the humic acid surface. However, the low oxygen content and hydrophobic nature of the asphalt surface were less efficient than humic acid surfaces in producing HONO, and substantial addition of hydrocarbons was needed to produce HONO, unlike in the case of humic acid. The observed HONO emission flux of 1.3 \(\times\) 10\({}^{9}\) molecules cm\({}^{-2}\) s\({}^{-1}\) under typical on-road conditions was much smaller than that of highly photoreactive mineral and organic surfaces, but it was in a similar range to that of other urban covered surfaces such as glass and soils. This implies that HONO production with heterogeneous photolytic NO\({}_{2}\) conversion from asphalt surfaces is a likely important source of daytime HONO in most urban areas. Numerically simulated vertical HONO profiles with obtained asphalt emission sources revealed that the asphalt surface may account for 13% of daytime HONO in the typical roadside precursor concentrations in Seoul. However, this number could be higher, given that NO\({}_{2}\) and hydrocarbon concentrations on the road asphalt surface are larger as they are directly exposed to emissions from vehicle exhaust and tailpipes. Overall, these results indicate a potentially significant daytime source of HONO in urban areas and propose a need for further analysis of the formation mechanism and its contributions to daytime HONO in real traffic conditions. Conceptualization and methodology, G.L., M.L. and J.Y.A. ; formal analysis, D.K. and J.K.; investigation, D.K., J.K., G.L. ; resources, G.L. and J.Y.A. ; data curation, D.K., J.K.; writing--original draft preparation, D.K., J.K. and G.L. ; writing--review and editing, M.L. ; visualization, D.K., J.K., G.L. All authors have read and agreed to the published version of the manuscript. This research was funded by National Research Foundation of Korea (NRF), grant number 2018R1A2B6005090. Not applicable. The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest.