# Assessment of Daytime HONO Emission Source from Asphal Surface to Urban Air
## 1 Introduction  
Photolysis of nitrous acid (HONO) is a primary daytime source of hydroxyl radicals (OH), which are dominant atmospheric oxidants and play a significant role in the formation of ozone and secondary organic aerosol (SOA) [1, 2]. The investigation of OH radical sources is crucial for understanding the tropospheric oxidation chemistry process. Therefore, many studies have investigated daytime HONO sources.  
Numerous recent studies measured daytime HONO concentrations in various urban sites around the world. The observed daytime levels were unusually high, from a few hundred ppt to a few ppb, especially in China [3, 4, 5, 6]. These results indicate that unknown dominant daytime sources of HONO, which quantitively correspond to or exceed the photolysis rate of HONO and other loss mechanisms, are yet to be defined. The NO\({}_{2}\) dependence of heterogeneous HONO production has been well studied along with various potential formation paths, such as the reaction of NO\({}_{2}\) with water (R1) or chemisorbed water via secondary heterogeneous conversion (R2), as well as excited NO\({}_{2}\)* from adsorbed HNO\({}_{3}\) photolysis (R3) [7].  
\[2 NO_{2}+H_{2}O\to HONO+HN\] (R1)\[2 NO_{2}+H_{2}O \xrightarrow[]{+hv,photocatalytic surface}HONO+HNO_{3}\] (R2) \[HNO_{3} \xrightarrow[]{h\gamma} NO_{2}^{*} HONO+OH\] (R3)  
In addition, several studies proposed HONO formation based on daytime HONO sources with large uncertainties as the mechanisms and parameters were not fully understood [2; 8; 9; 10; 11].  
Recent studies suggested that the additional potential sources for daytime HONO production in urban areas are ground-level heterogeneous hydrolysis reaction with nitrate (NO\({}_{3}\)-) on aerosols and building material surfaces, such as paint or glass with a thin layer of coated titanium dioxide (TiO\({}_{2}\)) [12; 13], in the presence of humidity and UV radiation [14]. The emission rate of HONO is relatively high, especially on the surfaces of materials used in building construction that contain TiO\({}_{2}\) particles. Other major ground-level sources reported in recent studies are HONO and NO emitted from natural soil or biological soil crusts, which are produced during nitrification and denitrification and vary with the soil pH, content of organic matter, and nutrients in the soil [15; 16]. Vertical gradient studies also suggested higher HONO concentrations at ground level than at higher altitudes [17; 18; 19; 20; 21].  
Forest canopy studies conducted in rural regions suggested a HONO production source at a higher altitude than soil emission. This source is namely the photochemical process that converts deposited HNO\({}_{3}\) into HONO and NO\({}_{x}\) through photolysis on the surface of tree leaves and depends considerably on the difference in acidity/pH of canopy surfaces [22]. Other daytime sources that have been reported are vehicular emissions [23], photosensitized reduction of NO\({}_{2}\) on humic acid surfaces [24], soot [25], and photolysis of nitrate and nitric acid (HNO\({}_{3}\)) [26; 27]. Previous studies reported that NO\({}_{2}\) uptake coefficients and the ambient aerosol surface areas for heterogeneous conversion of NO\({}_{2}\) were low enough to be excluded [28; 29; 30]. However, in one study conducted in Beijing [27], the average HONO production rate on aerosol was comparable to that of other sources, suggesting that aerosol significantly contributed to HONO production. Thus, HONO formation is still not fully understood.  
In this study, a continuous flow chamber system was constructed to quantify HONO production on an asphalt surface based on various concentrations of NO\({}_{2}\) and VOCs (toluene and hexane) in the presence of humidity and UV light. With the observed emission rates, the first-order conversion rate of NO\({}_{2}\) to HONO was estimated and compared with those of other studies. In addition, HONO contributions from the asphalt surface to the daytime summer concentrations in Seoul were determined.